Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Sat Apr 26 2025

Valid 12Z Tue Apr 29 2025 - 12Z Sat May 03 2025


...Heavy rainfall threat over the southern half of the Plains
Tuesday-Wednesday...

...Overview...

A cold front pushing across the northeastern quadrant of the lower
48 will hang back over the Plains while awaiting ejection of a
Southwest U.S. shortwave. This feature and dynamics coming in
behind it should ultimately generate a surface low that brings a
front through more of the East by the end of the week as upper
troughing amplifies. The Tuesday-Wednesday pattern will favor
episodes of heavy rainfall over and near the southern half of the
Plains before system progression spreads less pronounced rainfall
totals across the East. A trailing upper ridge should move into the
Northwest after early Wednesday, reaching the Plains by the start
of the weekend, while a Pacific upper trough should reach the West
Coast around Saturday with some precipitation over the Northwest.

...Guidance/Predictability Assessment...

Guidance is similar in principle for the forecast over the central
and eastern U.S. but with some typical differences and minority
stray solutions. There are still some timing differences as low
pressure crossing southeastern Canada Tuesday-Wednesday pushes a
trailing cold front through the Great Lakes/Northeast, with an
average providing a reasonable starting point. Then as Southwest
U.S. shortwave energy ejects northeastward, solutions still vary
for details of developing low pressure that tracks northeastward.
At least the surface low's position over the central-eastern Great
Lakes by Thursday night-Friday in the majority cluster has held up
rather well over the past day. In addition the latest machine
learning (ML) models seem to be gaining some definition for this
waviness compared to 24 hours ago. Through Thursday or so, the 12Z
CMC started to stray on the fast side while the 12Z UKMET lagged
other guidance for the ejecting Southwest upper feature. The 00Z
solutions have trimmed the extreme sides of the envelope. However
toward the end of the week there is a fair amount of
spread/variability for how the overall eastern North America upper
trough will evolve in both the dynamical and ML guidance, lowering
confidence in specifics. This favors a blended/ensemble mean
approach late in the period.

Over the West, some weak shortwave/upper low energy may briefly
reach the Southwest before an amplified upper trough with leading
cold front reaches the West Coast by early Saturday. Recent GFS
trends have led to improved clustering into Friday, though with
consensus as a whole adjusting a bit slower compared to yesterday.
By early Saturday the latest dynamical/ML majority suggests the
upper trough will remain fairly phased, versus some earlier model
runs that had various ideas for how flow could separate. However
multiple solutions show separation by later Saturday or Sunday.

Guidance comparisons based on 12Z/18Z guidance led to emphasizing
the 18Z GFS/12Z ECMWF along with a little 12Z CMC Tuesday-
Wednesday, followed by a transition toward an even weight of the
GFS/ECMWF and their corresponding ensemble means by next Saturday.

...Weather/Hazards Highlights...

During Tuesday-Wednesday, low pressure lifting away from Lake
Superior will push a cold front across the northeastern quadrant of
the lower 48 while the trailing part over the Plains stalls and
develops a wave as a Southwest U.S. shortwave ejects northeastward.
This pattern combined with anomalous moisture and sufficient
instability will favor a heavy rainfall threat over and near the
southern half of the Plains in this time frame. The Day 4/Tuesday
Excessive Rainfall Outlook maintains a Marginal Risk area from
parts of Texas into the lower Ohio Valley. There still seems to be
enough guidance spread within this area to favor waiting for
improved clustering before introducing an embedded Slight Risk.
Models/ensembles and first-guess fields generally strengthen the
heavy rainfall signal on Day 5/Wednesday. Thus the Day 5 ERO
introduces a Slight Risk area encompassing parts of Texas/Oklahoma
and nudging into far western Arkansas. The surrounding Marginal
Risk covers a similar area as the Day 4 ERO. The Storm Prediction
Center is also monitoring for severe threats on Tuesday from the
eastern Great Lakes and vicinity southwest into northern Texas.
Check the SPC outlooks for the latest info on severe threats.

After early Wednesday, continued progress of low pressure and its
trailing cold front should push rainfall across much of the East
but with lesser totals than forecast over the central U.S. Farther
west, shortwave energy and a frontal system should spread rain and
some high elevation snow from the Northwest through the northern
and central Rockies Tuesday-Thursday. By the end of the week some
rain may develop over the southern High Plains with the help of
easterly low level flow. An upper trough/cold front reaching the
West Coast by Friday-Saturday should bring rain and high elevation
snow into the Northwest.

Expect unseasonably warm temperatures over the eastern U.S. on
Tuesday ahead of an advancing cold front. Plus 10-20F anomalies
will equate to highs reaching into the 80s over the Ohio Valley.
Morning lows will be even a little more anomalous. Northern parts
of the East will trend cooler by midweek but locations farther
south should remain somewhat above normal through Friday. A more
amplified upper trough will likely push the next cold front farther
south, bringing temperatures over the East to within a few degrees
on either side of normal by Saturday. Meanwhile an upper ridge
drifting from the Northwest into the Plains after Wednesday will
support corresponding eastward progress of an area of highs 10-20F
above normal.

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