…Unsettled weather to bring lower elevation rains and heavy mountain
snowfall across the Pacific Northwest and the northern Rockies going
through the weekend…
…Strong atmospheric river to bring heavy rains and flooding concerns to
areas of central and northern California…
…New storm system ejecting out across the northern Plains will bring
accumulating snowfall to portions of the Upper Midwest, Great Lakes and
Northeast…
A switch to a very unsettled weather pattern is underway across the
Northwest and much of the West Coast as a more zonal flow pattern unfolds.
This will bring moist onshore Pacific flow with lower elevation rains and
increasingly heavy mountain snowfall for the Cascades and also the
northern Rockies. Going through Sunday, as much as 1 to 3 feet of new
snowfall is expected over the high terrain, and snow levels will actually
be falling with time as colder air also settles south from southwest
Canada. This allow temperatures to drop below normal with high
temperatures as much as 5 to 10+ degrees below average by the end of the
weekend.
As unsettled weather impacts the Northwest, strong deep layer Pacific flow
along with a quasi-stationary front will become situated over central and
northern California and this will set the stage for a persistent and
strong atmospheric river to impact the region. Heavy to excessive rainfall
is expected for the coastal ranges, including areas around the Bay Area,
and especially areas farther inland into the foothills and higher terrain
of the northern Sierra Nevada. Multi-day rainfall totals through Sunday
for the coastal ranges may reach see as much as 3 to 6 inches. However,
for the upslope areas of the Sierra Nevada, as much as 6 to 12 inches of
rain is forecast, and with high snow levels, there will be strong concerns
for flooding. This will include a threat for burn scar flash
flooding/debris flow activity locally. Heavy rains will be impacting the
Sacramento Valley and some flooding will be possible here as well. The
Weather Prediction Center has depicted a Slight Risk of excessive rainfall
across the region (level 2 of 4) to address the multi-day flooding
concerns associated with this impending atmospheric river activity.
Meanwhile, a new storm system related to the unsettled weather unfolding
across the West will begin to eject east out across the northern Plains
later Saturday through Saturday night this will bring a threat for several
inches of accumulating snow to the Upper Midwest. This system will then
begin to lift up into southeast Canada going through Sunday, but will
bring a threat for snowfall also across portions of the Great Lakes region
and the Northeast where there will be sufficient cold air in place. In the
wake of this system, much colder air will be settling southward across the
northern Plains which will allow temperatures to begin returning back to
below normal.
Elsewhere, very mild temperatures with above average highs can be expected
across much of the rest of the Plains region and the South. In some cases
across the Plains and Midwest, the temperatures this weekend will be as
much as 15 to 20 degrees above normal. Many areas of the Southeast and the
Southwest will also be warmer than normal with highs as much as average 5
to 10+ degrees above average.
Orrison
Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php
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