Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sat Mar 29 2025

Valid 12Z Tue Apr 01 2025 - 12Z Sat Apr 05 2025


...Multi-day heavy rain and severe threat for Lower/Middle
Mississippi, Tennessee, and Ohio Valleys next week...


...Overview...

The medium range upper pattern should trend more amplified through
the period and increasingly active. Troughing moving through the
West Tuesday will lead to a surface low emerging into the Plains
around midweek and tracking towards the Great Lakes. A building
upper ridge over the western Atlantic into the Gulf should keep the
associated frontal boundary from making much progress, especially
later in the week. This sets the stage for a multi-day heavy rain
and severe weather threat from the Lower Mississippi into the Ohio
Valley along this boundary. Some late-season snow is likely north
of this system from the northern Plains to Great Lakes and Interior
Northeast. The West should stay relatively cool underneath of
persistent troughing, while above normal temperatures progress from
the central U.S. and Midwest, eventually settling late week across
the Southeast and into the Mid-Atlantic.


...Guidance/Predictability Assessment...

Model guidance is relatively agreeable on the large scale pattern
next week into the weekend, but still somewhat uncertain with some
of the details, which could have sensible weather impacts. A
shortwave exiting the East on Tuesday will be replaced by a
building ridge over the western Atlantic to the Gulf that lasts
into next weekend. This should help amplify and reload troughing
over the West through the period. Shortwaves within that trough
show relatively more spread, but the first shortwave/compact upper
low and its associated surface low tracking from the Central U.S.
to Great Lakes Tuesday to Thursday shows enough agreement for a
multi-model blend. Energy will allow for the Western U.S. trough to
reload late week with a stationary surface boundary setting up
between the Southeast ridge and Western trough. There are subtle
differences in the exact placement of this front though, which has
significant implications on the axis of heavy rainfall expected to
accompany the front. Expect these to vary a bit in future
forecasts, though the general idea of heavy rain in parts of the
east-central U.S. is pretty locked in. The next shortwave digging
through the western side of the trough over the Northwest later
next week shows more uncertainty. ECMWF runs have been generally on
the western side with this digging energy compared to the better
consensus of the other guidance and even the EC-based AI/ML models,
so the ECMWF was not particularly favored by the late period.

The WPC forecast was able to utilize a deterministic model blend
featuring the ECMWF, GFS, CMC, and UKMET (from most to least in the
blend) through Day 4. By Day 5, began incorporating some ensemble
mean guidance to help temper the detail differences, for a half
models/half mean blend by Days 6 and 7. This approach maintained
relatively good agreement with the previous WPC forecast.


...Weather/Hazards Highlights...

Overall pattern amplification and a leading shortwave will support
a surface low pressure system forming in the central U.S. Tuesday
into Wednesday with widespread thunderstorms across the east-
central U.S. in its warm sector. On Tuesday evening and night,
ample upper-level energy and instability wrapping northward could
allow for severe thunderstorms to form per SPC, and heavy rainfall
rates in most model guidance could lead to isolated flooding issues
for parts of the central Plains and Middle Mississippi Valley. A
Marginal Risk is now in place for those areas for the Day 4/Tuesday
night period. As the shortwave/embedded upper low move northeast
Wednesday, moist and unstable inflow into the associated surface
fronts along with the strong upper level support will lead to
widespread severe weather and flash flooding concerns. SPC shows
severe probabilities from the south-central U.S. into the Midwest,
with embedded higher probabilities from portions of the
Lower/Middle Mississippi Valley and Tennessee/Ohio Valleys. The
greatest concern for flash flooding on Wednesday will stretch
across similar areas, as the front anchors storms for training
concerns. A Slight Risk remains in place in the Day 5/Wednesday ERO
from the Ohio Valley southwest across the Mid-South and extending
farther southwest than the previous issuance into Arkansas, due to
heavy rain amounts there in most model guidance and our typical
bias for EROs not being far enough southwest. This is considered a
higher-end Slight Risk as models already show 3 to 7 inches of rain
during the 12Z Wednesday to 12Z Thursday period alone. The
stagnating pattern will cause the front to stall and unfortunately
allow for heavy rain to likely fall in much the same place on
Thursday. While this is currently later than the ERO time frame,
Moderate Risks in future EROs are likely at this point, and High
Risks are not out of the question at some point as the event draws
nearer if the forecast remains on track. There is more uncertainty
by the end of next week in rainfall placement and possible severe
weather, with possibly a shift westward as the trough pivots, but
regardless multiple days of heavy rain and severe weather are
likely next week.

On the backside of the low track, some April snow is likely across
the northern Plains and Upper Midwest Tuesday-Wednesday and
spreading into the Interior Northeast Wednesday-Thursday. Snow
amounts have typical uncertainty, and an axis of freezing rain
and/or sleet is possible as well.

As troughing sets up over the West with multiple shortwaves moving
through, rounds of precipitation are possible from the West Coast
into the Intermountain West and Rockies. Locally heavy rain may be
possible across northern California and southwest Oregon into
Tuesday, but likely below Marginal Risk levels for flooding.
Generally lower elevation rain and higher elevation snow are
expected, with highest snow amounts in the Sierra Nevada on
Tuesday. Additionally, high winds are possible from the central
Great Basin Tuesday into the central Rockies Wednesday.

Above normal temperatures will shift from the Central/Southern
Plains on Tuesday into the Midwest on Wednesday, and settle across
much of the Eastern U.S. Thursday and beyond. Daytime highs 10 to
20 degrees above normal are possible. Meanwhile, the West should
trend and stay cooler underneath upper troughing through Friday. By
next weekend, temperatures may moderate somewhat while cooler
temperatures shift slightly east as eastern Pacific upper ridging
moves in towards the West Coast.


Tate/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