904
FXUS02 KWNH 310716
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
215 AM EST Fri Jan 31 2025

Valid 12Z Mon Feb 03 2025 - 12Z Fri Feb 07 2025

...Prolonged atmospheric river event will last into next week for
California...


...Overview...

Quasi-zonal flow is forecast across much of the lower 48 next
week, with subtropical ridging near the Bahamas/Cuba promoting
above normal temperatures across a majority of the country, aside
from colder than average temperatures in the northern stream.
Meanwhile a slow moving upper low drifting near/offshore Vancouver
Island and the Pacific Northwest will sustain a prolonged
atmospheric river across northern California into Monday and
gradually drifting south. Snow is likely farther inland with higher
amounts across higher elevations, and additional rounds of
precipitation are possible into much of next week. Farther east,
moisture return and a frontal system will lead to increasing
precipitation chances across the eastern third of the U.S. or so
into Wednesday-Thursday.


...Guidance/Predictability Assessment...

Model agreement is fairly good on the overall pattern, especially
early in the week, but the details differ especially with the
eastern Pacific upper low. A multi-model blend favoring the 12/18Z
GFS and 12Z ECMWF seemed to provide a good intermediate solution
through Monday-Tuesday. By Wednesday, the 12Z UKMET became farther
south with the southern extent of the upper low and affected its
precipitation distribution. There is more model divergence for the
latter half of the week with the upper low and the possibility of
another shortwave perhaps in the Northwest. The 12Z CMC was an
outlier in drawing the main upper low well offshore into the
Pacific by Thursday, but the new Model agreement is fairly good on
the overall pattern, especially early in the week, but the details
differ especially with the eastern Pacific upper low. A multi-model
blend favoring the 12/18Z GFS and 12Z ECMWF seemed to provide a
good intermediate solution through Monday-Tuesday. By Wednesday,
the 12Z UKMET became farther south with the southern extent of the
upper low and affected its precipitation distribution. There is
more model divergence for the latter half of the week with the
upper low and the possibility of another shortwave perhaps in the
Northwest. The 12Z CMC was an outlier in drawing the main upper low
well offshore into the Pacific by Thursday, but the new 00Z CMC
looks to be in better alignment. Meanwhile the 12Z ECMWF and 18Z
GFS pivoted the energy eastward, while the EC in particular dug
troughing across the interior West later week. The 12Z GFS and
several of the AI/ML models seemed to be the closest to the more
agreeable ensemble means with the timing of the trough. Thus the
WPC forecast quickly transitioned to a mean-heavy blend by the late
period due to the individual model differences. This also worked
for the timing/placement of the low pressure and frontal system
farther east into later week. Overall the incoming 00Z model suite
seems to be in fair alignment with the forecast blend.00Z CMC looks
to be in better alignment. Meanwhile the 12Z ECMWF and 18Z GFS
pivoted the energy eastward, while the EC in particular dug
troughing across the interior West later week. The 12Z GFS and
several of the AI/ML models seemed to be the closest to the more
agreeable ensemble means with the timing of the trough. Thus the
WPC forecast quickly transitioned to a mean-heavy blend by the late
period due to the individual model differences. This also worked
for the timing/placement of the low pressure and frontal system
farther east into later week. Overall the incoming 00Z model suite
seems to be in fair alignment with the forecast blend.


...Weather/Hazards Highlights...

A strong and long-lasting atmospheric river taking aim at northern
California will be ongoing as the period begins Monday. Multi-inch
rainfall totals may again be an issue on Monday after a few very
wet days in a row, and a Slight Risk remains delineated in the Day
4 Excessive Rainfall Outlook from the coast into the Sierra Nevada
foothills. Depending on how much rain falls in previous days in
similar locations, this may be considered a higher-end Slight Risk
and future upgrades are not out of the question. By Tuesday, the
atmospheric river finally starts to progress southward and shift
the heaviest rain southward across California, as well as becoming
a little broader/less focused. The Day 5/Tuesday ERO has a Marginal
Risk across central California as a starting point given the
faster movement by that time, but pending overlap with heavy rain
in previous days perhaps in the Sierra Foothills. At this point,
through Tuesday night, the heavy rain does not look to get as far
south as the Los Angeles metro area atop burn scars, but this will
continue to be monitored due to their sensitivity. Some moderate
rain could make it into southern California Wednesday as the
moisture plume continues to broaden while additional rounds come in
Wednesday-Thursday.

Snow will also be likely across much of the Intermountain West
with this system. Early next week, heaviest snow amounts are
forecast across the Sierra Nevada (as snow levels come down after
the weekend) into the northern Rockies, while the northern Great
Basin also receives snow. Some snow should extend into the Wasatch
and central Rockies for midweek and beyond while some moderate
snows last in the Sierra. The pattern will also promote strong
winds especially on mountain ridges like the Sierra and central
Rockies, but some gusty conditions may reach lower elevations of
the Great Basin and Four Corners too.

Meanwhile farther east, some lingering rain/snow is possible
across the Interior Northeast on Monday as a frontal system exits.
Then moisture return is forecast for Tuesday and especially
Wednesday-Thursday in the east-central U.S. while a frontal system
with an embedded wave of low pressure or two lifts, producing an
expanding area of precipitation. Best potential for some wintry
weather will be over the Northeast, while rainfall of varying
intensity will be possible farther south.

Colder than average temperatures are forecast for northwestern and
north-central parts of the lower 48 next week. Montana to the
Dakotas will be particularly cold, with anomalies of 20-35 degrees
below normal and highs barely reaching 0F in some locations early
in the week. These areas will likely remain below average but with
some moderation later next week. Meanwhile, other areas from the
Four Corners states through the East should see above to well above
normal temperatures during the period. Some areas will likely see
one or more days of min/max temperatures reaching 20 or more
degrees above normal and some daily records will be possible next
week. A cold front coming through the central U.S. will cause
temperatures to moderate there by late week.


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

$$