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FOUS30 KWBC 212109
QPFERD
Excessive Rainfall Discussion...Correction
NWS Weather Prediction Center College Park MD
508 PM EDT Fri Sep 21 2018
Corrected to Chickasha spelling
Day 1
Valid 2023Z Fri Sep 21 2018 - 12Z Sat Sep 22 2018
...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE
SOUTHERN PLAINS...
...Significant and locally life-threatening flash flood event
underway across portions of northern TX and southern OK...
...A potentially record heavy rain event may be onfolding for the
region...
...Southern Plains...
A surface frontal boundary sinking slowly southward into the
Southern Plains interacting the with mid-level remnants of
Tropical Depression 19-E has been the focus for potentially
widespread heavy to excessive rainfall totals, which should
continue through 12z. PW values in the vicinity of this front are
2.25-2.5" per recent GPS data as south southwesterly flow of ~25
kts (5-10 kts greater than the mean 850-400 hPa wind) converges
into the boundary. A shortwave trough pushing slowly eastward
along and north of this front in combination with an upper low in
the eastern Gulf of Mexico have enhanced divergence in the
vicinity, with ongoing rainfall strengthening frontogenesis. With
the above mentioned overall favorable pattern for heavy
rains...there is a strong model signal for this widespread heavy
to excessive rains, with confidence fairly high that it will occur
in the vicinity of this front over the Southern Plains. Given the
high moisture availability, CIN formation in the region should be
minimal at best, with the system importing 1000-2000 J/kg of ML
CAPE through 12z. There are some very high maxima in the various
models for this period ranging from 10 to 20"+, with these maxima
depicted over a broad geographic range in the consensus qpf axis.
For a storm total, 15-20" is possible similar to other tropical
cyclone remnant-related events from Norma 1981 (18.71" at Kingston
4 SSE OK), Tico 1983 (16.95" at Chickasha OK), and similar heavy
rain events from September 28-October 2, 1927 (14.59" at Dutton
AR), October 19-24, 1908 (16.23" at Meaker OK), and October 11,
1973 (15.68" at Enid OK). Isolated totals of 10" have already
occurred in OK, and local amounts to 20" are possible overall
which, if realized, would threaten tropical cyclone- (or their
remnants) related rain records for OK and AR. The risk areas have
shifted south and eastward to account for minimal elevated
convection overnight and the current radar trends.
Roth/Oravec
Day 2
Valid 12Z Sat Sep 22 2018 - 12Z Sun Sep 23 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER A PORTION OF
THE SOUTHERN PLAINS...LOWER MISSISSIPPI AND LOWER OHIO VALLEY ON
SATURDAY...
Update...
One of the larger adjustments was to extend the Slight Risk
further south across North East Texas and to bring the Marginal
Risk all the way to the Texas coast. The 12z guidance offered an
increasing signal for moderate to locally heavy amounts coincident
with the enhanced southerly inflow and the core of the deep
moisture (PWs around 2.25 inches) centered over the region. While
areal average accumulations are expected to come down this period,
a Moderate Risk centered near the ArkLaTex region may need to be
introduced in future updates, especially if the forecast for heavy
amounts in the prior period verifies.
Also following guidance trends, the risk areas were shifted
slightly further to the southeast across the lower Mississippi
into the Tennessee and Ohio valleys.
Previous Discussion...
...Southern Plains to Lower MS Valley...
Split flow develops in the wake of a large low shifting east
across eastern Canada. The trailing cold front drapes across the
southern Plains and Ohio Valley Saturday. Shortwave activity
ejecting east in the southern stream invigorates low level low
development over TX per the preferred blend of the 00Z ECMWF/GEFS
ensemble mean/3km NAM/UKMET. Ample gulf moisture will spread into
the boundary/low. The actual placement of the boundary will be
determined by the activity of today. Often guidance is too far
north with such boundaries from widespread precip dominated by
mesoscale forcing. The ECMWF and NAM 12km have been too bullish on
QPF, but the placement is noted with a westward shift up the Red
River Valley. Expanded the Slight Risk to the south of the River
over north TX.
...Central Appalachians into the Mid-Atlantic...
Surface high pressure in the wake of the large eastern Canada low
will shift east across the northeastern CONUS Saturday. A wide
latitudinal range in this east-west boundary is noted in 00Z
guidance with the UKMET near the Mason-Dixon line and the 3km NAM
over northern NC with the ECMWF in between. Given typical surface
ridge wedging situations with the Mid-Atlantic, will prefer the
farther south guidance. However, low FFG from Florence related
rains warrants a Marginal Risk across the central Appalachians to
the VA/northern NC Piedmont.
Pereira/Jackson
Day 3
Valid 12Z Sun Sep 23 2018 - 12Z Mon Sep 24 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE LOWER
MISSISSIPPI VALLEY TO THE CENTRAL APPALACHIANS...
Update...
Overall, forecast remains largely the same, with the update
reflecting mainly minor adjustments based on the 12z guidance.
Previous Discussion...
...Lower MS Valley to Central Appalachians...
Southern stream dynamics weaken under an amplifying ridge over the north-central CONUS Sunday per the 00Z ECMWF. This weakens the low
over TX and should lift the surface boundary east of there north.
The 00Z preference for Day 3 of the ECMWF/GEFS Mean/UKMET focuses
precip from Memphis to KY where an axis of 2 inch PW is expected.
The Slight Risk covers the threat area for heaviest rain over the
northern MS delta (where FFG is high) to lesser rains into the
central Appalachians which has low FFG from Florence related
rains. The placement of the front will depend on mesoscale
processes (like on Day 2) and is subject to change. Often a shift
south is noted in cases like these, so the Slight Risk covers
north-central portions of TN which is currently south of the model
consensus axis of heaviest rain.
Pereira/Jackson
Day 1 threat area:
http://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area:
http://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area:
http://www.wpc.ncep.noaa.gov/qpf/99epoints.txt
$$
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