Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EST Wed Dec 20 2023

Valid 00Z Thu Dec 21 2023 - 00Z Sat Dec 23 2023

...Heavy to excessive rainfall is expected tonight and Thursday across
southern California where there will be significant concerns for flash
flooding...

...Very mild temperatures for this time of the year will continue through
the remainder of the week for large areas of the country...

...Unsettled weather to arrive across the Pacific Northwest by Friday...

A slow-moving upper low and associated frontal system currently offshore
of California will be steadily advancing southeastward through Thursday
and will then advance inland across the Southwest on Friday. Strong
forcing associated with this coupled with a notable increase in Pacific
moisture out ahead of it will lead to bands of heavy showers and
thunderstorms through this evening and into the overnight hours across
portions of central and especially southern California. The heaviest rains
will be situated across the coastal ranges and especially the Transverse
Range tonight where the better moisture transport and heavier rainfall
rates are expected to set up. The heavy rainfall threat will persist
through Thursday across southern California, and over the next 24 to 36
hours, the rainfall totals across the Transverse Range may approach or
even locally exceed 4 to 8 inches.

These rains are likely to result in significant concerns for flash
flooding with a notable threat to the area burn scar locations where
debris flows and mudslides will be possible. Urban flash flooding will
also be possible in the lower elevations closer into the Los Angeles
basin. Some rainfall rates with this event may approach or locally exceed
1 inch per hour with the heavier showers and thunderstorms, and the
persistence of these rates will lead to much of the broader threat of
potentially significant flash flooding. The Weather Prediction Center has
highlighted a Moderate Risk of excessive rainfall to address these
concerns going through Thursday.

By Friday, this storm system will be accelerating off to the east and will
begin to overspread the Southwest where areas of locally heavy rain and at
least some isolated concerns for flash flooding will continue. As the
heavy rainfall threat begins to exit southern California, areas of central
and southern Arizona will begin to see these impacts. However, the
rainfall totals should be much more beneficial and modest by comparison.

In general, the threat of winter weather over the next couple of days will
be rather limited due to the lack of cold air across much of the country,
but the storm system coming into California and the broader Southwest will
foster some locally heavy accumulating snowfall tonight and Thursday for
the high terrain of the southern Sierra Nevada. Gradually the threat of
accumulating snowfall will increase also into the San Juan mountains of
southwest Colorado, with a much heavier threat of snow here expected by
this weekend.

Given the presence of a lot of Pacific air across the country, and general
lack of southward advancing Arctic air from Canada, the temperatures
across large areas of the country will be well above normal for this time
of the year. This will especially be the case across areas of the central
and northern Plains, and the Midwest, where high temperatures will be as
much as 15 to 25 degree above normal for the remainder of the week.

Relatively cooler and more unsettled weather should begin to arrive across
the Pacific Northwest by Friday as a new cold front arrives and advances
inland. Locally heavy showers with the front can be expected given the
influx of Pacific moisture, and this will translate into areas of heavy
snowfall for the higher elevations including the Cascades and eventually
the northern Rockies.

Orrison


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