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FOUS30 KWBC 020012
QPFERD

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

Day 1
Valid 01Z Fri Aug 02 2019 - 12Z Fri Aug 02 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN KANSAS...

The latest trends indicate a west nudge to the axis of expected
rainfall, following the 18z NAM Conus Nest, 18z Canadian regional
GEM, and 18-22z RAP/High Res Rapid Refresh. So the moderate risk
was taken out of MO and nudged west towards central KS. The 22z
RAP model run is showing development later tonight towards morning
in eastern KS within a 700 mb theta-e advection and
convergence/300 mb divergence maxima.

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 tonight appears a bit lower than
the more predictable event last night. Convection over the High
Plains this evening will progress eastward across NE/KS, with
another potential north northwest to south southeast oriented
training axis developing overnight. The risk was increased
slightly in western KS as activity moves east out of CO and NE
into the moist inflow, with dewpoints in the mid to upper 60s
helping fuel cell regeneration.

Petersen/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.








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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