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FOUS30 KWBC 040823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Sun Aug 04 2019

Day 1
Valid 12Z Sun Aug 04 2019 - 12Z Mon Aug 05 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST...SOUTHERN ROCKIES..SOUTHERN HIGH PLAINS...PORTIONS
OF THE NORTHERN PLAINS...SOUTHEAST AND COASTAL SOUTHEAST FLORIDA...

...Southwest...Southern High Plains...Southern Rockies...
No significant changes to the large scale pattern over the
Southwest as the upper high remains centered near the four
corners. Above average pw values 1.5 to 2 standard deviations
above the mean will support potential for additional locally heavy
and isolated excessive rainfall in what will likely be another
round of widespread scattered convection.  The marginal risk area
was not changed appreciably from the previous issuance...tracing
out the area of the higher HREF neighborhood probabilities for 1
and 2"+ rainfall amounts this period.

...Portions of the Southeast...
Two marginal risk areas were added across portions of the
Southeast for the upcoming day 1 period.  One is across northeast
Mississippi...northern Alabama and a small portion of southwest
Tennessee. These areas have lowered ffg values from locally heavy
rain that occurred over the past 24 hours.  The upper trof axis is
forecast to remain oriented from the Upper Ohio Valley into the
Tennessee Valley and Central Gulf Coast.  This will support
potential for additional convection across the above mentioned
regions that received heavy rains over the past 24 hours.  HREF
neighborhood probabilities for 2"+ amounts are high across this
area...generally in the 50-70%+ range.

The other marginal risk area over the Southeast was placed over
far northeast Florida...southeast to coastal Georgia into much of
coastal South Carolina. Similar to areas upstream...this region
will be in a favorable place to the east of the above mentioned
upper trof axis to have potential for active convection. Hi res
model consensus is for heavy rains across these areas with HREF
neighborhood probabilities for 2 and 3"+ amounts high..70-90% for
2"...50-80% for 3" and a few 20%+ area for 5"+ amounts.

...Coastal Southeast Florida...
A marginal risk was maintained over coastal southeast Florida for
isolated urban runoff issues in the 1800 utc Sunday to 0000 utc
Monday time frame.  A narrow axis of much above average pw values
2 to 3 standard deviations above the mean expected to persist
across South Florida Sunday with convection enhancing this
afternoon into early evening in this axis.  HREF neighborhood
probabilities for 2 and 3"+ amounts are high...80-90% and 70-80%
respectively...with even a few 20% probabilities for 5"+ totals.
The marginal risk area was confined to the urbanized areas of
southeast coastal Florida.

...Southeast North Dakota...northeast South Dakota...west central
Minnesota...
A surface frontal boundary dropping southeast across the Northern
Plains into the Upper Mississippi Valley will become the focus for
convection Sunday afternoon into Sunday evening and Sunday night.
While there is a large spread in qpf details among the latest 0000
utc models...there is consensus for moderate to locally heavy
precip totals in an axis of increasing instability Sunday
afternoon on...mu-cape values 2000-2500+ j/kg.  The previous
marginal risk area was expanded to cover the range of model
solutions and lower ffg value areas.

Oravec


Day 2
Valid 12Z Mon Aug 05 2019 - 12Z Tue Aug 06 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE PLAINS AND THE SOUTHEAST U.S. ON MONDAY...

...Central Rockies and Adjacent High Plains...
Monsoon moisture should still be located over portions of the
Central Rockies...trapped within the broad anticyclonic flow over
the region.  That should set the stage for another round of
scattered afternoon and evening convection.  Current thinking is
that the convection should largely being unfocused in terms of
synoptics...leaving forcing on the mesoscale or smaller as
important players in initating and sustaining convection.  The GFS
continued to show the highest Precipitable Water values along the
Colorado and New Mexico front range where PW values are forecast
to be at or above 1 inch.  The interior portions of the Great
Basin showing a drying trend with values generally under 0.75
inches.  Will maintain the Marginal Risk area from Day 3 with only
minor adjustments in the area.

The exception was to expand coverage across portions of southern
Arizona as another push of monsoon moisture makes its way
northward around the periphery of a mid/upper level high anchored
over New Mexico.  The GFS was most aggresive in terms of bringing
1.5 to 1.8 inch PW values along and just north of the internationa
border region...but it is not alone in producing forecast
soundings that show over a 30 degree F temperature/dewpoint spread
in the lowest levels while nearly saturated from 650 mb and above.
 The Marginal Risk area was extended as far west as where the GFS
was able to generate MUCAPES of several hundred J/kg and where PW
values reach the 75th percentile.

...Central Plains...
Maintained the previously issued Marginal Risk area over portions
of Nebraska and expanded the area northward and eastward in
advance of a cold front moving southward. The GFS and ECMWF both
has shown the idea that the front should become oriented more east
to west...allowing for the possibility of repeat convection or
training.  With Precipitable Water values running 1.6 to 1.75
inches...which is in the range of 1.0 to 1.5 standardized
anomalies greater than climatology...there should be a period
where rainfall rates of an inch or more per hour is possible
(especially from the afternoon into the evening).

...Southeast U.S....
A lingering surface boundary combined with higher than normal
precipitable water values (2.0-2.2 inches) across the eastern
Carolinas coastal plain will favor locally intense rainfall rates
Monday afternoon into the early morning hours on Tuesday. The
latest HREF continued to show probabilities of locally heavy
rainfall and the CSU first guess ERO fields suggested probability
of flash flooding in the 5-10 percent range, indicating potential
for some ponding problems within areas of poor drainage. Little
changes were made to the existing Marginal Risk from the previous
issuance.

Bann


Day 3
Valid 12Z Tue Aug 06 2019 - 12Z Wed Aug 07 2019

...SOUTHWEST UNITED STATES...

...Arizona to Colorado and New Mexico...
Maintained a Marginal Risk area over the Southwest states as
monsoon moisture continues to circulate around a sprawling upper
high anchored over New Mexico.  While the deepest moisture
gradually shifts out of the Colorado/New Mexico portion of the
region during the period...deep moisture will make its way
northward into portions of Arizona and even far southern Nevada
Tuesday night/Wednesday.  Highest Precipitable Water values will
be confined to areas close to the international border...where the
04/00Z GFS briefly produced PW values close to 2 inches on Tuesday
afternoon...with PWs over an inch reaching into southern by early
Wednesday morning.  These PW values are 1.5 to 2 standardize
anomalies greater than climatology and in the 75 to 90 percentile
range.  Low level flow is forecast to be weak...allowing for slow
cell motion.  Forecast soundings show enough dry air at low level
to suggest that cold pools may be able to form...but other factors
such as upslope flow will be important in initiating and
sustaining convection that can produce locally intense rainfall
rates.

Bann



Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.t...
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.t...
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.t...


$$




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