Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 PM EDT Fri Apr 18 2025

Valid 12Z Mon Apr 21 2025 - 12Z Fri Apr 25 2025


...Guidance/Predictability Assessment...

The guidance remains reasonably well clustered during much of next
week but still with some considerable differences in the details.
There are some timing differences with the initial surface low into
the Great Lakes, with the UKMET the fastest, but a general blend
of the deterministic guidance seemed to be a good starting point
for the WPC forecast. Upstream, another shortwave moves into the
Northwest around the middle of the week, and the ECMWF is slightly
deeper with this system, which maintains more flat flow into the
Northern Plains later in the week, rather than more ridging like
suggested in the GFS/CMC and ensemble means. An even deeper
shortwave/upper low will dive towards the Northwest coast next
Friday, and again, the ECMWF is farther south and more out of line
than the better consensus of guidance. The later week WPC forecast
skewed towards the ensemble means, with some modest contributions
still from the CMC and GFS.


...Weather/Hazards Highlights...

A protracted series of generally progressive upper features and
associated surface based systems are set to work over the lower 48
next week in multiple stream flows to locally focus April weather.

An organized lead system next week will bring a deepened main low
and moderate wrapping rains from the Great Lakes/Midwest through
the Northeast early next week. A trailing cold front will meanwhile
act to focus deeper convective potential across the South and the
Appalachians mainly Monday that could lead to some locally heavy
downpours and runoff issues to monitor. The front should be
generally progressive across this region though, limiting the
overall flash flood threat. Upstream upper energies and surface
response will also act to produce light to moderate precipitation
back over the Northern Rockies/Plains and vicinity with approach
and passage.

A WPC Excessive Rainfall Marginal Risk area for portions of the
Southern Plains for Day 5/Tuesday is in place given the potential
for some repeat/training of cells during the day and with overnight
activity with emerging moist and unstable return flow into/over
several frontal boundaries. Some portions of this area are also
forecast to have enhanced rainfall this weekend that would lead to
wet soil conditions. Heavy rain and runoff threats along with
strong convective potential in this region and into the Mississippi
Valley will also be monitored for mid-later next week given
lingering support and multi-day potential, albeit with greater
uncertainty on the details.

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