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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0053Z Sep 19, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Wed Sep 18 2019

Day 1
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. 

Bann


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
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...
...20Z Update...
Adjusted highest QPF axis northeast across northeast TX/southwest
AR. QPF amounts remain 1-2 inches with locally higher amounts. By
late Friday, the moisture will be fairly defuse as the approaching
trough helps pick up the mid-level features with the tropical
moisture feed diminishing fairly quickly.  Therefore, minor flash
flooding may occur across far northeast TX and southwest AR,
likely through the day Friday. 

...Previous Discussion...
The remnants of Tropical Depression Imelda will continue to plague
portions of east Texas as it finally begins to lift slightly more
rapidly to the north on Friday as it becomes embedded in the
westerlies ahead of a high amplitude trough digging through the
Northern Plains. Although the core and mid-level circulation will
begin to erode and dissipate, continued anomalous moisture noted
by PWATs of +2 standard deviations will exist and be drawn
northward ahead of the trough. While the overall forcing and
rainfall intensity should ease on Friday compared to earlier in
the week, there is still potential for a few inches of rainfall as
noted by ECENS probabilities as high as 20% for 3 inches. Some of
this may fall on top of areas that are pre-conditioned from heavy
rain on Thursday, so despite current high FFG (4"/3hrs), expect
these will lower and rain rates of 1-2"/hr could lead to flash
flooding. The guidance does differ into how quickly the remnants
will become absorbed to the north, and this produces some lowered
confidence into where the heaviest rain axis will occur, but the
mean position of the remnant circulation and most robust moisture
transport vectors overlap well the highest GEFS/ECENS
probabilities, and this was used to draw the SLGT risk.


...Northern Plains...
...20Z Update...
Minor refinements were made to the Slight Risk mainly to account
for the QPF adjustments across the Dakotas.  Otherwise, the
current forecast remains on track. 

...Previous Discussion...  
High amplitude trough digging through the Pacific Northwest will
move eastward into the Northern Plains with 500hPa height
anomalies of -1 to -1.5 standard deviations from the
climatological mean. This trough will be accompanied by robust
shortwave energy rotating through its base, driving a surface cold
front eastward into the Dakotas Friday night. Significant height
falls, strong PVA, and intense upper diffluence in the RRQ of a
jet streak lifting into Canada will provide ascent in an
increasingly moist column, characterized by PWATs as high as
1.75", +3-+3.5 standard deviations from the mean.

This moisture is being drawn northward on deep layer SW flow ahead
of the trough, with WAA on an intensifying LLJ producing MUCape
exceeding 3000 J/kg in the Dakotas and into Minnesota. Although
there exists considerable longitudinal spread in the axis of
heaviest rainfall, much of this region has experienced recent
significant rainfall noted by 14-day departures of more than 300%
of normal in SD and western ND. Models suggest widespread
convection developing along the front, and despite 0-6km mean
winds from the south at 30 kts or more, training along the
boundary is likely. With rainfall rates potentially reaching
1-2"/hr, and the ECENS showing modest probabilities for 24-hr
rainfall exceeding 3", have added a SLGT risk to align with the
global consensus for the axis of heaviest rainfall, surrounded by
a broad MRGL risk to account for the variability in eastward
expansion of the convection.


Pagano/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