389
FXUS02 KWNH 220657
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EST Wed Jan 22 2025

Valid 12Z Sat Jan 25 2025 - 12Z Wed Jan 29 2025


...Overview...

By the start of the period (Saturday), troughing will be exiting
the East coast with weak ridging developing behind for the weekend,
finally helping to moderate the bitterly cold short range
temperatures. Meanwhile, a shortwave dropping into the northern
Rockies late this week will split, sending northern stream energy
across the northern tier of the nation while trailing energy dives
south to form a closed low over the Southwest that will linger and
slowly drift east into the Southwest by early-mid next week. The
upper low will likely bring beneficial precipitation to parts of
southern California that have been dealing with devastating
wildfires. Moderate to locally heavy rainfall will develop
downstream across the Lower Mississippi Valley Sunday and Monday as
a slow moving frontal boundary moves through the region.


...Guidance/Predictability Assessment...

On the large scale, the guidance continues to show reasonable
agreement, but there remains plenty of uncertainty in the details
and timing of individual systems. Relatively good clustering early
period as the shortwave amplifies and a low cuts off over the West
Coast Saturday-Sunday. By Monday though, some guidance holds the
low farther west (CMC and UKMET), while other guidance (GFS, ECMWF)
pulls it farther east faster. Ensembles are split also, though the
ensemble means were a tad faster like their deterministic
counterparts. Mean northern stream troughing over the Northern
Plains-Great Lakes-Northeast shows uncertainty in the amplitude of
individual shortwaves dropping through and the GFS is the most
bullish with one shortwave from the northern Plains to the Midwest
next Wednesday-Thursday.

The WPC forecast was able to use a general model blend between the
GFS, ECMWF, CMC, and UKMET for the early period (Saturday/Sunday)
amidst decent agreement. For the Monday-Wednesday time frame,
gradually transitioned towards majority ensemble means (with the
GFS and ECMWF) which provided a good starting point for the
Southwest upper low evolution and northern stream
troughing/shortwaves.


...Weather/Hazards Highlights...

Colder than normal temperatures will linger through Saturday for
parts of the South and Southeast with daytime highs still 5-15F
below normal with overnight lows below freezing. Temperatures will
gradually moderate, returning to near normal Sunday through early
next week. Anomalously cold temperatures will also develop in the
High Plains, Rockies, and much of the West this weekend under the
influence of upper troughing. The coldest anomalies will be in the
High Plains and Rockies where high temperatures could be 10-20
degrees below normal. Late period ridging over the northern tier
could allow temperatures to rise above normal next week.

Precipitation wise, a couple clipper systems may produce some
light lake effect snows downwind of the Great Lakes. Meanwhile,
more widespread rain and mountain snows will be dropping through
the Great Basin to Southwest this weekend bringing beneficial rain
to Southern California over the weekend. A tight gradient behind
the cold front dropping south may produce an episode of strong
winds along parts of the Rockies late this week and over California
and Nevada on Saturday with offshore flow and modestly favorable
fire weather conditions possible again behind the upper low next
week. Farther east, Gulf moisture will interact with a front
approaching the Lower Mississippi Valley to Tennessee and Ohio
Valleys by this coming weekend/early next week. Latest guidance
continues to trend heavier with rainfall totals over parts of the
southern U.S., with the potential for heavy rain and thunderstorms
on Sunday across the Lower Mississippi Valley. There was enough
support for a marginal risk in the Day 5/Sunday Excessive Rainfall
Outlook from eastern Texas into Mississippi where storms may train
along a slow moving frontal boundary. Snow or a wintry mix is
possible on the northern side of this precipitation shield mainly
across the Ohio Valley, central Appalachians, and potentially the
Tennessee Valley as well. Precipitation may spread towards the East
Coast early next week as the frontal system progresses with some
redevelopment of rainfall across the Southern Plains as well.


Santorelli


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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