Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
328 AM EDT Fri Apr 26 2024

Valid 12Z Fri Apr 26 2024 - 12Z Sun Apr 28 2024

...An active weather pattern for much of the Plains into the Mid to Upper
Mississippi Valley regions...

...Thunderstorms to bring heavy rain, localized Flash Flooding and Severe
Weather to portions of the Plains...

...An elevated to Critical Fire Weather threat across the Southern High
Plains...

..Cooler than average temperatures expected from the Rockies to the West
Coast and from the Mid-Atlantic to the Northeast, while much above average
temperatures spread from the Southern Plains to the Great Lakes...


A dynamic weather system is poised to bring significant weather impacts
across the Plains extending into the Mid-to-Upper Mississippi Valley
regions this weekend. This system will usher in a variety of weather
phenomena including thunderstorms, heavy rainfall, and even severe weather
conditions. Thunderstorm activity is forecasted to intensify across
portions of the Plains, particularly in areas extending from the Southern
Plains to the Great Lakes. These storms are likely to bring heavy
rainfall, increasing the risk of localized flash flooding in low-lying and
urban areas. There's a Slight Risk (at least 15%) of Excessive Rainfall
leading to Flash Flooding over portions of eastern Oklahoma and
northeastern Texas into western Arkansas and southern Missouri today.
Another system emerging from the Rockies will bring the focus for heavy
rainfall and severe storms back over the Southern Plains on Saturday.
There's a Moderate Risk (at least 40%) of Excessive Rainfall over central
to northeastern Oklahoma on Saturday. Furthermore, some of these storms
may turn severe, with the potential for damaging winds, large hail, and
isolated tornadoes. To that end, the Storm Prediction Center issued an
Enhanced Risk (level 3/5) of Severe Thunderstorms over portions of eastern
Nebraska, northeastern Kansas, northwestern Missouri and southwestern Iowa
today Another Enhanced Risk area was issued across parts of southern Iowa,
eastern/central Kansas, southeastern Nebraska, northwestern Missouri,
central Oklahoma and north-central Texas for Saturday as another system
spawns another round of storms for the Central U.S..

While thunderstorms pose a threat to some areas, the Southern High Plains
face an Elevated to Critical Fire Weather threat through this weekend. Dry
and gusty conditions, coupled with low relative humidity levels, will
create favorable conditions for the rapid spread of wildfires. Outdoor
burning is strongly discouraged, and residents should exercise extreme
caution to prevent the ignition of fires. A cooler air mass will settle
over regions stretching from the Rockies to the West Coast and from the
Mid-Atlantic to the Northeast. Temperatures are forecasted to remain below
average for this time of year. Conversely, a swath of much above average
temperatures is expected to encompass areas from the Southern Plains to
the Great Lakes. Daytime highs will soar, potentially reaching
unseasonably warm levels. Meanwhile, unsettled weather, including
rain/snow showers and scattered to isolated thunderstorms are expected to
spread across much of the West through Saturday.


Kebede


Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Fri Apr 26 2024

Valid 12Z Mon Apr 29 2024 - 12Z Fri May 03 2024

...Potentially severe thunderstorms and heavy rain causing flash
flooding are forecast to continue in the Lower Mississippi Valley
Monday...


...Overview...

Upper troughing will support a surface frontal system with
widespread thunderstorms ahead of it across the central U.S. into
Monday. Heavy rain is likely to cause flash flooding in the Lower
Mississippi Valley in particular, while severe weather may also be
a threat. The upper pattern looks to become more zonal by Tuesday-
Wednesday, but uncertain shortwaves in the northern stream should
continue to push frontal systems across the Lower 48 with
additional showers and thunderstorms. More amplified troughing may
come into the West by late next week, ahead of which near normal to
warmer than normal temperatures will be the norm across much of
the country from the Intermountain West eastward.


...Guidance/Predictability Assessment...

Model guidance is reasonably agreeable at the start of the period
Monday, with compact upper and surface lows in the north-central
U.S. and upper ridging in the East while additional energy comes
into the Northwest. By Tuesday-Wednesday, though the overall flow
becomes more zonal, the northern stream jet shows some notable
differences in the timing of those potent shortwaves coming into
the northwestern to north-central U.S., leading to some out of
phase differences. There are generally two camps for these
features. The 12/18Z GFS and 12Z ECMWF, in a departure from their
previous runs, indicate some energy feeding faster into the
Northwest while some energy is held back over the Pacific. Most of
the ECMWF-based AI/machine learning models are generally similar to
this 12Z trend. However, some guidance like the CMC, UKMET, and
the AI FourCastNet stick with a solution like the older GFS/ECMWF
runs in holding most energy back and creating a deeper upper low in
the eastern Pacific. The 06Z GFS-based AI Graphcast was also in
this second camp, but the 12Z run flipped to more progressive
initial energy. Ensemble members vary somewhat but do generally
stick with their parent models more than preferred. The incoming
00Z CMC and UKMET still indicate more energy hanging back west than
the GFS/EC camp, but are not as deeply aggressive with the eastern
Pacific upper low as their 12Z runs. Overall, evidence was more
compelling to prefer the GFS/ECMWF type of solution, even though
this was a new trend. Models do still indicate troughing coming
into the West by Friday with ridging ahead of it in the central
U.S. and some troughing to the east of that, but with amplitude and
exact positioning dependent on earlier forecast problems. Thus the
WPC forecast was based on a blend heavily favoring the 12/18Z GFS
and 12Z ECMWF, but gradually reducing the deterministic runs in
favor of the GEFS and EC ensemble means especially by Day 7 given
increasing model spread.


...Weather/Hazards Highlights...

Ample moisture and instability will be present across much of the
central U.S. into Monday ahead of the upper trough and a surface
frontal system. The most likely focus for the heaviest convection
and rainfall is across the Lower Mississippi Valley, where a Slight
Risk is maintained in the Monday/Day 4 Excessive Rainfall Outlook.
This may be a higher-end Slight Risk if the heaviest model QPFs of
3-5 inches with rain rates over 2 inches per hour end up
verifying. By Tuesday the upper- level support is forecast to
weaken, so overall lower rainfall amounts and rain rates are
expected. However, some additional convection could develop, which
could lead to flash flooding concerns especially if storms develop
over areas that saw excessive rainfall amounts the previous day. A
Marginal Risk will serve as a starting point to cover this threat
on Tuesday/Day 5. There is also some potential for convection
farther north from generally the central Plains into the Mid-
Mississippi and Ohio Valleys. Model guidance is not agreeable with
where any heavier rainfall might occur and these elements may be
progressive anyway, precluding any ERO risk area there at this
point. By midweek and beyond, additional rounds of rain and
thunderstorms are likely for the central/southern Plains and
Mid-/Lower Mississippi Valley as moisture surges ahead of frontal
systems again.

Elsewhere, multiple shortwaves will lead to rounds of
precipitation across the northern tier. In the Northwest, some
higher elevation snow is possible with this activity, with the most
widespread coverage on Monday before snow levels may rise even
further. Mainly rain is expected farther east across the northern
Plains to Midwest/Great Lakes/Ohio Valley and into the
Northeast/Mid-Atlantic at times.

With the exception of the cooler West Coast and Northwest
especially Monday-Tuesday, the bulk of the lower 48 will have near
to warmer than average temperatures next week. An upper ridge over
the East Monday-Tuesday should help produce temperatures around
10-20F above normal, with temperatures in the 80s possibly as far
north as the Great Lakes region. Scattered daily record highs are
possible. Temperatures are likely to gradually moderate but stay
above average in the East until late next week. Meanwhile, parts of
the central U.S. will also see above normal temperatures with
anomalies around +10F Monday-Wednesday before cooling. The
Intermountain West to Rockies on the other hand should gradually
warm and see its greatest warm anomalies Thursday-Friday.


Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
359 AM EDT Fri Apr 26 2024

Valid 00Z Sat Apr 27 2024 - 00Z Sat May 04 2024

Breezy trades may hold through Friday with upper/surface highs
centered northeast of Hawaii, but these features weakening will
cause winds to weaken as well by Saturday. Meanwhile to the
northwest of the state, upper/surface lows will track slowly
eastward into this weekend and early next week. In response to
these features, winds are likely to veer a bit and may become
east-southeast. Winds are forecast to stay at weak to moderate
levels and perhaps become a hybrid trade and land/sea breeze
pattern. Minor moisture plumes may also be drawn into the state at
times, with the first Friday night into Saturday increasing
showers particularly over windward areas of the Big Island
according to the HREF. Generally windward showers will be the main
chance for precipitation, since the upper ridge east of Hawaii is
likely to block the western lows/associated frontal system and any
heavier precipitation from reaching even the northwestern islands
through early next week. What is left of the weakening trough may
move across Hawaii around midweek or so. By the later part of next
week, surface high pressure is forecast to move north of the state
and promote strengthening easterly trades.

Tate

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
735 PM EDT Thu Apr 25 2024
Day 1
Valid 01Z Fri Apr 26 2024 - 12Z Fri Apr 26 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
WESTERN HIGH PLAINS INTO THE MISSISSIPPI VALLEY...

01Z Update...
Expanded the northern end of the Slight Risk area into arts of
southwest Nebraska/northwest Kansas in response to continued
convective regeneration along axis of instability during the
evening hours. Refer to MPD 0172 issued for the area that is valid
until 26/0345Z for additional details. While the HRRR has not been
performing particularly well in the placement of the heaviest
rainfall this afternoon...it seems to be doing okay with the
convection along the warm front early this evening. The latest run
of the HRRR and the ARW suggest additional convection will be
developing along and ahead of the dryline over portions to Texas
that spreads eastward across parts of Oklahoma and Kansas later
tonight. Did not make any changes with the southern end of the
Slight Risk area because of the amount of soil saturation from
areas of heavy to excessive rainfall earlier in the day...and
additional rainfall should turn to run off more quickly in spots
that receive additional heavy rainfall tonight or where rainfall
rates of 1.5 to 2 inches per hour occur.

Bann


16Z update... The latest hi-res guidance is suggesting multiple
hours of rainfall rates up to 2 inches/hour to pass through
southeast Nebraska and northeast Kansas. 3-hr FFGs over this part
of the region are as low as 2 inches which given the new hi-res
trends, could reach or exceed soil saturation. The Slight Risk area
was expanded to the northeast to cover more of southeast Nebraska
and northeast Kansas. Meanwhile, latest observations and model
trends supported the small trimming of the backside of the Slight
Risk across portions of Oklahoma and central Kansas. The best
potential for the highest amounts and rainfall rates of 1.5-3
inches/hour remain across eastern Kansas to the borders of
Oklahoma/Arkansas/Missouri/Kansas.

Campbell

Convection will be ongoing at 12z this morning across portions of
the central and southern Plains. There remains some spread within
the 00z HREF guidance whether to favor a northern training axis
across portions of KS and MO, or a southern axis from northeast OK
into northwest AR. At this point it remains a bit unclear which
axis will become the dominant one, but both will likely see some
upscale convective development this morning within an area of
increasing moisture convergence in the 850-700mb layer. Storm
motions will favor some training and backbuilding, and thus some
flash flood risk will probably exist this morning from KS into
western MO, and from northeast OK into northwest AR.

Stronger mid and upper level forcing will eject east out of the
Rockies tonight, which will trigger additional convective
development over the Plains. Over KS and OK this convection should
be quite progressive off to the east. However high rainfall rates
are likely, and so any areas that see heavy rain from the first
round of convection this morning/afternoon could potentially see
areas of flash flooding tonight. Otherwise the activity should be
quick enough to pose only an isolated flash flood risk. There is a
better chance of a convective training tonight closer to the warm
front near the KS/NE border, which could drive an isolated to
scattered flash flood threat.

Overall will continue with a broad Slight risk that stretches from
near the NE/KS border southeast into northwest AR. This covers
both the expected convection this morning/afternoon, and the
development expected tonight with the arrival of stronger forcing
from the west.

Chenard

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Fri Apr 26 2024
Day 1
Valid 12Z Fri Apr 26 2024 - 12Z Sat Apr 27 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE SOUTHERN PLAINS INTO THE LOWER TO MIDDLE MISSISSIPPI VALLEY...

A Slight risk was maintained with this update from portions of
northeast TX into MO, although there remains some uncertainty
regarding the intensity and organization of this activity.
Convection will likely be ongoing at 12z this morning across
portions of TX and OK, with this activity gradually pushing
eastward across these two states and into portions of MO and AR
through the afternoon hours. With time today the better synoptic
forcing lifts to the north, resulting in a downward trend in
forcing across the southern half of the Slight risk, with
weakening lower level convergence and also less pronounced
mid/upper level ascent. These larger scale factors would generally
favor weaker and less organized convection with time today into
tonight over these southern areas. Countering this is a rather
favorable thermodynamic environment, with plentiful instability and
moisture. Thus it seems most likely that if convection this
morning over OK and TX can grow upscale enough to generate an
organized outflow/cold pool, then this mesoscale feature could be
enough to sustain an axis of more robust convection through today.
Any convection that is able to sustain and grow upscale may exhibit
some training and backbuilding characteristics into the strong low
level jet in place. More recent observational and HRRR trends
suggest portions of MO have the best chance of seeing multiple
convective rounds today with stronger upper forcing and WAA...with
one round this morning and another through the afternoon hours.
Thus we did expand the Slight risk into more of MO to account for
this risk.

Further north a Marginal risk is in place from northern MO into SD
and MN. Elevated showers and embedded thunderstorms will move
across this region this morning, but will be outrunning the better
instability...which should keep rainfall rates low enough to
prevent any more organized flood threat. As the surface low and
strong mid/upper forcing ejects into the Plains this afternoon
additional convective development is likely from eastern NE/SD into
portions of IA and MN. Some of this activity should be intense,
but generally should be moving at a quick enough pace to limit the
flash flood risk.

Chenard

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Fri Apr 26 2024
Day 2
Valid 12Z Sat Apr 27 2024 - 12Z Sun Apr 28 2024

...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
FAR NORTHERN TEXAS INTO OKLAHOMA AND FAR SOUTHEAST KANSAS...

A significant rainfall event still appears likely Saturday night
across portions of north central TX into central/eastern OK.
Overall not much has changed over the last couple model cycles,
and thus much of last nights discussion still holds true. Large
scale forcing really ramps up Saturday night with the approach of
the mid level trough and 250mb upper jet streak. With strong
ridging to the east, these features lift more northward than
eastward during the overnight hours, resulting in a prolonged
period of impressive upper level divergence centered over Oklahoma.
The persistence of this large scale forcing is typically
indicative of a favorable training convective setup.

In the lower levels we have a very strong 850mb jet in excess of
50 kts resulting in impressive moisture transport and convergence
near a slow moving cold front. This low level flow is nearly equal
and parallel to the deep layer mean flow, which typically supports
a backbuilding convective risk. The expectation is that with deep
layer mean flow nearly parallel to the low level boundary,
convective cells will tend to train and also backbuild into the
strong low level jet.

Convection should become intense and organized enough to develop
a stronger cold pool resulting in some eastward cell progression
with time. This eastward progression is the main wild card in
determining the magnitude of the flash flood risk with this event.
We do think there will be some eastward progression, but the
persistent forcing and strong low level jet both will be opposing a
significant eastward push...and thus tend to think we will see
enough persistence of convection to result in an organized flash
flood threat. Convection will be capable of 2-3"/hr rainfall, and
thus it will not take much persistence to start getting some
significant rainfall totals. While widespread 5"+ totals may not
occur, this event does appear to have the potential to produce a
swath(s) of as much as 4-8" of rainfall. This is expected to
result in an increasing, and locally significant, flash flood
threat Saturday night

Model guidance remains in decent agreement, all generally focusing
the highest swath of QPF from north central TX into
central/eastern OK. Although, as would be expected with any
convective forecast, there is still some uncertainty on where
exactly the highest QPF ends up, as the swath of the most excessive
amounts will likely be rather narrow. The GEM REG has been pretty
consistent with a swath heavy rainfall, and while the magnitudes
and areal extent are probably overdone in that model, do think it
represents a reasonable evolution of how things may play out
Saturday into Saturday night. The 00z ECMWF remains a bit further
southeast than the consensus. The 00z GFS has a secondary max over
eastern KS into western MO, and while heavy rain is expected here,
tend to think the GFS is overdoing this northern swath, and under
doing the swath over OK (this is a typical GFS bias). The
experimental machine learning GFS Graphcast actually appears to be
correcting this bias, and has an axis more similar to the GEM reg
(albeit lower magnitudes). The current placement of the MDT risk is
where the best overlap in ingredients and the model QPF consensus
resides, and was extended a bit more southwest into north central
TX with this update.

Across eastern KS into northwest MO and southern IA locally
excessive rainfall is also expected along an axis of stronger low
level convergence. But in general the thermodynamic ingredients
here are less favorable for a prolonged period of heavy rain
compared to the swath over OK, and thus a Slight risk should
suffice. A Marginal risk extends westward across NE into northeast
CO along/near the low track where convection near the warm front
may result in a localized flood risk.

Chenard

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Fri Apr 26 2024
Day 3
Valid 12Z Sun Apr 28 2024 - 12Z Mon Apr 29 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE SOUTHERN PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY...

Convection will likely be ongoing at 12z Sunday across portions of
OK and TX, a continuation of the Moderate risk day described above
for Saturday night. Current expectations are that convection will
have enough of a cold pool by this time to result in a decent
eastward propagation. Thus the flash flood risk should be
decreasing into Sunday morning, although some continued flash
flood risk is probable.

The bigger question becomes what happens on Sunday night. There is
some model disagreement on this, although the majority of solutions
would suggest an additional round of upscale convective development
is likely. This is also generally supported when looking at things
from an ingredients based perspective. With the main low lifting
off to the northeast over the Upper MS Valley, the portion of the
front over TX and AR should become nearly stationary. Meanwhile we
see a strong and persistent upper level divergence signature
centered near the TX/AR border, and a trailing mid level shortwave
ejecting into the southern Plains should only help to enhance lift
near the stalled low level convergence axis. Overall, ingredients
are similar to what we will see Saturday night over OK, thus
training and backbuilding convection appears probable.

Still think this event may very well eventually need a Moderate
risk given the setup and ingredients in place. However there
remains a bit of uncertainty with regards to where the best
convective training will be...with anywhere from northeast TX into
AR within the range of possibilities. Given this uncertainty, and
the fact that the overall synoptic setup, while similar to
Saturday night, is probably not quite as favorable (also noting
slightly lower model QPFs compared to day 2)...think sticking with
a higher end Slight risk is the best course of action for now.
Plus, rainfall Friday will likely play a role in if/where soil and
stream conditions will be more sensitive for flash flooding come
Sunday...so knowing how convection plays out Friday may help with
any eventual MDT risk placement Sunday. Thus the Slight risk
remains for now, and will continue to evaluate on future shifts.

A broad Marginal risk extends from MO northward into MN and WI,
generally along and east of the strong low and mid/upper forcing.
Convection here, while locally intense, should be quick moving,
limiting the extent of any flash flood risk. However this will be
the second strong low pressure and convective threat within a 3
day period...so can not rule out an eventual need for an embedded
Slight risk pending soil and stream response from round 1.

Chenard

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Fri Apr 26 2024


Ample moisture and instability will be present across much of the
central U.S. into Monday ahead of the upper trough and a surface
frontal system. The most likely focus for the heaviest convection
and rainfall is across the Lower Mississippi Valley, where a Slight
Risk is maintained in the Monday/Day 4 Excessive Rainfall Outlook.
This may be a higher-end Slight Risk if the heaviest model QPFs of
3-5 inches with rain rates over 2 inches per hour end up
verifying. By Tuesday the upper- level support is forecast to
weaken, so overall lower rainfall amounts and rain rates are
expected. However, some additional convection could develop, which
could lead to flash flooding concerns especially if storms develop
over areas that saw excessive rainfall amounts the previous day. A
Marginal Risk will serve as a starting point to cover this threat
on Tuesday/Day 5. There is also some potential for convection
farther north from generally the central Plains into the Mid-
Mississippi and Ohio Valleys. Model guidance is not agreeable with
where any heavier rainfall might occur and these elements may be
progressive anyway, precluding any ERO risk area there at this
point. By midweek and beyond, additional rounds of rain and
thunderstorms are likely for the central/southern Plains and
Mid-/Lower Mississippi Valley as moisture surges ahead of frontal
systems again.

Elsewhere, multiple shortwaves will lead to rounds of
precipitation across the northern tier. In the Northwest, some
higher elevation snow is possible with this activity, with the most
widespread coverage on Monday before snow levels may rise even
further. Mainly rain is expected farther east across the northern
Plains to Midwest/Great Lakes/Ohio Valley and into the
Northeast/Mid-Atlantic at times.

With the exception of the cooler West Coast and Northwest
especially Monday-Tuesday, the bulk of the lower 48 will have near
to warmer than average temperatures next week. An upper ridge over
the East Monday-Tuesday should help produce temperatures around
10-20F above normal, with temperatures in the 80s possibly as far
north as the Great Lakes region. Scattered daily record highs are
possible. Temperatures are likely to gradually moderate but stay
above average in the East until late next week. Meanwhile, parts of
the central U.S. will also see above normal temperatures with
anomalies around +10F Monday-Wednesday before cooling. The
Intermountain West to Rockies on the other hand should gradually
warm and see its greatest warm anomalies Thursday-Friday.


Tate

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Fri Apr 26 2024


Ample moisture and instability will be present across much of the
central U.S. into Monday ahead of the upper trough and a surface
frontal system. The most likely focus for the heaviest convection
and rainfall is across the Lower Mississippi Valley, where a Slight
Risk is maintained in the Monday/Day 4 Excessive Rainfall Outlook.
This may be a higher-end Slight Risk if the heaviest model QPFs of
3-5 inches with rain rates over 2 inches per hour end up
verifying. By Tuesday the upper- level support is forecast to
weaken, so overall lower rainfall amounts and rain rates are
expected. However, some additional convection could develop, which
could lead to flash flooding concerns especially if storms develop
over areas that saw excessive rainfall amounts the previous day. A
Marginal Risk will serve as a starting point to cover this threat
on Tuesday/Day 5. There is also some potential for convection
farther north from generally the central Plains into the Mid-
Mississippi and Ohio Valleys. Model guidance is not agreeable with
where any heavier rainfall might occur and these elements may be
progressive anyway, precluding any ERO risk area there at this
point. By midweek and beyond, additional rounds of rain and
thunderstorms are likely for the central/southern Plains and
Mid-/Lower Mississippi Valley as moisture surges ahead of frontal
systems again.

Elsewhere, multiple shortwaves will lead to rounds of
precipitation across the northern tier. In the Northwest, some
higher elevation snow is possible with this activity, with the most
widespread coverage on Monday before snow levels may rise even
further. Mainly rain is expected farther east across the northern
Plains to Midwest/Great Lakes/Ohio Valley and into the
Northeast/Mid-Atlantic at times.

With the exception of the cooler West Coast and Northwest
especially Monday-Tuesday, the bulk of the lower 48 will have near
to warmer than average temperatures next week. An upper ridge over
the East Monday-Tuesday should help produce temperatures around
10-20F above normal, with temperatures in the 80s possibly as far
north as the Great Lakes region. Scattered daily record highs are
possible. Temperatures are likely to gradually moderate but stay
above average in the East until late next week. Meanwhile, parts of
the central U.S. will also see above normal temperatures with
anomalies around +10F Monday-Wednesday before cooling. The
Intermountain West to Rockies on the other hand should gradually
warm and see its greatest warm anomalies Thursday-Friday.


Tate

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Fri Apr 26 2024

Valid 12Z Fri Apr 26 2024 - 12Z Mon Apr 29 2024


...Great Basin into the Rockies...
Days 1-2...

A mid-level shortwave will surge onto the CA coast with an
elongated vorticity lobe strung out along its southern periphery.
This entire feature will pivot eastward into the Great Basin by
Saturday morning, with amplification into a closed low expected
over the Four Corners. This will drive increasing ascent across
much of the Intermountain West, with the most intense lift moving
across the Central Rockies Saturday into Sunday. In this area,
forcing will be provided via an overlap of impressive height falls,
downstream divergence, and increasing LFQ diffluence as the
subtropical jet streak pivots around the base of the trough and
arcs poleward. This overlap of ascent will drive surface
cyclogenesis in the lee of the Rockies as well, with E/NE flow
behind the associated cold front also producing upslope flow into
the terrain of CO/WY. At the same time, mesoscale ascent will
maximize as a potent deformation axis and accompanying mid-level
fgen drive lift into the DGZ, which will fuel the potential for CSI
and even CI as reflected by cross-sections and good model
agreement of an axis of theta-e lapse rates less than 0C/km
collocated with -EPV. This indicates the likelihood for some areas
receiving snowfall rates in excess of 1"/hr, and the WPC prototype
snowband tool suggests locally 2+"/hr rates are possible.

All of this lift will work across an environment with widespread
PW anomalies above +1 sigma according to NAEFS as moisture lingers
from a recently departed low, and in response to persistent onshore
flow from the Pacific spreading moisture eastward. Although snow
levels will be moderate at around 8000 ft (using the NBM 25th
percentile as a guide), strong and dynamic ascent should permit at
least some snow into the lower elevations.

The heaviest snow is likely above 8000 ft, especially in the
vicinity of the Front Range on D2, where WPC probabilities for more
than 6 inches are 80-90% or more, and with sufficient upslope flow
wringing out the moisture, where it remains all snow, some areas
could see as much as 3 feet of accumulation when added up over D1
and D2. Other heavy snow areas in CO include the San Juans and
remaining CO Rockies where WPC probabilities D2 reach 50-70% for 6+
inches. Elsewhere across the Intermountain West, WPC probabilities
D1 for more than 6 inches are above 80% around Yellowstone NP and
across the Wind River Range, and 50-80% in the Uintas, Wasatch, and
higher terrain of eastern NV. During D2 the focus shifts into CO,
but some additional moderate snowfall accumulations are possible in
the Uintas and Wind Rivers.


...Pacific Northwest...
Day 3...

Confluent westerly flow across the Northern Pacific will gradually
back to the SW ahead of an approaching shortwave D3, and this
impulse is likely to pivot onshore the WA/OR coast late Sunday
night while amplifying. This shortwave will be accompanied by
modest upper level diffluence in the LFQ of a 90 kt jet streak,
enhancing ascent into the area. This confluent flow and overlapping
jet streak will also surge moisture eastward, with an arc of
150-250 kg/ms IVT supporting an expanding precipitation shield
beginning late Sunday aftn and expanding into Sunday night. Snow
levels across the Cascades should generally be above 4500 ft
limiting total impacts, but WPC probabilities for more than 6
inches peak above 60% in the northern WA Cascades and highest
terrain farther south towards OR.


The probability of significant icing is less than 10 percent.


Weiss

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Fri Apr 26 2024

Valid 12Z Mon Apr 29 2024 - 12Z Fri May 03 2024

...Potentially severe thunderstorms and heavy rain causing flash
flooding are forecast to continue in the Lower Mississippi Valley
Monday...


...Overview...

Upper troughing will support a surface frontal system with
widespread thunderstorms ahead of it across the central U.S. into
Monday. Heavy rain is likely to cause flash flooding in the Lower
Mississippi Valley in particular, while severe weather may also be
a threat. The upper pattern looks to become more zonal by Tuesday-
Wednesday, but uncertain shortwaves in the northern stream should
continue to push frontal systems across the Lower 48 with
additional showers and thunderstorms. More amplified troughing may
come into the West by late next week, ahead of which near normal to
warmer than normal temperatures will be the norm across much of
the country from the Intermountain West eastward.


...Guidance/Predictability Assessment...

Model guidance is reasonably agreeable at the start of the period
Monday, with compact upper and surface lows in the north-central
U.S. and upper ridging in the East while additional energy comes
into the Northwest. By Tuesday-Wednesday, though the overall flow
becomes more zonal, the northern stream jet shows some notable
differences in the timing of those potent shortwaves coming into
the northwestern to north-central U.S., leading to some out of
phase differences. There are generally two camps for these
features. The 12/18Z GFS and 12Z ECMWF, in a departure from their
previous runs, indicate some energy feeding faster into the
Northwest while some energy is held back over the Pacific. Most of
the ECMWF-based AI/machine learning models are generally similar to
this 12Z trend. However, some guidance like the CMC, UKMET, and
the AI FourCastNet stick with a solution like the older GFS/ECMWF
runs in holding most energy back and creating a deeper upper low in
the eastern Pacific. The 06Z GFS-based AI Graphcast was also in
this second camp, but the 12Z run flipped to more progressive
initial energy. Ensemble members vary somewhat but do generally
stick with their parent models more than preferred. The incoming
00Z CMC and UKMET still indicate more energy hanging back west than
the GFS/EC camp, but are not as deeply aggressive with the eastern
Pacific upper low as their 12Z runs. Overall, evidence was more
compelling to prefer the GFS/ECMWF type of solution, even though
this was a new trend. Models do still indicate troughing coming
into the West by Friday with ridging ahead of it in the central
U.S. and some troughing to the east of that, but with amplitude and
exact positioning dependent on earlier forecast problems. Thus the
WPC forecast was based on a blend heavily favoring the 12/18Z GFS
and 12Z ECMWF, but gradually reducing the deterministic runs in
favor of the GEFS and EC ensemble means especially by Day 7 given
increasing model spread.


...Weather/Hazards Highlights...

Ample moisture and instability will be present across much of the
central U.S. into Monday ahead of the upper trough and a surface
frontal system. The most likely focus for the heaviest convection
and rainfall is across the Lower Mississippi Valley, where a Slight
Risk is maintained in the Monday/Day 4 Excessive Rainfall Outlook.
This may be a higher-end Slight Risk if the heaviest model QPFs of
3-5 inches with rain rates over 2 inches per hour end up
verifying. By Tuesday the upper- level support is forecast to
weaken, so overall lower rainfall amounts and rain rates are
expected. However, some additional convection could develop, which
could lead to flash flooding concerns especially if storms develop
over areas that saw excessive rainfall amounts the previous day. A
Marginal Risk will serve as a starting point to cover this threat
on Tuesday/Day 5. There is also some potential for convection
farther north from generally the central Plains into the Mid-
Mississippi and Ohio Valleys. Model guidance is not agreeable with
where any heavier rainfall might occur and these elements may be
progressive anyway, precluding any ERO risk area there at this
point. By midweek and beyond, additional rounds of rain and
thunderstorms are likely for the central/southern Plains and
Mid-/Lower Mississippi Valley as moisture surges ahead of frontal
systems again.

Elsewhere, multiple shortwaves will lead to rounds of
precipitation across the northern tier. In the Northwest, some
higher elevation snow is possible with this activity, with the most
widespread coverage on Monday before snow levels may rise even
further. Mainly rain is expected farther east across the northern
Plains to Midwest/Great Lakes/Ohio Valley and into the
Northeast/Mid-Atlantic at times.

With the exception of the cooler West Coast and Northwest
especially Monday-Tuesday, the bulk of the lower 48 will have near
to warmer than average temperatures next week. An upper ridge over
the East Monday-Tuesday should help produce temperatures around
10-20F above normal, with temperatures in the 80s possibly as far
north as the Great Lakes region. Scattered daily record highs are
possible. Temperatures are likely to gradually moderate but stay
above average in the East until late next week. Meanwhile, parts of
the central U.S. will also see above normal temperatures with
anomalies around +10F Monday-Wednesday before cooling. The
Intermountain West to Rockies on the other hand should gradually
warm and see its greatest warm anomalies Thursday-Friday.


Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw