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FOUS30 KWBC 010844
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
443 AM EDT Thu Aug 01 2019

Day 1
Valid 12Z Thu Aug 01 2019 - 12Z Fri Aug 02 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN KANSAS, WESTERN MISSOURI, AND FAR NORTHWEST OKLAHOMA...


...Central Plains...

Short wave energy tracking over the ridge into the eastern portion
of the Central Plains 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 continues to be some west/east
differences with respect to the placement of the highest rainfall
axis, which certainly could lead to large repercussions in terms
of the greatest flash flood risk. In an attempt to mitigate these
differences, both the WPC QPF and Excessive Rainfall Outlook were
again based on a multi model blend.

Remnants of an MCS crossing eastern KS into western MO early in
the period could produce locally heavy rainfall, though will be on
the wane as the low level jet weakens and the elevated 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 30-40 knot low level southerly
flow injects 2.00+ inch precipitable water air (pre-convective
environment) 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 aoa 2.00 inches are expected where training
occurs, before the MCS tracks more southerly and picks up more
forward momentum (becomes more outflow dominated). Now fully
within the window of the high-res CAMs, there is a multi-model
signal for localized 3-6+ inches of rainfall during the day 1
period (which includes residual rain from early today). Based on
the most recent MCS, three hour flash flood guidance values have
now come down considerably across portions of eastern KS into far
western MO -- with a large area below 2.00" and a narrow stripe of
1.00" or less. Based on the antecedent wet conditions with
additional heavy rainfall forecast, per collaboration with
affected WFOs a Moderate Risk was hoisted over this area.


...Southwest/4 Corners Region/Rockies/Central High Plains...

There are two areas of concern for flash flooding across the
Southwest into Rockies and central-northern High Plains during Day
1.

The first is across portions of AZ/southeast NV into UT and far
western CO. The best synoptic scale forcing will exit this region
early, however deeper moisture and instability will linger 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
1.00-1.25 inch precipitable water air in place, which could
support hourly rainfall rates nearing an inch (per the 00z
high-res CAMS) 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 central AZ and portions of western NM up
through western and central CO.

The second area of concern is across eastern CO-WY into the high
plains of western NE-SD. 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-WY into
far western NE-SD. Model soundings showed a ribbon of 1000-2000
J/KG of MLCAPE from the Front Range into the High Plains, as
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 1.00" (which is depicted by most of the 00z
high resolution guidance), especially across eastern CO and
western NE. Considering that three hour flash flood guidance is as
low as 1.00-1.50 inches over portions of northeast CO, eastern WY,
and western NE-SD, an enhanced (Slight) flash flood risk exists
over this area.


...Lower Mississippi Valley/Southeast/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 1.75-2.00+ inch
precipitable water axis along the front should support storms
capable of producing hourly rainfall rates near 2.00 inches from
the mid Atlantic, much of the Southeast, and toward the central
Gulf Coast ahead of the stationary surface front. 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.

There continues to be multi-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 across these areas.

Hurley


Day 2
Valid 12Z Fri Aug 02 2019 - 12Z Sat Aug 03 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EASTERN KANSAS, NORTHEAST OKLAHOMA, AND FAR WESTERN MISSOURI...

...Central Plains into the Mid Mississippi Valley...

Eastern Kansas to far western Missouri and northeast Oklahoma
continues to be in the path for multiple rounds of convection and
heavy to excessive rainfall during both the day 1 and day 2
period. A weakening MCS at the beginning of day 2 (Friday morning)
will pave the way for a second round later in the period (Friday
night into early Saturday). A very moist airmass (with PWs as much
as two standard deviations above normal) with increasing
instability will be in place and hourly rainfall rates associated
with both MCS's could exceed 2 inches per hour. Models continue to
indicate a strong signal for two day rainfall totals of more than
5 inches along this axis and soils during the day 2 period will be
compromised from the day 1 period posing a significant flash flood
threat. Given this assessment, a moderate risk is in place on the
day 2 ERO across eastern Kansas to northeast Oklahoma, and far
western Missouri. This is a slight west and southward shift from
the previous outlook (yesterdays Day 3) based on the latest model
trends.

While there is very good agreement on the overall synoptic set-up,
there remains uncertainty on the exact axis of heaviest rainfall
with models continuing to show some longitudinal differences. This
is based on how the previous MCS's during day 1 track which in
turn will affect MCS activity on day 2. Given this, a high risk
was not included on this issuance of the ERO, but the potential
for a high risk will need to be monitored for future issuances.


...Southern and Central Rockies...

Deepening moisture within upslope directed flow across portions of
the Central and Southern Rockies along with marginal to moderate
instability could produce storms capable of heavy to locally
excessive rainfall during day 2. Storms could produce hourly
rainfall rates in excess of an inch with the potential for short
term training and cell mergers. Three hour flash flood guidance
values are low (around 1 to 2 inches along the higher terrain)
which leaves the door open for a low end flash flood threat. Based
on this, a marginal risk was maintained from the previous day 3
ERO across parts of central Colorado into northeast New Mexico.


...Mid Atlantic/Southeast...

A weak frontal boundary extending from the Mid-Atlantic coast into
the lower Tennessee Valley during day 2 will act as a focus for
deep moisture and moderate instability. Scattered convection is
likely to break out across this region, with the ingredients in
place for a low end flash flood threat. Given continued
uncertainty on where the heaviest QPF will end up, a broad
marginal risk was continued from the central Mid-Atlantic states
southward to the Southeast, and back into the Central/Southern
Appalachians.


...Florida...

There continues to be a good model signal for either a trough or
weak surface circulation to develop off the southwest FL coast
during the beginning of Day 2/Friday. Deep anomalous moisture (PWs
averaging around 2.25-2.5 inches) and convergent flow across the
state will support bands of convection capable of producing
localized 2-4+ inch rainfall totals, especially across the south
part of the Peninsula. Three hour flash flood guidance values are
generally above 3 inches, with the exception of far southwest FL
where values are as low as 2.5 inches. Given the potential for
heavy rainfall during the day 1 period as well, and the fact that
parts of the state have seen well above average rainfall already,
a marginal risk was continued across parts of central and southern
FL. This is a slight northward expansion from the previous shift
based on the latest model guidance.


Santorelli

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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