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FOUS30 KWBC 041936
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
335 PM EDT Sun Aug 04 2019

Day 1
Valid 1926Z 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, DAKOTAS,
AND SOUTHEAST...

...Southwest...Southern High Plains...Southern Rockies...
An upper high remains centered near the Four Corners. Above
average precipitable water 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.  Temperatures at 700 hPa
of ~15C indicate that locations closer to sea level in the deserts
should remain capped, with activity mostly confined to the
topography.  The marginal risk area was not changed appreciably
from the previous issuance and takes into account recent radar
trends.

...Southeast/Mid-South...
A broad marginal risk area exists across the region near a pair of
weak fronts.  The upper trough axis is forecast to remain oriented
from the Upper Ohio Valley into the Tennessee Valley and Central
Gulf Coast, supporting the 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 60-70%
range...which could occur in one hour given the moisture present.
Adjustments were made to the area to account for the more
significant instability pools which are leading to pockets of
heavy rain across KY, southwest TN, and northwest MS.

...Dakotas & West-central Minnesota...
A surface frontal boundary dropping southeast across the Northern
Plains into the Upper Mississippi Valley will remain the focus for
convection this afternoon into night.  While there is a large
spread in QPF details..there is consensus for moderate to locally
heavy precip totals in an axis of increasing instability...ML CAPE
values are currently 2000-3000 J/kg in eastern ND and western MN.
Adjustments were made to the area based on radar trends.

Roth/Chenard/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 initiating 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 aggressive in terms of bringing
1.5 to 1.8 inch PW values along and just north of the
international border region...but it is not alone in producing
forecast soundings that show over a 30 degree F temperature/dew
point 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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