Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Wed Dec 07 2022
Valid 16Z Wed Dec 07 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 AND SOUTHERN
...16Z Excessive Rainfall Outlook Discussion...
Preliminary look at the 12Z guidance and the ensemble guidance
that arrived after the initial issuance of Day 1 outlook suggests
that the on-going outlook was in good shape and only minor
adjustments were needed. The 09Z SREF showed a couple of members
with 2+ inch amounts in the deformation zone straddling the
Oklahoma/Kansas border...an idea supported more so by the GEFS.
However, higher resolution guidance like the HRRR was focusing the
axis of heaviest QPF farther south. As a result, opted to expand
the northern boundary of the MRGL just a bit northward. Farther
east...expanded the Marginal Risk area a bit more northward given
on-going rainfall over eastern TN and the signal from the
operational guidance for upwards of an addition inch where terrain
and lower FFG may be met. Given the modest deep-layer
instability...still think any problems should be isolated enough
to maintain the on-going Marginal Risk area.
...0830Z Excessive Rainfall Outlook Discussion...
...North Texas and Eastern Oklahoma eastward into the western
Models still suggest the potential for locally heavy to excessive
rainfall from parts of North TX into central-eastern Oklahoma and
across portions of the Lower MS Valley and TN Valley, though the
signal for anything other than an isolated (marginal) threat has
diminished considerably over the past couple of model cycles.
Aloft, a southern stream trough pivots across the Southwest U.S.
on Wednesday and across the High Plains Wednesday night. This
initial shortwave will be moving rather swiftly, given the
increased confluence (de-amplification of the shortwave with time)
by late Wednesday night. Given the flattening wave and more
west-east oriented front, the the degree and depth of
south-southwest low-mid layer flow will be somewhat restricted
(warm conveyor belt or WCB more compact). Moreover, limited
deep-layer instability (MUCAPEs 500-1000 J/Kg early dropping below
500 J/Kg later this afternoon and overnight) will also limit the
short-term rainfall rates. So despite a relatively quick bout of
robust low-mid layer moisture transport, with PW values topping
out between 1.5-1.7", the lack of instability along with
progressive nature of the favorable forcing (forward-propagating
Corfidi or MBE vectors) will lead to a more limited (isolated)
flash flood threat. In addition, per the latest NASA SPoRT
analysis, 0-100cm soil moisture percentiles are running between
10-30% of normal across much of the outlook area. As a result, the
Slight Risk area that was present in yesterday's Day 2 ERO has
been dropped to a Marginal.
Valid 12Z Thu Dec 08 2022 - 12Z Fri Dec 09 2022
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
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.
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