Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
432 AM EDT Thu Sep 19 2019

Day 1
Valid 12Z Thu Sep 19 2019 - 12Z Fri Sep 20 2019

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF FAR
SOUTHEAST TEXAS...
...SIGNIFICANT LIFE THREATENING FLASH FLOODING IS ONGOING AND WILL
CONTINUE INTO THE MORNING HOURS OVER FAR SOUTHEAST TX...

...Eastern Texas and Extreme Western Louisiana...
Significant and life-threatening flash flooding is ongoing early
this morning across far southeast TX within a nearly stationary
band of intense rainfall. This band is focusing well to the south
of the center of Imelda, within a low level convergence
axis...with persistent southerly flow into this axis supplying a
continuous influx of instability. Unfortunately this band is not
expected to move much through the morning hours. As Imelda lifts
slowly northward the background convergent flow will try to shift
northward as well. However, the convection will also be attempting
to push southward into the strong inflow and better instability
pool. Thus the situation favors a persistence of the band into the
morning hours without much movement. After 12z we may begin to see
a slow southward shift of the band into the Houston metro area,
but this is uncertain. Hopefully by later this morning into the
afternoon hours we will begin to at least see a
broadening/weakening of the heavy rain band. This scenario is
depicted by the high res guidance and fits the typical diurnal
trend with a general weakening/broadening of the low level inflow.
Thus, while not a sure bet, appears more likely than not that the
heavy rainfall threat becomes less organized this afternoon into
the evening hours. What happens tonight is uncertain...but at
least some threat we see another increase in the flash flood
threat overnight as low level inflow increases and tightens with
the diurnal cycle. By this time the circulation of Imelda will be
quite weak...so uncertain if there will be enough there to allow
for an intense banding scenario to evolve. There is certainly some
chance of this occurring somewhere over east TX tonight, so will
need to monitor trends. While the activity overnight should not be
able to match what we are seeing this morning over far southeast
TX, it still has the potential to bring a significant flash flood
threat and worth monitoring.

A HIGH risk will be carried into the new day 1 ERO for far
southeast TX. A dangerous and life-threatening situation is
ongoing and will continue into the morning hours across this
region. A moderate risk extends northward across east TX, to
account for rainfall closer to the center of Imelda and any
potential redevelopment tonight. We were able to trim back some on
the northern extent of the Slight and Moderate risk areas. Overall
it appears that rainfall near the center of Imelda will not be as
significant as some of the earlier guidance suggested. Instead the
significant rainfall will tend to remain focused south of the
center within slow moving intense bands within the convergent flow
south of the system.

...Plains...
Convection ongoing at 12z may pose an isolated flash flood threat
into the morning hours across portions of eastern NE into
IA...although the threat should generally be on a downward trend
through the morning hours. Redevelopment is likely this afternoon
from the TX Panhandle into western KS/NE along the dry line. This
activity may stay tied to the low level boundary long enough to
pose a localized flash flood threat. Do note at least some
mid/upper level connection to the tropical Pacific in the layered
PW fields...which may help increase rainfall efficiency.
Additional development appears likely over NM...and this may be
able to persist through the evening hours as it moves towards west
TX and encounters increasing southeasterly inflow. This may
continue the isolated flash flood potential over these areas into
the overnight.

Chenard


Day 2
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.


...Northern Plains...
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.


Weiss

Day 3
Valid 12Z Sat Sep 21 2019 - 12Z Sun Sep 22 2019

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MIDWEST...

...Kansas/Iowa/Missouri/Illinois...
A high amplitude trough will dig into the Central Plains driving a
surface cold front southeastward beneath it. Ahead of this trough,
low/mid level flow will become increasingly robust from the SW,
with winds between 850-500mb all climbing above 40 kts immediately
ahead of the cold front. In the upper levels, this will begin to
advect moisture from Hurricane Lorena near Mexico northeastward
into the central CONUS, while low level moist advection will tap
tropical moisture associated with Tropical Depression Imelda. The
combination of these two pools of moisture being advected into the
region will drive PWATs towards or in excess of 2 inches, 2-2.5
standard deviations above the climo mean, and above daily PWAT
records.

In this exceptionally moist airmass, deep layer ascent will be
prolific through low-level convergence along the front, mid-level
height falls, and increasing 300mb diffluence in the RRQ of a
strengthening 110 kt jet streak. Additionally, MUCape is forecast
to climb on low-level WAA through the evening despite what could
be considerable cloud cover, further enhancing the thermodynamic
environment supporting heavy rainfall.

Despite what should be rapid storm motions to the east on 30+ kts
cloud layer winds, this 0-6km mean flow parallel to the boundary
suggests that training is likely. Rainfall rates should exceed
1"/hr, and may approach 2"/hr the latter half of D3 as warm cloud
depths surge above 13kft to increase rainfall efficiency. There
exists a robust model signal for several inches of rainfall across
this area, and both the ECENS and GEFS 24-hr probabilities
indicate already modest probabilities for more than 3" of rain.
FFG across this area is relatively high due to 14-day antecedent
rainfall that has been generally less than 75% of normal, but
training of these excessive rain rates could lead to flash
flooding, and a SLGT risk has been introduced where confidence is
highest in this occurring.

Weiss


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