Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Fri May 02 2025

Valid 12Z Fri May 02 2025 - 12Z Sun May 04 2025

...Upper-level omega block to form across the CONUS, leading to periods of
unsettled weather and large temperatures contrasts heading into the
weekend...

...Heavy rain, flooding, and severe weather threats to stretch from the
Southern Plains northeast into the Ohio Valley Friday...

The mid- to upper level flow pattern will become increasingly amplified
over the next few days with an omega block forming from California,
eastward into the Ohio Valley. This pattern will bring periods of
unsettled weather first to portions of the eastern and southern U.S., and
eventually to the western U.S. heading into the weekend. Southerly flow
supporting a moist airmass along a cold front stretching from the Lower
Great Lakes southwestward through the Ohio Valley, Middle/Lower
Mississippi Valley, and southern Plains will help to focus the first area
of showers and thunderstorms. The boundary is forecast to slow through
Friday and become quasi-stationary in some areas, bringing repeated rounds
of storms as storms will have a tendency to move generally parallel to the
boundary. The plentiful moisture will lead to locally heavy downpours, and
there is a Slight Risk of Excessive Rainfall (level 2/4) across portions
of the Lower Mississippi Valley/Mid-South, ArkLaTex, and southern Plains
where scattered instances of flash flooding will be possible. In addition,
increasing upper-level winds as an upper-level trough drops southward over
the very moist, buoyant airmass will lead to some more intense clusters of
thunderstorms. The Storm Prediction Center has outlined an Enhanced Risk
of severe weather (level 3/5) stretching from the Ohio Valley
southwestward through the Tennessee Valley, the Lower Mississippi Valley,
and into portions of Texas for the threat of damaging winds, large hail
(especially in Texas), and perhaps a tornado or two. The front will make
some progress south and eastward into Saturday, bringing shower and
thunderstorm chances to the Northeast, Mid-Atlantic, Appalachians, and
Southeast. Moderate to locally heavy rainfall is expected with some
additional isolated instances of flash flooding possible.

To the West, some scattered showers and thunderstorms will be possible
along a lingering frontal boundary through the Four Corners region
Saturday before a second upper-level trough solidifying the omega block
drops southward from the northeastern Pacific and over the West Coast by
late Friday/early Saturday. Precipitation chances will first pick up over
the Pacific Northwest Friday evening before spreading into the Great Basin
and northern Rockies Saturday. Some snow will be possible for some of the
higher mountain peaks, particularly in the Sierra Nevada.

The increasingly amplified pattern stretching west to east across the
CONUS will bring large temperatures contrasts heading into the weekend.
Forecast highs from the Ohio/Tennessee Valleys east to the East Coast will
remain above average Friday, with the greatest anomalies focused across
portions of the Mid-Atlantic. Forecast highs generally range in the 70s
across the Northeast and Ohio Valley with 80s for the Mid-Atlantic and the
Tennessee Valley/broader Southeast. The cold front will bring cooler
temperatures to the Ohio Valley and much of the Southeast Saturday as
highs fall into the 60s and 70s. Highs along the immediate East Coast will
remain well above average and into the 70s north with 80s further south.
Portions of the Great Lakes/Midwest and much of the Plains will be well
below average the next couple of days under the eastern trough, with highs
generally in the 50s and 60s for the Great Lakes/Midwest and 60s and 70s
for the central/southern Plains outside of southern Texas. Much of the
West will be well above average Friday, especially across the northern
Great Basin/Rockies, as highs rise into the 70s and 80s for most
locations. The well above average temperatures will shift eastward on
Saturday as the second upper-level trough drops southward along the West
Coast. Highs across the northern Plains, northern/central Rockies, and
eastern portions of the Great Basin will remain and rise into the 70s and
80s. Much cooler temperatures will come to the West Coast under the
influence of the trough with highs dropping into the 50s and 60s.

Putnam

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 AM EDT Fri May 2 2025

Valid 12Z Mon May 05 2025 - 12Z Fri May 09 2025


...Blocky pattern with closed lows over the West and the East with
areas of heavy rainfall potential early to mid next week...


...Overview...

A blocky pattern over the lower 48 will be in place by the
beginning of the medium range period on Monday. The dominant
features will be two closed upper lows, one over the West/Southwest
and one over the East, that will both slowly drift around
weakening through Wednesday/Thursday. These systems are expected to
produce areas of significant rainfall over portions of the Central
and Southern Plains, Northeast, and Mid- Atlantic and high
elevation snows in the West. Eventually, the eastern upper low
should interact with a northern stream trough that will swing
across southern Canada, resulting in the low being swept up towards
the Canadian Maritimes later next week. The Western low will
likely remain isolated as it drifts east over the Plains and
weakens. The remnants of the Western low may eventually combine
with another trough developing over the East Coast late Thursday
into Friday. By Wednesday, another trough and frontal system will
approach the West Coast.

...Guidance/Predictability Assessment...

Models and ensembles remain in good agreement on the presence of a
blocky pattern to start next week, though subtle differences in
exactly where the upper lows set up over the Southwest and the Ohio
Valley will directly impact the distribution and locations of
heaviest rainfall. In the East, the GFS was displaced slightly east
and this resulted in a quicker push off the East Coast mid next
week. Meanwhile, a northern stream shortwave swinging into the
Great Lakes was much more pronounced in the CMC with another closed
low over the East, which was very much an outlier solution at this
point. The ECMWF (with the ensemble means and UKMET early in the
period) was preferred for this system, but a little faster exit
like depicted in the GFS is not out of the question.

Out West, there remains some notable differences in energy
rotating around the upper low early in the week, but general
agreement that this low should drift slowly eastward as it weakens
into the Central Plains and eventually gets absorbed into some
broader troughing over the East late next week. A general model
compromise, with the ensemble means later in the period worked well
as a starting point, outside of the CMC due to the outlier upper
low over the East late next week.


...Weather/Hazards Highlights...

Early next week, two occluded frontal systems are expected to
bring moderate to locally heavy rainfall to portions of the West
and East. In the West, anomalous moisture accompanying the upper
low will support heightened precipitation from the Great Basin to
the Central and Southern Rockies and Plains. By monday, the system
will become stationary over the Plains, allowing moisture to
continuously pool over the Plains, which will likely result in a
multi-day heavy rainfall event across the Central and Southern
Plains and into the lower Mississippi Valley Monday-Wednesday.
Portions of the Plains states will be especially vulnerable to
heavy rain/flooding due to recent heavy rains and saturated soils.
This activity is highlighted in the Day 4-5 Excessive Rainfall
Outlooks (EROs) with a slight risk area highlighted across the
Texas and Oklahoma panhandles on Monday, and another slight risk
area on Tuesday for parts of the Lower Mississippi Valley/ArkLaTex
vicinity. Marginal risks surround these areas.

Under the eastern upper low, the frontal system will very slowly
push eastward early next week, resulting in a stream of onshore
moisture over the Mid-Atlantic and Northeast and persistent
precipitation across these regions through Tuesday. Initially,
rainfall could be beneficial and bring some relief to drought
conditions, but repeated rounds of rainfall could produce locally
heavy totals and may result in localized flooding in urban and poor
drainage areas. The Day 4-5 EROs show a Marginal Risk area across
these regions for Monday and Tuesday. By Wednesday, the frontal
system should accelerate to the east as the upper low interacts
with the northern stream, shifting precipitation farther into the
Northeast.

The upper lows over the East and Southwest early in the week will
result in below to well below normal temperatures, especially the
Southwest where daytime highs could be 15-20 degrees below normal.
This should moderate with time as the East returns to near normal
by Tuesday and the Southwest area slowly shifts eastward. Above
normal temperatures to begin the week over the Northern Plains may
expand into the Upper Great Lakes. Upper ridging building over the
west as the upper low shifts east will allow for expanding coverage
of above normal temperatures next week.


Santorelli


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
322 AM EDT Fri May 2 2025

Valid 00Z Sat 03 May 2025 - 00Z Sat 10 May 2025

Guidance continues to show a persistent trade pattern over the
next week or so with precipitable water values tending to be near
or somewhat below climatology. Upper ridging should prevail over
and north of the main islands into midweek, possibly weakening by
next Thursday as weak troughing approaches from the east. Thus
generally windward/mountain- focused showers should be fairly
light. The modest decrease in heights aloft may lead to a subtle
increase in rainfall around the end of the period. Initially
moderate wind speeds should become brisk Friday onward due to one
high passing by to the north on Friday and then another high to
the northwest/north around the middle to latter part of next week.

Santorelli

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Thu May 1 2025
Day 1
Valid 01Z Fri May 02 2025 - 12Z Fri May 02 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN
OKLAHOMA AND FAR WESTERN ARKANSAS, SOUTH-CENTRAL TEXAS, AS WELL AS
FOR PORTIONS OF THE CENTRAL APPALACHIANS...

...01Z Update...

A Slight Risk was added to portions of south-central Texas, from
the U.S.-Mex border eastward through the Austin-San Antonio metros
to College Station, where slow-moving, heavy-rainfall producing
storms have increased in coverage this evening. Supported by moist,
southeasterly low level inflow and high instability, some of these
storms have shown rainfall rates of 2-3 in/hr, with MRMS indicating
estimated totals of over 4 inches in the past 3 hours. There is
concern that conditions will remain favorable for continued
development, with the heavy rainfall threat persisting longer and
across a broader area than most of what the CAMs, including the
HRRR, indicate. Given the observed rainfall rates and the rainfall
already received in some areas, a Slight Risk was added. Refer to
WPC MPD #206 for additional details concerning the threat across
parts of this area.

Also added a Marginal Risk across portions of central and southern
California, where an upper level low interacting with PWs of
0.75-1 inch is supporting showers and thunderstorms, mostly along
the high terrain from the southern Sierra Nevada to the coastal
ranges, including the San Gabriel and Sierra Madre. Some of these
storms are producing torrential downpours and isolated flooding
concerns. Storms are expected to diminish later this evening, but
may pose additional concerns, especially across areas recently
impacted by wildfires.

Elsewhere, more minor adjustments were made to the previous outlook
based on current observations and CAM guidance.

Pereira

...16Z Update...

Only very minor tweaks were needed to the Slight Risk areas across
the Southern Plains and central Appalachians. For the southern
Plains, the 12Z HiRes guidance shifted very slightly (10-20 miles)
north with tonight's expected MCS development across Oklahoma.
Given the high sensitivity of that entire area between Dallas and
Oklahoma City for heavy rainfall, the Slight Risk was maintained.
The 12Z HRRR followed previous runs suggesting there may be
training showers that get hung up on the southern end of the MCS
just north of the Red River, but by then the instability should be
largely used up and therefore rates should come down a bit. Given
that, the Slight was maintained with no upgrades...though it
wouldn't take much of an uptick in future guidance to possibly need
a targeted Moderate, depending on if the guidance can come into
better agreement on where the axis of heaviest rain will set up.

Over to the Central Appalachians, the guidance unfortunately
remains in poorer agreement than in the southern Plains. Expect
more widely scattered convection to impact the area starting this
afternoon, developing into one or more lines of storms by the
evening. Each line will be very progressive/fast-moving, so
individual storms will not have much of a flooding impact. However,
with multiple rounds of showers and storms expected in an area
recently hard hit with heavy rain, the multiple rounds of storms
may collectively add up to widely scattered instances of flash
flooding into this evening. The inherited Slight was trimmed in
MD/WV out of areas east of the Allegheny Front, which are
significantly drier and with higher FFGs than areas along and west.

Wegman

...Previous Discussion...

...Southern Plains...

Shortwave ejection out of the Southern Rockies will interact with a
cold front pressing south through the Central and Southern Plains
by the late-afternoon and evening hours Thursday through Friday
morning. A relevant theta_E differential will be aligned within
the confines of the boundary providing a suitable thermodynamic
alignment such that area convection anchors and pushes along the
west to east oriented instability gradient across Southern OK.
Majority of CAMs are in consensus of a heavy rainfall footprint
along and south of I-40, generally configured to the timing of the
cold front, shortwave interaction, and LLJ convergence signal. This
is primarily right around 00z when the convective threat enhances
with the integration of the nocturnal LLJ, a timing historically
prevalent for this time of year and this setup is no exception.

00z HREF neighborhood probs for >1" are relatively high (50-80%)
within the corridor extending from Southwest OK across the Red
River Basin towards the AR border. >2" is highest just north of the
river from I-35 on east, a testament to the timing of the
shortwave arrival and upscale growth of the convective threat as it
maneuvers to the east-southeast. Rainfall rates between 1-2"/hr are
most likely when assessing the hourly rate probs from the both the
NBM and HREF output, a rate assessment that is more than suitable
considering the blanket low FFG intervals due to the antecedent
moist soils from the past 36-48 hrs of rainfall. The areal QPF
footprint of >1" aligned within those lower FFG's pretty much
perfectly, a scenario that lends to a greater credence of a touch
higher risk category that what is normal the case for this setup.
The previous SLGT risk was relatively maintained with minor
adjustments on the edges pertaining to recent hi-res QPF and in
conjunction with the ML First Guess Fields.

...Interior Mid Atlantic...

Shortwave trough currently positioned over the Southern Plains will
lift northeast through the Mississippi Valley with sights on the
Great Lakes region by the second half of the D1. Downstream, an
improved diffluent signature will align over the Ohio Valley and
points northeast allowing for regional ascent to boost convective
initiation in-of places within that diffluent core. The most
notable area for convective development lies within an established
theta_E tongue riding northward through the Appalachians Front
with poleward expansion into the southeastern Great Lakes. This
allowed for moisture rich air exuding PWAT anomalies between +2 and
+3 within the corridor extending through much of WV up through
Eastern OH and Western PA to the Lake Erie border. SBCAPE between
1500-2500 J/kg will be a common occurrence during peak diurnal
destabilization creating a corridor of enhanced thermodynamic
prospects to coincide with the arrival of the better mid-level
forcing downstream of the approaching trough. Regional shear will
be improving as well, especially within the confines of a shallow
warm front that will be lifting northward through the course of the
afternoon across the interior Mid Atlantic. Cell initiation will
begin between 18-00z today with greater multi-cell cluster coverage
likely to occur towards the back end of the above time frame due
to anchoring along the warm front as it approaches. Areas across
Western PA have been hit relatively hard recently with convective
rainfall adding to the areal decoupling of the FFG indices meaning
the prospects for flash flooding have increased with the arrival of
this next disturbance.

00z HREF neighborhood probs for >2" are between 30-50% across a
large section of Western PA extending south into the northern High
Country of WV, rooted in the Appalachians Front, an area notorious
for flashy rivers and smaller towns located within the valleys.
There was enough consensus to warrant an upgrade considering the
antecedent conditions, atmospheric environmental favorability, as
well as the increased probs denoting a locally heavy rain prospect
over the region. A SLGT risk has been added to much of Western PA
down into portions of the Central Appalachians.

Kleebauer


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Fri May 2 2025
Day 1
Valid 12Z Fri May 02 2025 - 12Z Sat May 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN PLAINS AND MID-SOUTH AND LOWER MISSISSIPPI...

Another round of thunderstorms is expected to develop along and
ahead of a southward moving cold front on Friday...with guidance
still focusing the greatest chances for excessive rainfall from
portions of the Mid-South into the Southern Plains Friday into
Friday night as weak waves propagate northeastward along the front.
Southerly low level flow will draw Gulf moisture northward...helping
increase precipitable water values ahead of the front to 1.5 to
1.75 inches at the time of the peak heating in a stripe from Texas
into southern Arkansas. Forecast soundings depict an atmosphere
supportive of rainfall rates exceeding 2 inches per hour. As
mentioned previously...the potential for a prolonged period of
training is fairly minimal so a Slight Risk still seems to cover
the potential. 00Z HREF neighborhood probabilities of 1-hour QPF
exceeding 2 inches were over 30 percent in parts of southeast TX
with lower probabilities extending into parts of Arkansas.

A Marginal Risk area surrounding the Slight Risk area extends as
far northeastward as parts of the Great Lakes. Precipitable water
values were generally at or below 1.25 inches ahead of the
advancing cold front but guidance still showed enough instability
to support showers and thunderstorms over an area that was still
had lingering hydrologic sensitivity following recent heavy
rainfall. In addition...the area will be underneath a right
entrance region of an upper level jet stream aiding upper level
support during the afternoon.

Farther west...there is also a Marginal risk of excessive rainfall
from late day and evening convection over the terrain of New
Mexico. The QPF is fairly modest but even modest rainfall rates
could result in isolated problems with run off...especially if the
rain falls on recent burn scars.

Bann

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Fri May 2 2025
Day 2
Valid 12Z Sat May 03 2025 - 12Z Sun May 04 2025

....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE GULF
COAST REGION INTO PARTS OF THE EASTERN GREAT LAKES...

The focus for excessive rainfall will shift eastward as a closed
mid-level circulation center takes shape over the nation's mid-
section. The mid- and upper-level flow backs with time from the
Gulf coast northward across parts of the Tennessee and Ohio
Valleys. Initially...this allows deeper moisture to stream
northward. But it also results in the surface/low level front to
move eastward more quickly later in the period...with generally a
low-end risk of flooding from the Gulf coast northward. The 02/00Z
suite of numerical guidance hint that there could be some 1 inch to
perhaps 2 inch amounts where frontogenetic forcing aligns
favorably in between the Gulf and the Ohio Valley...but the signal
for excessive rainfall showed little run to run / model to model
consistency.

There is also a Marginal Risk of excessive rainfall over far
southern portion of Texas...where the southern end of the cold has
stalled its southward progression and return flow of Gulf moisture
has begun. Several of the High Res ensemble highlight this area
with QPF in the upper percentiles although the consensus is less
than unanimous in amounts or placement. With soundings showing
warm cloud processes through a deep layer and the CAPE profile is
tall and skinny...introduced a Marginal risk here.

Bann

Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Fri May 2 2025
Day 3
Valid 12Z Sun May 04 2025 - 12Z Mon May 05 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
EASTERN UNITED STATES AND SOUTHERN ROCKIES ...

The risk of excessive rainfall persists across parts of the Mid-
Atlantic and Southeast US as a broad upper trough continues to make
its way eastward on Day 3. Deepest moisture of 1.25 to 1.5 inches
will be eventually be shunted off-shore late in the period. Until
then...isolated downpours are possible with spaghetti plots of
2 and 3-inches of rain in 24 hours mainly being produced by models
with the ARW core...but enough members from the NMM core and from
the GEFS to warrant a Marginal.

There is also a Marginal Risk area over parts of New Mexico again
as moisture continue to spread northward and westward from the
Gulf. After 00Z...the flow aloft between the westerly subtropical
jet and a jet streak and low- and mid-levels become increasingly
diffluent with height falls beginning ahead of an approaching
closed low from the west. This...along with moisture and a low
level boundary already in place...should support late day and
evening storms that may produce locally heavy rainfall rates.

Bann

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 AM EDT Fri May 2 2025


Early next week, two occluded frontal systems are expected to
bring moderate to locally heavy rainfall to portions of the West
and East. In the West, anomalous moisture accompanying the upper
low will support heightened precipitation from the Great Basin to
the Central and Southern Rockies and Plains. By monday, the system
will become stationary over the Plains, allowing moisture to
continuously pool over the Plains, which will likely result in a
multi-day heavy rainfall event across the Central and Southern
Plains and into the lower Mississippi Valley Monday-Wednesday.
Portions of the Plains states will be especially vulnerable to
heavy rain/flooding due to recent heavy rains and saturated soils.
This activity is highlighted in the Day 4-5 Excessive Rainfall
Outlooks (EROs) with a slight risk area highlighted across the
Texas and Oklahoma panhandles on Monday, and another slight risk
area on Tuesday for parts of the Lower Mississippi Valley/ArkLaTex
vicinity. Marginal risks surround these areas.

Under the eastern upper low, the frontal system will very slowly
push eastward early next week, resulting in a stream of onshore
moisture over the Mid-Atlantic and Northeast and persistent
precipitation across these regions through Tuesday. Initially,
rainfall could be beneficial and bring some relief to drought
conditions, but repeated rounds of rainfall could produce locally
heavy totals and may result in localized flooding in urban and poor
drainage areas. The Day 4-5 EROs show a Marginal Risk area across
these regions for Monday and Tuesday. By Wednesday, the frontal
system should accelerate to the east as the upper low interacts
with the northern stream, shifting precipitation farther into the
Northeast.

The upper lows over the East and Southwest early in the week will
result in below to well below normal temperatures, especially the
Southwest where daytime highs could be 15-20 degrees below normal.
This should moderate with time as the East returns to near normal
by Tuesday and the Southwest area slowly shifts eastward. Above
normal temperatures to begin the week over the Northern Plains may
expand into the Upper Great Lakes. Upper ridging building over the
west as the upper low shifts east will allow for expanding coverage
of above normal temperatures next week.


Santorelli

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 AM EDT Fri May 2 2025


Early next week, two occluded frontal systems are expected to
bring moderate to locally heavy rainfall to portions of the West
and East. In the West, anomalous moisture accompanying the upper
low will support heightened precipitation from the Great Basin to
the Central and Southern Rockies and Plains. By monday, the system
will become stationary over the Plains, allowing moisture to
continuously pool over the Plains, which will likely result in a
multi-day heavy rainfall event across the Central and Southern
Plains and into the lower Mississippi Valley Monday-Wednesday.
Portions of the Plains states will be especially vulnerable to
heavy rain/flooding due to recent heavy rains and saturated soils.
This activity is highlighted in the Day 4-5 Excessive Rainfall
Outlooks (EROs) with a slight risk area highlighted across the
Texas and Oklahoma panhandles on Monday, and another slight risk
area on Tuesday for parts of the Lower Mississippi Valley/ArkLaTex
vicinity. Marginal risks surround these areas.

Under the eastern upper low, the frontal system will very slowly
push eastward early next week, resulting in a stream of onshore
moisture over the Mid-Atlantic and Northeast and persistent
precipitation across these regions through Tuesday. Initially,
rainfall could be beneficial and bring some relief to drought
conditions, but repeated rounds of rainfall could produce locally
heavy totals and may result in localized flooding in urban and poor
drainage areas. The Day 4-5 EROs show a Marginal Risk area across
these regions for Monday and Tuesday. By Wednesday, the frontal
system should accelerate to the east as the upper low interacts
with the northern stream, shifting precipitation farther into the
Northeast.

The upper lows over the East and Southwest early in the week will
result in below to well below normal temperatures, especially the
Southwest where daytime highs could be 15-20 degrees below normal.
This should moderate with time as the East returns to near normal
by Tuesday and the Southwest area slowly shifts eastward. Above
normal temperatures to begin the week over the Northern Plains may
expand into the Upper Great Lakes. Upper ridging building over the
west as the upper low shifts east will allow for expanding coverage
of above normal temperatures next week.


Santorelli

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 AM EDT Fri May 2 2025

Valid 12Z Fri May 02 2025 - 12Z Mon May 05 2025


...Sierra Nevada...
Days 2-3...

An anomalous closed mid-level low (700-500mb heights reaching -3
sigma by Sunday aftn) will dive along the CA coast before turning
into the Four Corners states Sunday night. Impressive height falls
and downstream divergence will combine with a sharpening jet streak
to produce deep layer ascent across CA and into the Great Basin
late Saturday through late Sunday. This will result in expanding
coverage of precipitation, with snow likely in the higher terrain
of the Sierra. Initially, snow levels will be quite high, generally
8000-9000 ft, but as the closed low pivots south of the region,
colder air will funnel in and drop snow levels to 7000-7500 ft.
This will keep most of the heavy snow accumulations above the area
Passes, but at least light accumulations are possible even as low
as Donner Pass before precip wanes late Sunday. WPC probabilities
across the Sierra peak around 50% for 6+ inches of snow D2 into D3,
with the highest amounts expected in the higher terrain of the
southern Sierra where locally 8-10 inches is possible.


...Central Rockies...
Day 3...

A complex mid-level synoptic evolution will occur late in the
forecast period across the Intermountain West as the extremely
amplified flow across the CONUS exhibits minimal change into early
next week. Dual closed lows, one over the Ohio Valley and another
across the Four Corners will meander with little movement, as flow
stalls over the country. The important low for winter purposes will
be the one drifting across the Four Corners, which will shed
multiple lobes of vorticity around it as nearly a carousel of
shortwaves rotates southward from Canada and towards Baja and
around the primary closed low. The interaction of these shortwaves
with the primary low and secondary developing subtropical jet
streak will result in expanding precipitation, with snow likely in
the higher terrain of the San Juans before the close of D3 /12Z
Tuesday/. More significant precipitation, including more widespread
heavy snow across the Rockies, is possible beyond this forecast
period as a cold front drops through the region as well, but at
this time, for D3, WPC probabilities indicate a 70-90% chance of 6+
inches, focused in the higher elevations of the San Juans.



The probability of significant freezing rain across the CONUS is
less than 10%.

Weiss

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 AM EDT Fri May 2 2025

Valid 12Z Mon May 05 2025 - 12Z Fri May 09 2025


...Blocky pattern with closed lows over the West and the East with
areas of heavy rainfall potential early to mid next week...


...Overview...

A blocky pattern over the lower 48 will be in place by the
beginning of the medium range period on Monday. The dominant
features will be two closed upper lows, one over the West/Southwest
and one over the East, that will both slowly drift around
weakening through Wednesday/Thursday. These systems are expected to
produce areas of significant rainfall over portions of the Central
and Southern Plains, Northeast, and Mid- Atlantic and high
elevation snows in the West. Eventually, the eastern upper low
should interact with a northern stream trough that will swing
across southern Canada, resulting in the low being swept up towards
the Canadian Maritimes later next week. The Western low will
likely remain isolated as it drifts east over the Plains and
weakens. The remnants of the Western low may eventually combine
with another trough developing over the East Coast late Thursday
into Friday. By Wednesday, another trough and frontal system will
approach the West Coast.

...Guidance/Predictability Assessment...

Models and ensembles remain in good agreement on the presence of a
blocky pattern to start next week, though subtle differences in
exactly where the upper lows set up over the Southwest and the Ohio
Valley will directly impact the distribution and locations of
heaviest rainfall. In the East, the GFS was displaced slightly east
and this resulted in a quicker push off the East Coast mid next
week. Meanwhile, a northern stream shortwave swinging into the
Great Lakes was much more pronounced in the CMC with another closed
low over the East, which was very much an outlier solution at this
point. The ECMWF (with the ensemble means and UKMET early in the
period) was preferred for this system, but a little faster exit
like depicted in the GFS is not out of the question.

Out West, there remains some notable differences in energy
rotating around the upper low early in the week, but general
agreement that this low should drift slowly eastward as it weakens
into the Central Plains and eventually gets absorbed into some
broader troughing over the East late next week. A general model
compromise, with the ensemble means later in the period worked well
as a starting point, outside of the CMC due to the outlier upper
low over the East late next week.


...Weather/Hazards Highlights...

Early next week, two occluded frontal systems are expected to
bring moderate to locally heavy rainfall to portions of the West
and East. In the West, anomalous moisture accompanying the upper
low will support heightened precipitation from the Great Basin to
the Central and Southern Rockies and Plains. By monday, the system
will become stationary over the Plains, allowing moisture to
continuously pool over the Plains, which will likely result in a
multi-day heavy rainfall event across the Central and Southern
Plains and into the lower Mississippi Valley Monday-Wednesday.
Portions of the Plains states will be especially vulnerable to
heavy rain/flooding due to recent heavy rains and saturated soils.
This activity is highlighted in the Day 4-5 Excessive Rainfall
Outlooks (EROs) with a slight risk area highlighted across the
Texas and Oklahoma panhandles on Monday, and another slight risk
area on Tuesday for parts of the Lower Mississippi Valley/ArkLaTex
vicinity. Marginal risks surround these areas.

Under the eastern upper low, the frontal system will very slowly
push eastward early next week, resulting in a stream of onshore
moisture over the Mid-Atlantic and Northeast and persistent
precipitation across these regions through Tuesday. Initially,
rainfall could be beneficial and bring some relief to drought
conditions, but repeated rounds of rainfall could produce locally
heavy totals and may result in localized flooding in urban and poor
drainage areas. The Day 4-5 EROs show a Marginal Risk area across
these regions for Monday and Tuesday. By Wednesday, the frontal
system should accelerate to the east as the upper low interacts
with the northern stream, shifting precipitation farther into the
Northeast.

The upper lows over the East and Southwest early in the week will
result in below to well below normal temperatures, especially the
Southwest where daytime highs could be 15-20 degrees below normal.
This should moderate with time as the East returns to near normal
by Tuesday and the Southwest area slowly shifts eastward. Above
normal temperatures to begin the week over the Northern Plains may
expand into the Upper Great Lakes. Upper ridging building over the
west as the upper low shifts east will allow for expanding coverage
of above normal temperatures next week.


Santorelli


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