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FOUS30 KWBC 010818
QPFERD

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

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

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

The Day 2 outlook will be updated by 0830Z.


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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