117
FXUS02 KWNH 010701
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Tue Apr 1 2025

Valid 12Z Fri Apr 04 2025 - 12Z Tue Apr 08 2025


...Multi-day and significant heavy rain/flooding and severe threat
likely for Lower/Middle Mississippi, Tennessee, and Ohio Valleys
through next Saturday...


...Overview...

The medium range upper pattern will be amplified, blocky, and
active through much of the period. A frontal boundary parked over
the Ohio Valley to Southern Plains in between an amplified western
trough and strong Atlantic to Gulf upper ridge, will continue to be
the focus for an ongoing multi-day heavy rain and severe weather
event from the Lower Mississippi Valley to the Ohio Valley, which
starts during the short range period. Significant flooding and
flash flooding is increasingly likely for portions of this area. A
gradually amplifying Canada into northern U.S. upper trough nearing
the East by Sunday- Monday may finally help to push out the
frontal boundary. Most of the West should stay relatively cool
underneath persistent troughing with some progression eastward
early next week into the Central U.S., while upper ridging brings a
warming trend to the Northwest during the weekend. The East will
see above normal temperatures late this week and then a cooler
trend that moves in from the north and west.


...Guidance/Predictability Assessment...

The latest model guidance continues to agree fairly well on most
aspects of the forecast for Friday and Saturday. There are
differences still with some embedded lower-predictability details,
which would have sensible weather impacts. A building and blocky
ridge over the Atlantic to Gulf should help amplify troughing over
the West. Shortwaves and energy perturbations exiting that trough
show more spread, and may play a role in exactly where the heaviest
rainfall occurs. Either way, models have been extremely consistent
in showing a potentially significant flooding event from the lower
Mississippi Valley into the Ohio Valley. Energy digging through
the western trough on Friday should interact with a northern stream
shortwave next weekend to eventually help push the wavy front and
heavy rain threat eastward. This shortwave should amplify over the
East by early next week. There are some differences in the weekend
interactions between this shortwave and western U.S. energy and
then timing of surface low pressure/cold front through the East
next week. Any leftover troughing over the West should weaken and
shift east as an upper ridge moves into the Northwest next Monday-
Tuesday.

WPC was able to use a general deterministic model compromise for
about the first half of the period. Transitioned to more ensemble
means, eventually comprising 60 percent of the blend, by Day 7.
This helps mitigate the smaller scale differences and also
maintains good agreement with the previous WPC forecast.


...Weather/Hazards Highlights...

By Friday, a wavy frontal boundary will be stuck between a strong
upper ridge to the east and amplified troughing over the West.
Moist and unstable inflow pooling along the front will lead 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 several inch rainfall
totals, especially across Arkansas into western Kentucky where
24-hour totals in excess of 5-8 inches could be realized in some
spots (and this is on top of several inches of rainfall in the
short range as well). For Friday, the heaviest rainfall axis looks
to be from eastern Oklahoma northeastward into the Ohio Valley,
with some slight shift to the east by Saturday, but there has been
some wobble in exactly where the heaviest rain sets up. Regardless,
the potential for high impact and life-threatening flash flooding
is there. As such, moderate risks are in place for this general
region for both the Day 4/Friday and Day 5/Saturday ERO periods.
High Risks are likely in future EROs as details become clearer,
especially into the timeframe of the CAMs. 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 as well along the front.

A possible upper low in the Four Corners region on Friday could
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 uncertainty
at this point in if/how much snow could spill into lower elevations
of the High Plains. Elsewhere by next Monday, the cold upper
trough reaching the Great Lakes should lead to unsettled weather
over the region with rain or snow depending on location and time of
day, while the upper shortwave/front reaching the West Coast may
bring some light activity to far western Washington/Oregon and
northwestern California.

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. Temperatures should
moderate early next week as the upper ridge shifts eastward and
troughing moves into the East. Meanwhile, the West should stay
cooler underneath upper troughing through Friday. By next weekend,
temperatures may moderate somewhat along the West Coast while
eastern Pacific upper ridging moves in. Parts of the Northwest
could see highs reach up to 10 to 15 degrees above normal which
eventually expands into the northern Plains next week. This shifts
cooler temperatures eastward, and daytime highs across parts of the
southern Rockies and High Plains could be 10 to 25 degrees below
normal next weekend. Below normal temperatures also shift into the
central U.S. and Midwest by next Monday, including highs as low as
10 to 15 degrees below normal over the Great Lakes/Ohio Valley.


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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