678
FXUS02 KWBC 020657
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM 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 into the West on
Sunday-Monday could bring some light precipitation to parts of the
Northwest. After Monday, the medium range period should trend much
drier across the country as the overall pattern deamplifies.


...Guidance/Predictability Assessment...

Latest model guidance agrees fairly well on the main aspects of
the forecast through medium range time scales and have good
ensemble and machine learning model support. There are differences
still with some embedded lower predictability details, which would
have sensible weather impacts. Shortwaves and energy perturbations
exiting thae western trough do still show some spread, and will
play a role in exactly where the heaviest rainfall occurs. Either
way, models have been extremely consistent in showing a significant
flooding event from the lower Mississippi Valley to the Ohio
Valley lasting through Saturday. Timing and aspects of the Great
Lakes to Northeast trough have also improved, but there is still
some uncertainty on how quickly southern stream energy finally
ejects east. Another shortwave into the Northwest shows some timing
uncertainties as well (CMC is slower than the ECMWF and GFS) with
bigger questions on how this evolves downstream mid next week and
how strong ridging is over the Southwest.

The WPC forecast for tonight utilized a deterministic model blend
the first half of the period amidst minimal model spread. By Day 5
and beyond, gradually increased the ensemble means to help mitigate
differences in the deterministic guidance, though still maintained
some operational models (anchored by the ECMWF) for added system
definition.

...Weather/Hazards Highlights...

A wavy and stuck 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. 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.

Out West, an upper low dug into the Southwest into Saturday is
expected to lead to 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, with
some snow possibly spilling into lower elevations of the High
Plains as enhanced by 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.

Meanwhile upstream, upper shortwave/front expected to reach the
West Coast into early next week offers seemingly modest moisture
transport, but will likely bring some light to terrain enhanced
moderate activity to 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 25 degrees below
normal are possible this weekend. Moderated below normal
temperatures also shift into the central U.S. and Midwest Monday
and the East mid next week. 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.

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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