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FXUS02 KWBC 211859
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Fri Feb 21 2025

Valid 12Z Mon Feb 24 2025 - 12Z Fri Feb 28 2025


...Wet Pacific Northwest pattern lasts into Monday...


...Overview...

The upper flow pattern should remain rather progressive through
much of the medium range period. By Monday, a northern stream
shortwave will shift through the Northeast, with a southern stream
wave through the Gulf and Florida. Meanwhile, a shortwave
approaching the Pacific Northwest will maintain an atmospheric
river coming into the region for a heavy rain threat continuing
into Monday. That shortwave should move inland by Tuesday and helps
deepen a trough over the East by late week. Upstream an amplifying
ridge will form over the West, though with a southern stream
trough or cutoff low approaching California by next Friday.


...Guidance/Predictability Assessment...

Model guidance shows reasonable agreement in the overall pattern
described above, though with typical detail differences that could
have sensible weather impacts at times. The shortwave moving
through the northern tier Monday-Wednesday was clustered well
enough in guidance to use a multi-model blend early in the period--
the 00Z ECMWF ended up deeper with it than other guidance, but
seemed useful as part of the blend.

There are two notable areas of uncertainty the second half of the
week. First, there is some spread with the way troughing digs
across the east-central U.S., which leads to surface low placement
and magnitude differences. The newer 12Z models seem to favor a
weaker surface low moving through the Midwest Wednesday compared to
the 00/06Z cycle, but at least are a bit better clustered with the
low`s position in the Northeast or northern Mid-Atlantic on
Thursday than the deterministic 00Z/06Z models were. Second, the
eastern Pacific southern stream upper low shows variability in its
position as it moves toward California Thursday-Friday. After a
couple of cycles of slower GFS runs a day ago, now GFS runs have
switched to their more typical bias of a faster progression of the
upper low while the non-NCEP models are slower. The GFS-based and
EC-based AI Graphcast models are both in between the slow EC and
faster GFS with the upper low position, suggesting some model
physics differences leading to the spread. The WPC forecast
gradually reduced the proportion of the deterministic guidance in
favor of the ensemble means as the period progressed, reaching half
means Day 6 and over half Day 7 amid increasing model variability.


...Weather/Hazards Highlights...

A consistent feed of moisture will be ongoing into the Pacific
Northwest as the period begins Monday, with moderate to heavy lower
elevation rain and high elevation snows along with windy
conditions. The Day 4/Monday WPC Excessive Rainfall Outlook
continues a Marginal Risk threat area given the cumulative effect
of the multi-day episode which begins in the short range period.
Expanded the Marginal eastward toward the western foothills of the
Cascades in this forecast cycle for some rains working inland and
to better align with the short range outlooks. Enhanced snows will
reach the northern Intermountain West/Rockies as the system
translates eastward. But the Northwest will trend drier midweek and
beyond as upper ridging builds in. Gusty winds are also possible
across parts of the northwestern to north-central U.S. at times
next week. The most persistent highest winds are likely to be along
the northern/central Rockies ridges, while spilling into the
Dakotas at times.

Downstream, shortwave energy over the Gulf and Florida could
support moderate to heavy rainfall across south Florida on Monday.
Ample moisture and instability should be in place for some heavy
rain rates. However, convection looks to be moving quickly enough
to keep the chances of flash flooding lower than the 5 percent
threshold for a Marginal Risk. But an ERO risk area is possible in
future cycles especially if models converge in showing heavy
rainfall over the more sensitive urban corridors. Digging of
northern stream shortwave energy into Monday-Tuesday will spread
light precipitation from the Great Lakes region through New
England. Then moderate cyclogenesis and frontogenesis are likely
over the northern Plains/Mississippi Valley Wednesday and work into
the East by Thursday and Friday. This could lead to another round
of rain and northern tier snow making its way from the north-
central to east-central U.S. midweek (extending farther south into
the Ohio and Tennessee Valleys than the early week round) and
across the East by Thursday. The Interior Northeast has the best
probabilities for plowable snow on Thursday at this point.

By early next week, most locations west of the Mississippi River
will warm to much above normal with decent coverage of plus 10-25
degree anomalies. Best potential for highest anomalies will be near
the northern Plains. The East should also warm to above normal
early to mid week, before cooling off again back to normal (or
slightly below) late week underneath of troughing.


Tate/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




$$