Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Wed Apr 30 2025

Valid 12Z Wed Apr 30 2025 - 12Z Fri May 02 2025

...Episodes of heavy rain and severe thunderstorms will keep heightened
flash flooding concerns across the south-central U.S. through early
Thursday...

...Showers and embedded severe thunderstorms expected to shift east into
the Mid-Mississippi, Ohio, and Tennessee Valleys on Thursday...

...Chance of strong to severe thunderstorms return to the south-central
Plains by early Friday morning...

A stalled frontal boundary interacting with upper-level disturbances
ejecting from the southern Rockies will remain the focus for additional
episodes of heavy rainfall and severe thunderstorms across the
south-central U.S. through today and into early Thursday. The highest
threat of heaviest rain is forecast to shift only slightly to the east
today across southeastern Oklahoma into northeastern Texas and western
Arkansas where a moderate risk of flash flooding is anticipated. An
enhanced risk of severe thunderstorms is also embedded within this general
area. A slight risk of severe thunderstorms and flash flooding is
expected for a larger portion of the south-central U.S. for today into
tonight. By tonight into Thursday, a low pressure system forming along
the front will begin to track northeastward across the Mid-Mississippi
Valley into the Midwest, which will effectively end the heavy rainfall and
severe weather threats across the south-central U.S. on Thursday. On the
other side of the system, showers and embedded severe thunderstorms are
forecast to increase and expand across Mid-Mississippi, Ohio, and
Tennessee Valleys. By early Friday morning, the low pressure system
should be moving through the Great Lakes with scattered showers and
embedded thunderstorms across the lower Great Lakes and spreading into New
England.

Meanwhile, a Pacific frontal system moving across the northern
Intermountain West, northern Rockies into the northern High Plains is
bringing a round of lower elevation rains and accumulating wet snow in the
mountains. In the wake of the system and cold front, drier weather moves
in later Wednesday into Thursday as high pressure takes control. By early
Friday morning, the cold front will dip south into the southern High
Plains and intersect moisture returning from the western Gulf to provide
increasing chance of strong to severe thunderstorms extending east into
the southern Plains.

Across the Northeast, cooler and drier air will arrive today with gusty
winds behind a cold front. High pressure will be in control on Thursday
before showers and thunderstorms arrive ahead of the low pressure system
approaching from the Great Lakes Friday morning. The Mid-Atlantic will
have the best chance of showers and thunderstorms today and Thursday as
the front stalls across the region where afternoon temperatures are
expected to reach well into the 80s. Florida will enjoy mostly fine and
warm to hot weather appropriate for the Sunshine State. The Desert
Southwest will remain dry as well.

A strong ridge of high pressure extending across much of the eastern U.S.
will bring well above average warmth to much of the region through mid to
late week. Forecast highs in the 70s and 80s will be common. Behind the
front, portions of the Great Lakes and Northeast cool down briefly today
before warming back up on Thursday. Overall near average temperatures are
in store for the western and central U.S. Wednesday into Thursday before
warming up some across the Pacific Northwest heading into the latter half
of the week.

Kong/Miller


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
317 AM EDT Wed Apr 30 2025

Valid 12Z Sat May 03 2025 - 12Z Wed May 07 2025


...Overview...

Latest guidance continues to show the general theme of an
increasingly blocky pattern across the Lower 48, and with better
relative agreement for the upper low closing off over California
during the weekend and drifting gradually inland versus the pattern
over the East. The western upper low will support a broad area of
rain and high elevation snow across the region, as well as an
increasing threat for areas of heavy rain across parts of the
central/southern Plains next week. Meanwhile solutions rapidly
diverge over the East, with new 00Z runs adding to recent trends
toward closing of an upper low well inland--leaving the more
progressive GFS/GEFS that build a ridge into the east-central U.S.
in the vast minority. The majority scenario increases the
probability of a wetter pattern over portions of the East.

...Guidance/Predictability Assessment...

For the developing western upper low, the new 00Z UKMET strays a
little southwest of consensus while GFS/GEFS runs have sometimes
been leaning a bit fast and north later in the period. Recent
machine learning (ML) runs have been favoring the southern half of
the envelope, while diverging a little for timing later in the
forecast. Multi-day trends have generally been slower but the 00Z
CMC may be too slow as it holds the upstream Pacific upper trough
(which should bring a cold front into the Northwest Tuesday-
Wednesday) west of consensus. Recent ECMWF/ECens runs have been
most consistent with the upper low's path and timing with relative
support from ML models.

Guidance through the 12Z/18Z guidance cycles maintained the
tremendous spread for eastern U.S. evolution from Saturday onward.
ML models starting with the 12Z/28 cycle have been showing
potential for initial Mississippi Valley energy to close off an
upper low as far west as the Tennessee/Ohio Valleys, with the ECMWF
trending that way over the past day. The 12Z UKMET brought its
closed low into the Carolinas while the 12Z CMC was progressive
until using Canadian energy to close off a low over the Northeast
(but the CMCens mean was a compromise with the ECMWF). On the most
progressive extreme were the GFS/GEFS (albeit with a heavy rain
producing trough/upper low passing over Florida). ML model
consistency along with some trends in non- GFS/GEFS guidance
favored some trending toward a more inland upper low, but not yet
to the point of matching the ECMWF given the wide guidance spread.
By midweek the ML models suggest the upper low would either drift
into the northern Mid-Atlantic or lift into the Great Lakes and
phase with a Canadian trough. These solutions do not support the
new 00Z CMC that drops its upper low to the Southeast Coast. The
new 00Z UKMET has joined the latest ECMWF runs. The full array of
00Z runs so far could allow for some added westward trending of the
upper low and gradual enhancement of expected rainfall.

Combining guidance preferences for the western upper low and the
eastern U.S. evolution, the first half of the forecast started with
a blend of 12Z and 00Z/29 ECMWF runs, along with input from the
12Z UKMET, ECens, and CMCens. ECens/CMCens weight increased a
little after early Monday to make up for the UKMET. Local
unavailability of the 12Z ECens valid 12Z Tuesday required greater
weight of the CMCens at that exact time.

...Weather/Hazards Highlights...

Latest ML models and non-GFS dynamical models are steadily showing
more pronounced clustering toward closure of an upper low that may
persist for a couple days or so to the west of the Appalachians.
This would support some areas of moderate to perhaps locally heavy
rainfall depending on the orientation and persistence of any
particular bands. There is still a lot of spread for rainfall
specifics across areas with varying soil moisture, so for now have
opted not to depict any Marginal Risk areas in the Days 4-5
Excessive Rainfall Outlooks. It appears likely that one or more
risk areas would become warranted as soon as a little better detail
clustering emerges. The drier scenario, aside from some heavy rain
potential over Florida/Southeast coast, represented by the GFS
seems to be trending lower in probability.

Easterly flow and adequate moisture/instability over the far
southern Plains should maintain the threat for some locally heavy
rainfall over the southwestern half of Texas on Saturday, though
with a diminished signal compared to the prior day. The current
Marginal Risk over that area in the Day 4 ERO appears reasonable.
Meanwhile guidance has been consistent in showing a band of
potentially heavy rainfall aligned over west- central Nevada on
Saturday as the upper low closes off over California, so that
Marginal Risk remains nearly unchanged as well. By Day 5/Monday the
rainfall signal over the West looks a little more diffuse but with
some guidance clustering toward enhanced rainfall over the
sensitive terrain in the eastern half of New Mexico as gradual
progress of the Southwest upper low pushes a cold front toward the
region. Thus the Day 5 ERO introduces a Marginal Risk area for this
threat. Additional gradual drift of the Southwest upper low should
raise the potential for areas of heavy rainfall over parts of the
central/southern Plains from Monday onward, with details depending
on exact path and timing of the upper low. Otherwise, the system
moving into the West will bring a broad area of rain and high
elevation snow across the region, from the Sierra Nevada and inland
Northwest into the northern-central Rockies.

Well above normal temperatures should extend from the northern
Rockies and eastern Great Basin into the northern Plains on
Saturday, with anomalies gradually moderating as the warmth moves
eastward toward the Upper Midwest. Plus 15-25F anomalies should be
most common over far northern areas during Saturday-Sunday. In
contrast, the upper low tracking into the Southwest will bring much
cooler temperatures to the region. Highs should be 10-20F below
normal on Sunday-Monday, with some record cold highs possible near
the upper low's path especially on Sunday. Persistent clouds and
precipitation should keep the southern High Plains highs below
normal as well. The potentially unsettled pattern over the East may
favor temperatures within a few degrees of normal after some
modest warmth along the East Coast on Saturday.

Rausch


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
337 AM EDT Wed Apr 30 2025

Valid 00Z Thu 01 May 2025 - 00Z Thu 08 May 2025

On Wednesday the eastward departure and opening of a nearby upper
low will lead to dissipation of the surface trough that has been
over the state early this week. Expect trades to be light to
moderate initially, but trending more brisk Friday onward with
influence from one surface high passing by to the north around
Friday and another next Tuesday-Wednesday. Guidance is consistent
in showing precipitable water values settling at below normal
values from Friday into at least early next week while upper
ridging prevails over/north of the state. Thus anticipate mostly
light to moderate showers focused primarily over windward/mountain
areas.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Wed Apr 30 2025
Day 1
Valid 16Z Wed Apr 30 2025 - 12Z Thu May 01 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST OKLAHOMA, NORTH TEXAS, AND WESTERN ARKANSAS...

The threat of significant flash flooding continues today, with the
focus increasingly shifting to portions of north Texas based on
observational trends and the latest 12Z hi-res model guidance.

A bowing convective line continues to advance eastward through
S OK, with a trailing line of convection extending into N TX on
the southern periphery of strong cold pool and mesohigh situated
over SC OK. Over the past couple hours, the convective line in N
TX has become increasingly oriented NE-SW, or even ENE-WSW; in
other words, more parallel to the mean flow vector. This has
increased the potential for training and backbuilding, especially
into the afternoon hours as the LLJ begins to weaken and low-level
winds start to veer to the southwest. For a more detailed mesoscale
analysis, please refer to MPD #202 issued at 14Z for this region.

The Moderate Risk area was maintained in SE OK and W AR, even
though confidence in flash flooding has decreased somewhat due to
the fairly steady forward progress of the bowing line, and how thin
the convective line has become. However, it was maintained out of
deference to continuity and the potential to reinvigorate the
convection in the next few hours with some clearing occurring ahead
of the line, which could increase rain rates again.

Greater concern now exists in N TX. The forward progression of the
line and associated cold pool in S OK and overall convective
evolution has led to far more west-east orientation to the
convective line in N TX than models had originally been
advertising. This certainly increases flash flood potential and
over the next few hours this places the Dallas-Ft. Worth metro area
squarely in the greatest risk. Hourly rainfall could exceed 2
inches per hour, with unobstructed inflow from a very moist and
unstable air mass (MLCAPE 1500-2000 j/kg with minimal CINH; PWs
1.6 to 1.8 inches per RAP analysis.) In an urban environment, this
could lead to significant and life-threatening flash flooding,
particularly if the rain rates are sustained for a couple hours,
which is a distinct possibility in this case.

As the afternoon and evening progress, the hi-res models do
indicate the potential for the southwestern periphery of the
convective line to continue to slow or stall. While they don't
necessarily agree on the specifics, many HREF members, as well as
the RRFS, indicate backbuilding and training convection could
become more likely in the corridor from Waco to Longview. This
seems to be well supported by observational trends, and in fact
many hi-res members are: (1) already not extending the convective
line far enough southwest based on current radar trends, and (2)
have the convective line oriented far too much in a N-S fashion.
Therefore, they may be underestimating rainfall potential a bit,
and also underselling the potential for a significant rainfall and
flash flood event further to the southwest. Probabilities and
associated risk categories were increased a bit down the I-35
corridor between DFW and AUS, and this will be monitored for
further updates based on observational trends. Rain rates in much
of C/N TX could reach or exceed 2 inches per hour given the
available instability and moisture.

The other biggest change for this outlook was to extend the Slight
Risk a bit further northeast through MO and into W IL based on more
elevated HREF probabilities of exceeding 1hr and 3hr flash flood
guidance and a fairly consistent signal in the guidance for a
corridor of enhanced rainfall associated with an MCV and
corresponding surface low.

Lamers


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Wed Apr 30 2025
Day 2
Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A BROAD
AREA FROM THE GREAT LAKES TO MID MISSISSIPPI VALLEY TO WESTERN GULF
COAST...

Surface cyclogenesis across the Southern Plains will lead to a
steady low progression to the northeast up through the Mississippi
Valley into the Great Lakes towards the end of the forecast cycle.
A cold front will trail the vacating low allowing for some frontal
convergence to occur across the above areas with the greatest
convective coverage likely to be within the warm sector of the Ohio
and Tennessee Valley into the southern Great Lakes. PWAT anomalies
between +2 and +3 deviations will be present for much of the CONUS
situated east of the Mississippi lending to a fairly rich
environment conducive for moist convective cores from the Gulf
coast up to the Great Lakes region. The reasoning for the heavier
convective threat north is the added sheared profiles within the
bounds of the warm sector creating better mesocyclone maturity and
stronger storms. All this to say....heavy rain prospects for longer
periods more likely within the zone encompassing the Tennessee and
Ohio Valleys. Current forecast totals region-wide over the
Southeast up into the Ohio Valley are pretty consistent with an
areal average between 0.5 to 1" with some locally enhanced totals
reaching upwards of 2-2.5" in the hardest hit locations. Given
rainfall rates peaking between 1-1.5"/hr when assessing the
probabilistic output of both the NBM and 00z HREF, there's a cap on
the higher end potential of any flash flood scenario leaving this
overall threat in the broad MRGL risk coverage, as inherited.

There is another area of interest down across the Rio Grande
extending from the Big Bend down to about Laredo for tomorrow
evening as a shortwave perturbation ejects east off of Coahuila
with some convection spawning over the Serranias del Burro. Some of
the CAMs are pretty significant in the overall convective
development, but much of the CAMs reach the end of their runs prior
to the activity making its way towards the Rio Grande Valley. Other
deterministic and ensemble bias corrected means tell the story in
full with some respectable QPF outputs within the above bounds
leading to potentially some localized flooding within some of the
urbanized zones between Del Rio down to Laredo. Considering the
environment to be favorable and this scenario having a long history
of potential, opted to keep the MRGL risk from previous forecast
issuance to outline the threat for a quick 1-3" of rainfall with
locally higher amounts plausible pending cold pool convergence
evolutions.

Kleebauer


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Wed Apr 30 2025
Day 3
Valid 12Z Fri May 02 2025 - 12Z Sat May 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
CENTRAL TEXAS...

A cold front progression stemming from a trailing cold front and
pressing surface high over the Northern Plains will lead to
significant low-level convergence regime over Central TX by Friday
afternoon. Return flow around the western flank of a broad surface
ridge positioned over the Western Atlantic will reach back towards
the southern half of TX leading to a convergent pattern in
conjunction with slow-moving cold front moving north to south.
Models have been steadily increasing precip coverage within the
confines of the boundary once it makes headway into the Hill
Country of the Lone Star State by late-Friday morning, carrying
through the rest of the period as the front encounters higher
theta_E airmass within the western periphery of that surface ridge.
PWATs between +1 and +2 deviations will be present once the front
makes its presence across the region, a sufficient environment for
deep moist convective development along and ahead of the boundary
as it slowly migrates south. Ensemble means have upped precip
output in recent runs with a solid 1-2" areal average now
encompassing a large portion of the state from the Stockton
Plateau, east through Hill Country, the I-35 corridor, all the way
back towards the Piney Woods area of East TX. Favorable environment
with likely training echoes in proxy to the cold front will
generate locally enhanced QPF maxima, especially within the bounds
of the terrain where an added weak upslope component could yield
greatest results (3+" totals). The convergent pattern can be seen
as far east as the Southeastern CONUS, but the greatest surface
frontogenesis signal is situated back over the Southern Plains,
south of I-20.

A SLGT risk was added across the above area with a broad MRGL
encompassing the higher risk as scattered convection will still be
present up towards I-20 extending east through the Lower
Mississippi Valley up through Northern AL and Eastern TN. Will be
monitoring the setup closely as the setup will likely be most
prolific within one of the flashier areas of the country.

Kleebauer

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
317 AM EDT Wed Apr 30 2025


Latest ML models and non-GFS dynamical models are steadily showing
more pronounced clustering toward closure of an upper low that may
persist for a couple days or so to the west of the Appalachians.
This would support some areas of moderate to perhaps locally heavy
rainfall depending on the orientation and persistence of any
particular bands. There is still a lot of spread for rainfall
specifics across areas with varying soil moisture, so for now have
opted not to depict any Marginal Risk areas in the Days 4-5
Excessive Rainfall Outlooks. It appears likely that one or more
risk areas would become warranted as soon as a little better detail
clustering emerges. The drier scenario, aside from some heavy rain
potential over Florida/Southeast coast, represented by the GFS
seems to be trending lower in probability.

Easterly flow and adequate moisture/instability over the far
southern Plains should maintain the threat for some locally heavy
rainfall over the southwestern half of Texas on Saturday, though
with a diminished signal compared to the prior day. The current
Marginal Risk over that area in the Day 4 ERO appears reasonable.
Meanwhile guidance has been consistent in showing a band of
potentially heavy rainfall aligned over west- central Nevada on
Saturday as the upper low closes off over California, so that
Marginal Risk remains nearly unchanged as well. By Day 5/Monday the
rainfall signal over the West looks a little more diffuse but with
some guidance clustering toward enhanced rainfall over the
sensitive terrain in the eastern half of New Mexico as gradual
progress of the Southwest upper low pushes a cold front toward the
region. Thus the Day 5 ERO introduces a Marginal Risk area for this
threat. Additional gradual drift of the Southwest upper low should
raise the potential for areas of heavy rainfall over parts of the
central/southern Plains from Monday onward, with details depending
on exact path and timing of the upper low. Otherwise, the system
moving into the West will bring a broad area of rain and high
elevation snow across the region, from the Sierra Nevada and inland
Northwest into the northern-central Rockies.

Well above normal temperatures should extend from the northern
Rockies and eastern Great Basin into the northern Plains on
Saturday, with anomalies gradually moderating as the warmth moves
eastward toward the Upper Midwest. Plus 15-25F anomalies should be
most common over far northern areas during Saturday-Sunday. In
contrast, the upper low tracking into the Southwest will bring much
cooler temperatures to the region. Highs should be 10-20F below
normal on Sunday-Monday, with some record cold highs possible near
the upper low's path especially on Sunday. Persistent clouds and
precipitation should keep the southern High Plains highs below
normal as well. The potentially unsettled pattern over the East may
favor temperatures within a few degrees of normal after some
modest warmth along the East Coast on Saturday.

Rausch

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
317 AM EDT Wed Apr 30 2025


Latest ML models and non-GFS dynamical models are steadily showing
more pronounced clustering toward closure of an upper low that may
persist for a couple days or so to the west of the Appalachians.
This would support some areas of moderate to perhaps locally heavy
rainfall depending on the orientation and persistence of any
particular bands. There is still a lot of spread for rainfall
specifics across areas with varying soil moisture, so for now have
opted not to depict any Marginal Risk areas in the Days 4-5
Excessive Rainfall Outlooks. It appears likely that one or more
risk areas would become warranted as soon as a little better detail
clustering emerges. The drier scenario, aside from some heavy rain
potential over Florida/Southeast coast, represented by the GFS
seems to be trending lower in probability.

Easterly flow and adequate moisture/instability over the far
southern Plains should maintain the threat for some locally heavy
rainfall over the southwestern half of Texas on Saturday, though
with a diminished signal compared to the prior day. The current
Marginal Risk over that area in the Day 4 ERO appears reasonable.
Meanwhile guidance has been consistent in showing a band of
potentially heavy rainfall aligned over west- central Nevada on
Saturday as the upper low closes off over California, so that
Marginal Risk remains nearly unchanged as well. By Day 5/Monday the
rainfall signal over the West looks a little more diffuse but with
some guidance clustering toward enhanced rainfall over the
sensitive terrain in the eastern half of New Mexico as gradual
progress of the Southwest upper low pushes a cold front toward the
region. Thus the Day 5 ERO introduces a Marginal Risk area for this
threat. Additional gradual drift of the Southwest upper low should
raise the potential for areas of heavy rainfall over parts of the
central/southern Plains from Monday onward, with details depending
on exact path and timing of the upper low. Otherwise, the system
moving into the West will bring a broad area of rain and high
elevation snow across the region, from the Sierra Nevada and inland
Northwest into the northern-central Rockies.

Well above normal temperatures should extend from the northern
Rockies and eastern Great Basin into the northern Plains on
Saturday, with anomalies gradually moderating as the warmth moves
eastward toward the Upper Midwest. Plus 15-25F anomalies should be
most common over far northern areas during Saturday-Sunday. In
contrast, the upper low tracking into the Southwest will bring much
cooler temperatures to the region. Highs should be 10-20F below
normal on Sunday-Monday, with some record cold highs possible near
the upper low's path especially on Sunday. Persistent clouds and
precipitation should keep the southern High Plains highs below
normal as well. The potentially unsettled pattern over the East may
favor temperatures within a few degrees of normal after some
modest warmth along the East Coast on Saturday.

Rausch

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Wed Apr 30 2025

Valid 12Z Wed Apr 30 2025 - 12Z Sat May 03 2025


...Rockies...
Days 1-2...

Positively-tilted trough over the northern Rockies this morning
with a vort lobe over the Bitterroots will drift south through
Thursday. Focus for precip is along a surface boundary ahead of the
upper trough axis with light to moderate rates over WY today and CO
tonight Thursday. Snow levels will be elevated - around 8000ft
in WY and 9000ft in CO. Day 1 snow probs for >6" are around 30% in
the southern Absarokas/Wind River and 40-60% over the Bighorns. Day
1.5 snow probs for >6" are 40-70% through the Front Range and the
Mosquito Range.


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


Jackson

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
317 AM EDT Wed Apr 30 2025

Valid 12Z Sat May 03 2025 - 12Z Wed May 07 2025


...Overview...

Latest guidance continues to show the general theme of an
increasingly blocky pattern across the Lower 48, and with better
relative agreement for the upper low closing off over California
during the weekend and drifting gradually inland versus the pattern
over the East. The western upper low will support a broad area of
rain and high elevation snow across the region, as well as an
increasing threat for areas of heavy rain across parts of the
central/southern Plains next week. Meanwhile solutions rapidly
diverge over the East, with new 00Z runs adding to recent trends
toward closing of an upper low well inland--leaving the more
progressive GFS/GEFS that build a ridge into the east-central U.S.
in the vast minority. The majority scenario increases the
probability of a wetter pattern over portions of the East.

...Guidance/Predictability Assessment...

For the developing western upper low, the new 00Z UKMET strays a
little southwest of consensus while GFS/GEFS runs have sometimes
been leaning a bit fast and north later in the period. Recent
machine learning (ML) runs have been favoring the southern half of
the envelope, while diverging a little for timing later in the
forecast. Multi-day trends have generally been slower but the 00Z
CMC may be too slow as it holds the upstream Pacific upper trough
(which should bring a cold front into the Northwest Tuesday-
Wednesday) west of consensus. Recent ECMWF/ECens runs have been
most consistent with the upper low's path and timing with relative
support from ML models.

Guidance through the 12Z/18Z guidance cycles maintained the
tremendous spread for eastern U.S. evolution from Saturday onward.
ML models starting with the 12Z/28 cycle have been showing
potential for initial Mississippi Valley energy to close off an
upper low as far west as the Tennessee/Ohio Valleys, with the ECMWF
trending that way over the past day. The 12Z UKMET brought its
closed low into the Carolinas while the 12Z CMC was progressive
until using Canadian energy to close off a low over the Northeast
(but the CMCens mean was a compromise with the ECMWF). On the most
progressive extreme were the GFS/GEFS (albeit with a heavy rain
producing trough/upper low passing over Florida). ML model
consistency along with some trends in non- GFS/GEFS guidance
favored some trending toward a more inland upper low, but not yet
to the point of matching the ECMWF given the wide guidance spread.
By midweek the ML models suggest the upper low would either drift
into the northern Mid-Atlantic or lift into the Great Lakes and
phase with a Canadian trough. These solutions do not support the
new 00Z CMC that drops its upper low to the Southeast Coast. The
new 00Z UKMET has joined the latest ECMWF runs. The full array of
00Z runs so far could allow for some added westward trending of the
upper low and gradual enhancement of expected rainfall.

Combining guidance preferences for the western upper low and the
eastern U.S. evolution, the first half of the forecast started with
a blend of 12Z and 00Z/29 ECMWF runs, along with input from the
12Z UKMET, ECens, and CMCens. ECens/CMCens weight increased a
little after early Monday to make up for the UKMET. Local
unavailability of the 12Z ECens valid 12Z Tuesday required greater
weight of the CMCens at that exact time.

...Weather/Hazards Highlights...

Latest ML models and non-GFS dynamical models are steadily showing
more pronounced clustering toward closure of an upper low that may
persist for a couple days or so to the west of the Appalachians.
This would support some areas of moderate to perhaps locally heavy
rainfall depending on the orientation and persistence of any
particular bands. There is still a lot of spread for rainfall
specifics across areas with varying soil moisture, so for now have
opted not to depict any Marginal Risk areas in the Days 4-5
Excessive Rainfall Outlooks. It appears likely that one or more
risk areas would become warranted as soon as a little better detail
clustering emerges. The drier scenario, aside from some heavy rain
potential over Florida/Southeast coast, represented by the GFS
seems to be trending lower in probability.

Easterly flow and adequate moisture/instability over the far
southern Plains should maintain the threat for some locally heavy
rainfall over the southwestern half of Texas on Saturday, though
with a diminished signal compared to the prior day. The current
Marginal Risk over that area in the Day 4 ERO appears reasonable.
Meanwhile guidance has been consistent in showing a band of
potentially heavy rainfall aligned over west- central Nevada on
Saturday as the upper low closes off over California, so that
Marginal Risk remains nearly unchanged as well. By Day 5/Monday the
rainfall signal over the West looks a little more diffuse but with
some guidance clustering toward enhanced rainfall over the
sensitive terrain in the eastern half of New Mexico as gradual
progress of the Southwest upper low pushes a cold front toward the
region. Thus the Day 5 ERO introduces a Marginal Risk area for this
threat. Additional gradual drift of the Southwest upper low should
raise the potential for areas of heavy rainfall over parts of the
central/southern Plains from Monday onward, with details depending
on exact path and timing of the upper low. Otherwise, the system
moving into the West will bring a broad area of rain and high
elevation snow across the region, from the Sierra Nevada and inland
Northwest into the northern-central Rockies.

Well above normal temperatures should extend from the northern
Rockies and eastern Great Basin into the northern Plains on
Saturday, with anomalies gradually moderating as the warmth moves
eastward toward the Upper Midwest. Plus 15-25F anomalies should be
most common over far northern areas during Saturday-Sunday. In
contrast, the upper low tracking into the Southwest will bring much
cooler temperatures to the region. Highs should be 10-20F below
normal on Sunday-Monday, with some record cold highs possible near
the upper low's path especially on Sunday. Persistent clouds and
precipitation should keep the southern High Plains highs below
normal as well. The potentially unsettled pattern over the East may
favor temperatures within a few degrees of normal after some
modest warmth along the East Coast on Saturday.

Rausch


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