Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Wed Oct 20 2021

Valid 00Z Thu Oct 21 2021 - 00Z Sat Oct 23 2021

...Back to back Pacific storms to bring heavy rain and mountain snow to
Northern California and the Pacific Northwest...

...Severe thunderstorms possible in association with a mature low pressure
system bringing rain to the Midwest, Eastern Seaboard, and parts of the
Southern Plains through Friday...

...Moderately warm temperatures expected in the East, cooler over the
Norther Plains and along the West Coast...

A series of Pacific storms will move onshore over northern California and
the Pacific Northwest during the latter half of the work week, bringing
some much needed rain to the regions. The first, which had already pushed
itself ashore earlier today, will predominantly generate modest rainfall
throughout northern California and to the west of the Oregon and
Washington Cascades. Any snowfall will be isolated to higher elevations,
particularly above 5000 ft in the Cascades and above 8000 ft in the
northern Sierra Nevada. Precipitation will quickly diminish overnight as
the system rapidly decays, providing a brief reprieve before the approach
of the second storm on Thursday morning. Carrying within it an abundance
of moisture in the form of a moderately strong Atmospheric River, this
second system will deliver a considerably greater amount of rain to the
area, particularly in northern California. While a welcome sight in light
of the ongoing drought, periods of heavy rainfall could be cause for
concern, especially in the mountainous areas of northern California
riddled with burn scars. Given the potential for rapid runoff and debris
flows in these sensitive areas, the Weather Prediction Center has issued a
Slight Risk of Excessive Rainfall for portions of northern California
effective Thursday morning through Friday morning. In addition to the rain
some isolated heavy snowfall will be possible over portions of the
Cascades, Olympics, and northern Sierra Nevada, with Mt. Shasta possibly
receiving nearly 2 ft of snow.

Further east, a deepening low pressure system is moving through the Upper
Midwest, with its trailing cold front passing through the Middle
Mississippi Valley and the Southern Plains. Showers and thunderstorms will
remain primarily focused around the low pressure center and its associated
cold front as they progress east, lift north into Canada Thursday evening,
and move offshore late Friday. Rainfall rates and totals are expected to
be relatively moderate with this system, however, the risk for the
development of severe thunderstorms will persist through Friday morning.
The areas at greatest risk for severe weather overnight include southern
Minnesota and northern Iowa, where a Marginal Risk has been issued due to
the possible formation of large hail. On Thursday the severe weather
potential will move east with the progression of the storm system into the
Ohio/Tennessee Valleys and Central/Southern Appalachians, where a Marginal
Risk has been issued to account for the possible development of isolated
strong to damaging wind gusts. Another Marginal Risk area has also been
put in place over west Texas, where any storms that form may pose a hail
and strong/gusty wind threat.

Temperatures will generally remain near or moderately above normal
throughout the country during the short range period. Exceptions to this
trend include regions that fall immediately behind the progressing low
pressure system and the West Coast. The former will experience marginal to
modest temperature drops as cool Canadian air is ushered in behind the
deepening low. The most extreme anomalies are expected in the Northern
Plains, where daily highs are forecast to drop into the 40s, falling 20
degrees below normal. As for the West Coast, persistent cloud cover from
the series of systems moving onshore will generally keep the region 10 to
15 degrees cooler than average for this time of year.

Zavadoff

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