Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
356 PM EDT Fri May 03 2024

Valid 00Z Sat May 04 2024 - 00Z Mon May 06 2024

...Heavy rain, significant flooding, and severe weather concerns remain
across parts of eastern Texas and the southern Plains through the
weekend...

...Heavy mountain snow and gusty winds to enter much of the West, Great
Basin, and Rockies over the next few days...

...Below average temperatures and showers impact the Mid-Atlantic and
Northeast into the end of the weekend...

Slow-moving thunderstorms containing intense rainfall rates are ongoing
this afternoon throughout parts of southeastern Texas just north of the
Houston metro area, which are leading to a continuing threat of flash
flooding. Ongoing significant to catastrophic flooding throughout parts of
eastern Texas from prior heavy rain will only exacerbate the flash, urban,
and river flooding threat from additional rainfall into this evening.
Newly formed thunderstorms developing off a southern High Plains dryline
could lead to isolated to scattered flash flooding across central Texas as
well tonight, but the main hazard associated with these storms are
forecast to be associated with giant hail, damaging wind gusts, and a few
tornadoes. Farther north, a potent cold front exiting the central High
Plains and progressing into the mid-Mississippi Valley by Saturday is also
expected to spark a line of showers and thunderstorms capable of producing
heavy rain and severe thunderstorms, mainly throughout parts of Kansas and
Nebraska.

By this weekend, the aforementioned cold front is expected to continue
progressing eastward through the Midwest and Ohio Valley. The tail end of
of this frontal boundary reaching westward to the southern Plains will be
the focus for additional thunderstorm activity and the potential for flash
flooding. A Slight Risk (level 2/4) of Excessive Rainfall has been issued
for central and north-central Texas on Saturday, with the threat area
shifting east on Sunday, extending from eastern Texas to southwest
Missouri and far southeast Kansas. Additionally, severe thunderstorms are
possible, with the greatest risk anticipated on Saturday in the Permian
Basin of West Texas. Several storms may produce large to giant hail and a
couple strong tornadoes.

An anomalously strong upper-level low entering northern California on
Saturday night is forecast to spread moderate to heavy precipitation and
gusty winds into the western U.S. and Great Basin this weekend. Heavy
mountain snow in the Sierra Nevada and Oregon Cascades is likely to add up
to over a foot (mainly above 6000 feet) on Saturday. As the system
continues to swing eastward, gusty southerly winds up to 60 to 80s mph are
possible in the central Great Basin and the Sierra Nevada, where High Wind
Warnings have been issued. The associated precipiation and gusty winds
will shift eastward as the upper-level low crosses the Intermountain West
on Sunday. Residents and visitors through are advised to check local
conditions before traveling, especially throughout exposed elevated
roadways.

Showers are also expected to dampen the first weekend in May throughout
much of the Great Lakes and eastern United States. Most precipitation
should remain scattered, with thunderstorm chances in the Southeast, Ohio
and Tennessee valleys. A backdoor cold front sinking through the
Mid-Atlantic today will allow for additional shower activity and below
average temperatures this weekend. Highs are only expected to reach the
50s and 60s as cloud cover and cool air streaming off the Atlantic keep
temperatures 10 to 20 degrees below average between the Mid-Atlantic and
New England when compared to climatology for the dates.

Snell


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Fri May 3 2024

Valid 12Z Mon May 06 2024 - 12Z Fri May 10 2024


...Northern Plains to Midwest Heavy Rain Threat early next week...

...Possible heavy rain across the interior South later next week...


...Overview...

Models and ensemble means continue to indicate a synoptic pattern
that swings upper-level troughs/lows onshore across the western
U.S. that would favor cyclogenesis over the mid-section of the
country followed by gradual lifting of the troughs toward New
England. The beginning of the medium-range period (Monday morning)
will likely feature one such cyclogenesis over the central to
northern High Plains. This will support cooler than normal
temperatures, gusty winds, and precipitation including higher
elevation snow in the West early in the week. Rain that could be
locally heavy shifts into the northern Plains and downstream
through the Midwest. Meanwhile farther south, mean upper ridging
will lead to warming pre-frontal temperatures for much of the
central and eastern U.S. that could yield some record values.
Upstream, Pacific upper trough energy will work inland over an
unsettled West and to the south- central U.S.. This may interact
with a wavy and stalling lead front and combine with pooling
moisture and instability to fuel rain and thunderstorms over the
south-central states/Mid-South and onward into later next week,
still also under some influence from the lead upper low/trough.

...Guidance/Predictability Assessment...

Model and ensemble solutions overall seem reasonably clustered for
Monday-Wednesday and generally in step with National Blend of
Models (NBM) guidance. This bolsters forecast confidence in a
composite GFS/ECMWF/Canadian model blend to provide maximum system
detail as consistent with feature predictability. Opted to edge
toward the ECMWF and EC mean by Days 6 and 7 in favor of the more
southern placement of the heavier rainfall across the interior
South for next Thursday and Friday. WPC product continuity is
decently maintained in this manner. The WPC forecast package was
based on 40% blend of the 00Z ECMWF/EC mean, 40% from the 06Z
GFS/GEFS, and 20% from the 00Z Canadian/GEM mean, increasing to 55%
from the EC solutions by Day 7.

...Weather/Hazards Highlights...

As a main/closed upper low tracks out of the West with additional
energies reaching the West Coast, widespread precipitation will
include some enhanced snow potential given cooled temperatures. The
higher elevations of the Great Basin into the Intermountain
West/Rockies will see May snow as the medium-range period begins
Monday morning. Vigorous troughing aloft in conjunction with a
potent cold front will lead to windy conditions and a threat of
high winds from much of the Rockies to High Plains as the
cyclogenesis should be well under way across the northern High
Plains into Tuesday. Precipitation is forecast to be ahead of the
system as it spreads across the northern Plains to Midwest Monday
and Tuesday with some timing differences among models (faster with
the ECMWF). Rainfall on the west side of the strong surface low
could be heavy across eastern Montana into the Dakotas given strong
upper- level support for lift with a gradual slowdown of the storm
track. However, this region is not particularly sensitive to
flooding concerns, and instability should be limited wrapping
around the low. A WPC Excessive Rainfall Outlook (ERO) Marginal
Risk area remains in place in the Day 4/Monday and into Day
5/Tuesday for this area given the potential for protracted heavy
rain, but likely without widespread significant flooding impacts.
The Marginal Risk extends southeast into the Upper/Mid- Mississippi
Valley, where more instability will be in place for higher rain
rates, but storms will likely be faster-moving. The Storm
Prediction Center also has portions of the Plains delineated with
potential for severe weather on Monday. Convective rainfall is
forecast to shift east into the Great Lakes and Ohio Valley Tuesday
into Wednesday with system progression and intercepting return
flow. The Marginal Risk ERO was maintained farther east into the
Great Lakes, Ohio Valley and down toward the mid-Mississippi Valley
for Day 5 as the fronts push farther toward the east and
northeast.

Upstream, aforementioned Pacific upper trough energy will work
inland over an unsettled West and over the south-central U.S. into
later next week, riding under the base of the lingering main upper
low/trough. These will interact with wavy and stalling fronts and
combine with pooling moisture and instability to fuel a favorable
pattern for rain and thunderstorms across the south-central
states/Mid-South and onward to the East where downstream energy
track may support moderate late next week coastal low development.

Much cooler than average maximum temperatures will slowly moderate
while spreading next week from the West to the northern Plains
as the closed upper low ejects eastward. But farther southeast,
mean upper ridging will meanwhile spread quite warm pre-frontal
temperatures from the South through the Appalachians/Mid-Atlantic
to threaten record seasonal values. 90s are forecast to become more
widespread across the southern tier, with temperatures over 100F
(and even higher heat indices) into south Texas and the possibility
for the first excessive heat days of the year.

Kong/Schichtel


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
325 AM EDT Fri May 03 2024

Valid 00Z Sat May 04 2024 - 00Z Sat May 11 2024

Surface highs strengthening and consolidating to the north of
Hawaii will favor breezy to locally strong trade flow through the
weekend into early next week. Meanwhile model guidance remains in
fairly good agreement and a favored composite shows a mid-upper
trough/low centered west of Hawaii late week and then coming
overhead by early next week. This could provide some instability
for increased showers, which would favor windward and mauka areas
given the increasing trades. Slightly above average moisture
anomalies may peak around Monday-Tuesday. Rainfall totals do not
appear to be significantly high though. This wet trade wind
pattern looks to continue into much of next week, though trades
may decrease slightly later next week.

Schichtel

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Fri May 3 2024
Day 1
Valid 01Z Sat May 04 2024 - 12Z Sat May 04 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN AND CENTRAL PLAINS, SOUTHEAST, AND IN THE VICINITY OF
WEST VIRGINIA...

...Southeast Texas...
Convection across central TX, or any outflows emanating from such
which could spur new convection, is likely to track/propagate
east- southeast to southeast across portions of south-central TX
towards the Middle TX Coast to the south of Houston/Galveston.
This area has been much drier than areas farther north and the 18z
HREF is not emphatic about the potential. This update drops the
risk level across portions of TX to a Marginal Risk, as a
precaution, should another convection complex form and develop some
isolated to widely scattered flash flood potential not seen
through the whole of the mesoscale guidance suite, whose
performance has been okay but not stellar lately.


...Central Plains...
The Marginal Risk area in Kansas has been shifted south in
deference to the 18z HREF guidance, which in itself seems a bit far
to the north based on radar reflectivity trends. A squall line is
moving across the state and there's some evidence in the mesoscale
guidance and radar reflectivity that some cell mergers with
activity that forms in advance of the feature would raise some
flash flood potential. There's always a chance that the southwest
flank of the line could slow and allow for cell training, so the
Marginal Risk accounts for such potential.


...Southeast...
An MCV from earlier activity across TX and LA is making a turn
east-northeast and gaining some convection while doing so. In case
a warm advection convective band or cell mergers with less
organized convection to its northeast occurs, extended the Marginal
Risk to the southwest to account for such potential.


...In and near West Virginia...
Active convection with heavy rainfall is expected to shift somewhat
eastward into the Blue Ridge mountains of western VA before fading
or forward propagating thereafter. The previous Marginal Risk has
been shifted somewhat eastward as a result.

Roth

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Fri May 3 2024
Day 2
Valid 12Z Sat May 04 2024 - 12Z Sun May 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF
CENTRAL AND NORTH TEXAS...

...2030Z Update...

...Southern Plains...

Model guidance has shifted south quite a bit with the most recent
updates. The general consensus now is for storms to form along the
dry line in West Texas, then develop upscale and towards the east
in central Texas. As the storms develop, there will be cell mergers
and other interactions, as well as possible training that could
lead to local rainfall totals to 5 inches. Further, soils in this
area have been saturated due to rainfall from thunderstorms as
recently as yesterday. Thus, the Slight Risk area was expanded
south to include much of central Texas. The surrounding Marginal
was expanded further south to account for likely guidance changes
prior to the event. Most of the flash flood producing rainfall is
expected overnight Saturday night, continuing into Sunday morning.

On the northern side of the Slight, what happens further south will
be critical as to how much rain falls along the Red River. Since
this area too has nearly saturated soils, the northern side of the
Slight was trimmed less on the north side than it was expanded on
the south. Nonetheless, the northern side of the Slight has more
uncertainty.

...Sacramento Valley of California...

The inherited Marginal was shifted a few miles west to move the
area off of the higher elevations of the Sierras. Unseasonably cold
air associated with an upper level low will keep most of the
precipitation at the higher elevations and passes through the
Sierras as snow. Any rainfall at the lower elevations will be
confined to the valley. The event is a strong one for this time of
year, and so abundance of moisture still keeps the area as a low
end flash flooding threat.

Wegman

...Previous Discussion...

Height falls aloft ejecting eastward from the Southern Rockies and
the presence of a dryline over West Texas will foster an
environment for thunderstorms capable of producing heavy rainfall
by later in the day on Saturday that persists into Saturday
night/early Sunday morning. Precipitable water values will have
increased to nearly 1,5 inches...some 2 to 2.5 standardized
anomalies greater than climatology...over portions of central and
west central Texas by 00Z Sunday in response to a period of
southeasterly flow off of the Gulf of Mexico. Consensus of model
guidance QPF in the range of 1 to 3 inches seems
reasonable...although the UKMET was a notable outlier.

...Northern Sierra Nevada Foothills...

A fairly robust IVT pulse will slide down the west coast with
southwesterly flow aimed orthogonally to the northern and central
Sierra's by Saturday. Higher elevations will be greeted with more
snow, but the adjacent foothills will remain warm enough to benefit
from all rainfall. Rates will be borderline overall, but the
prospects for over 1" of precip has grown in the past succession
of runs with the ensemble bias corrected guidance now upwards of 2"
now forecast within the elevations below 8000' MSL, falling mainly
in a span of 8-12 hours. The threat is on the lower end of MRGL. Maintained
continuity from previous forecast and will assess whether it needs
adjusting, or removal pending future trends in guidance.

Bann

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Fri May 3 2024
Day 3
Valid 12Z Sun May 05 2024 - 12Z Mon May 06 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHERN
MISSOURI TO NEAR THE UPPER TEXAS COAST...

...2030Z Update...

As the guidance has shifted southward on Saturday, the same has
occurred on Sunday. Storms will likely be ongoing in portions of
east Texas at the start of the period Sunday morning, and will
continue pushing east into the morning. It appears likely that
there will be an east-west-oriented boundary along which storms
will be training on the south end of a more progressive line of
storms. There is inherent and typical uncertainty with where that
line will set up, especially with recent large southward shifts in
the guidance. Thus, to cover the large changes in the guidance, as
well as very recent and ongoing heavy rain across southeast Texas,
the Slight was expanded well south of inherited, and likely will be
upped at least to a higher-end Slight in Texas if the current
shifts in the guidance hold, with a distinct possibility for a
Moderate upgrade with hopefully agreeable CAMs guidance.

For the Ozarks region, significant uncertainty also exists, as the
area will be on the northern side of the strong shortwave trough
driving all of the convective activity. Much like much of the lower
Mississippi and points west, the area also has average to above
average soil moisture, which in turn will support the development
of flash flooding given enough rainfall. The northern side of the
storm complexes often get cut off from the Gulf moisture supporting
them in favor of storms further south and closer to the Gulf. This
increases the uncertainty in the forecast for this area. For now,
guidance spread has led to an expansion of the Slight into
south-central Missouri and north-central Arkansas...as well as the
surrounding Marginal as far east as Alabama. These are likely to
change due to the highly variable forecasts of convection for this
area.

Wegman

...Previous Discussion...

The potential for excessive rainfall makes its way eastward on
Sunday in association with a closed low aloft and a cold front at
the surface. Model QPF was not as heavy as forecast on Day 2 as
shifting low level flow results in weaker low level moisture flux
convergence along the front. Despite model QPF generally being 2
inches or less...opted for a Slight Risk given the wet antecedent
conditions over portions of eastern Texas and eastern Oklahoma.

Bann

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Fri May 3 2024


As a main/closed upper low tracks out of the West with additional
energies reaching the West Coast, widespread precipitation will
include some enhanced snow potential given cooled temperatures. The
higher elevations of the Great Basin into the Intermountain
West/Rockies will see May snow as the medium-range period begins
Monday morning. Vigorous troughing aloft in conjunction with a
potent cold front will lead to windy conditions and a threat of
high winds from much of the Rockies to High Plains as the
cyclogenesis should be well under way across the northern High
Plains into Tuesday. Precipitation is forecast to be ahead of the
system as it spreads across the northern Plains to Midwest Monday
and Tuesday with some timing differences among models (faster with
the ECMWF). Rainfall on the west side of the strong surface low
could be heavy across eastern Montana into the Dakotas given strong
upper- level support for lift with a gradual slowdown of the storm
track. However, this region is not particularly sensitive to
flooding concerns, and instability should be limited wrapping
around the low. A WPC Excessive Rainfall Outlook (ERO) Marginal
Risk area remains in place in the Day 4/Monday and into Day
5/Tuesday for this area given the potential for protracted heavy
rain, but likely without widespread significant flooding impacts.
The Marginal Risk extends southeast into the Upper/Mid- Mississippi
Valley, where more instability will be in place for higher rain
rates, but storms will likely be faster-moving. The Storm
Prediction Center also has portions of the Plains delineated with
potential for severe weather on Monday. Convective rainfall is
forecast to shift east into the Great Lakes and Ohio Valley Tuesday
into Wednesday with system progression and intercepting return
flow. The Marginal Risk ERO was maintained farther east into the
Great Lakes, Ohio Valley and down toward the mid-Mississippi Valley
for Day 5 as the fronts push farther toward the east and
northeast.

Upstream, aforementioned Pacific upper trough energy will work
inland over an unsettled West and over the south-central U.S. into
later next week, riding under the base of the lingering main upper
low/trough. These will interact with wavy and stalling fronts and
combine with pooling moisture and instability to fuel a favorable
pattern for rain and thunderstorms across the south-central
states/Mid-South and onward to the East where downstream energy
track may support moderate late next week coastal low development.

Much cooler than average maximum temperatures will slowly moderate
while spreading next week from the West to the northern Plains
as the closed upper low ejects eastward. But farther southeast,
mean upper ridging will meanwhile spread quite warm pre-frontal
temperatures from the South through the Appalachians/Mid-Atlantic
to threaten record seasonal values. 90s are forecast to become more
widespread across the southern tier, with temperatures over 100F
(and even higher heat indices) into south Texas and the possibility
for the first excessive heat days of the year.

Kong/Schichtel

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Fri May 3 2024


As a main/closed upper low tracks out of the West with additional
energies reaching the West Coast, widespread precipitation will
include some enhanced snow potential given cooled temperatures. The
higher elevations of the Great Basin into the Intermountain
West/Rockies will see May snow as the medium-range period begins
Monday morning. Vigorous troughing aloft in conjunction with a
potent cold front will lead to windy conditions and a threat of
high winds from much of the Rockies to High Plains as the
cyclogenesis should be well under way across the northern High
Plains into Tuesday. Precipitation is forecast to be ahead of the
system as it spreads across the northern Plains to Midwest Monday
and Tuesday with some timing differences among models (faster with
the ECMWF). Rainfall on the west side of the strong surface low
could be heavy across eastern Montana into the Dakotas given strong
upper- level support for lift with a gradual slowdown of the storm
track. However, this region is not particularly sensitive to
flooding concerns, and instability should be limited wrapping
around the low. A WPC Excessive Rainfall Outlook (ERO) Marginal
Risk area remains in place in the Day 4/Monday and into Day
5/Tuesday for this area given the potential for protracted heavy
rain, but likely without widespread significant flooding impacts.
The Marginal Risk extends southeast into the Upper/Mid- Mississippi
Valley, where more instability will be in place for higher rain
rates, but storms will likely be faster-moving. The Storm
Prediction Center also has portions of the Plains delineated with
potential for severe weather on Monday. Convective rainfall is
forecast to shift east into the Great Lakes and Ohio Valley Tuesday
into Wednesday with system progression and intercepting return
flow. The Marginal Risk ERO was maintained farther east into the
Great Lakes, Ohio Valley and down toward the mid-Mississippi Valley
for Day 5 as the fronts push farther toward the east and
northeast.

Upstream, aforementioned Pacific upper trough energy will work
inland over an unsettled West and over the south-central U.S. into
later next week, riding under the base of the lingering main upper
low/trough. These will interact with wavy and stalling fronts and
combine with pooling moisture and instability to fuel a favorable
pattern for rain and thunderstorms across the south-central
states/Mid-South and onward to the East where downstream energy
track may support moderate late next week coastal low development.

Much cooler than average maximum temperatures will slowly moderate
while spreading next week from the West to the northern Plains
as the closed upper low ejects eastward. But farther southeast,
mean upper ridging will meanwhile spread quite warm pre-frontal
temperatures from the South through the Appalachians/Mid-Atlantic
to threaten record seasonal values. 90s are forecast to become more
widespread across the southern tier, with temperatures over 100F
(and even higher heat indices) into south Texas and the possibility
for the first excessive heat days of the year.

Kong/Schichtel

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
436 PM EDT Fri May 3 2024

Valid 00Z Sat May 04 2024 - 00Z Tue May 07 2024

...Cascades through Sierra Nevada, Intermountain West, and
Rockies...
Days 1-3...

Sprawling low over the northeast Pacific west of BC this afternoon
will deepen further from reinforcing shortwave energy tonight as it
reaches the OR/CA coast before shifting east over far northern CA
through Saturday night. This is an anomalously deep system for
early May, with 500/700mb heights dropping 3 standard deviations
below normal across southern Oregon and northern California. Warm
conveyor moisture brings high elevation snow tonight to the
Shasta/Siskiyou and southern OR Cascades before notable height
falls and strong moisture influx along with left-exit region upper
jet forcing and strong topographic lift brings a swath of heavy
snow with rates locally exceeding 2"/hr down the Sierra Nevada
Saturday afternoon and evening. Initial snow levels on the Sierra
of over 7000ft Saturday morning will quickly drop below 5000ft
during the heavy precip rates.

This will generally be a heavy/wet snow with lower SLRs adding a
snow load risk to the snow amounts which continue to trend upward.
Day 1.5 WPC PWPF shows high probabilities for 8 inches or more
falling on the CA Cascades and the northern/central Sierra in areas
above 5000ft. Heavier accumulations of 1-2ft can be expected above
about 7000ft. In addition to the falling snow, strong/gusty winds
and blowing snow are forecast to contribute to moderate to locally
major winter storm impacts as indicated by the WSSI.

The system slowly fills/weakens as it moves from the northern CA/NV
border Sunday morning across northern UT Sunday night. Snow levels
still drop to around 5000ft with the low over the Great
Basin/Intermountain west. Day 2.5 PWPF for more than 8 inches are
40-80% over the higher ranges of eastern OR/northeast NV, central
ID, northern UT (including the Wasatch where local maxima are
expected) to northwest WY.

However, redevelopment is forecast by the 12Z model consensus as
it moves over WY on Monday with Gulf-sourced moisture streaming in
from the Plains enhancing snowfall for the eastern slopes of the
northern Rockies. Day 3 PWPF for more than 8 inches are 30-70% for
the northern CO Rockies, Absarokas and Bighorns in WY and into
ranges of southwest MT.

Furthermore, the next trough off the Pacific crosses the PacNW on
Monday with renewed snow on the Cascades with snow levels around
4000ft. Day 3 PWPF for more than 8 inches are 40-80% north from the
central OR Cascades through the western WA Cascades.


The probability of significant icing is less than 10 percent.


Jackson

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Fri May 3 2024

Valid 12Z Mon May 06 2024 - 12Z Fri May 10 2024


...Northern Plains to Midwest Heavy Rain Threat early next week...

...Possible heavy rain across the interior South later next week...


...Overview...

Models and ensemble means continue to indicate a synoptic pattern
that swings upper-level troughs/lows onshore across the western
U.S. that would favor cyclogenesis over the mid-section of the
country followed by gradual lifting of the troughs toward New
England. The beginning of the medium-range period (Monday morning)
will likely feature one such cyclogenesis over the central to
northern High Plains. This will support cooler than normal
temperatures, gusty winds, and precipitation including higher
elevation snow in the West early in the week. Rain that could be
locally heavy shifts into the northern Plains and downstream
through the Midwest. Meanwhile farther south, mean upper ridging
will lead to warming pre-frontal temperatures for much of the
central and eastern U.S. that could yield some record values.
Upstream, Pacific upper trough energy will work inland over an
unsettled West and to the south- central U.S.. This may interact
with a wavy and stalling lead front and combine with pooling
moisture and instability to fuel rain and thunderstorms over the
south-central states/Mid-South and onward into later next week,
still also under some influence from the lead upper low/trough.

...Guidance/Predictability Assessment...

Model and ensemble solutions overall seem reasonably clustered for
Monday-Wednesday and generally in step with National Blend of
Models (NBM) guidance. This bolsters forecast confidence in a
composite GFS/ECMWF/Canadian model blend to provide maximum system
detail as consistent with feature predictability. Opted to edge
toward the ECMWF and EC mean by Days 6 and 7 in favor of the more
southern placement of the heavier rainfall across the interior
South for next Thursday and Friday. WPC product continuity is
decently maintained in this manner. The WPC forecast package was
based on 40% blend of the 00Z ECMWF/EC mean, 40% from the 06Z
GFS/GEFS, and 20% from the 00Z Canadian/GEM mean, increasing to 55%
from the EC solutions by Day 7.

...Weather/Hazards Highlights...

As a main/closed upper low tracks out of the West with additional
energies reaching the West Coast, widespread precipitation will
include some enhanced snow potential given cooled temperatures. The
higher elevations of the Great Basin into the Intermountain
West/Rockies will see May snow as the medium-range period begins
Monday morning. Vigorous troughing aloft in conjunction with a
potent cold front will lead to windy conditions and a threat of
high winds from much of the Rockies to High Plains as the
cyclogenesis should be well under way across the northern High
Plains into Tuesday. Precipitation is forecast to be ahead of the
system as it spreads across the northern Plains to Midwest Monday
and Tuesday with some timing differences among models (faster with
the ECMWF). Rainfall on the west side of the strong surface low
could be heavy across eastern Montana into the Dakotas given strong
upper- level support for lift with a gradual slowdown of the storm
track. However, this region is not particularly sensitive to
flooding concerns, and instability should be limited wrapping
around the low. A WPC Excessive Rainfall Outlook (ERO) Marginal
Risk area remains in place in the Day 4/Monday and into Day
5/Tuesday for this area given the potential for protracted heavy
rain, but likely without widespread significant flooding impacts.
The Marginal Risk extends southeast into the Upper/Mid- Mississippi
Valley, where more instability will be in place for higher rain
rates, but storms will likely be faster-moving. The Storm
Prediction Center also has portions of the Plains delineated with
potential for severe weather on Monday. Convective rainfall is
forecast to shift east into the Great Lakes and Ohio Valley Tuesday
into Wednesday with system progression and intercepting return
flow. The Marginal Risk ERO was maintained farther east into the
Great Lakes, Ohio Valley and down toward the mid-Mississippi Valley
for Day 5 as the fronts push farther toward the east and
northeast.

Upstream, aforementioned Pacific upper trough energy will work
inland over an unsettled West and over the south-central U.S. into
later next week, riding under the base of the lingering main upper
low/trough. These will interact with wavy and stalling fronts and
combine with pooling moisture and instability to fuel a favorable
pattern for rain and thunderstorms across the south-central
states/Mid-South and onward to the East where downstream energy
track may support moderate late next week coastal low development.

Much cooler than average maximum temperatures will slowly moderate
while spreading next week from the West to the northern Plains
as the closed upper low ejects eastward. But farther southeast,
mean upper ridging will meanwhile spread quite warm pre-frontal
temperatures from the South through the Appalachians/Mid-Atlantic
to threaten record seasonal values. 90s are forecast to become more
widespread across the southern tier, with temperatures over 100F
(and even higher heat indices) into south Texas and the possibility
for the first excessive heat days of the year.

Kong/Schichtel


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