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FOUS30 KWBC 011938
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
337 PM EDT Thu Aug 01 2019

Day 1
Valid 16Z 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 NORTHEAST OKLAHOMA...

16z Update: Not much change with this update. A narrow significant
rainfall event occurred last night into the morning hours over
eastern KS, with an axis of 6-12" of rain. Confidence in how
things evolve this afternoon into tonight appears a bit lower than
the more predictable event last night. Model spread is increasing
with regards to the magnitude and location of heavy rain tonight.
Convection over the High Plains this afternoon will progress
eastward across NE/KS this evening, with another potential north
northwest to south southeast oriented training axis evolving
overnight. With last nights activity, the NAM nest, ARW2 and NMMB
did a pretty good job with the axis over eastern KS...with the ARW
not handling it well. This makes it a bit easier to downplay the
unimpressive ARW forecast for tonight...however it is worth noting
that the NMMB is also quite isolated with higher totals tonight.
The NAM Nest and ARW2 remain quite aggressive with rainfall
magnitudes with another round of elevated training cells. Given
the moisture convergence and overall setup tonight...tend to favor
something closer to the more aggressive 12z NAM Nest...however as
noted above...confidence is only average. Given the model spread,
saw no reason to make any significant changes to the going Risk
areas...and will continue to monitor. A small scale significant
flash flood event is still certainly a possibility tonight...and
there is some chance that tonights activity could overlap with
last nights...which would make flooding concerns even more
significant. So will need to continue to closely monitor trends
through the day.

Chenard


...Previous Discussion...

...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 FAR SOUTHEAST NEBRASKA, EASTERN KANSAS AND NORTHEAST OKLAHOMA...

...Central/Southern Plains into the Lower Mississippi Valley...

Multiple rounds of mesoscale convective systems (MCSs) will impact
the Central and South Plains into the Lower Mississippi Valley Day
1 through Day 3 as the mid-level ridging continues across the
Plains. An MCS will be on going early on day 2 (Friday morning),
eventually diminishing through the mid/late morning. This round
will place residual boundaries across the vicinity which will act
as a catalyst for the next round of convection later in the period
(Friday night into early Saturday). A very moist airmass, as noted
by precipitable water values above 2 inches (which is over two
standard deviations above the mean), combined with increasing
instability (MUCAPE values >2000 J/kg) will result in hourly
rainfall rates exceeding 2 inches per hour. Models continue to
indicate a strong signal for two day (Day 1 and Day 2) rainfall
totals of more than 5 inches along an axis across southeast NE,
eastern KS into northeast OK. While there is very good agreement
on the overall synoptic setup, there remains uncertainty on the
exact axis of heaviest rainfall with models continuing to show
some longitudinal differences. This is largely due to mid-level
impulses, previous MCS residual boundary placement and
rebounding/axis of instability that develops by which the next MCS
will likely move along.  Given the very heavy rainfall received
over the past 48+ hours within this region (a narrow coordinator
of 8-12+ inches), significant flash flooding could occur. This is
a potentially high impact, low confidence event with the main
question being- will the heaviest rainfall expected on Day 2
coincide with the heaviest rainfall received as of late.  The
coordinator is quite narrow for this to occur and thus, feel we
will maintain the Moderate Risk across across far southeast NE,
eastern KS to northeast OK.


...Southern and Central Rockies...

Deepening moisture within upslope directed flow across portions of
the Central and Southern Rockies along with marginal to moderate
instability (MUCAPE 1000-2000 J/kg) could produce storms capable
of heavy to locally excessive rainfall Friday afternoon into the
evening hours. Storms could produce hourly rainfall rates in
excess of an inch with the potential for short term training and
cell mergers. Storm motions will also be slow around time of
initiation. 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 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 Friday will act as a focus for deep
moisture (1.5-2 inch precipitable water) and moderate instability
(1000-2000 J/kg MUCAPE). Scattered convection is likely to break
out across this region during the afternoon, with the potential
for cell mergers. This combined with the aforementioned
ingredients will result in a  low end flash flood threat. Given
continued uncertainty on where the heaviest QPF will occur, 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
(precipitable waters 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.


Santorelli/Pagano

Day 3
Valid 12Z Sat Aug 03 2019 - 12Z Sun Aug 04 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
EASTERN OKLAHOMA INTO CENTRAL ARKANSAS...

...Southern Plains and Lower Mississippi Valley...

An ongoing MCS at the end of day 2/beginning day 3 will be the
main feature of concern as it tracks southward from southeast KS
into eastern OK Saturday morning. Luckily this next round of
convection is finally going to shift south of the coordinator
where the heaviest rain has occurred and is likely to occur per
Day 1/2.  Deep moisture (precipitable water of 2-2.25),
instability (1000-2000 J/kg MUCAPE) and mid-level vorticity will
allow for heavy to excessive rainfall capable of producing flash
flooding, mainly for the first half of Saturday. There is good
model agreement on the overall synoptic setup with some
uncertainty remaining in terms of overall rainfall amounts and
placement. The flash flood threat is highly dependent on the heavy
rainfall observed from tonight and tomorrow nights MCS.
Regardless, most of the global models indicate 24-hour QPF of at
least 1 to 2+ inches (with a few in excess of 4 or 5 inches in
some locations). Based on this, a general model blend was used for
the Day 3 QPF, though the 12Z NAM was not taken into account given
it is a far eastern outlier. The Slight Risk was maintained and
was shifted a bit westward based on the latest QPF with the notion
that the ridge is expected to break down down across this region
due to the height falls/MCS activity. As a result the mid-level
energy and resultant convection activity should occur farther to
the west, quiescently across eastern OK.

...Southwest...

Monsoonal moisture combined with upslope flow along the favored
terrain will bring another day of convection capable of producing
heavy to excessive rainfall across parts of the Southern and
Central Rockies into the southwest Saturday afternoon into the
overnight. Areal average precipitation will be around 0.5-1.25+
inches along the slopes of the highest terrain. Three hour flash
flood guidance values across this region are low, and given the
potential for heavy rains in the days 1 and 2 period, a Marginal
Risk was maintained across much of New Mexico into central
Colorado, and back into parts of eastern Arizona. With some
uncertainty in terms of amounts and antecedent conditions, an
upgrade to Slight Risk was not made at this update.  However, if
confidence increases and model continuity improves, expect a
Slight Risk area may be warranted for a portion of this area given
vulnerable soils and residual burn scars.

...Central/Southern Appalachians...

A weakening frontal boundary from the southern Mid-Atlantic into
the Tennessee Valley will still be in place on Day 3. Upslope flow
into the central and southern Appalachians may result in enhanced
rainfall along the higher terrain. Areal average precipitation of
0.5-1.5 inches with locally higher amounts are expected as storms
become slow to move across this region. The setup is marginal at
best, but given the boundary will be present during day 2 with
moderate to localized heavy rainfall amounts expected, opted to
maintain the Marginal Risk area on the new Day 3 ERO across the
higher elevations of western North Carolina and Virginia.

...Florida...

An area of disturbed weather/surface trough currently being
monitored by the National Hurricane Center is likely to produce
widespread rain across parts of FL on Day 3. While the heaviest of
the rain should remain offshore of the eastern Florida coast and
the Southeast, models show that some heavier rainfall may extend
into parts of southeast FL and some into southwestern FL. There is
still significant uncertainty with this, and current 3 hourly
flash flood guidance values are quite.  However with precipitable
water values near 2.5 inches, instability and surface convergence
present, continual precipitation activity could result in a
marginal risk for flash flooding. In addition, the FFG values may
come down with heavy rainfall expected during the day 1 and 2
periods, thus the Marginal Risk area was maintained.

Santorelli/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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