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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0757Z Aug 15, 2019)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 AM EDT Thu Aug 15 2019

Day 1
Valid 12Z Thu Aug 15 2019 - 12Z Fri Aug 16 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS...

...Central Plains into the Mid MS Valley...
Multiple rounds of convection expected today across the Central
Plains. Models are split on convective evolution this morning into
early afternoon. Based on latest radar/observational trends,
leaned against the dry solutions shown by the ARW/NMMB and earlier
runs of the HRRR. Seems probable that the convection over western
SD as of 07z will survive as it dives southeastward into NE and
northern KS through the morning hours...with something closer to
the 06z HRRR, ARW2 and GFS preferred here. Generally think this
activity will remain progressive enough to keep the flash food
threat low.

The ingredients are in place for another round of even more robust
convection by later this afternoon into the overnight hours. The
exact evolution of this second round is at least somewhat
dependent on what happens this morning...so inherently there is
some level of uncertainty on the exact axis of heaviest rains with
the overnight storms. With that said, given the increasing
synoptic forcing (shortwave and upper jet), increasing 850mb
moisture transport, and plentiful instability...seems likely that
we end up with a pretty strong convective complex moving
southeastward across the Plains and towards the Mid MS Valley
tonight. This activity may very well stay progressive...however
there are some hints in the guidance of a period of backbuilding,
and the evolution/orientation of the 850mb moisture transport
corridor would also support some training/backbuilding potential.
Given this potential, along with the extreme upstream instability
forecast, still seems like at least some flash flood threat will
exist by tonight. Again, some uncertainty with exactly where the
highest threat is, but for now it still looks like northeast KS
into southeast NE and far northwest MO...and thus no changes were
needed to the Slight risk area. A Marginal will surround the
Slight...with areas to the north having stronger forcing...and
areas to the south over KS having stronger instability.

...Southeast...
Areas along and east of a stationary front will again be a focus
for scattered convection across much of the Southeast coastal
areas. Slow cell motions and cell merging will likely lead to some
localized heavy rainfall rates and magnitudes...with isolated 3-5"
amounts probable. Amounts of this magnitude will be on a small
scale, and thus overall still thinking any flash flooding will
remain rather localized in nature. Thus think a Marginal risk
should suffice. Models are suggesting a more concentrated area of
heavy rainfall along the northwest FL coast...where a greater
coverage of 2-3"+ amounts appears likely. This appears to be more
of a longer duration event, with periodic heavy rainfall rates
embedded within a broad area of light to moderate rainfall. Given
the drier antecedent conditions here and higher FFG...a Marginal
risk should still cover the threat, with flash flooding expected
to stay localized. This event may continue beyond day 1...and with
time as soils saturate, flash flooding could become more of a
concern...but for day 1 it would appear this area should generally
be able to handle the expected rainfall amounts.

Did opt to expand the Marginal Risk up into portions of the Mid
Atlantic and southern NY with this issuance. Activity may try to
focus along/near a backdoor boundary that will setup across
portions of NC/VA/PA...with localized flash flooding a possibility
if/where cells briefly anchor or merge. Forcing is a bit stronger
as you get into northern PA and southern NY, and evidence of
another boundary to potentially focus convection...thus could see
a bit better coverage of storms here...and can not rule out an
isolated instance of flash flooding here as well.

...New Mexico...
Isolated convection is again expected from portions of eastern AZ
into NM. Not expecting anything too intense with only marginal
PWs/CAPE and isolated coverage...however cell motions will be slow
and portions of this area have been anomalously wet of late...thus
isolated instances of flash flooding across more susceptible
locations will again be possible.

Chenard


Day 2
Valid 12Z Fri Aug 16 2019 - 12Z Sat Aug 17 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE CENTRAL
PLAINS INTO THE MID MISSISSIPPI VALLEY...AS WELL AS THE BIG BEND
AREA OF FLORIDA...

...Central Plains into the Mid Mississippi Valley...
An approaching shortwave from the Dakotas during the day Friday is
expected to spur the development of an MCS during the late
afternoon and persist into the overnight hours Friday night.  This
feature will interact with deepening moisture as precipitable
water values increase to over 1.75 inches as the low level jet
increases over eastern Kansas and northern Missouri.  The
convection developing in the instability axis is expected to grow
into an organized MCS that will likely track towards the southeast
over the course of the evening and overnight hours within the axis
of greatest instability.

There has been a southward trend in the model guidance with
respect to the axis of heaviest convective rainfall, with the 00Z
NAM among the most intense solutions.  After analyzing the latest
trends, and also taking into account that these types of events
often verify south of the multi-model consensus, the WPC forecast
for this cycle is roughly 50 miles farther south with the QPF axis
compared to the previous forecast, with the potential for
additional southeastward adjustments if future model runs continue
this trend.  The Day 2 Slight Risk encompasses the region from
near Kansas City to south-central Illinois.  The existing Marginal
Risk area was also extended to the southeast to include southern
Missouri and western Kentucky.  Ground conditions will likely be
more saturated over eastern Kansas and western Missouri owing to
significant rainfall expected during the Day 1 period.


...Northern Florida and the Coastal Southeast...
A plume of anomalously deep tropical moisture situated ahead of a
gradually weakening frontal boundary across northern Florida and
the Southeast Coast is expected to support multiple rounds of
intense convection from the Big Bend of Florida to the greater
Jacksonville area.  Aloft will be a broad trough axis that will
aid in providing synoptic scale lift near the front, along with
potential embedded mesoscale convective vortices. 

Atmospheric parameters appear to be conducive to warrant a Slight
Risk of excessive rainfall between the greater Tampa and
Tallahassee metro areas, particularly across the Big Bend region
of the state.  Low to mid level flow should be nearly parallel to
the front, which would create an environment favorable for
training convection.  Precipitable water values on the order of
2.2 to 2.5 inches will result in a deep warm cloud layer with
highly efficient rainfall processes, along with 1500 to 2500 J/kg
of MLCAPE south of the front.  High res guidance is suggesting the
potential for 4 inches of rainfall over a six hour period on a
localized scale, and broader 1-3 inch totals from the global
guidance.  The mitigating factor here are the initially higher
flash flood guidance values, but these will likely be reduced some
given expected rainfall during the day 1 forecast period.  The
existing Marginal Risk area was maintained for coastal sections of
the Southeast U.S.

Hamrick

Day 3

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