Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Thu Oct 21 2021

Valid 00Z Fri Oct 22 2021 - 00Z Sun Oct 24 2021

...Back to back Pacific storms to bring heavy rain and gusty winds to the
Pacific Northwest...

...Severe thunderstorms possible along a cold front moving through the
Appalachians today and Eastern Seaboard tomorrow...

...Moderate to heavy rain and severe thunderstorms expected in the Central
Plains Saturday in association with a developing low pressure system...

...Above normal temperatures will continue in the Central and Southern
Plains; more seasonal temperatures to return to the Intermountain West and
East Coast by week's end; cooler than average in the Midwest 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 is currently
spinning offshore with an intense central pressure of 951 mb, is forecast
to move inland later today, bringing heavy rain and gusty winds to
portions of northern California and southern Oregon. Combined with the
effects of upslope flow, ample moisture within this system in the form of
a moderately strong Atmospheric River will allow for high rain rates and
an additional 3 to 4 inches of rain to fall over the area. 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 and far southwest Oregon through Friday morning. Flash Flood
Watches are also in effect in portions of the northern Sierra Nevada. In
addition to the rain some isolated heavy snowfall will be possible,
primarily in mountain ranges above 8000 ft including the Cascades,
Olympics, and northern Sierra Nevada. As the storm system pushes inland
moderate rain and high elevation snow will spread to the Intermountain
West and the Northern/Central Rockies on Friday. On Saturday a second
Pacific system will move onshore, bringing another round of rainfall over
already sensitive and saturated soils. Given the fact that most of the
rain is forecast to fall over areas that are also covered in burn scars,
the Weather Prediction Center has issued a Marginal Risk of Excessive
Rainfall for portions of northern California and southwest Oregon to
account for the risk of additional rapid run off and debris flows.

Further east, a deepening low pressure system is moving through
southeastern Canada, with its trailing cold front passing through the
Appalachian Mountains and the Southern Plains. Showers and thunderstorms
will remain primarily focused around the cold front as it progresses east
and moves offshore late Friday night. 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
the Ohio/Tennessee Valleys and Central/Southern Appalachians, where a
Marginal Risk has been issued by the Storm Prediction Center 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
through the early evening. On Friday the severe weather potential will
move east with the progression of the cold front into eastern North
Carolina, where a Marginal Risk area has been introduced where
thunderstorms could produce locally strong damaging wind gusts.

On Saturday a developing low pressure system is forecast to bring showers
and thunderstorms to the Central Plains and Middle Mississippi Valley
ahead of a lifting warm front. Though rainfall rates and totals are not
expected to pose any threats at this time, some of the thunderstorms that
develop over the region may become severe and produce large hail Saturday
night. As such, the Storm Prediction Center has introduced a Marginal Risk
for Severe Thunderstorms over portions of Nebraska, Kansas, Iowa, and
Missouri effective Saturday morning.

Temperatures will generally remain near or above normal in the
south-central part of the country during the short range period. As cold
air is ushered in behind the Canadian low modest temperature drops will be
felt from the Upper Midwest to the Northeast. The most extreme anomalies
are expected in the Upper Midwest and Ohio Valley on Friday, where daily
highs are forecast to drop into the low to mid 50s, around 10 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. However, in areas of
higher elevation such as the northern Sierra Nevada, temperature
departures are expected to be more extreme, sitting 20 to 25 degrees below
normal.

Zavadoff

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