991
FXUS02 KWBC 091859
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM 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. Another developing upper trough nearing
the West Coast by next Sunday, with one or more surface systems,
should bring additional precipitation to the West next weekend. The
leading 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...

The updated forecast maintains good continuity in principle, with
an early-period operational model composite (more 06Z GFS/00Z ECMWF
relative to the 00Z UKMET/CMC) and then a model/mean mix (reaching
40 percent total weight of the 06Z GEFS/00Z ECens by Day 7 Sunday)
providing a good representation of guidance ideas through late
week and accounting for rapidly increasing detail spread near the
West Coast by next Sunday.

During Wednesday-Friday recent guidance has been steadily trending
toward better agreement for the gradually weakening southern tier
shortwave (looking fairly close to yesterday`s 12Z model average)
as well as for the Northeast/Mid-Atlantic pattern. Latest guidance
runs are reinforcing the idea of fairly strong high pressure
crossing New England and supporting a more pronounced southward
push of a backdoor front into the Mid-Atlantic, in contrast to
pre-12Z/08 ECMWF runs that had a surface wave crossing the
Northeast.

Ahead of the East Pacific upper trough heading into the West,
latest trends have tilted toward the weaker/faster side for surface
low pressure forecast to reach near Vancouver Island by early
Wednesday--due to a weaker and more open depiction of supporting
small-scale shortwave energy. This solution is closer to some CMC
and machine learning (ML) runs from recent days versus some ECMWF
runs that were very deep and somewhat slower.

By Friday-Saturday, guidance overall shows better than average
track and timing agreement for deepening central High Plains into
Upper Midwest low pressure. The average of ML models and a relative
majority of members from the various ensemble systems suggest a
most likely depth in the 980s mb as of early Friday (GFS/GEFS
leaning a bit deeper. Then over the Upper Midwest by early Saturday
the best dynamical model/ensemble clustering falls in the 970s mb
range while the ML model average is in the low 980s. The manual
forecast maintains continuity with an implied depth in the low 980s
and then upper 970s. By Sunday there is more spread for surface
low latitude in Canada as well as for depth, but agreement is still
better than average for timing of the trailing cold front pushing
through the eastern U.S.

Forecast issues over the East Pacific into western U.S. toward
next Sunday have some resemblence to what had been causing problems
near the West Coast in forecasts valid Wednesday, namely uncertain
evolution of small-scale shortwave energy within a more
predictable medium to larger scale upper trough. Sunday`s shortwave
detail differences yield a 20-30 mb difference in strength of
potential low pressure (12Z CMC being the deep extreme), in
addition to various ideas for track and timing. The manual forecast
stays on the conservative side depth-wise given the wide spread
among dynamical and ML guidance at this time.


...Weather/Hazards Highlights...

A weak to moderate atmospheric river will be taking aim mainly at
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. Closer in time to
this event, high resolution guidance may signal where localized
focus could be pronounced enough to warrant an upgrade in risk
level on either day. 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. During the weekend this precipitation could be on the
moderate/heavy side at times over some locations across the Pacific
Northwest into parts of California, but with ongoing uncertainty
in specifics.

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.
Guidance signals thus far suggest that this activity should not be
intense enough to warrant any ERO risk area. 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. At this time SPC is highlighting the best severe
potential from parts of the Midwest southward almost to the west-
central Gulf Coast on Friday, and from portions of the Tennessee
Valley southward on Saturday. 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. In addition southerly winds in the
warm sector could get fairly strong over/near the Appalachians and
East Coast next weekend.

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. Daily record highs look fairly isolated, but
there could be more widespread record warm lows from the Midwest
through the East Friday-Sunday if the morning mins hold for the
calendar day. 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 the region
mid- late 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.


Rausch/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






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