Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Wed Sep 18 2019

Day 1
Valid 16Z Wed Sep 18 2019 - 12Z Thu Sep 19 2019

...A HIGH RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
UPPER TEXAS COAST...

...East TX/Southwest LA...
1600 UTC Update -- Shifted the High Risk area a little farther to
the east, while extending the eastern periphery of the Moderate
eastward as well, based on the 12Z guidance/trends (including
high-res CAMs). 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. Currently, the CDO continues to result in a
relative min in instability toward Imelda's center (thus lower
short term rainfall rates). However, as the TD lifts slowly
northward, so will those outer inflow bands.

Hurley

Previous discussion...  

Very heavy rainfall associated with Tropical Depression Imelda
will continue as the system slowly makes its way northward. 
Precipitable water values of 2-2.6" were indicated by recent GPS
data near the depression with inflow at 850 hPa of 25-30 kts --
2-3 times the mean 850-400 hPa wind -- which should lead to
precipitation efficiency.  With the low- to mid-level flow
unidirectional and cyclonic up to at least 500 hPa, spiral banding
should remain the heavy rain threat on the southern and eastern
side of the system.  Hourly rain totals to 3" are possible in any
bands or cell mergers. Flash flooding would result in areas where
bands stagnate, cell mergers occur, or core convective bursts
reach 3" an hour (which the guidance is not sold upon and has not
occurred this morning).  The mesoscale guidance has a strong
signal for 9"+ tonight into Thursday morning, although some of
this stems from their slower movement of the overall system. 
While it has dawdled as of late, the depression is expected to
slowly pick up the pace to the north and north-northwest over the
next couple of days as it moves around a strengthening ridge to
its northeast.  Trending towards the more northerly GFS/ECMWF
solutions for the heavy rainfall, though some west to east spread
remains in the guidance.  Coordination with HGX/the
Houston-Galveston TX forecast offices led to an upgrade to a high
risk where it has rained heavily thus far.  The LCH/Lake Charles
LA forecast office was also coordinated with concerning the
ongoing risk area.

...Midwest...
The guidance continues to show a bit of spread -- both north-south
and east-west -- with a heavy rain axis across IA/WI/NE.  Both the
GFS and NAM show persistent low-level inflow of 25-50 knots
diagonally across IA this period with embedded precipitable water
values of 1.5-1.75", with the edge of a good mid-level cap across
the central Plains where CAPE of 2000-3000 J/kg is expected to
build.  Any organized convection across NE and IA should have a
tendency to move east to east-southeast per the 1000-500 hPa
thickness pattern and forward propagating convective vectors.  The
WRF NSSL QPF was closest to the manual forecast in this region,
which was within the southwest side of the envelope of solutions,
which makes sense since the atmosphere in this region looks
relatively uncapped.  Coordination with the DMX/Des Moines IA and
OAX/Valley NE offices led to an upgrade to a slight risk in this
region.  Hourly rain totals to 2" are possible here, which would
be problematic as this would be more than a match for three hourly
flash flood guidance values.

Roth


Day 2
Valid 12Z Thu Sep 19 2019 - 12Z Fri Sep 20 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN TEXAS AND THE TEXAS COAST...

...Eastern Texas and Extreme Western Louisiana...
...20Z update...
Minor changes to adjust for the guidance shift to the east. But
overall the coverage and QPF amounts are in line with the previous
forecast. One feature of concern remains with the potential of a
convective band developing well south of the depression which
could result in a narrow swath of high rainfall amounts.  It is
hard to discern if/where this feature may develop, but it is a
continued concern that it may coincide with the Houston/Galveston
area; however, that remains less certain at this point. The timing
of this band of heavy rain will likely occur after 06Z tonight
(Thursday) into mid-morning as the 850 mb southerly inflow
interacts with the instability gradient along the coast.  With a
higher degree of certainty, the wrap around associated with Imelda
will occur across eastern TX with little movement overall. Though
the instability will be less and thus rain rates not nearly as
impressive as the convective banding to the south. Generally
speaking 2-4 inches with locally higher amounts of 6+ inches can
be expected through Thursday night.  Only minor changes made to
the ERO to account for the shift in QPF.  The overall threat for
significant flash flooding remains across portions of northern TX
coast/eastern TX and extreme west LA. 

...Previous Discussion...
The remnants of Tropical Depression Imelda will continue to decay
across eastern Texas while slowly lifting to the north. The
guidance has shifted subtly eastward and northward this morning
with the axis of heaviest rainfall, and the WPC QPF has been
modified to reflect the best consensus.

Although the system will be very weak by Thursday, persistent
moist advection and increasing instability will fuel exceedingly
heavy rainfall rates which could reach 3"/hr at times. This is
most likely near the core of the remnant low, but may also occur
in a secondary band south of the low center. This secondary band
could produce even more intense rain rates, and low-level
confluence in an exceptionally moist environment (PWAT over 2.25")
combine with increasing instability as cloud cover erodes to the
south. This is a common development in systems such as this, and
the high-res guidance including the preferred NMM and NAMNest
early on D2 suggest this occurring. Near the slow moving core, and
within this potential banded structure to the south of the center,
rainfall rates of 2-3" hr could produce 3-5" of rainfall, with
isolated amounts above 6" possible. This rainfall will occur on
top of heavy rain from Monday and Tuesday, so current FFG is
likely not representative of the rainfall needed to produce flash
flooding. Incorporating rainfall from D1, the MDT risk has been
extended northward almost to the Red River Valley, and the SLGT
has been extended into far SE OK. There still remains uncertainty
into where the axis of heaviest rain will occur, but is is likely
that flash flooding will be significant across portions of east
Texas due to several days of heavy rainfall.


...Corn Belt...
...20Z Update...
Minimal changes to QPF amounts and the ERO.  There has been a slow
trend south with the expected convection which may limit the flash
flood issues given FFG is higher across southern IA. Otherwise,
QPF totals and messaging are still in line.

...Previous Discussion...
Weak front will be decaying across Nebraska and Iowa early on
Thursday, with lingering convection/heavy rainfall likely through
the morning. A shortwave is progged to lift northeast across the
region early Thursday to interact with the front, and likely lead
to persistent convection even as the LLJ begins to weaken and veer
to reduce moisture transport. PWATs will still hover around
1.5-1.75" due to at least a subtle connection to moisture from TD
Imelda, +1.5 standard deviations above the climatological mean,
and ascent via PVA, low-level convergence, and subtle upper
diffluence will allow thunderstorms to persist into the late
morning/early aftn. This is well modeled by the global consensus,
and rain rates could reach 1-2"/hr. Have carried a MRGL risk
despite the relatively short temporal window for flash flooding,
as heavy rainfall from D1 may sufficiently pre-condition the soils
to lower FFG and allow for isolated flash flooding.


Pagano/Weiss

Day 3

The Day 3 outlook will be updated by 2030Z.


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