633
FXUS02 KWBC 021859
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Wed Apr 2 2025

Valid 12Z Sat Apr 05 2025 - 12Z Wed Apr 09 2025


...Multi-day and potentially catastrophic heavy rain/flooding and
severe threat likely for Lower/Middle Mississippi, Tennessee, and
Ohio Valleys into Saturday...


...Overview...

A significant and potentially historic flooding and flash flooding
event will be ongoing as the period begins on Saturday along a
wavy frontal boundary from the Lower Mississippi Valley to the Ohio
Valley. This front is sandwiched between a strong Atlantic to Gulf
upper ridge and an amplified western U.S. trough. A gradually
amplifying Canada into northern U.S. upper trough nearing the East
into Monday and eventual southern stream energy ejection and
phasing will finally help to push the frontal boundary and rainfall
east with some heavy rainfall potential along the central Gulf
Coast and parts of the Southeast. A shortwave reaching the West on
Sunday-Monday could bring mostly light precipitation to parts of
the Northwest. After Monday, the medium range period should trend
much drier across the country as the overall pattern briefly
becomes flatter and more progressive on the way toward a western
ridge/eastern trough configuration.

...Guidance/Predictability Assessment...

Within the agreeable large scale pattern, primary differences of
note involve the timing of energy ejecting from the
Southwest/southern Plains/northern Mexico late weekend onward,
specifics of upper trough energy digging into the Great Lakes early
next week (with significant influence on surface evolution and
precipitation), and low-predictability specifics of shortwave
energy progressing from the West Coast into central U.S. Monday-
Wednesday. Dynamical and machine learning (ML) guidance comparisons
led to starting the updated forecast with 40 per 00Z ECMWF and an
even remaining split of the 06Z GFS/00Z UKMET/00Z CMC early-mid
period, followed by a rapid increase of 06Z GEFS/00Z ECens input
that reaches 60 percent total by next Wednesday. The forecast
phased out the GFS after Monday and maintained more ECMWF versus
CMC through the end of the period.

The first guidance difference that arises is with the energy
ejecting from the southern part of the large scale trough from late
weekend into early next week, with the GFS/GEFS leaning a bit
faster than most other solutions. 00Z/06Z ML models show some
variance as well and on average are not quite as slow as the ECMWF
cluster. At the surface, ML models show a decent signal for a
Tennessee (Sunday) through Mid-Atlantic (Monday) wave with other
guidance showing some trends in that direction.

Latest GFS runs have been most aggressive with the degree to which
digging Great Lakes trough energy closes off an upper low by
Tuesday, leading to a much stronger/wrapped up surface system along
with more significant snowfall over parts of the Northeast. Recent
ML models generally favor keeping the trough more open, though
with the potential for a neutral to negative tilt for a time. Some
ICON runs and the 12Z CMC develop a closed low somewhat east of the
GFS, so the ultimate evolution over the Northeast remains
uncertain.

Low predictability favors a conservative approach for Monday West
Coast energy that may reach the Plains by midweek. The strong side
of the envelope (such as in some GFS runs) would produce meaningful
precipitation over the central U.S. but consensus recommends
weaker energy and a much drier pattern.

...Weather/Hazards Highlights...

A persistent wavy frontal boundary will continue to focus moist
and unstable inflow leading to widespread flash flooding concerns
which should begin during the short range period and continue
through about Saturday. For several days now, models have been
indicating significant multi-day rainfall totals, especially across
Arkansas into western Kentucky where 24-hour totals on Saturday
alone in excess of 4-5 inches could be realized in some spots (and
this is on top of potentially 5-10 inches of rainfall in the short
range as well). A catastrophic and life-threatening flash flood
event is likely and the Day 4/Saturday ERO continues to show a
large moderate risk from northeast Arkansas into western
Kentucky/southern Indiana, with a likely high risk upgrade needed
once this moves into the short range period. Latest guidance
recommends maintaining continuity for the Day 4 ERO. By Sunday, the
heavy rain threat should finally begin to shift eastward with
moderate to heavy rains possible across the Gulf Coast states into
the Tennessee Valley/Southeast, and rainfall farther north into the
Mid-Atlantic and Northeast near the front. The Sunday ERO shows a
Marginal Risk for this region with an embedded Slight Risk farther
south across parts of southern Alabama and Mississippi where recent
heavy rainfall has primed soils and the greatest instability
should be located. Guidance shows potential for a band of somewhat
heavy QPF to extend farther northeast across northern Georgia into
or near the far southern Appalachians, but latest first-guess
fields plus neutral ground moisture (and likely drier by then given
no rain forecast in the shorter term) favor keeping this region in
Marginal Risk for the time being.

Farther west, an upper low that digs into the Southwest by
Saturday should produce enhanced precipitation totals across the
southern and central Rockies and Plains. Snow is likely in higher
elevations of the Rockies in Colorado and northern New Mexico, and
some snow may spill into lower elevations of the High Plains within
cooled post-frontal upslope flow. Elsewhere by Monday, a cold
upper trough reaching the Great Lakes should lead to unsettled
weather over the region with rain or snow. This precipitation
should extend into the Northeast as well. Latest guidance shows
significant differences for surface/upper evolution over the
Northeast early- mid week, with the more extreme side of the spread
producing some significant snowfall over parts of New England in
contrast to much lighter and scattered snow in the greater
proportion of solutions. Probabilities of 0.25 inch liquid in the
form of snow reach no higher than 10-15 percent or so during the
Tuesday-Tuesday night period, reflecting the guidance majority.

Meanwhile upstream, the upper shortwave/front expected to reach
the West Coast into early next week will be accompanied by a brief
increase of moisture anomalies but the system will be fairly
progressive. Thus expect mostly light precipitation but with some
terrain-enhanced moderate activity over western Washington/Oregon
and northwestern California, with decreasing amounts inland over
the Northwest.

Above normal temperatures will linger across much of the East
through the weekend with daytime highs 10 to 20 degrees above
normal. Best potential for daily records will be across the
South/Southeast into southern Mid-Atlantic. Meanwhile, well below
normal temperatures will be in place across parts of the southern
Rockies and High Plains where daytime highs 10 to 30 degrees below
normal are possible on Saturday and just a little warmer on Sunday.
Moderated below normal temperatures will also shift into the
central U.S. and Midwest Monday and the East by Tuesday-Wednesday.
Highs over and near the Ohio Valley could be up to 15 to 20 degrees
below normal over and near the Ohio Valley on Tuesday. Warmer than
average temperatures initially over the Northwest this weekend
should expand into the remainder of the West and the
northern/central Plains by next week. A strengthening upper ridge
will likely expand coverage of plus 10 to 20 degree anomalies over
the West on Wednesday.

Rausch/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















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