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FOUS30 KWBC 311949
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
348 PM EDT Wed Jul 31 2019

Day 1
Valid 16Z Wed Jul 31 2019 - 12Z Thu Aug 01 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
ARIZONA AND UTAH, AND THE LOWER MISSOURI VALLEY...


...Southwest to the Central Great Basin and Central Rockies...
Large mid-level ridge spinning nearly in place will continue to
drive Monsoon flow across the Southwest. The southerly flow on the
western periphery of this ridge will continue to drive robust
moist advection into the region, with morning PWATs around 2" in
southwestern Arizona, an axis around 1.5" into eastern Nevada as
well as 1.25" in central Utah. These values represent anomalies of
2-2.5 standard deviations above the climatological mean. Despite
the ample degree of deep-layer moisture, the the amount of
instability observed during the day will be the wild card.
Considerable cloudiness is inhibiting strong destabilization, at
least across areas along or near vort lobes rounding the ridge.
Areas along the periphery of the denser cloud shield have a much
better chance to destabilize, hence the western expansion of the
Slight Risk.

Although storm motions may be a bit too quick for widespread flash
flooding noted by 0-6km mean wind of 10-15 kts, the 12Z HREF still
indicates scattered 40-60% probabilities of >1"/hr rainfall rates,
with 10-20% probabilities for 2"/hr. These rain rates across areas
with FFG as low as <1"/1hr, should support flash flooding, and
will be most likely where the mid-day instability peaks in
conjunction with the larger scale forcing. Additionally, some
upslope enhancement is possible in portions of the terrain of
northern AZ into UT and northwest CO, further expanding the
potential for flash flooding.


...Central Plains into the Mid Mississippi Valley...
Guidance is in very good agreement that a shortwave impulse riding
atop the ridge and swinging southeast towards Missouri will spawn
an MCS Wednesday night. This MCS will develop in response to a
compact area of upper divergence out ahead of the vort lobe,
interacting with an elevated confluence boundary on the nose of an
intensifying 850mb LLJ which may exceed 35 kts. This LLJ will
additionally transport ample moisture into this region, with PWATs
progged to rise towards 1.75-2 inches, about 2 standard deviations
above the climatological mean.

This setup will be highly favorable for MCS development, likely to
ride SE along the sharp instability gradient noted by MUCAPE
dropping from nearly 3000 J/kg over KS, to less than 500 J/kg in
eastern MO. Despite rapid motion of the MCS noted by Corfidi
vectors reaching 35 kts, there is some potential for training as
the mean flow becomes parallel to the elevated boundary. This is
reflected by high-res guidance painting streaks of 3+ inches of
QPF across the area, along with HREF probabilities of 40-60% that
greater than 3 inches will fall within 6 hours.

Eventually the MCS should become cold pool dominated as its heads
further south, causing faster progression and a reduction in flash
flood potential. Maintained the placement of the Marginal and
Slight Risks.


...Mid-Atlantic to Northeast...
A long wave trough will continue to drive a cold front toward the
northeastern seaboard through tonight, with forward progression
slowing as the upper trough begins to tilt negatively. Deep SW
flow ahead of the surface boundary will drive warm moist advection
into the region, noted by PWATs exceeding 1.5 inches and MUCAPE up
to 2500 J/Kg. In addition to the low-level convergence, broad
upper diffluence will drive ascent, which in the favorable
thermodynamic environment will support scattered to widespread
afternoon thunderstorms.

12Z HREF continues to depict spotty 2 to 3". However, the lowest
FFG is generally north of the highest progged QPF, so a Marginal
will continue for now.


...Gulf Coast of TX/LA...
Ongoing Gulf breeze convection will persist along the TX/LA Gulf
coast on the tail end of the baroclinic zone/front. Slow storm
motions and rain rates which may exceed 2"/hr at times given such
a highly-favorable thermodynamic environment (4000+ J/Kg SBCAPE)
support an isolated flash flood threat today, thus the
continuation of the Marginal Risk.

Jackson


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/Central Plains...

...20Z Update...
Expanded the Slight Risk to incorporate northeastern CO, eastern
WY, southwestern SD and west NE given the potential for heavy
rainfall to exceed flash flood guidance.  Hourly rain rates may
exceed 1.5 inches leading to localized flash flooding, especially
if these higher QPF values fall over recent burn scars.  Expect
convection to be most impactful into the Plains Thursday afternoon
into the overnight.

...Previous Discussion...
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...

...20Z Update...
Slight Risk was expanded a bit to incorporate uncertainty in terms
of the MCS track and also residual boundaries from the previous
MCS and afternoon/convection. Though models are coming into better
agreement on QPF amounts and potential training across far western
MO into eastern KS, with the hi-res models highlighting Topeka's
CWA.  Based on close coordination with the impacted offices, there
was discussion about a possible upgrade to a Moderate Risk due to
the expected MCS tonight dropping heavy rain in the vicinity.
However, given uncertainty in where the heaviest rain will fall
tonight and the MCS on Day 2, felt the Slight Risk should suffice
at this update.

...Previous Discussion...
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 that could lead to large repercussions for flash
flooding on Day 3. 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...

...20Z Update...
Minor adjustments made to the Marginal Risk with minimal changes
to the forecast philosophy as shown below.

...Previous Discussion...
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/Pagano

Day 3

The Day 3 outlook will be updated by 2030Z.


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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