Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EST Fri Nov 12 2021

Valid 00Z Sat Nov 13 2021 - 00Z Mon Nov 15 2021

...Heavy rain and strong to severe thunderstorms across the Northeast to
move offshore overnight...

...Flash flooding potential to persist over the Pacific Northwest through
the weekend as onslaught of heavy rainfall continues...

...An Alberta clipper will deliver another around of snow from the
Northern Plains to the Great Lakes Saturday into Sunday...

The occluded low pressure system that brought snow and high winds to the
Northern Plains and Upper Midwest will progressively weaken today as it
slowly moves across the Great Lakes. In spite of this weakening gusty
winds will continue to blow on the southwest side of the system through
the afternoon while any lingering light snowfall transitions into mixed
precipitation. Meanwhile, the easternmost cold front associated with this
system will pass through the Eastern Seaboard, bringing showers and
thunderstorms to portions of the Northeast and south Florida. Isolated
instances of severe weather in the form of damaging wind gusts and a
tornado or two will be possible over southern New England this afternoon,
prompting the issuance of Marginal Risk of Severe Thunderstorms by the
Storm Prediction Center through Saturday morning. In addition to the
severe weather these storms may produce heavy rainfall over the Northeast,
where high rain rates could lead to isolated instances of runoff and flash
flooding. As such, the Weather Prediction Center has issued a Marginal
Risk of Excessive Rainfall for portions of the Northeast effective through
Saturday morning.

Strong cyclonic flow around the deep Great Lakes low pressure system will
serve to transport colder Canadian air southward into the central US,
allowing for the development of a secondary cold front extending from the
Ohio Valley to the Southern Plains. This influx of cooler air will force
temperatures to drop well below normal over the eastern half of the
country throughout the weekend. The greatest hit to daily high
temperatures will occur on Saturday in the Ohio and Tennessee Valleys
where highs in the 40s and low 50s are expected, which translates to a
departure from normal of around 15 to 20 degrees. Overnight lows are also
expected to take a steep drop to near or below freezing in the Southern
Plains and Southeast this weekend, prompting the issuance of Freeze
Warnings and Freeze Watches in these regions, respectively.

Shifting focus out west, onshore moisture flow from yesterday's
atmospheric river will continue to wind down this afternoon, however, the
potential for high rainfall rates of 0.5-0.75 inches per hour and flooding
associated with these heavy downpours will persist, especially across
steep terrain and burn scarred areas. As such, the Weather Prediction
Center has issued a Marginal Risk of Excessive Rainfall for portions of
the Pacific Northwest. A brief reprieve from the rain will occur early
Saturday before a second, stronger atmospheric river reaches the coastline
Saturday night, this time delivering 3-5 inches of rain in 24 hours to the
region. Given the already saturated soils from the previous days' rain and
the concern for more widespread rapid run-off and urban flooding, the
Weather Prediction Center has upgraded to a Slight Risk of Excessive
Rainfall for portions of extreme northwest Washington beginning Sunday
morning.

Further inland, a new low pressure center will organize over Alberta,
Canada tonight as the energy from the dissipating atmospheric river
progresses east. Reinforced by a shortwave trough aloft, this system will
deepen overnight as it moves southeast into the Northern Plains. Trailing
to the south of the low an associated warm front is expected to serve a
dual purpose: allow temperatures throughout the west and Great Plains to
rise 10 to 15 degrees above normal and provide instability for another
round of snow across the Northern Tier. Widespread snowfall totals of 2-4
inches are expected from eastern North Dakota to Michigan, while a
precipitation transition zone further west will allow for the development
of freezing rain and light ice accumulations from northeast Montana to
western Minnesota.

Elsewhere in the Central/Southern Plains, dry and breezy conditions
coinciding with low values of relative humidity has prompted the issuance
of an Elevated Risk of fire weather by the Storm Prediction Center through
Saturday morning, as these conditions could allow any fires that ignite to
spread rapidly.

Zavadoff


Graphics are available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php