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FOUS30 KWBC 030056
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Fri Aug 02 2019

Day 1
Valid 01Z Sat Aug 03 2019 - 12Z Sat Aug 03 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER THE
SOUTHERN PLAINS...

...Southern Plains...
Using observational trends and the hi-res model QPFs valid through
12Z, we maintained a small Moderate Risk area over parts of KS/OK.
Expected deep layer ascent is not especially well defined, which
brings the upcoming overnight event somewhat into question. But
models remain insistent that at least one narrow band of heavy
rain will occur, most likely centering on central to eastern
Oklahoma, along a CAPE gradient and just to the cool side of a
warm front. Low level jet wind speeds are forecast to be very
modest given a weak 850-mb height gradient. By 03-05Z, however,
there should be enough speed convergence to promote organized
thunderstorm development north and east of Oklahoma City. This
evening the HRRR along with the NSSL Warn on Forecast System
(WoFS) continued to support the going forecast of a QPF / heavy
rainfall maximum to occur north through east of Oklahoma City
proper.

The event does not appear to be aided by much of an upper level
difluence signature, but a subtle mid level shortwave trough
dropping into the area should give a boost to lapse rates and
lift. Models do suggest some potential for very local scale,
high-end rainfall amounts above 5 inches, hence the Moderate Risk.
The 00Z sounding from Norman, OK, sampled strong inhibition, so
wherever the event sets up it may have difficulty migrating to the
south and west.

...Southern Atlantic Coast...
The upper trough over the Central Gulf of Mexico will continue to
transport tropical moisture across the Florida Peninsula into the
coastal Southeast. PW anomalies 2 to 3 standard deviations above
the mean are expected to persist. Mid to upper level circulations
on the eastern side of the Gulf of Mexico trough will accentuate
lift...supporting near-shore convection overnight up through the
Carolinas. Should any convergence bands set up onshore, several
inches of rain could fall in a short amount of time on local
scales.

...Portions of the Southwest into the Southern Rockies...
Scattered monsoonal convection will persist into the late evening.
Storms in Arizona had prompted a few flash flood warnings. Storms
in Colorado were proving to be more progressive, so we trimmed the
coverage of Marginal Risk there. Isolated flash flooding owing to
a few cell mergers will be possible in parts of AZ/NM through
about 03Z.

Burke/Oravec


Day 2
Valid 12Z Sat Aug 03 2019 - 12Z Sun Aug 04 2019

20Z Update: For this update, the main focus was across the
south-central Plains where convection is expected to ongoing at
the start of the period. Ingredients remain favorable during the
early morning hours with anomalous moisture and sufficient
instability in place with a diminishing threat by mid morning. The
latest trends in the forecast guidance showed a nudge westward
with the heaviest QPF, across central Oklahoma into portions of
north-central Texas. Accordingly, the Day 2 Excessive Rainfall
Outlook (12Z Sat - 12Z Sun) was adjusted westward.

Minimal changes were made to the ERO across the Southwest U.S.
where increasing monsoonal moisture combined with upslope flow and
daytime heating will initiate convection across the favored areas.
The latest QPF was based on a multi-model consensus with higher
weight given to the latest hi-res models which seem to simulate
the favored terrain areas better.

Across the Appalachians into portions of central PA and southern
NY, the Marginal Risk was expanded northward where model guidance
suggests locally heavy rainfall will be possible in the
slow-moving, high moisture environment.

Finally, the Marginal Risk across southern/southeast Florida had
minor adjustments. Given the south to southwest wind flow and
anomalously high moisture for the area (90th percentile), sea
breeze convection combined with lingering tropical moisture in the
area should promote widespread thunderstorm activity that will be
capable of producing locally heavy rainfall.

Taylor

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
EASTERN OKLAHOMA INTO CENTRAL ARKANSAS...

...Southern Plains and Lower Mississippi Valley...

Still expecting convection to be lingering into the early part of
the Day 2 period across portions of the Southern Plains and the
Lower Mississippi Valley as shortwave energy from the Day 1 period
sags southward.  There should be plenty of moisture
available...with the GFS and ECMWF both depicting Precipitable
Water values in the 2 to 2.25 inch range...with MUCAPES of 1500 to
2500 J/kg.  Overall, the model agreement is good with the
placement and amounts...although the NAM appeared to be suffering
from convective feedback that manifest itself as a much-larger
than consensus QPF and it produced a mid-level low/vort center
much stronger than the other models.  Convergence and dynamics
weaken and CAPES diminish Saturday night/Sunday morning with
models showing a corresponding drop in rainfall amounts/rainfall
rates.

...Southwest...

Monsoonal moisture combined with upslope flow along the favored
terrain will bring another day of convection capable of producing
heavy to excessive rainfall across parts of the Southern and
Central Rockies and Southern Great Basin on Saturday afternoon
into the overnight. Areal average precipitation will be around
half an inch, although localized rainfall amounts of 1.00 to 1.25
inches will be possible in the upslope region of the complex
terrain.  Still not confident enough to hoist a Slight Risk.  As
it stands, thinking is that the burn scars areas have the greatest
potential for flooding.

...Central/Southern Appalachians...

A weakening frontal boundary from the southern Mid-Atlantic into
the Tennessee Valley will still be in place on Day 2. Upslope flow
into the central and southern Appalachians may result in enhanced
rainfall along the higher terrain. Areal average precipitation of
0.5-1.5 inches with locally higher amounts are expected as storms
become slow to move across this region. The setup is marginal at
best, but given the boundary will be present and the Precipitable
Water values should generally range from 1.7 to 2.0
inches...moderate to localized heavy rainfall amounts are
possible.  Parts of the Southern Appalachians and adjacent areas
of the Tennessee Valley have received heavy rainfall over the past
few days...so opted to make few changes to the on-going Marginal
Risk area.

...Florida...

An area of disturbed weather/surface trough with its associated
area of rain should generally remain off the Florida
coast...although the atmosphere across the peninsula has deep
moisture and instability that can be released to produce locally
heavy rainfall rates and amounts as the dynamics move across the
peninsula.  Flash Flood Guidance is high...but there could still
be instances of flooding or problems resulting from runoff or
ponding in areas of poor drainage.

Bann


Day 3
Valid 12Z Sun Aug 04 2019 - 12Z Mon Aug 05 2019

20Z Update: Minimal changes were needed to the Excessive Rainfall
Outlook for Day 3 (valid 12Z Sun-12Z Mon). The latest WPC QPF was
based on a multi-model consensus with the heaviest weight given
toward a local bias-corrected blend. Monsoonal moisture will
remain in place across the Southwest into the central Rockies
where local amounts around 1 inch remain possible in the favored
terrain areas. Across the southeast U.S. into the southern
Appalachians, lingering frontal boundary combined with daytime
heating and ample moisture should promote the development of
scattered/numerous thunderstorms of the pulse to loosely organized
variety during the afternoon and early evening hours. Over areas
that have lower flash flood guidance, some runoff problems may
develop and the Marginal Risk in place there was not changed
significantly from the previous issuance.

Taylor

...MARGINAL RISK OF FLASH FLOODING OVER PORTIONS OF THE SOUTHERN
ROCKIES AND ACROSS THE SOUTHERN APPALACHIANS...

...Southwest U.S....
Monsoonal moisture will still be present over parts of the
Southwest U.S. on Sunday...although the global models were showing
a drying trend over parts of the Desert Southwest where
Precipitable Water Values will start the period above 1.5 inches
but decrease to under 1.25 inches (and in some cases drop under
1.00 inches).  At the same time, PW values will be generally be
steady farther north in portions of Utah and Colorado.  Thus,
think the afternoon and evening convection will be spreading a bit
northward and eastward compared with QPF for Saturday.  Highest
areal average amounts should be under half an inch...although
local amounts approaching 1 inch are possible in areas of the most
active convection, in regions of upslope flow in complex terrain,
and over burn scars.

...Southern Appalachians...
WPC QPF showed scattered convection on Day 3 with the highest
amounts along and near the spine of the Southern Appalachians and
adjacent portions of the Southeast U.S.. With no real change in
airmass...and with lingering mid-level dynamics...over some
locations where Flash Flood Guidance had been lowered by moderate
to heavy rainfall amounts recently have opted to keep a Marginal
Risk in place from Sunday into early Monday. With light flow in
the low levels and PW values gradually climbing to values at or
slightly above 2.0 over parts of GA and the Carolinas, thinking is
that cells capable of producing locally intense rainfall rates
will move little before dissipating.

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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