…Back-to-back powerful Pacific storm systems to impact the West Coast
through the end of this week with heavy rain, life-threatening flooding,
strong winds, and higher elevation mountain snow…
…Heavy snow is likely throughout parts of the central Appalachians
beginning on Thursday, with a separate burst of snowfall possible across
northeast Pennsylvania and neighboring regions of the Northeast Thursday
night into Friday…
An active pattern remains in place this week as multiple systems bring
heavy rain, life-threatening flooding, and higher elevation mountain snow
to the West and the season’s first heavy snowfall to the Appalachians over
the next couple of days. In the West, back-to-back systems from the
Pacific and an accompanying strong Atmospheric River will bring waves of
very heavy rain and high elevation snow to the Pacific Northwest/northern
California, with the first wave continuing through Wednesday evening into
early Thursday and a second wave peaking Thursday night. The most intense,
heavy rainfall will focus on northern California/southwestern Oregon, with
a Moderate Risk of Excessive Rainfall (level 3/4) in place along the
northern California coast through Thursday morning where the heaviest
amounts and greatest chance for flooding is expected. A broader Slight
Risk (level 2/4) extends inland along the northern Sierra ranges. The
second wave Thursday evening is expected to bring even more intense
rainfall on top of already saturated soils, with a High Risk (level 4/4)
in place along the northern California coast and a Moderate Risk inland
along the northern Sierra ranges. Storm total rainfall may reach as high
as 12-16″, with dangerous flash flooding, rock slides, and debris flows
likely. In addition, heavy wet snow is expected for the Cascades and far
northern California. Snowfall rates of 2-3″/hr and wind gusts up to 65 mph
will result in whiteout/blizzard conditions and near impossible travel at
pass level. Snowfall will increasingly focus at higher elevation in
northern California as snow levels rise ahead of the continued mild, very
moist inflow of Pacific air. Moisture will also spread further inland
bringing lower elevation rain and high elevation mountain snow to the
northern Great Basin and northern Rockies through Friday. Some locally
heavy snowfall totals are most likely in the mountains across central
Idaho and along the U.S/Canadian border.
To the east, another low pressure system will track eastward from the
northern Plains into the Great Lakes region this evening, sweeping a
trailing occluded/cold front and a line of showers and thunderstorms
eastward through the Appalachians, Mid-Atlantic, and settling into the
Northeast by Thursday morning. Meanwhile, a broad area of precipitation
continues behind the system under the influence of a deep upper-level
cyclone, expanding eastward in tandem with the low pressure system. Heavy
snow and blizzard conditions over the northern Plains will wind down
through this evening as a wintry mix picks over the Great Lakes overnight
Wednesday and into the day Thursday. Some light to moderate accumulations
are expected, most likely in vicinity of Lake Michigan. Then, rainfall
over New England southwest through the interior Northeast and into the
Appalachians will begin to change over to snow for higher elevations as
the upper-low moves eastward over the region. Winter weather-related
Warnings/Advisories have been issued through the northern and central
Appalachians as heavy wet snowfall accumulations are expected through
Thursday night and into the day Friday. Winds across the region will also
be rather blustery.
Elsewhere, conditions will be mostly dry with the exception of some
possible showers/thunderstorms for south Florida Wednesday evening. Cooler
more seasonable Fall temperatures will return to much of the eastern U.S.
following the cold front passage, as highs drop from the 60s and 70s for
many locations Wednesday to the 40s and 50s Thursday. In contrast, the
West will see rising temperatures as a ridge builds northward over the
region, with highs running generally 5-15 degrees above average through
the end of the work week.
Putnam
Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php
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