879
FXUS02 KWBC 061923
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
223 PM EST Thu Mar 6 2025

Valid 12Z Sun Mar 09 2025 - 12Z Thu Mar 13 2025


...Overview...

Overall, the medium range forecast period is looking fairly
tranquil to start before turning more active late, especially
across the Western U.S. There are caveats, however. Sunday into
Monday, a deep, closed, and positively tilted upper level low will
track from the ArkLaTex region south and east along the northern
Gulf Coast. This feature will force the development of a surface
low that will bring widespread showers and thunderstorms to
portions of the Southeast, some of which could contain heavy
rainfall. In the Northeast, a fast-moving clipper will dive south
and eastward across the northern New England states Sunday night
into Monday, bringing a brief period of light to moderate snow and
mixed rain/snow showers. Meanwhile, farther west, a large, high
amplitude ridge will shift eastward into the central and southern
Plains, bringing mainly dry and much above normal temperatures
along with it. This springlike weather eventually slides into the
Eastern U.S. Tuesday into Wednesday. Finally, a large and
potentially deep Pacific storm, accompanied by an atmospheric
river, is looking increasingly likely to bring widespread rain and
snow to much of the West for the middle of next week.

...Guidance/Predictability Assessment...

The latest deterministic and ensemble model guidance remains in
decent overall agreement for the evolution of the large scale
pattern through the medium range period. However, subtle
differences emerge when looking closer at smaller scale features. A
majority of guidance has come into much better agreement with the
strength and speed of the upper level low tracking from the
southern Plains and along the northern Gulf Coast Sunday into
Monday. Most models keep the upper low closed, compact, and slower,
which translates to a stronger surface low and heavier, more
widespread precipitation. The set of guidance that had not quite
caught on were the 00Z and 06Z GFS, which opened the closed low
into an open wave and sheared it apart as it quickly exited the
Southeast U.S. coast. Machine Learning (ML) guidance, particularly
the AIFS, was also in the stronger and slower camp. This anti-GFS
consensus gave high enough confidence to completely exclude the GFS
from the favored blend.

Out West, models were in generally good agreement with a leading
closed low making landfall across southern California, before
eventually opening it up into an open wave. That lead wave is then
followed by a much stronger and deeper upper trough, with varying
differences in speed and strength. With that being said, agreement
is quite good for a Day 5-7 forecast, with the big picture showing
an active pattern with the growing potential for widespread heavy
rain along the California coast and heavy snow in the high
elevations of the West and Intermountain West.

The initial model blend consisted of the 00Z ECMWF, 00Z CMC and
00Z UKMET. By mid-period, the UKMET was dropped and replaced with
increased weighting of the GEFS and EC ensemble means through the
end of the extended period.

...Weather/Hazards Highlights...

Aside from the GFS, guidance has come into much better agreement
with the evolution of the upper low and surface low in the
Southeast Sunday into Monday. Guidance continues to highlight the
potential for heavy showers and thunderstorms on Day 4, with the EC
mean showing the potential for 2+ inches of rainfall across
portions of southern Georgia and northern Florida. Given better
agreement, decided to introduce a Marginal Risk for excessive
rainfall for that area.

The West Coast will be ramping up for a multi-day wet period. Over
the weekend an eastern Pacific trough will direct an atmospheric
river into Vancouver Island with some precipitation extending into
northwestern Washington state. An upper low closing off near
California by early Monday will move across southern California,
becoming an open wave as it does so. This wave will bring light to
moderate rain and snow to the low and high elevations respectively.
In the wake of that system, a stronger and deeper Pacific storm
with a potentially moderate atmospheric river will likely bring
heavier and more widespread precipitation and locally strong winds
to much of the West and Intermountain West later in the period.

Above average temperatures will expand eastward through the
northern Plains, the Midwest, and eventually into the Eastern U.S.
through the Days 4-7 period as a large ridge builds overhead and
slides east. These regions will likely see several days with highs
pushing 15-30F above early March averages. This warmth may push
highs well into the 60s from the Dakotas into Iowa and into the 70s
as far north as the Middle Mississippi Valley and parts of the
Mid-Atlantic. In response to increasing clouds and precipitation
across the West, below normal temperatures become more common,
particularly Wednesday and Thursday.

Miller

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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