Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EST Wed Nov 20 2024
Valid 00Z Thu Nov 21 2024 - 00Z Sat Nov 23 2024
...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
