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FOUS30 KWBC 310821
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
421 AM EDT Wed Jul 31 2019

Day 1
Valid 12Z Wed Jul 31 2019 - 12Z Thu Aug 01 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
ARIZONA, NEW MEXICO & LOUISIANA...

...Southwest...
Mid-level ridge centered east of the 4 Corners Region will remain
nearly stationary through tonight, continuing to drive greater
than average moisture for late July into the Southwest. Meanwhile,
upper level divergence maxima
continue to revolve around the closed mid/upper high, crossing
much of southern and central AZ tonight.
PWATs as high 2.2" will drift north across southern Az in response
to persisting low/mid level southerly flow.

Across southern and central AZ, a Slight Risk remains due to the
high available moisture along with the lifting from the upper
divergence and possibility of merging outflow boundaries to
provide additional lift.

The high-res CAMs indicate scattered areas with max 1 hour
rainfall rates of 1.5-2.5+ inches, which given the soils
(percolation vs. runoff rates), would lead to the possibility of
isolated flash flooding.

...Southeast TX...

An area of enhanced upper divergence was rotating around the high
centered over the southern high plains, with showers/storms moving
south across eastern TX in the deep layer northerly flow.
As the activity moves southwest into a region with lower mixed
layer CAPEs as the evening progresses, coverage and amounts are
forecast to decline.  With steady forward progression following
spreading cold pools/outflow boundaries, the duration of higher
rainfall rates is forecast to be limited, so the area was
downgraded to a marginal risk.

....Eastern KY and TN to adjacent southwest VA/southern WV...

Deep-layer instability over the rest of the evening will combine
with upper divergence maxima ahead of the upper trough to produce
lift in the pre-frontal moist environment. There is expected to be
occasional mergers of cells and clusters that produce brief
periods of heavy rain or activity over areas that received earlier
heavy downpours, resulting in isolated flooding concerns.  Short
training bands are expected to be the main heavy rain threat in
this area.

Roth/Hurley/Petersen



Day 2
Valid 12Z Thu Aug 01 2019 - 12Z Fri Aug 02 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN
COLORADO INTO WESTERN NEBRASKA...AS WELL AS THE CENTRAL PLAINS
INTO THE MID MISSISSIPPI VALLEY...

...Southwest/Rockies...
There are two areas of concern for flash flooding across the
Southwest into Rockies and northern High Plains during Day 2.

The first is across portions of AZ/southeast NV into UT and far
western CO. The best synoptic scale forcing is expected to exit
the region during the first part of Day 2, but deeper moisture and
instability remain behind the short wave energy. Cloudiness early
could affect how quickly instability rebounds across the area, but
differential heating would suggest that the terrain could support
convective initiation first during the late morning into early
afternoon.

Between 500-1500 J/KG of MLCAPE is expected along the terrain from
central AZ across UT into western CO, peaking between 01/18z and
02/00z. Storms developing in the axis of instability tap the 1.00
inch precipitable water air in place, which could support hourly
rainfall rates nearing an inch (per the 00z NAM CONUS Nest) over
the terrain. These rates are high enough to support an isolated to
widely scattered flash flood threat in these areas, mainly through
02/06z. Based on the above, a Marginal Risk was stretched from AZ
across UT into western and central CO.

The second area of concern is across eastern CO into the high
plains of western NE. As a frontal boundary becomes banked up
along the Front Range in CO, the low level east to northeast flow
transports 1.00/1.25 inch precipitable water air (which is about
two standard deviations above the mean) across eastern CO,
southeast WY into far western NE. Model soundings showed a ribbon
of 2000 J/KG of MLCAPE from eastern CO into western NE, and
convective initiation is expected after 01/18z. The upslope flow
could result in slow cell motions with the storms in the terrain,
which track across the plains after cold pools become established.

Before that occurs, the deepening moisture could support hourly
rainfall rates nearing 1.25 inches (which is depicted by some of
the 00z high resolution guidance), especially across eastern CO
and western NE. The 00z NAM CONUS Nest was the most aggressive
with rainfall amounts, with spotty 3.00 inch amounts in these
locations (which seems plausible where cell mergers or short term
training occurs). Considering that three hour flash flood guidance
is as low as 1.00/1.50 inches over portions of eastern CO, there
appears to be an enhanced flash flood threat here. With this in
mind, a Slight Risk was extended across eastern CO into western NE
for Day 2.


...Central Plains into Mid Mississippi Valley...
Short wave energy tracking over the ridge into the eastern portion
of the Central Plains and Mid MS Valley interacts with deepening
moisture on the nose of a strengthening low level jet and
instability to form an MCS that tracks across eastern KS into
western MO, mainly after 02/06z. While there is generally good
model agreement with the overall synoptic setup, there are some
west/east differences with respect to the placement of the highest
rainfall axis. In an attempt to mitigate these differences, both
the WPC QPF and Excessive Rainfall Outlook were based on a multi
model blend.

Remnants of an MCS crossing eastern KS into western MO early on
Day 2 could produce locally heavy rainfall, as the low level jet
weakens and instability become exhausted. As the morning debris
cloudiness exits, an axis of 2000/3000 J/KG of MLCAPE is expected
to develop along and north of the front extending across KS into
far eastern OK. The instability should be sufficient to support
scattered convection in the instability axis before 02/00z,
generally on the edge of the mid level capping. The storms are
expected to be scattered enough to pose a low end flash flood
threat.

As short wave energy tracking from MT and the western Dakotas
drops southeast, the low level jet backs and strengthens over
eastern NE into eastern KS. The 35/40 knot low level southerly
flow injects 2.00 inch precipitable water air into the instability
axis. In the presence of synoptic scale lift associated with the
short wave energy, an MCS is expected to form over eastern NE
between 02/00z and 02/06z, which moves southeast along the edge of
the cap. On the southwest side of the MCS, where the propagation
vectors align with the 850/400 mb flow, resulting in training.

Hourly rainfall rates near 2.00 inches are possible where training
occurs, before the MCS moves southeast and becomes more outflow
dominated. There is a multi model signal for local 2.00/3.00
inches rainfall amounts with system. Three hour flash flood
guidance values are generally above 2.50 inches, but heavy
rainfall during Day 1 could pre conditional the soils, resulting
in a higher flash flood threat. Based on the above, a Slight Risk
was placed over eastern KS into western MO. Much of the high
resolution guidance indicated the highest rainfall totals over
eastern KS, but this may be too far west, depending on the
placement of the best instability axis.


...Mid Atlantic...
Deep moisture and instability pool along a frontal boundary across
the Mid Atlantic states during Day 2. As the best mid level
support for the frontal shears out to the north across New
England, the front is expected to slow, and could become quasi
stationary across portions of NC into after 01/18z. The moisture
and instability should be sufficient to support at least scattered
storms along and south of the front, where MLCAPE values are
expected to range between 1500/2500 J/KG.

Increasing warm cloud depths could allow storms to be fairly
efficient rainfall makers, and given the
2.00+ inch precipitable water axis along the front should support
storms capable of producing hourly rainfall rates near 2.00
inches, especially over far southern VA into much of central NC.
Storm motions could be less than 10 knots, especially across
western VA/western NC, which may foster an environment for short
term training and cell mergers. A weak long wave trough extending
from the Ohio Valley to the central Gulf Coast would result in
steepening lapse rates, but overall, there may not be sufficient
shear for cell maintenance.

There is some regional/global model support for local 3.00 inch
rainfall amounts, particularly where training occurs. Three hour
flash flood guidance values are generally between 2.50/3.50
inches, and training in the higher moisture airmass could allow
some locations to approach these values. Based on this, a Marginal
Risk was placed over much of southern VA and NC for Day 2.


Hayes

Day 3

The Day 3 outlook will be updated by 0830Z.


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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