702
FXUS02 KWBC 171853
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 PM EDT Thu Apr 17 2025

Valid 12Z Sun Apr 20 2025 - 12Z Thu Apr 24 2025


...Heavy rainfall and severe weather in parts of the central U.S.
into Sunday...


...Overview...

As the medium range period begins early next week, upper troughing
and a surface low will pivot northeast from the Plains to Midwest.
Rain that could be heavy is likely along the low track Sunday,
while showers and thunderstorms that could be severe are possible
in the Mississippi Valley. Modest rainfall should shift into the
east-central U.S. by Monday as the front moves through. Overall the
upper pattern is forecast to relax for more zonal flow as the week
progresses. However, troughs poking into the northern tier could
spread rounds of precipitation there. Additionally, return flow
could lead to additional rain and thunderstorms in the south-
central U.S. by Tuesday-Wednesday.


...Guidance/Predictability Assessment...

The main system in the first half of the medium range period is a
surface low as it lifts from the Southern Plains on Sunday into the
Midwest/Great Lakes on Monday. There remains some smaller scale
timing/placement/intensity differences with the low center itself,
but a general model blend served as a good starting point for the
first half of the medium range forecast. Elsewhere, a couple of
weak shortwaves will move through the Northwest. There is generally
good agreement on the presence of these, but uncertainty in the
timing and evolution of them downstream. With one shortwave around
Wednesday, the CMC was notably slower than the GFS and ECMWF (and
ensemble means) and so it was not included in the late period
blend. The late period blend for the WPC forecast was based on an
equal weighting of the GFS, ECMWF, and ensemble means. Overall,
maintained good agreement with the previous WPC forecast.


...Weather/Hazards Highlights...

By Sunday, the upper trough and surface low pressure system moving
northeastward will spread some rain and thunderstorms across much
of the Mississippi Valley. Moisture and instability levels may
decrease from Saturday (likely the biggest day of the heavy rain
event) but remain above normal. The relatively faster movement of
the low should also limit rainfall totals by Sunday. However, there
is still some threat of heavy rainfall along the low track and
just to the east with the frontal system. A Marginal Risk in the
Day 4/Sunday ERO focuses in the Middle Mississippi Valley, with
some adjustments on the day shift cycle to pull it a little more
south into the Lower Mississippi Valley along the front where the
better instability should be located. These areas are north and
west of the most sensitive areas of the Mississippi/Ohio River
Valleys where streamflows remain high after recent flooding.
However, some rain atop recently flooded areas, and runoff from
other areas into these rivers, could exacerbate ongoing flooding
issues in some areas, so continue to monitor. Severe weather is
another potential threat from eastern Texas into the Lower/Middle
Mississippi Valley on Sunday per the Storm Prediction Center. Then
on the northern side of the precipitation shield there is a chance
of accumulating snow in northern Minnesota and Wisconsin through
Sunday night. By Monday, rain chances are forecast to spread across
the eastern third of the U.S. as the front pushes east, but with
generally lower amounts. There could be convection in the Southeast
with localized heavy rain rates, but this appears to be below
thresholds for any risk in the ERO. Meanwhile, an upper trough
pushing into the Northwest could cause precipitation there early
next week. Higher elevations of the northern Rockies can expect
snow while moderate rain is forecast in the northern Plains and
eventually the Midwest. By Tuesday- Wednesday, moist inflow from
the Gulf and meandering fronts could allow for another round of
moderate to heavy rain in the southern half of the Plains and
Mississippi Valley once again, but with still quite a bit of
uncertainty, so continue to monitor this potential.

On Sunday, cooler than average temperatures could persist across
portions of the Four Corners states into the central U.S. under the
lingering upper trough. Meanwhile warmer than normal temperatures
by 10-20 degrees are forecast across the Tennessee Valley into the
Mid-Atlantic. Temperatures may stay warmer than average in parts of
the Southeast as the workweek begins. Temperatures should also
quickly warm in the central U.S. by Monday, but be a few degrees
cooler than normal for highs in the Northwest under upper
troughing. By Tuesday and beyond with the quasi-zonal upper
pattern, most areas across the lower 48 should be near to slightly
above average.


Santorelli/Tate


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




$$