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FOUS30 KWBC 302008
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
407 PM EDT Tue Jul 30 2019

Day 1
Valid 1814Z Tue Jul 30 2019 - 12Z Wed Jul 31 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
ARIZONA, NEW MEXICO, TEXAS, & 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,
a mesoscale/convectively aided vort max over northwest Mexico
south early this morning continue to revolve around the closed
mid/upper high, tracking up along the eastern Gulf of California
and toward southwest AZ Tuesday night.  PWATs as high 2+" will
pool near the Mexico/AZ border before advecting northward in
response to persisting low/mid level southerly flow along with the
aforementioned shortwave rotating around the periphery of the
ridge.

Across southern and central AZ, a Slight Risk remains due to the
uptick in both areal-average QPF along with HREF/experimental HREF
probabilities of >1"/hr rainfall rates. While not over the same
area, all 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
more scattered vs. isolated flash flooding.  Recent coordination
with FGZ/the Flagstaff AZ forecast office led to some refinements
to the northern portion of the slight risk area.

The other area of concern would be across southeastern CA and
southern NV. Areal-average QPF is quite low over this region,
however given the pooling of greater deep-layer instability on the
western periphery of the deeper cloud canopy, expect an isolated
flash flood risk to branch out a bit farther west-northwest than
the latest guidance would suggest.


...Western-Central Gulf Coast northeast through the TN and OH
Valleys, Appalachians, Lower Great Lakes, and interior portions of
the upper Mid Atlantic and Northeast...
A broad Marginal Risk was noted ahead of the slow-moving upper
trough/surface cold front. Thermodynamics are rather modest --
with PWs of 1.5-1.75" north and near 2.00" south high for this
time of year. MUCAPEs between 2000-3000 j/kg from the Gulf Coast
through TN Valley would support more intense rainfall rates, as
evidenced by the higher 1-2+ inch hourly rainfall probabilities
per the HREF and experimental HREF.  The low-level inflow/moisture
flux into the frontal boundary, with 0-6km bulk shear values near
25 kts, could be problematic.  However, a lack of a mid-level
capping inversion implies productive cold pools/outflow boundaries
and thus limit the duration of the more intense rainfall rates.

A localized excessive rainfall/flash flood risk is expected across
most areas, though the most likely area for a more organized heavy
rainfall threat would be across eastern TX into much of LA and far
southwest MS per a multi-CAM blend, recent radar trends, and the
available ingredients (sufficient effective bulk shear and higher
instability).  There remains a sufficient heavy rainfall signal to
justify the Slight Risk, which was expanded to cover the Houston
Metropolitan Area per coordination with the HGX/Houston-Galveston
TX forecast office.

Farther north (OH Valley-upper Mid Atlantic and areas north),
deep-layer instability will be sufficient (MUCAPEs 2000-3000 j/kg)
with stronger forcing ahead of the upper trough (enhanced upper
divergence and low-level frontogenesis via the right entrance
region of the broad upper jet streak) fostering widespread,
organized pre-frontal convection, especially with the orographic
enhancement on the western slopes of the Appalachians.  Due to the
mobility expected, short training bands are expected to be the
main heavy rain threat in this area.

Roth/Hurley



Day 2
Valid 12Z 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 CENTRAL PLAINS...

...Four Corners Area...
Large mid-level ridge spinning nearly in place will continue to
drive Monsoon flow into the Southwest. The southerly flow on the
western periphery of this ridge will continue to drive robust
moist advection into the region, with PWATs climbing above 2" in
southern Arizona, and as high as 1.5" into central Utah. These
values represent anomalies as much as 3 standard deviations above
the climatological mean. In addition to this moisture, instability
will increase during the afternoon, potentially rising as high as
2000 J/kg. However, this will likely be dependent on how much
residual cloud cover exists behind a possible morning MCV lifting
through western/central Arizona. At this time expect there will be
at least a slow lag in heating to drive instability, but not
enough to preclude sufficient MUCape during the afternoon to drive
convection.

In this favorable thermodynamic environment, one or two shortwaves
of varying intensity will lift clockwise around the periphery of
the ridge to enhance lift, aided by the presence of the favorable
RRQ of a jet streak lifting off to the northeast. These features
together will provide deep layer ascent, which in the moist and
unstable environment will likely produce scattered to widespread
thunderstorms across the area.

Although storm motions may be a bit too quick for widespread flash
flooding noted by 0-6km mean wind of 10-15 kts, HREF probabilities
for 1"/hr rain rates are as high as 60%, 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, further
expanding the potential for flash flooding. The Slight Risk has
been extended both South and North of the previous issuance to
best overlap the high PWATs to the south and better synoptic
forcing to the north. This was coordinated with
VEF/SLC/FGZ/PSR/TWC, and some further refinement is possible into
day 1 if the focus can be better determined.


...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 the
shortwave aloft 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 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" of QPF
across the area, and low probabilities for this amount in the
ECENS probabilities. Eventually the MCS should become cold pool
dominated as its heads further south, causing faster progression
and a reduction in flash flood potential. However, the strong
model consensus suggests the continuation of the SLGT risk is
needed.


...Northeast...
A long wave trough will drive a cold front into New England during
day 2, 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 forecast to rise as high as 2000
J/kg into New England. 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.

Guidance shows a spotty signal for up to 3" of rainfall where any
training can occur despite fast storm motions, with the best
overlap occurring across NJ. There was some coordination with
PHI/OKX about potentially hoisting a SLGT risk for day 2, but due
to fast storm motions and that the lowest FFG is displaced a bit
north of the highest QPF, a MRGL risk was strong enough to cover
the isolated flash flood risk.

...Gulf Coast of TX/LA...
Residual thunderstorms may persist into day 2 along the Gulf Coast
of TX/LA on the tail end of the baroclinic zone/front.
Additionally, outflows from offshore convection, or residual
boundaries from convection on Tuesday, may serve as a focus in a
very unstable and moist atmosphere. Slow storm motions and rain
rates which may exceed 2"/hr at times suggest an isolated flash
flood risk continuing from day 1 into day 2, and a MRGL risk was
added. The best chance for any flash flooding will be the first
half of the period.

Weiss

Day 3
Valid 12Z Thu Aug 01 2019 - 12Z Fri Aug 02 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID MISSISSIPPI VALLEY...


...Mid Mississippi Valley...
19Z Update: In a pattern that looks to be almost a repeat of day
2, a potent shortwave topping the ridge to the west will dive
southeast across the area. This feature will interact with another
elevated boundary due to an intense 850mb LLJ reaching as high as
40 kts, driving PWATs over 2 inches. This should produce another
MCS dropping along the MUCape boundary, potentially mirroring the
MCS from day 2. There remains some spread among the guidance, but
the overall consensus for another MCS with training potential to
move across soils pre-conditioned from the first MCS. Although FFG
is quite high here, 2.5-3.5"/3hrs, this does not account for
forecast rainfall of 1-3" on day 2, and it is expected that FFG
will be much lower by day 3. For this reason have upgraded the
MRGL to a SLGT for parts of the area most likely to see
back-to-back excessive rain events.

Previous Discussion:
Convection following an axis of instability and deepening moisture
on a backing low level flow could feed clusters or a small MCS
tracking from NE into western MO, mainly after 02/00z. A ribbon of
1000/1500 J/KG of MLCAPE (which become more elevated with time) on
the edge of the mid level capping extends from SD across NE into
western MO, which peaks between 02/00z and 02/06z. The instability
should be sufficient for the development of storms on the axis,
which track southeast with the mid level flow.

There is some model support for short wave energy to track over
the ridge position from the western Dakotas into the Mid MS Valley
during the same time frame, which would allow the convective
elements to become more organized with time. The low level flow
backs TO the south and southeast ahead of the short wave (and the
front boundary or surface trough across central KS), feeding 2.00
inch precipitable water air into the developing convection.
Several regional/global models indicate the potential for an MCS
to form on this axis, tracking across eastern KS/western MO before
02/12z.


...Mid Atlantic...
19Z Update: Moisture and instability pooled ahead of a stalling
front will provide fuel for scattered to widespread thunderstorms
on Thursday. PWATs forecast to climb to 1.75-2 inches are slightly
anomalous, but with instability rising above 2000 J/kg, and mean
cloud layer wind falling to 10 kts or less, slow moving storms are
likely to produce heavy rainfall. FFG across this area is
generally more than 3"/3hrs, but slow moving storms with rain
rates rising as high as 2"/hr could locally exceed this threshold,
especially where any mergers or training takes place. A MRGL risk
was maintained for good model consensus and a signal for pockets
of rainfall of 3" despite basin average that will likely be
considerably less.

Previous Discussion:
Deep moisture and instability pool along a frontal boundary across
the Mid Atlantic states during Day 3. 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.

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.


...Southwest...
A continuation of the Monsoon flow will produce heavy rain
producing thunderstorms on Thursday. The overall forcing is less
robust than Wednesday, but continued high PWAT of 1.25-1.75
inches, and ample instability in weak flow will persist the
potential for flash flooding. The mid-level ridge shifts slightly
eastward, and the synoptic ascent due to diffluence aloft weakens
from Wednesday, but weak mid-level impulses and airmass
instability will still support localized flash flooding where
rates can exceed 1"/hr.

Hayes/Weiss




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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