Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Thu Sep 19 2019
Valid 01Z Thu Sep 19 2019 - 12Z Thu Sep 19 2019
...A HIGH RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
UPPER TEXAS COAST...
...East TX/Southwest LA...
The threat of heavy rainfall will continue overnight across
southeast Texas and parts of southwestern Louisiana. Saw little
need to make significant changes to the Excessive Rainfall Outlook
following the coordination call held earlier today...especially in
light of the fact that the numerical guidance is depicting similar
rainfall amounts and placement with their earlier runs. Still a
bit concerned with the strongest instability offshore; however,
some of those outer inflow bands are drawing 2000-3000 j/kg of
mixed CAPE, which earlier had led to hourly rainfall rates of
2.5-3.0+ inches along portions of the Upper Texas Coast. Mesoscale
guidance still has a strong signal for an additional 5 to 9 inches
(or more) of rainfall later tonight into early Thursday
morning...with the area immediately along the coast being favored.
Trending towards the more northerly GFS/ECMWF solutions for the
heavy rainfall with a nod to the guidance which had a history of
taking the system eastward given the eastward shift it made
earlier this afternoon.
...Upper Mississippi River Valley...
One round of convection from earlier in the day has been
weakening. A few of the models have been depicting redevelopment
later tonight that continues beyond 12Z Thursday...so opted to
maintain a Marginal Risk area there. Think that the overall risk
will be low given steady cell motion, although the flow pattern
suggests that some short-term training may occur in the pre-dawn
hours on Thursday. Instability remains on either side of a
boundary moving in from the northwest and precipitable water
values were on the order of 1.6 inches upstream from the area. As
a result, will keep a small Marginal Risk area overnight.
Valid 12Z Fri Sep 20 2019 - 12Z Sat Sep 21 2019
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL NEAR THE ARKLATEX
AND THE NORTHERN PLAINS...
...Northeast Texas into Southwest Arkansas...
The remnants of Tropical Depression Imelda will be very slowly
lifting to the N/NE on Friday through eastern Texas and towards
the Arklatex as it begins to become absorbed into the westerlies
ahead of an anomalously deep trough. Although the system will be
gradually dissipating and shearing to the northeast, it will
maintain a center with copious tropical moisture noted by forecast
PWATs approaching 2.25" surging northwards from the Gulf of
Mexico. PWATs of this magnitude for mid-September are well above
the 90th percentile, and will approach daily records at LZK and
SHV. Although Imelda is forecast to begin to dissipate on day 2,
the warm moist air being drawn northward ahead of this trough will
drive warm cloud depths above 14kft, and fuel showers and
thunderstorms with torrential rain rates. The highest threat for
these intense rain rates will be east of the center where moisture
confluence will be focused, but also to the south due to greater
instability from lesser cloud cover. Although forcing and
instability are expected to be weaker than earlier in the week,
there is good model consensus for several inches of rainfall, some
of which may fall on top of areas that receive heavy rain on
Thursday as well. Recent rainfall across much of this area has
been minimal, and 14-day rainfall departures are near 0% which
will somewhat limit the flash flood potential. However, soils will
likely be pre-conditioned across at least parts of the risk area
from the rainfall on Thursday, so an additional 1-3" of rainfall,
with locally higher amounts could exacerbate runoff to lead to
flash flooding. With rain rates of 1-2"/hr possible, this rain
could fall over a very short period of time, and the SLGT risk has
been focused where the combined D1/D2 rainfall totals are highest,
and where ECENS probabilities for 1" are greatest.
Anomalously deep upper trough characterized by 500mb height
anomalies of -1 standard deviation will translate from the
Northwest Friday and Friday night driving a surface cold front
eastward. This trough will be accompanied by an increasingly
coupled jet structure as one jet streak lifts into Central Canada
to be followed quickly by a secondary jet max rotating through the
base of the trough. This will drive increasing diffluence across
the Northern Plains Friday night, which combined with height falls
and potent PVA will produce large scale ascent, focused the latter
half of day 2.
Ahead of this trough, deep layer 925-500mb flow will become
southerly, driving tremendous moist advection northward noted by
large 850mb moisture transport vectors and PWATs surging to
1.75+", more than 3 standard deviations above the climatological
mean. As the 850mb wind reaches 40-50 kts immediately ahead of the
cold front, the accompanying WAA pushes MUCape to 3000 J/kg, and
widespread showers and thunderstorms are likely from the Dakotas
into Minnesota. Although storm motions are forecast to be rapid to
the north as evidenced by 0-6km mean wind vectors of 25-35kts,
deep layer unidirectional shear parallel to the length of the cold
front suggests training of echoes with heavy rainfall is likely.
ECENS and GEFS 24-hr rainfall probabilities show a high
likelihood, greater than 80%, for 1 inch of rainfall, with up to
30% probabilities now existing for 3". Although the global
consensus for the heaviest rainfall and best moisture confluence
is just east and north of the most saturated soils, where the
highest probabilities exist for excessive rainfall, 3-hr FFG is
still as low as 1-2". This has prompted a slight eastward
expansion of the SLGT risk, surrounded by a relatively large MRGL
risk to account for uncertainty and variability in QPF placement.
While not currently expected, it is possible if the guidance were
to increase their probabilities, especially in SD or further west
into ND where 14-day rainfall has been 300-600% of normal, a MDT
risk may be needed.
The Day 3 outlook will be updated by 0830Z.
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