944
FXUS02 KWNH 170659
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM 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...

While model guidance has converged somewhat on the track and
timing of the main upper trough/low and surface low moving over the
central U.S. early next week, there are still some smaller scale
differences that could have sensible weather impacts like rainfall
timing and positioning. With the 12/18Z model suite, the 12Z ECMWF
in particular showed the surface low strengthening quickly by
Sunday evening and a bit north of consensus. GFS runs were slightly
on the eastern side with the low track, though certainly better
than GFS runs from a day ago that were significantly east of other
guidance, taking the low east through the Ohio Valley. Fortunately
the newer 00Z guidance looks better clustered with the surface low
track, but some minor waffles in the low position can be expected.
Models show the troughing continuing to lift Monday-Tuesday, with
00Z guidance not as deep with the trough over the Mid-Atlantic as
the 12/18Z GFS runs were.

Meanwhile, models agree that the pattern becomes more zonal
overall after the trough lifts, but this allows for some model
spread in the details including position of frontal systems. Some
relatively shallow upper troughing looks to be in place over the
Northwest early next week and gradually move into the northern
Plains Tuesday-Wednesday. A surface low could develop just ahead of
it, with some spread in its position and strength. Then by midweek
and beyond, there is some signal for a slightly deeper broad
trough in the West. There are ample differences aloft upstream in
the northern Pacific, so expect this pattern may take more time to
resolve.

The WPC forecast began with an even blend of the 18Z GFS and 12Z
ECMWF, CMC, and UKMET to provide a middle ground forecast for the
low track early next week. With time, increased the proportion of
ensemble means, reaching just over half by Day 7 as spread
increased with the details of shortwaves and surface fronts.


...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 modest adjustments northward compared to continuity based on
model trends. 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,
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.


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


$$