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FOUS30 KWBC 031557
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Sat Aug 03 2019

Day 1
Valid 16Z Sat Aug 03 2019 - 12Z Sun Aug 04 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN PLAINS FROM SOUTH CENTRAL OKLAHOMA INTO NORTH CENTRAL
TEXAS AND OVER PORTIONS OF NEW MEXICO AND SOUTHERN COLORADO...

16z Update: Expanded the Slight risk over NM with this update.
Localized flash flooding, especially within sensitive areas, will
be possible over much of the Southwest today. However, there does
appear to be a signal for a bit better focus across portions of NM
into far southern CO. Convective coverage should be a little
higher here, with broad convergent flow expected around 700 mb.
Weak mean flow supports slow cell motions and eventual movement
along outflows, which may lead to some short duration cell
merging. With PWs remaining anomalous, any cells will be capable
of heavy rainfall rates...and think the expected coverage/slow
cell motions...suggests that there may be a bit higher coverage of
flash flood issues today, especially over any more sensitive
locations.

Will maintain a Slight risk from portions of southern OK into
northern TX. Convection is generally on a weakening trend this
morning, however should see another uptick this evening from near
the Red River into north TX. A wave dropping southward should help
focus an area of 850 mb moisture convergence to its east
tonight...which should act as a forcing mechanism for the
nocturnal development. Unclear exactly where this activity sets up
and what the magnitude of rainfall will be...but the setup does
support the potential for at least small scale training along the
moisture convergence axis. There is some potential that totals
could become quite significant on a localized scale somewhere over
northeast TX...but at this time tend to think these more extreme
totals would be localized in nature, and confidence is not all
that great. Thus think a Slight risk best characterizes the
threat, but will continue to monitor.

Convective development on the northeastern periphery of the
southward dropping wave over the Southern Plains may also pose a
localized flash flood threat today. These cells from near the
KS/OK border into far southwest MO and northwest AR will tend to
be slow moving, dropping locally heavy rain. Generally not
expecting much organization to these cells, and given their small
nature, any flash flood threat may stay localized enough to be
covered by just a Marginal risk at this time.

Chenard

...Previous Discussion...

...South Central Oklahoma into North Central Texas...
A north to south axis of above average pw values will continue
over the Southern Plains on the eastern side of the Four Corners
upper high.  Shortwave energy dropping south in the mid to upper
level north to northwesterly flow over the Southern Plains will
support additional generally north to south oriented convection
from eastern Oklahoma into northeast Texas.  Early morning
activity oriented northwest to southeast over east central
Oklahoma may persist for a short time after 1200 utc prior to
weakening.  Concerns will be for additional activity to form in a
general north to south direction late Saturday afternoon over
eastern Oklahoma and drop south Saturday night into early Sunday
into north central Texas.  Not a lot of confidence in hi res
guidance handling of this convection..with these models having a
bias over the past few nights of being too slow to form the
convection and too slow to push it downstream to the south and
southeast.  There will be potential for training of cells in a
north to south or northwest to southeast direction from whatever
additional activity does form from late Saturday afternoon into
the early morning hours of Sunday.  In areas of training...hourly
rates of 2"+ possible.  FFG values are relatively high across
large portions of the Southern Plains...with only a slight risk
depicted for the training potential.

...Southwest...Southern High Plains...Southern Rockies...
The upper high center will remain over the Four Corners this
period with slow moving shortwaves rotating around its
peripheries.  Expect another round of widespread...potentially
slow moving scattered convection affecting northern to eastern
Arizona...much of New Mexico into south central Colorado.  The
previous marginal risk area was expanded slightly into far
southeast Arizona...far southwest New Mexico and eastward along
the NM/TX border. A slight risk area was also added across
portions of eastern NM where an axis of anomalous pw values...1 to
2 standard deviations above the mean will be sinking south in
association with post frontal easterly flow.  Upslope flow into
terrain from in the vicinity of LVS to ALM will support potential
for heavy totals.  These areas are also in an axis of relatively
low ffg values.

...Appalachians...
Widespread scatters diurnal convection will likely fire through
the Appalachians Saturday afternoon.  While confidence is low with
qpf details...there is potential for locally heavy rains and
isolated runoff issues.  A marginal risk area was maintained from
far north GA through western NC...Southwest to western VA...far
eastern WV...western MD...central to northeast PA into southeast
NY state. This area corresponds well with the axis of the higher
HREF neighborhood probabilities for 1, 2 and 3"+ precip day
1...with the greatest threat likely in the 1800 utc Saturday to
0000 utc Sunday time frame.

Oravec


Day 2
Valid 12Z Sun Aug 04 2019 - 12Z Mon Aug 05 2019

...MARGINAL RISK OF FLASH FLOODING OVER PORTIONS OF THE SOUTHERN
ROCKIES AND UPPER MIDWEST...

...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 at or 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 holding steady farther north in portions of Utah and Colorado
where scattered afternoon and evening pulse convection is once
again anticipated in and near the complex terrain...with local
enhancement to rainfall rates in regions of upslope flow from
orographic ascent.  Over areas that have lower flash flood
guidance, some runoff problems may develop and a Marginal Risk
area was maintained with only minor modifications from the
previous Day 3 forecast.

...Upper Midwest...
Hoisted a Marginal Risk area mainly for the eastern portion of
North Dakota and a small portion of Minnesota in vicinity of a
cold front making its way into and across the region.  The concern
for Flash Flooding will be in the event that the front becomes
aligned east to west late in the period which would allow for
repeat convection and training.  The area ahead of the front will
be characterized by CAPE values on the order of 2000 to 3000 J/kg
and convergent midlevel flow.  If the front becomes aligned east
to west, continued southerly flow will be forced isentropically as
it moves north of the boundary.  WPC QPF and model QPF is pretty
modest here, but the HREF has shown some potential rainfall rates
approaching FFG values...and the area has been slow to dry after
recent rains.

Bann

Day 3
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.  With highest
Precipitable Water values closest to the Rockies front range and
lesser values over the interior portion of the Great Basin...am
expecteing the best chance for heaviest rainfall in and near the
complex terrain along the front range nearer the higher PWs (at or
above 1 inch) and low level upslope flow helps the forced ascent
of the deeper moisture.

...Central Plains...
Opted to issue a Marginal Risk area over portions of Nebraska as a
cold front approaches from the north.  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.  The atmosphere should be unstable enough to support
strong updrafts...with pre-frontal Precipitable Water values in
the 1.8 to 2.0 inch range on Monday evening which is nearly 2.5
standardized anomalies greater than climatology. Thinking is that
the better potential for heavy to potentially excessive rainfall
will be later in the forecast period.

...Southeast U.S....
Parts of the Carolinas will be in proximity to a boundary that
will provide the focus for locally intense rainfall rates late in
the day on Monday into late night Monday/early morning hours on
Tuesday.  The HREF showed some low probabilties for heavy rain.
The big mitigating factor is the Flash Flood Guidance values are
high in the reason, suggesting that there may be some problems
with ponding in low lying and areas of poor drainage...but the
risk of Flash Flooding is Marginal.

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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