Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 AM EDT Thu Sep 20 2018
Day 1
Valid 12Z Thu Sep 20 2018 - 12Z Fri Sep 21 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE UPPER
GREAT LAKES INTO THE UPPER MISSISSIPPI VALLEY AND A SLIGHT RISK OF
EXCESSIVE RAINFALL OVER THE SOUTHERN HIGH PLAINS...
...Upper Great Lakes into the Upper Mississippi Valley...
Surface low pressure expected to deepen Thursday across the
Central Plains into the Upper Mississippi Valley in response to
strong upstream height falls pushing out of the Northern Rockies
and into the Northern Plains. The west to east oriented frontal
boundary currently stretching from portions of the Central Plains
into the Mid Mississippi Valley will be lifting northward ahead of
the above mentioned deepening low. This will re-focus the next
round of organized overrunning convection Thursday afternoon into
Thursday night farther to the north across the Upper Mississippi
Valley into the Upper Great Lakes from Southern to northeast MN
into northern WI and the U.P. of MI. With strong south southwest
low level flow of anomalous pw values...2.5 to 3 standard
deviations above the mean...into this west to east front...along
with favorable right entrance region jet dynamics across this
region...widespread heavy precip values likely in this overrunning
zone. Model consensus is for areal average 1.25-2+" across these
areas...with locally heavier totals in the 3-5"+ range where
convection maximizes. In addition to overrunning convection to
the north of the warm front...convection should also enhance
Thursday afternoon along and ahead of the trailing cold front from
NW KS...eastern NE into NW IA and southern MN. This activity
likely will be progressive but the above mentioned favorable right
entrance region jet dynamics and above average pw values will also
extend south along and ahead of the trailing cold
front...supporting potential for a well defined squall line across
these areas. Changes to the previous outlook for this period were
to extend the slight risk area farther to the south into southeast
MN...NW IA and far eastern NE to account for the convection along
and ahead of the trailing cold front. No significant changes made
to the slight risk area over the Upper MS Valley into the Upper
Great Lakes.
..Southern High Plains...
Much above average pw values...3+ standard deviations above the
mean...will be spreading northeastward from Northwest Mexico in
the vicinity of weakening T.D. 19-E and into the Southern High
Plains day 1. Models indicate strong upper difluence likely to
spread northeastward across this anomalous pw axis from NM
into...far SW TX...NW to west TX...western OK and western to
central KS day 1--supporting potential for widespread moderate to
heavy precip totals across these regions. WPC qpf leaned toward
the href mean and latest nssl wrf for qpf details this period.
While the heaviest totals are depicted over NW TX into NW
OK...there is potential for heavy to locally excessive amounts
across large portions of the Southern High Plains this period. A
slight risk was added for this forecast period from the previous
outlook to cover this potential across southeast NM...far
southwest TX into NW TX/NW OK.
Oravec
Day 2
Valid 12Z Fri Sep 21 2018 - 12Z Sat Sep 22 2018
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF
THE SOUTHERN PLAINS ON FRIDAY/FRIDAY NIGHT...
...Southern Plains...
Upgraded to a Moderate Risk of Excessive Rainfall over portions of
Oklahoma on Friday. There is a Slight Risk area which extends
over parts of the Texas Panhandle and north Texas near the Red
River.
There were increasing signals in the models for excessive amounts
of rainfall over Oklahoma as a cold front makes its way eastward
into a region of anomalously deep moisture and moisture transport.
In addition, shortwave energy approaching from the Southwest U.S.
will also be a factor in helping initiate and sustain convection
that moves across parts of West/North Texas into Oklahoma. A deep
fetch of moisture from the eastern North Pacific and a second
fetch from the western Gulf of Mexico is another signal pointing
towards heavy to potentially excessive rainfall in the area.
The 00Z suite of numerical guidance showed an uptick in their
respective QPF over the Southern Plains but some spread lingered
in terms of QPF placement. Some typical model biases showed up
with the ECMWF being slower than the GFS, but we tended to put
more weight on the ECMWF solution considering the amount of
support it had from the ensembles and high resolution models. The
model spread isn't too great at the beginning of the period so
confidence is highest Friday morning into Friday evening. If the
GFS is correct, the convection may be moving out of
central/eastern Oklahoma more quickly than forecast. This may
necessitate adjustments in the eastern boundary of the outlook
area by future shifts.
Bann
Day 3
Valid 12Z Sat Sep 22 2018 - 12Z Sun Sep 23 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHERN PLAINS ON SATURDAY...
...Southern Plains...
A Slight Risk area was placed over the southern Plains as a system
continues to track eastward over the southern Plains from Day 2.
Conditions favoring heavy to excessive rainfall will become less
favorable during the day as low level flow off the Gulf of Mexico
becomes more southwesterly...which results in weaker moisture flux
convergence along the front. In addition, the upper level support
becomes less favorable. For these reasons, felt a Slight Risk was
still warranted (mainly early in the Day 3 period) in areas
immediately downstream from the Moderate Risk area on Day 2.
Confidence here is pretty low considering the model spread
involved and may be adjusted assuming that model agreement becomes
better.
Bann
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