929
FXUS02 KWBC 011855
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 PM EDT Tue Apr 1 2025

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


...Multi-day and potentially catastrophic 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 and quite stormy
this week. A wavy frontal boundary settling from the Ohio Valley
to Southern Plains, holding 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 to catastrophic
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 into Monday and eventual southern stream
energy ejection and phasing will finally help to push out the
frontal boundary. Most of the West this week 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. Detailed WPC
forecasts are linked below.

...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 that trough show more 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. One aspect of the machine learning models to consider is
their depiction of a slightly narrower axis of heaviest rainfall on
a given day, albeit with locally higher amount potential daily compared
to the NBM. WPC QPF was trended in this manner.

Energy digging through the western trough heading into the weekend
should with lingering uncertainty eventually interact with a more
amplifying northern stream upper trough into 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.

Overall reasonable GFS/ECMWF/UKMET/Canadian guidance clustering
and continuity aspects allowed for usage of a composite model blend
through medium range timescales to produce a solid starting
forecast basis in line with the NBM. This solution plan maintains
good WPC product continuity and consistent hazards messaging.

...Weather/Hazards Highlights...

It generally still remains the case that 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
multiple 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 upgrades
may well occur in future EROs as details become clearer, especially
into the timeframe of the CAMs. This would be consistent with
highly anomalous and protracted IVT/ARI values and training risks.
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.

Out West, an upper low dug into the Southwest into Friday/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
uncertainty on how much snow could spill 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. 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/Schichtel


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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