Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Sat May 04 2024

Valid 00Z Sun May 05 2024 - 00Z Tue May 07 2024

...A deep upper low will bring heavy mountain snow and windy conditions
across a large portion of the western U.S. through the next couple of
days...

...A slight chance of severe thunderstorms across Texas and upper Midwest
into Sunday will shift into the central Plains by later on Monday...

...Critical fire danger is forecast for the southern Rockies...

A rather vigorous and deep upper-level low currently approaching the West
Coast will result in the next wave of active and stormy weather to quickly
sweep through the western U.S. and into the Great Plains during the next
couple of days. Moderate to heavy rain and high-elevation heavy wet snow
are already reaching into the West Coast in earnest, especially across
central to northern California. The precipitation will expand quickly
inland across the Great Basin and a good portion of the Pacific Northwest
through tonight, reaching into the northern Rockies on Sunday, followed by
the central Rockies on Monday. This system will bring quite a bit of
wind, especially across southern California and Nevada through tonight,
from the Four Corners region to Wyoming on Sunday before overspreading
much of the High Plains on Monday. Two feet or more of heavy wet snow is
expected to accumulate along the Cascades in Oregon, with a foot along the
Sierra Nevada, and a foot or more for portions of the northern Cascades,
Great Basin, as well as northern and central Rockies through Monday. In
addition, severe thunderstorms are expected to develop over the central
Plains later on Monday ahead of a potent cold front. Meanwhile, the
center of an elongated deep low pressure system is forecast to track
across the northern Plains with wind-swept rain impacting the northern
High Plains on Monday.

Meanwhile, across the eastern two-thirds of the country, the most active
weather will be found over Texas where strong to severe thunderstorms can
be expected through tonight as an upper-level disturbance arriving from
northern Mexico begins to interact with a stationary front. Additional
energy setting up by the upper trough is expected to trigger another round
of strong to severe thunderstorms across central Texas to Oklahoma on
Sunday, before the potent cold front associated with the intensifying
elongated low shifts the focus of severe thunderstorms farther north
across the central Plains later on Monday.

Still farther east, some showers and possibly severe thunderstorms are
forecast across the Midwest to upper Mississippi Valley through tonight
near and ahead of a low pressure wave developing along a cold front. This
system will advance eastward and gradually merge with the large area of
scattered rain and embedded thunderstorms over the eastern U.S. in
association with a back-door cold front. These rain/showers will spread
farther northeast into New England on Sunday, and become more widely
scattered on Monday. Meanwhile, the Mid-South will see a higher chance of
showers and thunderstorms on Monday.

The Mid-Atlantic northern and central Plains will be cool in contrast with
warm conditions over the High Plains and Ohio Valley. Meanwhile, much
cooler weather will surge into and penetrate the western U.S. with the
arrival of the deep and vigorous upper trough along with windy and
inclement weather closer to the elongated low pressure system. Finally,
windy and dry conditions are forecast to raise the danger of wild fires
across the southern Rockies beginning on Sunday.

Kong


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
231 PM EDT Sat May 4 2024

Valid 12Z Tue May 07 2024 - 12Z Sat May 11 2024


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

...Mid-South Excessive Rainfall Threat into next Midweek...


...Overview...

It remains the case that recent models and ensembles indicate a
synoptic pattern that swings upper-level troughs/lows onshore
across the western U.S. that lead to and subsequently favor
downstream upper focus and cyclogenesis over the northern mid-
section of the country followed by gradual lifting of the troughs
toward the Northeast. The beginning of the medium-range period
(Tuesday morning) will likely feature one such cyclogenesis over
the 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 is likely to
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/eastward into
later next week, still also under some influence from the lead
upper low/trough.

...Guidance/Predictability Assessment...

Latest 00z and 06z guidance continues to show good agreement for
the overall synoptic pattern, though plenty of uncertainty still
in the details which has impactful implications on QPF and surface
features. A general compromise of the deterministic solutions
offered a good starting point for the Tuesday-Thursday/Day 3-5
forecast. After this, there remain significant differences between
model guidance and run to run continuity on how far south and east
the lingering northern Plains energy gets into the Great
Lakes/Northeast. Also uncertainty out west with the evolution of
energy into the Great Basin and a blocky upper ridge/high to the
north. The WPC blend trended towards more ensemble mean influence
late period to mitigate these concerns. This is consistent both
with previous WPC continuity as well as the ECMWF-initialized AI/ML
models.

...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
Tuesday 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 Tuesday
into midweek. 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 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 Day 4/Tuesday and into Day 5/Wednesday for this
area given the potential for protracted heavy rain, but likely
without widespread significant flooding impacts. 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 remains farther
east into the Great Lakes, Ohio Valley and down toward the mid-
Mississippi Valley for Day 4/5 as the fronts push farther toward
the east and northeast. A Slight Risk threat area remains in the
vicinity of the Mid-South Day 5/Wednesday as energy interacts with
a wavy and stalling front and combines with pooling moisture and
instability. The Storm Prediction Center also highlights a broad
area of the Midwest to Mississippi Valley/parts of the Southern
Plains for severe weather potential Tuesday and Wednesday. Beyond
Wednesday, a favorable pattern will continue for rain and
thunderstorms across the south-central states/Mid-South and
broadly onward to the East where downstream energy track may
support moderate late next week coastal low development, likely
off New England.

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 Tuesday-Thursday
before moderation by Friday and Saturday.

Santorelli/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
300 AM EDT Sat May 04 2024

Valid 00Z Sun May 05 2024 - 00Z Sun May 12 2024

Guidance agrees that 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 also 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. This
would be especially so if the latest solutions of the 00 UTC
Canadian and trends from the 00 UTC UKMET come to pass with a much
more robust digging of upper trough energy southward toward the
islands later next week. While ensemble support is much more in
line with the 00 UTC GFS/ECMWF with this feature, there is at
least an amplified upstream upper ridge in support of digging to
monitor over the next few guidance cycles.

Schichtel

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
354 PM EDT Sat May 4 2024
Day 1
Valid 12Z Sat May 04 2024 - 12Z Sun May 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL TODAY AND TONIGHT
ACROSS PORTIONS OF THE SOUTHERN PLAINS...

..Southern Plains and Gulf Coast...

16Z update... Very small adjustments were made to the Marginal's
northern bounds and the southern bounds of the Slight Risk to
adjust for a very small southward trend with the QPF footprint.
Consensus still favors the highest amounts to focus near the Texas
Hill country and surrounding locations to the immediate east and
north. Hi-res guidance is showing several hours where the hourly
rainfall rates will be 2-2.5 inches/hr with a couple instances
where it surges to 3-4 inches/hr.

Campbell

The focus of attention becomes the Southern Plains as a mid- and
upper level trough emerges from the southern Rockies later today
and encounters an atmosphere becoming increasingly moist and
unstable over much of Texas given persistent flow off the Gulf of
Mexico. The general consensus of guidance opinion is that storms
will be forming along the dry line in West Texas and 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. In addition...soils in this area have been saturated due
to rainfall from thunderstorms as recent 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 tonight/early
Sunday morning with a gradual northward and eastward expansion
across the Plains. Also made an expansion of the Slight Risk
southward towards southeast Texas in deference to signals from the
00Z HRRR of a complex that bows out and spreads yet more rain into
the area that does not need any more late tonight/early Sunday
morning.

Bann


...Eastern Ohio and Tennessee Valleys, Appalachians and Mid-
Atlantic...

Convection is firing up across portions of the Carolinas/Mid-
Atlantic region as confluent deep moisture streams interact with
the shortwave energy lifting out of the Southeast. Scattered areas
of heavy rainfall (1.5 to 2 inches/hr) and localized flash flooding
will be possible. There may be isolated max accumulations nearing
3 inches where storm motion is slow. Further north across the
eastern portions of the Tennessee and Ohio Valleys there will be
ample instability and mid-level forcing to support stronger cells
to move southward, especially into flashy prone areas. A Marginal
Risk area was hoisted for this period, spanning from central
Georgia north to eastern Ohio and southern Pennsylvania.

Campbell


...Sacramento Valley of California...

Maintained the Marginal Risk area with few changes as a deep and
cold low moves into the Pacific Northwest and spread rain into
the lower elevations of northern California. Latest guidance
continues to advertise 1 to 3 inches of rain possible in the
foothills of the Sacramento Valley and west aspect of the
mountains at lower elevations. This continues to be a low-end
Marginal...but excessive rainfall is possible in the event the
rainfall rates are enhanced by any thunderstorms.

Bann

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
354 PM EDT Sat May 4 2024
Day 2
Valid 12Z Sun May 05 2024 - 12Z Mon May 06 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST
TEXAS...

21Z update... Given the uptick in QPF trends over the water-logged
locations of southeast Texas, a Moderate Risk was raised for this
period. Areas of flooding are ongoing and additional rainfall will
only serve to aggravate or worsen conditions. Hi-res guidance is
depicting severals hours of rainfall rates pulsing from 1 to 3+
inch/hour as the cells cross the state. Over half of these
solutions show the heavy rains tracking across areas south and
east of of the Houston metro, so there may be the need for an
expansion of the Moderate Risk with future updates.

Campbell

They system over the Plains today will continue to move eastward on
Sunday. Storms will likely be ongoing in portions of
eastern/southeastern Texas at the start of the period Sunday and
will continue pushing eastward with training of cells appearing
likely along an east- west- oriented boundary 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 and a fairly large
spread within any suite of guidance. Thus, to cover the large
changes in the guidance, as well as very recent and ongoing heavy
rain across southeast Texas, maintained the large Slight Risk area.
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. The big concern is whether
or not the active convection farther south will disrupt the feed of
moisture and instability. For the moment...will not make too many
changes to the guidance that led to an expansion of the Slight
into parts of Missouri and Arkansas earlier.

Bann

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
354 PM EDT Sat May 4 2024
Day 3
Valid 12Z Mon May 06 2024 - 12Z Tue May 07 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL AND NORTHERN PLAINS ON MONDAY AND MONDAY NIGHT...

21Z update... There is a growing consensus for much of eastern
Montana and surrounding areas to get a soaking rain for this
period. Much of this region has nearly saturated soils which has
reduced the 3-hr FFG to as low as 1 inch. Portions of the Bear Paw
Mountains in north-central Montana still have liquid equivalent of
1 to 2.5 inches in the snowpack. Rain over this snowpack will
likely speed up snowmelt while increasing the amount of water
available for runoff. Areas of local flash flooding will be
possible.

For the Central Plains, the latest guidance and WPC forecast
shifted the QPF to the east/northeast about 100-150 miles. This
resulted in trimming the back edge of the Marginal to central
South Dakota to northeast Oklahoma and expanding the eastern
boundary into southern Minnesota and eastern Iowa. The Slight Risk
was shifted eastward and now encompasses eastern Kansas/Nebraska,
southeast South Dakota, western Iowa and western Missouri.

Campbell

Heavy rainfall that results in flash flooding is possible from
convection that is expected to develop over the Plains on Sunday as
an upper trough ejects northeastward across the northern/central
Plains on Monday. Precipitable water values of 1.2 to 1.6 inches
will be drawn northward on 850 mb flow of 30 to 45 kts and modest
height falls ahead of the trough aloft and a surface dry line.
Rainfall rates over 1.5 inches per hour appear reasonable given the
amount of instability to aid updraft strength. The broadly
diffluent aloft that forms could result in repeated rounds of
heavy rainfall in some cases but broader/larger scale organization
may keep individual cell motion progressive enough to mitigate at
least some of the excessive rainfall potential.

Bann

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
231 PM EDT Sat May 4 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
Tuesday 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 Tuesday
into midweek. 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 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 Day 4/Tuesday and into Day 5/Wednesday for this
area given the potential for protracted heavy rain, but likely
without widespread significant flooding impacts. 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 remains farther
east into the Great Lakes, Ohio Valley and down toward the mid-
Mississippi Valley for Day 4/5 as the fronts push farther toward
the east and northeast. A Slight Risk threat area remains in the
vicinity of the Mid-South Day 5/Wednesday as energy interacts with
a wavy and stalling front and combines with pooling moisture and
instability. The Storm Prediction Center also highlights a broad
area of the Midwest to Mississippi Valley/parts of the Southern
Plains for severe weather potential Tuesday and Wednesday. Beyond
Wednesday, a favorable pattern will continue for rain and
thunderstorms across the south-central states/Mid-South and
broadly onward to the East where downstream energy track may
support moderate late next week coastal low development, likely
off New England.

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 Tuesday-Thursday
before moderation by Friday and Saturday.

Santorelli/Schichtel

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
231 PM EDT Sat May 4 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
Tuesday 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 Tuesday
into midweek. 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 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 Day 4/Tuesday and into Day 5/Wednesday for this
area given the potential for protracted heavy rain, but likely
without widespread significant flooding impacts. 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 remains farther
east into the Great Lakes, Ohio Valley and down toward the mid-
Mississippi Valley for Day 4/5 as the fronts push farther toward
the east and northeast. A Slight Risk threat area remains in the
vicinity of the Mid-South Day 5/Wednesday as energy interacts with
a wavy and stalling front and combines with pooling moisture and
instability. The Storm Prediction Center also highlights a broad
area of the Midwest to Mississippi Valley/parts of the Southern
Plains for severe weather potential Tuesday and Wednesday. Beyond
Wednesday, a favorable pattern will continue for rain and
thunderstorms across the south-central states/Mid-South and
broadly onward to the East where downstream energy track may
support moderate late next week coastal low development, likely
off New England.

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 Tuesday-Thursday
before moderation by Friday and Saturday.

Santorelli/Schichtel

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 PM EDT Sat May 4 2024

Valid 00Z Sun May 05 2024 - 00Z Wed May 08 2024

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

A large vertically stacked low is moving ashore near the
California-Oregon border this afternoon. The storm is characterized
by low heights indicative of an unseasonably cold air mass. As of
this writing, snow levels are falling through the 5,000s ft above
sea level, resulting in snow ongoing across much of the Sierra
Nevada range of California. The peak precipitation rates over the
next 3 days are happening now into the Sierra Nevada in association
with the strong cold front demarcating the leading edge of the much
colder air mass.

The surface low will rapidly weaken as it moves inland and is
sheared apart by the mountains. Thus, while overall precipitation
rates will diminish, the upper level forcing will continue pressing
eastward, allowing the associated cold air to do the same. Thus,
snow will continue to move inland into the Intermountain West
through Day 2/Monday, with nearly all mountain ranges in the
western states outside of Arizona and New Mexico picking up at
least some snow. The greatest snowfall amounts will be in the
Sierra Nevada broadly, but over the next 3 days, the highest peaks
of the Cascades down to the Sierras could easily top 4 feet of new
snow, again with the heaviest snow ongoing through tonight before
gradually tapering to more of a long-duration light snow.

Atmospheric moisture amounts are about 3 sigma above normal across
the Central Valley this afternoon. While these levels will taper
as the storm moves into the Intermountain West, the unseasonably
cold air to follow keeps the snow going through the mountains and
passes, causing travel delays with major impacts nearly certain in
the probabilistic winter storm severity index across the Sierras
with potential for near major impacts by Tuesday into the Little
Belt and Highwood Mountains of central Montana.

The upper level low will track eastward and open up into a
negatively tilted trough over Colorado and Wyoming by early Monday
morning. The vigorous energy associated therewith will allow the
development of a new surface low over the High Plains of eastern
Montana Sunday night. The low will quickly intensify as the upper
level energy wraps back on itself along the ND/MT border Monday.
This will allow the comma-head region of the low to draw in Gulf
moisture as an extended LLJ reaches from the waterlogged upper
Texas Coast all the way into the Canadian Prairies. Due to it being
almost mid-May by this point, most areas will get rain, but the
higher elevations of Montana and the Bighorns of Wyoming will see
upwards of a foot of new snow on Monday as a result.

The probability of significant icing is less than 10 percent.

Wegman

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
231 PM EDT Sat May 4 2024

Valid 12Z Tue May 07 2024 - 12Z Sat May 11 2024


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

...Mid-South Excessive Rainfall Threat into next Midweek...


...Overview...

It remains the case that recent models and ensembles indicate a
synoptic pattern that swings upper-level troughs/lows onshore
across the western U.S. that lead to and subsequently favor
downstream upper focus and cyclogenesis over the northern mid-
section of the country followed by gradual lifting of the troughs
toward the Northeast. The beginning of the medium-range period
(Tuesday morning) will likely feature one such cyclogenesis over
the 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 is likely to
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/eastward into
later next week, still also under some influence from the lead
upper low/trough.

...Guidance/Predictability Assessment...

Latest 00z and 06z guidance continues to show good agreement for
the overall synoptic pattern, though plenty of uncertainty still
in the details which has impactful implications on QPF and surface
features. A general compromise of the deterministic solutions
offered a good starting point for the Tuesday-Thursday/Day 3-5
forecast. After this, there remain significant differences between
model guidance and run to run continuity on how far south and east
the lingering northern Plains energy gets into the Great
Lakes/Northeast. Also uncertainty out west with the evolution of
energy into the Great Basin and a blocky upper ridge/high to the
north. The WPC blend trended towards more ensemble mean influence
late period to mitigate these concerns. This is consistent both
with previous WPC continuity as well as the ECMWF-initialized AI/ML
models.

...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
Tuesday 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 Tuesday
into midweek. 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 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 Day 4/Tuesday and into Day 5/Wednesday for this
area given the potential for protracted heavy rain, but likely
without widespread significant flooding impacts. 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 remains farther
east into the Great Lakes, Ohio Valley and down toward the mid-
Mississippi Valley for Day 4/5 as the fronts push farther toward
the east and northeast. A Slight Risk threat area remains in the
vicinity of the Mid-South Day 5/Wednesday as energy interacts with
a wavy and stalling front and combines with pooling moisture and
instability. The Storm Prediction Center also highlights a broad
area of the Midwest to Mississippi Valley/parts of the Southern
Plains for severe weather potential Tuesday and Wednesday. Beyond
Wednesday, a favorable pattern will continue for rain and
thunderstorms across the south-central states/Mid-South and
broadly onward to the East where downstream energy track may
support moderate late next week coastal low development, likely
off New England.

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 Tuesday-Thursday
before moderation by Friday and Saturday.

Santorelli/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