Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
308 AM EDT Sat Apr 27 2024

Valid 12Z Sat Apr 27 2024 - 12Z Mon Apr 29 2024

...A very active weekend in store for parts of the Central U.S....

...Excessive Rainfall and Severe Weather concerns from Southern Plains to
Mississippi Valley...

...Critical Fire Weather Risk over portions of the Southern High Plains...

...Warmer than average temperatures spreads from Midwest into Mid-Atlantic
on Sunday while well below average temperatures shift from the Four
Corners and Rockies into the Northern Plains...


A highly amplified and slow-moving upper-level pattern will support a very
active period across the Central U.S. this weekend. Today, thunderstorms
will develop along a cold front and dryline extending north-south across
the Southern Plains and along a quasi-stationary front draped across the
Middle Mississippi Valley and into the Midwest. The Storm Prediction
Center issued an Enhanced Risk (level 3/5) of Severe Thunderstorms across
parts of the Southern/Central Plains, where large hail, damaging winds and
multiple tornadoes will be possible. Some of these storms may produce
intense rainfall rates at times, which could cause flash flooding of
roads, underpasses and urban areas. There's a Moderate Risk (at least 40%)
of Excessive Rainfall leading to Flash Flooding over parts of the
Central/Southern Plains and Middle Mississippi Valley today. Gusty winds
and very low dew points will support fire weather concerns on the western
side of the aforementioned dry-line. The Storm Prediction Center issued a
Critical Risk of Fires over much of the Southern High Plains as a result.

The focus for Excessive Rainfall and Severe Storms will expand in coverage
into the Mississippi Valley on Sunday. The Storm Prediction Center issued
a Slight Risk (level 2/5) of Severe Thunderstorms capable of producing
damaging winds, hail and a few tornadoes from east Texas into the Upper
Mississippi Valley. Some of these storms may be capable of wringing out
several inches of rain over vulnerable surfaces--from today's storms.
Thus, another Moderate Risk (at least 40%) of Excessive Rainfall is in
effect for the ArkLaTex on Sunday. Continued dry and windy conditions over
the southern periphery of the slow-moving upper trough axis will support
another Critical Fire Weather day across much of the Southern High Plains
on Sunday.

Meanwhile, cold continental air will pour into the Rockies on the backside
of the amplified upper trough and interact with Pacific and Gulf moisture
to produce heavy snow over portions of the Colorado Rockies today.
Snowfall totals of 8-12 inches are likely with isolated higher amounts
possible over higher elevations. Cooler air spreads east into the Northern
Plains on Sunday behind the ascendant and negatively tilting upper trough
trudging through the Plains. Strong southerly flow into the Mississippi
Valley and Midwest today will see high temperatures climb into the 70s and
80s, which is well above average for some places. Cool and cloudy
conditions over the Mid-Atlantic today should give way to the warmer
conditions on Sunday.


Kebede


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Sat Apr 27 2024

Valid 12Z Tue Apr 30 2024 - 12Z Sat May 04 2024

...Rounds of convection are likely in the central U.S. for much of
next week...


...Overview...

As the medium range period begins Tuesday, quasi-zonal upper flow
will be in place over much of the lower 48, but embedded shortwaves
could push frontal systems through the central and eastern U.S. and
cause some showers and thunderstorms. Mean troughing is forecast to
amplify in the Intermountain West Wednesday-Thursday and provide
support for increasing convection over much of the central U.S.,
with heavy rain and flash flooding possible hazards. A gradual
shift east with the convection is likely into late week.
Precipitation including some higher elevation snow is possible
across the Northwest into the northern/central Rockies, but with
rather low predictability.


...Guidance/Predictability Assessment...

Recent model guidance is fortunately in better agreement than a
day ago regarding energy across the northwestern U.S./British
Columbia on Tuesday. There is a general theme of some energy
developing a closed low and pressing east near the U.S./Canadian
border Wednesday-Thursday while secondary energy causes a trough to
dig into the Intermountain West. Operational models and AI/machine
learning models show some variability in how separate or phased
these features may be, but generally within typical spread for the
medium range. There is also some model spread in how deep the
trough may dig in the Intermountain West and pushing toward the
Plains by Friday. The 18Z GFS was quite aggressive with a
deeper/neutrally (rather than positively) tilted trough and a
resultant surface low track farther west compared to other
guidance. Thus for these features, preferred a model blend of the
12Z ECMWF/GFS/CMC. The newer 00Z GFS looked more in line with other
guidance.

Upstream, there are even larger model differences across the
northwestern U.S. by Thursday that lead to dynamical
models/ensembles/ML models showing out of phase solutions. Early
Thursday, most 12/18Z deterministic models indicated a ridge over
the West Coast while they held an upper low farther north into
western Canada compared to previous runs. Most of the 12Z ML models
also supported this ridge solution. On the other hand, the
ensemble means persisted in indicating troughing atop the Northwest
more similar to the previous runs. Individual ensemble members
show ample spread. Having to choose a camp, preferred the
deterministic and ML type solutions with ridging over the
Northwest, thinking that the ensemble means were lagging the trend.
The WPC model blend maintained more deterministic components than
normal for the latter part of the period, especially the ECMWF.
However, this may have been the wrong choice given the incoming 00Z
models generally show troughing (though think the 00Z CMC may be
too aggressive with its compact closed low). There may need to be
some changes made with frontal and precipitation timing in future
forecasts because of this. By Friday-Saturday, there are additional
uncertainties with northern Pacific and Canadian flow affecting
the West. Thus predictability and confidence are somewhat low for
the pattern in the western and central U.S. after midweek.


...Weather/Hazards Highlights...

On Tuesday, lessening upper-level support should lead to lower
rainfall amounts and rainfall rates in the central U.S. compared
earlier and later in the week. However, some convection is still
likely to occur in a moist and unstable environment. Currently the
best chance for rain with isolated flooding potential is across the
south-central Plains to Mid-Mississippi Valley as a front stalls.
Within that region there is some spread in location of locally
heavy convective potential--GFS runs have been persistent in
showing convection near the Red River while other models indicate
local maxima farther north. A Day 4 Marginal Risk is in place in
the ERO to cover these areas and will be refined with time.
However, rainfall amounts have trended down across the board
farther south in Texas and Louisiana, so a Marginal Risk that was
in place there on what is now Day 4 was removed. By Wednesday, the
upper flow (with troughing to the West and a southern stream
shortwave) will become more supportive of more widespread heavy
rain and thunderstorms for the central/southern Plains and
Mid/Lower Mississippi Valley as moisture surges ahead of a wavy
frontal system. Once again there is spread in the exact position of
the heaviest totals, but rain amounts of 3-5 inches are likely,
and some areas of heavy rain are likely to overlap with
current/short range heavy rainfall locations. A large Slight Risk
serves as a starting point for the Day 5/Wednesday ERO especially
given the likely sensitivity of these areas due to wet antecedent
conditions. Expect a slow trend of the rain/thunderstorms eastward
Thursday and Friday as the front presses east.

Elsewhere, one or more shortwaves and upper lows will lead to
rounds of precipitation across the northern tier. On Tuesday, ample
moisture with precipitable water values over the 90th or even 95th
percentile will be in place across the northeastern U.S., with
rain and thunderstorms likely. A shortwave aloft moving through and
some instability in place could lead to heavy rainfall causing
isolated flooding concerns, so added a Marginal Risk centered in
upstate New York with its footprint stretching a bit into the
northern Mid-Atlantic and western New England on Day 4/Tuesday.
Meanwhile the Northwest and into the northern and central Rockies
could get rounds of precipitation through the week, including
higher elevation snow. Some guidance suggests the potential for a
period of enhanced northern Rockies/Plains precipitation that will
be worth monitoring. Timing and amounts of precipitation are
dependent on the eventual uncertain evolution of the upper and
surface pattern.


Temperatures across the south-central U.S. all the way to the
Eastern Seaboard will be warmer than average by generally 10-15F on
Tuesday and Wednesday, while cooler than normal temperatures are
likely for the northwestern quadrant of the lower 48. Then upper
troughing and a cold front pushing from the Northwest should cool
temperatures to near or below normal across the central U.S. by
Thursday and reaching the east-central U.S. Friday and Saturday.
There is a possibility of a warmup across the West late week, but
the guidance spread is still yielding lower than average confidence
in the temperature forecast there.


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
400 AM EDT Sat Apr 27 2024

Valid 00Z Sun Apr 28 2024 - 00Z Sun May 05 2024

Upper and surface lows are located to the northwest of Hawaii and
are forecast to track slowly eastward this weekend and early next
week. In response to these features, winds are likely to veer a
bit and may become east-southeast, though staying weak enough to
allow for a hybrid trade and land/sea breeze pattern over the next
few days. Some increased moisture is in place and will likely hold
through Saturday, allowing for increasing shower coverage directed
by the winds. Somewhat drier conditions may come in early next
week. What is left of the weakening trough may move across Hawaii
around Tuesday.

By the later part of next week, it is most likely that surface
high pressure should move north of the state and promote
strengthening easterly trades. Some notable model differences
start to arise late week though. The ECMWF is showing a closed
upper low setting up west of the state drawing significant
moisture into the northwestern islands, unlike other guidance.
This type of pattern seems unlikely as the ECMWF is deeper than
basically all ensemble members from multiple model suites with
this feature. So this will continue to be monitored but prefer a
solution away from the ECMWF at this time.


Tate

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Sat Apr 27 2024
Day 1
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 CENTRAL AND EASTERN OKLAHOMA, SOUTHEAST
KANSAS, SOUTHWEST MISSOURI, AND FAR NORTHWEST ARKANSAS...

A significant rainfall event still appears likely Saturday night
across portions of north central TX into central/eastern OK and
portions of southeast KS and southwest MO. Numerous instances of
flash flooding are expected, some of which will likely be
significant in nature.

Overall not much has changed with the expected setup over the past
couple days. The environmental ingredients remain in place for
training/backbuilding convection, especially during the overnight
hours. Persistent and strong mid/upper level forcing, a
significant ramp up in 850mb moisture transport into a stalled
boundary, mean flow parallel to the boundary and weak Corfidi
vectors (due to the strong low level jet aligned parallel to the
mean flow)...all are favorable for training/backbuilding
convection. The overlap of impressive upper level divergence and
strong 850mb moisture convergence , both of which persist upwards
of 6-12 hours, combined with substantial instability and well
above average PWs...all point to a numerous flash flood threat,
ramping up by later in the day into the overnight hours.

Convection will likely get going pretty early in the day and then
persist into the overnight hours. The initial convection should be
more scattered in nature and moving at a decent clip...so would
expect just an isolated flash flood risk initially. However as the
low level jet and moisture transport really ramp up this evening
into the overnight expect we will see upscale growth of convection
into one or more training lines. Of course, the early convection
could play a role in where this boundary and most favorable
training corridor ends up, but overall models remain fairly well
clustered with an axis from north central TX into central/eastern
OK.

Probabilities from the 00z HREF are pretty impressive. EAS
probabilities of exceeding 3" are rather broad and as high as
50-80%. Given EAS probabilities can be considered a smooth point
probability...this indicates an event with a widespread 3"+ QPF
footprint is probable. Embedded within this 3" area will likely be
higher totals...with 5" neighborhood probabilities over 60%, and 8"
probabilities around 30%. This generally is in line with
expectations from previous shifts...that this event has the
potential to drop a swath of 4-8" of rain (isolated 10" max
possible)...with the most likely location from just north of the
Red River into central/eastern OK..including the Norman to Tulsa
corridor. Overall think the 00z HREF QPF and probabilities are
reasonable...although tend to think the southwest flank of
convection could over perform relative to the HREF...with the 00z
GEM Reg and FV3LAM potentially showing plausible outcomes of a QPF
max a bit southwest of the 00z HREFbm max. The northeast extent of
the MDT risk over Southeast KS and southwest MO may see a bit
lower QPF than areas further southwest, however with wetter
antecedent conditions here, flash flood impacts are still likely.

One thing to continue to consider is that convection should become
intense and organized enough to develop a stronger cold pool
resulting in some eastward cell progression with time. While we do
think there will be some eastward progression, 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 the organized and
significant flash flood threat described above.

Further north, a Marginal risk extends from northeast CO, across
NE and into IA and portions of WI/IL/IN. Gave some consideration to
a Slight risk upgrade over portions of NE and western IA given the
heavy rainfall over these areas on Friday. However the extent of
convective training is a bit more uncertain here, and the higher
probabilities of heavy rain end up a bit south of areas hardest hit
Friday. Thus tend to think flash flooding will be more isolated in
nature today, which fits more into the Marginal risk category.
However will continue to monitor.


Chenard

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Sat Apr 27 2024
Day 2
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
eastern OK and TX, into western AR/MO. Current expectations are
that convection will have enough of a cold pool by this time to
result in a decent eastward propagation, while also running into a
less unstable airmass with time. While some lingering flash flood
risk into Sunday morning is likely over these areas, the risk
should be on a decreasing trend.

The bigger question becomes what happens by later in the day into
the overnight hours. Ingredients do appear to be in place for
upscale convective growth and some training/backbuilding
potential. There is a slowing front over the area, with strong and
persistent upper level divergence moving over top of this
boundary. Moisture transport is not as strong as what we will see
Saturday night over the Plains, but still note a pretty strong low
level jet and 850mb moisture convergence signature.

Overall this event remains as a higher end Slight risk from
northeast TX into much of AR, and would expect to see scattered
flash flooding across this corridor. A more focused and
concentrated area of more numerous and significant flash flooding
is still a possibility. Based on the ingredients in place, the most
likely location for this appears to be in the vicinity of the
Arklatex. This is also where the 00z GEM reg, UKMET, ECMWF and
FV3LAM have a relative max in QPF. However there remains some
spread with this scenario, and model QPFs are generally lower than
what we are seeing on Saturday night over the Plains. Thus this
event will probably have a bit less flash flood coverage. Still
think a MDT risk upgrade may eventually be needed, with the
Arklatex region the most likely location. However we will have
much more high res guidance to evaluate over the the next 24 hours
than we do now, so would prefer to hold off on any upgrade and
continue to evaluate trends. Either way some flash flooding is
expected, some of which could be locally significant in nature.

A broad Marginal risk extends from MO northward into MN, WI and MI,
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, some of these
areas will have seen heavy rain on previous days...so can not rule
out an eventual need for an embedded Slight risk pending soil and
stream response from this earlier rain.

Chenard

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Sat Apr 27 2024
Day 3
Valid 12Z Mon Apr 29 2024 - 12Z Tue Apr 30 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...

The convective and heavy rain threat shifts eastward into the lower
MS Valley on Monday. Still seeing an impressive combination of
mid/upper forcing, low level convergence, instability and moisture
to support a flash flood risk. At this time these ingredients look
a bit more progressive than previous days, which should result in
quicker cell motions and an overall lowered flash flood threat.
Nonetheless, rainfall rates should still be quite high given the
instability and moisture in place, so 2"+/hr remains probable in
spots. This should be enough to drive an isolated to scattered
flash flood risk, especially over urban areas.

Models are in pretty good agreement on the axis of heaviest
rainfall, generally trending a bit further east compared to
previous runs. The greater uncertainty lies with where to place
the northern end of the Slight risk. The ECMWF remains further
north than the GFS, GEM reg and UKMET. For now will keep the
northern edge of this Slight risk over western TN, as this better
aligns with the model consensus, with even the machine learning EC
AIFS further south than the deterministic ECMWF.

Chenard

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Sat Apr 27 2024


On Tuesday, lessening upper-level support should lead to lower
rainfall amounts and rainfall rates in the central U.S. compared
earlier and later in the week. However, some convection is still
likely to occur in a moist and unstable environment. Currently the
best chance for rain with isolated flooding potential is across the
south-central Plains to Mid-Mississippi Valley as a front stalls.
Within that region there is some spread in location of locally
heavy convective potential--GFS runs have been persistent in
showing convection near the Red River while other models indicate
local maxima farther north. A Day 4 Marginal Risk is in place in
the ERO to cover these areas and will be refined with time.
However, rainfall amounts have trended down across the board
farther south in Texas and Louisiana, so a Marginal Risk that was
in place there on what is now Day 4 was removed. By Wednesday, the
upper flow (with troughing to the West and a southern stream
shortwave) will become more supportive of more widespread heavy
rain and thunderstorms for the central/southern Plains and
Mid/Lower Mississippi Valley as moisture surges ahead of a wavy
frontal system. Once again there is spread in the exact position of
the heaviest totals, but rain amounts of 3-5 inches are likely,
and some areas of heavy rain are likely to overlap with
current/short range heavy rainfall locations. A large Slight Risk
serves as a starting point for the Day 5/Wednesday ERO especially
given the likely sensitivity of these areas due to wet antecedent
conditions. Expect a slow trend of the rain/thunderstorms eastward
Thursday and Friday as the front presses east.

Elsewhere, one or more shortwaves and upper lows will lead to
rounds of precipitation across the northern tier. On Tuesday, ample
moisture with precipitable water values over the 90th or even 95th
percentile will be in place across the northeastern U.S., with
rain and thunderstorms likely. A shortwave aloft moving through and
some instability in place could lead to heavy rainfall causing
isolated flooding concerns, so added a Marginal Risk centered in
upstate New York with its footprint stretching a bit into the
northern Mid-Atlantic and western New England on Day 4/Tuesday.
Meanwhile the Northwest and into the northern and central Rockies
could get rounds of precipitation through the week, including
higher elevation snow. Some guidance suggests the potential for a
period of enhanced northern Rockies/Plains precipitation that will
be worth monitoring. Timing and amounts of precipitation are
dependent on the eventual uncertain evolution of the upper and
surface pattern.


Temperatures across the south-central U.S. all the way to the
Eastern Seaboard will be warmer than average by generally 10-15F on
Tuesday and Wednesday, while cooler than normal temperatures are
likely for the northwestern quadrant of the lower 48. Then upper
troughing and a cold front pushing from the Northwest should cool
temperatures to near or below normal across the central U.S. by
Thursday and reaching the east-central U.S. Friday and Saturday.
There is a possibility of a warmup across the West late week, but
the guidance spread is still yielding lower than average confidence
in the temperature forecast there.


Tate

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Sat Apr 27 2024


On Tuesday, lessening upper-level support should lead to lower
rainfall amounts and rainfall rates in the central U.S. compared
earlier and later in the week. However, some convection is still
likely to occur in a moist and unstable environment. Currently the
best chance for rain with isolated flooding potential is across the
south-central Plains to Mid-Mississippi Valley as a front stalls.
Within that region there is some spread in location of locally
heavy convective potential--GFS runs have been persistent in
showing convection near the Red River while other models indicate
local maxima farther north. A Day 4 Marginal Risk is in place in
the ERO to cover these areas and will be refined with time.
However, rainfall amounts have trended down across the board
farther south in Texas and Louisiana, so a Marginal Risk that was
in place there on what is now Day 4 was removed. By Wednesday, the
upper flow (with troughing to the West and a southern stream
shortwave) will become more supportive of more widespread heavy
rain and thunderstorms for the central/southern Plains and
Mid/Lower Mississippi Valley as moisture surges ahead of a wavy
frontal system. Once again there is spread in the exact position of
the heaviest totals, but rain amounts of 3-5 inches are likely,
and some areas of heavy rain are likely to overlap with
current/short range heavy rainfall locations. A large Slight Risk
serves as a starting point for the Day 5/Wednesday ERO especially
given the likely sensitivity of these areas due to wet antecedent
conditions. Expect a slow trend of the rain/thunderstorms eastward
Thursday and Friday as the front presses east.

Elsewhere, one or more shortwaves and upper lows will lead to
rounds of precipitation across the northern tier. On Tuesday, ample
moisture with precipitable water values over the 90th or even 95th
percentile will be in place across the northeastern U.S., with
rain and thunderstorms likely. A shortwave aloft moving through and
some instability in place could lead to heavy rainfall causing
isolated flooding concerns, so added a Marginal Risk centered in
upstate New York with its footprint stretching a bit into the
northern Mid-Atlantic and western New England on Day 4/Tuesday.
Meanwhile the Northwest and into the northern and central Rockies
could get rounds of precipitation through the week, including
higher elevation snow. Some guidance suggests the potential for a
period of enhanced northern Rockies/Plains precipitation that will
be worth monitoring. Timing and amounts of precipitation are
dependent on the eventual uncertain evolution of the upper and
surface pattern.


Temperatures across the south-central U.S. all the way to the
Eastern Seaboard will be warmer than average by generally 10-15F on
Tuesday and Wednesday, while cooler than normal temperatures are
likely for the northwestern quadrant of the lower 48. Then upper
troughing and a cold front pushing from the Northwest should cool
temperatures to near or below normal across the central U.S. by
Thursday and reaching the east-central U.S. Friday and Saturday.
There is a possibility of a warmup across the West late week, but
the guidance spread is still yielding lower than average confidence
in the temperature forecast there.


Tate

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 AM EDT Sat Apr 27 2024

Valid 12Z Sat Apr 27 2024 - 12Z Tue Apr 30 2024

...Central Rockies...
Days 1...

A surface low pressure consolidating in the lee of the Rockies
early Saturday will deepen briefly through the day before slowly
weakening as it ejects to the northeast and into the Central Plains
Saturday night. This low will deepen in response to impressive
synoptic ascent through LFQ upper diffluence as an upper jet
rotates through the base of an amplifying longwave trough, with
this trough deepening into a closed low before advecting into the
Plains late D1. As this low deepens and shifts to the east, it will
be accompanied by an increasingly intense deformation axis on its
NW side, which will overlap effectively with mid-level fgen to
drive ascent into the DGZ, and increasing upslope flow on easterly
winds into the Front Range and other CO Rockies. In this area,
there is additionally good consensus for CSI/CI as mid-level
theta-e lapse rates fall to below 0C/km in conjunction with pockets
of -EPV. This suggests snowfall rates will likely exceed 1"/hr, and
in the more intense convection could be 2-3"/hr as noted by the WPC
prototype snowband tool. This deformation axis will pivot in the
vicinity of the Front Range much of Saturday, and although snow
levels will likely hover around 8,000 ft, impressive cold air
dragging down to as low as 6,000 ft is probable which could bring
significant accumulations into the foothills and Palmer Divide as
well before everything shuts off Sunday morning. For the I-25 urban
corridor from Cheyenne to Colorado Springs, it is possible even the
lower elevations could mix with snow or even get some light
accumulations, but the marginal thermal structure should prevent
significant impacts east of the terrain except along the Palmer
Divide.

WPC probabilities have climbed once again this morning for the
Front Range, now exceeding 80% for the eastern slopes and into the
higher terrain, with a secondary maximum near Pike's Peek. The
bigger change this morning, however, has been a noted increase
along the Palmer Divide which now features probabilities as high as
60% for 6+ inches, especially west of I-25. Additional WPC
probabilities exceeding 50% for 6+ inches exist across other
portions of the CO Rockies above 8000 ft, and in the San Juans,
northern Sangre de Cristos, and portions of the Wasatch in UT.


...Pacific Northwest and Northern Rockies...
Days 2-3...

Persistent onshore flow will spread eastward into the Pacific
Northwest and Northern Rockies D2 and D3 as confluent mid-level
flow streams across the Pacific and into the region. Although the
available moisture will be near normal, there is forecast to be a
subtle increase in IVT within this confluent flow as reflected by
GEFS probabilities reaching 50-70% for 150+ kg/ms, highest on D2.
Despite the overall modest moisture, ascent will intensify as dual
shortwaves race eastward embedded within the pinched flow, to
enhance ascent to wring out the available PW, with PVA maximized
late Sunday and again late Monday. This ascent combined with the
favorable upslope flow component into the Cascades, Olympics, and
Northern Rockies due to the zonal flow will result in periods of
moderate to heavy snow, with snow levels falling to as low as 2500
ft Monday.

WPC probabilities for more than 6 inches ramp up during D2 across
the OR and WA Cascades, as well as the Olympic Range, where they
reach 50-80% in the higher elevations above 5000 ft. By D3 these
extend eastward into the Northern Rockies including in the vicinity
of the Salmon River and Sawtooth Ranges, with lighter probabilities
reaching as far east as the Absarokas. With snow levels falling,
several inches of snow is possible at the Cascade Passes, including
Snoqualmie and Stevens Passes, and especially by D3, resulting in
hazardous travel potential.


The probability of significant icing is less than 10 percent.


Weiss

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Sat Apr 27 2024

Valid 12Z Tue Apr 30 2024 - 12Z Sat May 04 2024

...Rounds of convection are likely in the central U.S. for much of
next week...


...Overview...

As the medium range period begins Tuesday, quasi-zonal upper flow
will be in place over much of the lower 48, but embedded shortwaves
could push frontal systems through the central and eastern U.S. and
cause some showers and thunderstorms. Mean troughing is forecast to
amplify in the Intermountain West Wednesday-Thursday and provide
support for increasing convection over much of the central U.S.,
with heavy rain and flash flooding possible hazards. A gradual
shift east with the convection is likely into late week.
Precipitation including some higher elevation snow is possible
across the Northwest into the northern/central Rockies, but with
rather low predictability.


...Guidance/Predictability Assessment...

Recent model guidance is fortunately in better agreement than a
day ago regarding energy across the northwestern U.S./British
Columbia on Tuesday. There is a general theme of some energy
developing a closed low and pressing east near the U.S./Canadian
border Wednesday-Thursday while secondary energy causes a trough to
dig into the Intermountain West. Operational models and AI/machine
learning models show some variability in how separate or phased
these features may be, but generally within typical spread for the
medium range. There is also some model spread in how deep the
trough may dig in the Intermountain West and pushing toward the
Plains by Friday. The 18Z GFS was quite aggressive with a
deeper/neutrally (rather than positively) tilted trough and a
resultant surface low track farther west compared to other
guidance. Thus for these features, preferred a model blend of the
12Z ECMWF/GFS/CMC. The newer 00Z GFS looked more in line with other
guidance.

Upstream, there are even larger model differences across the
northwestern U.S. by Thursday that lead to dynamical
models/ensembles/ML models showing out of phase solutions. Early
Thursday, most 12/18Z deterministic models indicated a ridge over
the West Coast while they held an upper low farther north into
western Canada compared to previous runs. Most of the 12Z ML models
also supported this ridge solution. On the other hand, the
ensemble means persisted in indicating troughing atop the Northwest
more similar to the previous runs. Individual ensemble members
show ample spread. Having to choose a camp, preferred the
deterministic and ML type solutions with ridging over the
Northwest, thinking that the ensemble means were lagging the trend.
The WPC model blend maintained more deterministic components than
normal for the latter part of the period, especially the ECMWF.
However, this may have been the wrong choice given the incoming 00Z
models generally show troughing (though think the 00Z CMC may be
too aggressive with its compact closed low). There may need to be
some changes made with frontal and precipitation timing in future
forecasts because of this. By Friday-Saturday, there are additional
uncertainties with northern Pacific and Canadian flow affecting
the West. Thus predictability and confidence are somewhat low for
the pattern in the western and central U.S. after midweek.


...Weather/Hazards Highlights...

On Tuesday, lessening upper-level support should lead to lower
rainfall amounts and rainfall rates in the central U.S. compared
earlier and later in the week. However, some convection is still
likely to occur in a moist and unstable environment. Currently the
best chance for rain with isolated flooding potential is across the
south-central Plains to Mid-Mississippi Valley as a front stalls.
Within that region there is some spread in location of locally
heavy convective potential--GFS runs have been persistent in
showing convection near the Red River while other models indicate
local maxima farther north. A Day 4 Marginal Risk is in place in
the ERO to cover these areas and will be refined with time.
However, rainfall amounts have trended down across the board
farther south in Texas and Louisiana, so a Marginal Risk that was
in place there on what is now Day 4 was removed. By Wednesday, the
upper flow (with troughing to the West and a southern stream
shortwave) will become more supportive of more widespread heavy
rain and thunderstorms for the central/southern Plains and
Mid/Lower Mississippi Valley as moisture surges ahead of a wavy
frontal system. Once again there is spread in the exact position of
the heaviest totals, but rain amounts of 3-5 inches are likely,
and some areas of heavy rain are likely to overlap with
current/short range heavy rainfall locations. A large Slight Risk
serves as a starting point for the Day 5/Wednesday ERO especially
given the likely sensitivity of these areas due to wet antecedent
conditions. Expect a slow trend of the rain/thunderstorms eastward
Thursday and Friday as the front presses east.

Elsewhere, one or more shortwaves and upper lows will lead to
rounds of precipitation across the northern tier. On Tuesday, ample
moisture with precipitable water values over the 90th or even 95th
percentile will be in place across the northeastern U.S., with
rain and thunderstorms likely. A shortwave aloft moving through and
some instability in place could lead to heavy rainfall causing
isolated flooding concerns, so added a Marginal Risk centered in
upstate New York with its footprint stretching a bit into the
northern Mid-Atlantic and western New England on Day 4/Tuesday.
Meanwhile the Northwest and into the northern and central Rockies
could get rounds of precipitation through the week, including
higher elevation snow. Some guidance suggests the potential for a
period of enhanced northern Rockies/Plains precipitation that will
be worth monitoring. Timing and amounts of precipitation are
dependent on the eventual uncertain evolution of the upper and
surface pattern.


Temperatures across the south-central U.S. all the way to the
Eastern Seaboard will be warmer than average by generally 10-15F on
Tuesday and Wednesday, while cooler than normal temperatures are
likely for the northwestern quadrant of the lower 48. Then upper
troughing and a cold front pushing from the Northwest should cool
temperatures to near or below normal across the central U.S. by
Thursday and reaching the east-central U.S. Friday and Saturday.
There is a possibility of a warmup across the West late week, but
the guidance spread is still yielding lower than average confidence
in the temperature forecast there.


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