Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
717 PM EST Wed Dec 07 2022

Day 1
Valid 01Z Thu Dec 08 2022 - 12Z Thu Dec 08 2022

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE SOUTHERN PLAINS EAST TO THE TENNESSEE VALLEY...

Frontal boundary draped east/west across portions of the S. Plains
toward the S. Appalachians will continue to be the focus for
showers and thunderstorms. This activity is along an axis of
higher moisture with the latest blended TPW product showing PWs of
1.3" to near 1.5", and should continue for a few more hours before
dissipating as energy focuses back to the west. Until then, hourly
totals between 0.5-1" are likely and this may cause localized
flooding due mainly to the recent heavier rainfall that has now
pushed soil saturation in the top 40 cm layer above 80 percent
(NASA SPoRT) and resulted in lower FFGs.

Further west across the S. Plains, shortwave energy lifting out of
western Texas will create favorable forcing for ascent to produce
a round of showers and elevated thunderstorms through early
Thursday morning. Slightly anomalous moisture characterized by PWs
approaching 1.5" will work with fairly limited instability (MUCAPE
under 500 J/kg) for isolated heavy periods of rainfall. Most
recent hi-res guidance and 18Z HREF show isolated signals for 1"+
hourly totals between 06Z-12Z across central Oklahoma, northwest
Arkansas, and southwest Missouri and some repeating rounds will be
possible given the favorable parallel mean flow to storm motions.
However, the overall dryness in the soils and progressive movement
of the showers/storms should limit any flash flood potential to
just isolated instances.

Taylor


Day 2
Valid 12Z Thu Dec 08 2022 - 12Z Fri Dec 09 2022

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Fri Dec 09 2022 - 12Z Sat Dec 10 2022

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE COAST
AND COASTAL RANGES IN NORTHWEST CALIFORNIA...

2030Z Excessive Rainfall Discussion...
The only change made to the on-going outlook was to remove the
Oregon portion of the Marginal Risk area and keep it in northern
California where the models show better on-shore flow into the
terrain as shortwave energy embedded aids in the re-amplification
of a longwave trough along the west coast.  The strength of the
onshore flow allows for anomalous low- to mid-level moisture flux
with modest elevated CAPE...so previous thinking that there could
be isolated rainfall rates that results in flooding (with
burn-scars being the most vulnerable) remains on track.

Bann


0830Z Excessive Rainfall Discussion...

Another vigorous upper level jet streak and associated shortwave
energy dropping off the Pacific NW coast will allow the longwave
trough to re-amplify toward the end of day 3 (Friday night), with
robust exit region upper level forcing (DPVA/upper level difluence
and strengthening low-mid level moisture transport) working into
the outlook area. While not a classic AR setup given the broad
central Pacific ridge and mid/upper level trajectories advecting
down from the northern Pacific instead of the subtropical regions
(limiting the TPW potential), the strength of the onshore flow
(sw/ssw 45-55 kts at 850mb along the coast after 00Z Sat) will
average between 4 and 5 standard deviations above normal per the
GEFS and SREF. The anomalous degree of the low-mid layer moisture
flux, with PWs between 0.75-1.00" and marginal elevated CAPEs of
~200-400 J/Kg, will likely result in isolated areas of 0.50+
inch/hr and/or 1.5+ inch/3hr rainfall rates west of the more
elevated terrain where the precip will fall as snow. These rates
may lead to isolated areas of rapid inundation, especially over
burn scars. As a result, a Marginal Risk was included in the Day 3
ERO across these areas in southwest OR and northwest CA.



Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt