839
FXUS02 KWNH 210707
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
305 AM EDT Mon Apr 21 2025

Valid 12Z Thu Apr 24 2025 - 12Z Mon Apr 28 2025


...Multi-day Heavy rain threat for the South-Central U.S....


...Guidance/Predictability Assessment...

Guidance offers decent larger scale agreement on the overall upper
flow evolution over much of the next week, but with considerable
uncertainty in the details of individual systems. Energy
progression from the southern Plains to Mississippi/Tennessee
Valleys mid-late week will take until the short range to resolve
details, but have implications on heavy rainfall distribution and
amounts for this area. Big differences arise uncharacteristically
early by Thursday/Friday as an upper low from the Gulf of Alaska
drops southward along the coast, with some energy likely to split
from the main low. Latest 00 UTC model guidance remains mixed.

Accordingly,the WPC forecast used a blend of GEFS/ECMWF/Canadian
ensemble mean guidance through medium-range time scales. Overall,
this acts to smooth the less predictable details and offers best
possible WPC product continuity given lingering uncertainties.

...Weather/Hazards Highlights...

An ill-defined series of generally progressive upper features and
surface based systems will continue to work over the lower 48 this
week to focus local weather/convection, but there does remain a
varied signal to amplify the upper flow with troughing from the
northeast/eastern Pacific into the West heading into next weekend.

The WPC Excessive Rainfall Outlook (ERO) offers Marginal Risk
areas from parts of the south-central Plains into the Mississippi
and Tennessee Valleys for Day 4/Thursday and Day 5/Friday, but
there is still considerable uncertainty on exactly where heaviest
amounts will be. Regardless, the pattern supports a moist and
unstable flow for a favorable, at least local, flash flood setup.
Recent and ongoing heavy rainfall over portions of these regions
will make some particularly susceptible to additional rains, so an
ERO Slight Risk upgrade may be needed in future updates. But with
the model uncertainty, held off on any upgrades at this time.
Uncertain central to eastern U.S. heavy rain and runoff threats
with strong convective potential near lingering boundaries with
slow system translations through the weekend will be monitored.

Meanwhile, the extent of upper trough amplifications and digging
off the West Coast this week is not a done deal in guidance, but
the prospect of surface system genesis and slow but steady system
translation from the northeast/eastern Pacific to the West Coast
Friday and inland over the West next weekend still offers increased
light to moderate widespread rain chances and mountain snows.

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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