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FOUS30 KWBC 311950
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 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
Valid 12Z Fri Aug 02 2019 - 12Z Sat Aug 03 2019

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

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

...20Z Update...
Expanded the Slight and Marginal Risk area to account for
uncertainty.  Also nudged the Moderate Risk slightly south as
model guidance has come into better agreement in terms of the
position of the MCS. The 12Z NCEP models were first on board
trying to erode the ridge with the MCS activity, which seemed to
make sense given the ridge is not all too strong.  In addition,
with this weakness being created the troughing in the east tries
to build west.  Therefore, this would shift the mid-level impulses
farther south and west in time.  The 12Z ECMWF came on board with
the UKMET and CMC showing similar signals. Therefore, confidence
increased in terms of shifting the highest QPF south and west.
This is reflected in the changes to the Moderate Risk. Though a
lot of uncertainty remains given the MCS tonight, on Day 2 and any
lingering residual boundaries.  This will all play a role in terms
of antecedent conditions across a portion of this region.

...Previous Discussion...
Eastern KS and western MO appear to be in the path of multiple
days of MCSs along and north of a front boundary extending from KS
into eastern OK, posing an increasing threat for significant flash
flooding here during Day 3. There was generally good model
agreement concerning the overall synoptic setup, though there was
some longitudinal differences in the placement of the highest
rainfall amounts. In an attempt to mitigate some of the
differences, the WPC QPF and Excessive Rainfall Outlook was based
on a multi model blend.

An MCS is expected to be in the weakening phase across eastern KS
into western MO early on Day 3 as the low level jet weakens,
during the diurnal instability minimum. After the debris
cloudiness clears, an axis of 1500/2500 J/KG of MLCAPE is expected
to develop near and just north of the frontal boundary across KS
into OK (though there is some spread on when this occurs because
of the morning cloudiness). Scattered convection develops in the
axis of instability through 03/00z.

After that time, the low level jet increases to 30 knots and back
to the south in advance of short wave energy dropping southeast
down the mid level ridge located over the Rockies into the
Southwest. The combination on increased moisture with the
strengthening low level jet (as precipitable water air jumps to
2.00 inches, which is about two standard deviations above the
mean) and some synoptic scale ascent is expected to induce the
development of an MCS over eastern NE before 03/06z. Once the MCS
forms, it will track along the best instability axis over eastern
KS and western MO in the 03/06z and 03/12z time frame.

On the southwest side of the MCS, where the best threat for flash
flooding is expected (as the propagation vectors align with the
mid level flow), hourly rainfall rates could exceed 2.00 inches.
There is a solid model signal for 3.00/5.00 inch rainfall amounts
along this axis, with the 00z NAM showing amounts closer to 9.00
inches over southeast KS. As mentioned earlier, there is some
longitudinal differences with the placement of the highest
rainfall amounts, but two day rainfall totals ending 03/12z could
top 6.00 inches over portions of eastern KS into western MO. Since
soils will be compromised by MCSs during Day 1 and 2, the
additional rainfall poses a significant flash flood threat in
these areas. After collaborating with WFOs
OAX/TOP/EAX/ICT/SGF/TSA, a Moderate Risk was placed here for Day
3.


...Southern and Central Rockies...

...20Z Update...
Previous forecast looks good with very minor changes to QPF and
ERO.

...Previous Discussion...
Deepening moisture on an upslope flow across portions of the
Central and Southern Rockies combines with marginal to moderate
instability to produce storms capable of heavy to locally
excessive rainfall during Day 3.

An easterly to northeast low level easterly flow banks 1.00/1.25
inch precipitable water air (which is about two standard
deviations above the mean) up across the terrain and plains of
eastern CO into eastern NM, mainly before 03/00z. Differential
heating should favor the terrain for convective initiation in the
02/18z to 02/21z time frame, and the low level upslope flow could
hold storms in the terrain before they become more outflow
dominated and move out of the terrain in the plains.

The airmass could support hourly rainfall in excess of an inch,
especially across eastern CO. While there could be some short wave
energy dropping through the ridge position, the convection could
be mostly scattered after leaving the terrain, with short term
training and cell mergers resulting in hourly rates this high.
Three hour flash flood guidance values are as low as 1.00/1.50
inches, particularly from the Front Range across southeast CO.

The lower flash flood guidance opens a window of opportunity for a
low level flash flood threat through about 03/03z, after which
time the activity is expected to wane as the instability is
exhausted. Based on this, a Marginal Risk was placed over much of
the eastern portion of CO and NM for Day 3.


...Mid Atlantic/Southeast...

...20Z Update...
Minimal changes made to QPF/ERO as the previous forecast looks on
track.

...Previous Discussion...
A weak frontal boundary extending from the Mid Atlantic coast into
the lower TN Valley during Day 3. The front will act as a focus
for deep moisture and moderate instability to pose a low end flash
flood threat across much of the Mid Atlantic into the Southeast
states.

A broad and weakening mid level trough extending from the Ohio
Valley to the central Gulf coast will allow lapse rates to steep
over a large portion of the Mid Atlantic and Southeast, mainly
during the afternoon and early evening hours. Model soundings
across the region showed between 1000 and 2000 J/KG of MLCAPE,
peaking before 03/00z. Along and south of the front lies a large
reservoir of 1.75/2.00 inch precipitable water air which is tapped
by developing convection. The convection could form into clusters
where outflow initiates additional activity, but for the most
part, with weak forcing, the storms should be scattered to broken
in coverage.

In this airmass, hourly rainfall rates could approach 2.00 inches,
especially where storms train or cell mergers occur. For the most
part, three hour flash flood guidance values are 2.50 inches or
higher, but heavy rainfall during Day 3 could lower these values
in spots. Without a clear model signal favoring one location over
another for flash flooding, a large Marginal Risk was placed over
much of the Mid Atlantic and Southeast for Day 3.


...Florida...

...20Z Update...
No changes made to the Marginal Risk ERO with only minor changes
made to the QPF. Previous forecast is on track with several models
showing very heavy rain near the south peninsula of Florida with a
lot of uncertainty remaining.

...Previous Discussion...
Deep moisture and a low level convergent flow across south FL
during Day 3 will support convection that is capable of producing
heavy to locally excessive rainfall. There is an increasing model
signal for either a trough or weak surface circulation to develop
just off the southwest FL coast during the first part to the
period. The low level east to southeast flow supports convergent
bands of convection across much of south FL, where steepening
lapse rates in association with the weak long wave trough
extending into the central Gulf of Mexico could allow the storms
to be more long lived.

Storm motions are expected to be fairly slow, with a general
southwest to northeast motion under 10 knots. This could support
cell mergers, as well as short term training. Any storms training
in the plume of 2.25/2.50 inch precipitable water air (which is
about three standard deviations above the mean), which could
support hourly rainfall rates between 2.00/3.00 inches. Some of
the regional/global models suggest that local 4.00/5.00 inch
rainfall amounts are possible, and given the depth of moisture
expected to be in place. these values seems plausible.

Three hour flash flood guidance values are generally above 3.50
inches across much of south FL. The exception here is southwest
FL, where values are as low as 2.50 inches. These numbers could be
modulated by heavy rainfall during Day 2, and parts of the
peninsula have seen 300+ percent of normal rainfall over the last
seven days. Based on this, a Marginal Risk was placed over the
southern third of the FL Peninsula for Day 3.

Hayes/Pagano



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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