884
FXUS02 KWNH 090742
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
342 AM EDT Sun Mar 9 2025

Valid 12Z Wed Mar 12 2025 - 12Z Sun Mar 16 2025


...Potent system brings precipitation to the West Wednesday-
Thursday, the central U.S. Friday, and toward the east-
central/eastern U.S. next weekend...


...Overview...

A leading southern stream compact closed upper low/shortwave will
progress from the southern Rockies midweek eastward, spreading
light to moderate rain to the Lower Mississippi Valley into
Southeast Wednesday-Thursday before weakening. Upstream, confidence
remains high in a deep and phased eastern Pacific upper trough
reaching near the West Coast by early Thursday along with a leading
surface low/frontal system and atmospheric river. This system will
bring widespread rain and higher elevation snow across the West in
the mid-late week time frame. Most guidance suggests the upper
trough will close off an embedded upper low over the Plains toward
the end of the week, supporting strong surface low development in
the central High Plains Friday and moving northeastward into the
Upper Midwest and Great Lakes region over the weekend. Expect
rainfall to increase in coverage/intensity over and east of the
Mississippi Valley by Friday-Saturday, while snow is possible in
the northern half of the Plains into the Upper Midwest to the
northwest of the surface low track. Meanwhile areas of strong winds
will be possible and the Storm Prediction Center is monitoring
severe weather potential. The Pacific trough crossing the West will
bring below normal highs to that region while the leading ridge
(and possibly a weaker one farther east) should promote above
normal temperatures over the central/eastern U.S.


...Guidance/Predictability Assessment...

Model guidance remains reasonably agreeable on the large scale
with more model variability on the smaller scale for various
features. A model average worked well for the initial shortwave
pushing across the southern tier along with a Plains surface low
that weakens as it moves east. There is still good model agreement
in the track and timing of the deep and phased eastern Pacific
upper trough moving toward the West Coast, but embedded energies
including the potential formation of an upper low Wednesday have
been a source of model spread as it affects the depth of a surface
low. The 12Z GFS was on the strong side of the envelope with the
low near Vancouver Island after the 12Z ECMWF became weaker (closer
to consensus of the other operational and AI/ML guidance). New 00Z
model runs are generally weaker. Regardless, the upper trough will
pivot east Thursday and reach the Rockies by Friday, spinning up a
deep surface low in the central High Plains. The CMC was a bit
northeast of the GFS/ECMWF agreeable placement. Just how deep the
low will be is still somewhat questionable. The 18Z GFS became
stronger than most as it fell into the 960s (though in between the
12Z surface progs). The low seems more likely to fall somewhere in
the 970s and this forecast does show a sub-980mb low for 12Z
Saturday. Upstream of the main trough, the pattern becomes more
questionable as energy feeds into the Pacific Northwest. CMC runs
become out of phase over the weekend as vorticity dives into the
western side of the trough to form broader troughing while the
GFS/ECMWF have a ridge behind the trough and the vorticity gets
held back.

The WPC forecast began with a blend of deterministic guidance
favoring the 12Z ECMWF and 18Z GFS early in the period. As the
period progressed, gradually increased the proportion of GEFS and
EC ensemble means to over half by Day 7 amid increasing spread.


...Weather/Hazards Highlights...

A weak to moderate atmospheric river will be taking aim at
particularly California on Wednesday ahead of the deep eastern
Pacific/West trough. Some heavy rain could cause flooding in lower
elevations, warranting a Marginal Risk in the Day 4/Wednesday ERO.
The relatively fast movement of the AR could limit the flooding
potential to stay below Slight Risk levels, but will continue to
monitor for any localized enhanced areas that could flood, with
particular concern for burn scars. A Marginal Risk is also in place
for Day 5/Thursday in southern California for the tail end of the
AR and for possible convective showers behind it. Higher elevation
snow could be heavy across the Sierra Nevada with the AR. More
broadly, areas of rain and higher elevation snow are forecast to
spread across much of the West Wednesday-Thursday. Widespread high
winds will also be a concern. Additional rounds of precipitation
are likely in the West late this week into next weekend as another
frontal system or two approach.

Farther east, rain is forecast to spread across the Lower
Mississippi and Tennessee Valleys and Southeast Wednesday-Thursday
ahead of the initial shortwave and a couple of frontal systems.
Farther north there remains some model spread with shortwaves and
frontal systems that could cause precipitation, but with a general
focus for light precipitation from the Great Lakes region to
Interior Northeast.

As the primary upper trough moves into the central U.S., there is
high confidence in surface low development/consolidation in the
central Plains by Friday with strong fronts. On the backside of the
low, snow could spread across the central High Plains and into the
Dakotas on Friday and into the Upper Midwest on Saturday. A
transition zone of ice is also possible. Rain chances emerge ahead
of the cold front across the Mississippi Valley Friday, and heavier
rain is likely by Saturday, with the current forecast showing it
centered in the Tennessee Valley and vicinity, possibly extending
as far south as the Gulf Coast. Monitor Storm Prediction Center
outlooks for the latest information regarding potential severe
weather threats. Winds are another possible concern near the deep
low track, as well as in the Plains behind the cold front.
Currently the guidance shows the potential for highest wind speeds
over the southern Rockies/High Plains around Friday, likely
producing fire weather concerns.

Above average temperatures are likely over most areas east of the
Rockies during the period. Temperatures of 15-20F or so above
normal are forecast across the north-central U.S. to Ohio Valley on
Wednesday, warming even further on Thursday-Friday to plus 20-30F
anomalies. Highs are forecast to reach the 60s and 70s.
Temperatures in the south-central U.S. could reach the 80s while
localized areas like far South Texas can expect 90s, even
approaching 100F. The Plains are likely to cool late week behind
the cold front, and the above normal temperatures will reduce in
scope to the Great Lakes and then Eastern Seaboard over the
weekend. In the West, the upper troughing along with clouds and
precipitation will promote below normal temperatures spreading
across into later week. Highs are forecast to be generally below
average by 15-20 degrees for the central Great Basin and Southwest,
while lows of 5-15 degrees below normal are likely. Temperatures
should moderate somewhat over the weekend as the trough pulls away.


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




$$