Quantitative Precipitation Forecast
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FOUS30 KWBC 232018

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 PM EDT Fri Aug 23 2019

Day 1
Valid 16Z Fri Aug 23 2019 - 12Z Sat Aug 24 2019


...East Coast...
Progression of a deep layer trough through the eastern U.S. will
bring gradual height falls and an environment favorable for
thunderstorms ahead of a slowly sinking cold front. Models have
been fairly steady in their placement of an axis of locally heavy
rainfall over southern Virginia and much of central and eastern
North Carolina. This speaks to the synoptic forcing, with
energetic westerly flow and moderate CAPE likely to result in some
organized storm modes and efficient regeneration of cells along
boundaries. Predicted cell motions do favor some west to east
training at times, and precipitable water values are well above
even the summer norms, at near 2.00 inches. Models easily crank
out hourly rain rates greater than 2.00 inches today, and
scattered event totals above 4 inches. The southern Mid Atlantic
and North Carolina have not been especially wet lately, so soils
are not much of a factor, but the intense short term rain rates
are likely to cause rapid surface runoff in support of a Slight
Risk of excessive rainfall.

...Far Eastern Oklahoma and Northwestern Arkansas...
Conditions will remain favorable for locally heavy rainfall
through Friday evening into the overnight.  Model signal for
locally heavy amounts has increased, with the consensus of the 12Z
hi-res guidance showing convection redeveloping near the northern
Oklahoma-Arkansas border and western Arkansas 00-12Z Saturday.
Guidance shows showers and storms developing once again along an
axis of deeper moisture (PWs 1.75-2 inches) - supported by
increasing southerly flow into a nearly stationary boundary
extending across the region.  These storms may impact regions
already affected by recent heavy rains and where flash flood
guidance are relatively low.  For the 16Z update, a Slight Risk
was added to far eastern Oklahoma and northwestern Arkansas -
centering over a region where the 12Z HREF Mean 40km neighborhood
probabilities are largely in excess of 50 percent for additional
amounts of 2-inches or more during the 00-12Z Saturday period.

Thunderstorms with heavy rain are expected once again along an
effective frontal zone in the lower Mississippi Valley and
southern Plains. Post-frontal upslope comes into better focus
today along the central High Plains. And an area of height falls
ahead of an increasingly active northern stream upper jet should
promote additional activity in the northern Plains. Instability
becomes especially focused over eastern Colorado and adjacent
NE/KS down into northwest Oklahoma. QPF signal has increased over
these areas. Southeasterly 850-mb inflow strengthening to 30 knots
should slow down cell motion, at least until upscale growth
occurs. With PW running well above an inch in NE/KS and around an
inch at the high elevation in eastern Colorado, there seemed to be
plenty of reason to upgrade to a Slight Risk here - especially as
14-day and 30-day antecedent rainfall is well above average from
highway 96 northward.

...South Florida...
An easterly wave positioned over the Bahamas will drift into the
Florida peninsula Friday and Friday night. With inverted troughing
setting up through the center of the southern FL peninsula by
midday, convection forming in the moderately unstable air near the
trough axis will drift west to west-southwest. This could force a
scenario in which convergence and cell mergers increase near the
Gulf Coast sea breeze over southwest Florida this afternoon.
Eventually cell motions to the west and/or exhaustion of the
instability should yield a change in character, but perhaps not
before rain rates of 1.00 to 2.50 inches per hour cause some rapid
accumulation of water and potential flash flooding.

Meanwhile, steady southeasterly inflow will set up along the
southeast coast of Florida, and this is where hi-res models place
some of the heaviest rainfall overall. Solutions such as the 00Z
NAM CONUS Nest and 00Z WRF-ARW2 depict training convection over
the population centers in southeast Florida during the nighttime
hours, and if this sets up onshore there could be some isolated
5-plus inch totals through 12Z Saturday.

...Western Gulf Coast...
In the early morning issuance we chose to maintain a sliver of a
Marginal Risk area over coastal southeast Texas and southern
Louisiana. This primarily covers thunderstorm activity expected
early in the day as coastal convergence occurs along the leading
edge of a rich tropical airmass marked by precipitable water
values over 2.25 inches. Ongoing cells may then merge with a few
other cells forced by the sea breeze later in the morning. Most
guidance depicts a decrease in activity by afternoon as outflows
push the focused convergence both offshore and also inland toward
a more stable environment. Flash flood guidance values are
somewhat lower than usual in this region owing to spotty heavy
rainfall as of late. Still, the duration and intensity of the
event today seems only to warrant Marginal Risk probabilities.

Slow moving storms produced reported flash flooding late Thursday
in southeast Arizona. A repeat is possible Friday as deep layer
moisture content and wind profiles remain relatively unchanged.
The outlined Marginal Risk area is based primarily on QPF output
from the NAM CONUS Nest, WRF-NMMB, and the University of Arizona
WRF. The NCEP hi-res models generate hourly rain rates of a little
more than an inch later today. Some mesonet observations around
southeast Arizona sampled shorter term rates that equate to at
least 1.5 inch per hour on Thursday evening.


Day 2
Valid 12Z Sat Aug 24 2019 - 12Z Sun Aug 25 2019



20Z update... A slight uptick of PW from previous forecast (1.75")
will be drawn into the Central Plains where a shortwave energy
will be passing through the frontal boundaries. Moderately strong
CAPE values of 2000 to 4000 J/kg will be present from the
foothills of the Southern/Central Rockies to the Central Plains.
Height falls over the western portions of Kansas and Oklahoma will
support more QPF over this area than the previous forecast, as
such the Slight Risk was adjusted accordingly.

Previous Discussion... Moisture and instability focused on frontal
boundaries across the Plains during Day 2 will feed convection
that produces heavy to excessive rainfall during Day 2. For the
most part, there was generally good model agreement with the
overall synoptic setup, so the WPC QPF and Excessive Rainfall
Outlook were based on a blend of the 00z ECMWF/GFS (as well as
some high resolution guidance, especially across the Central and
Southern Plains).

A strengthening long wave trough extending across the Northern
Plains pushes a cold front across the Northern Plains during Day
2. Ahead of the trough and surface front, a 35 knot low level
southwest flow transports 1.50 inch precipitable water air (which
is about two standard deviations above the mean) across SD into
ND. Model soundings showed 500/1000 J/KG of MLCAPE in a narrow
ribbon near the low level jet, and convection is expected to
develop in the instability axis between 24/18z and 25/00z. For the
most part, the convection should remain progressive ahead of the
low and mid level trough, and this could mitigate the flash flood
threat to some degree.

Most of the 00z model solutions showed an area of 0.75/1.25 inches
of rainfall here. Three hour flash flood guidance is as low as
1.50 inches over portions of
SD/ND, so there is at least a low end flash flood threat. Based on
that, a Marginal Risk was placed over the Northern Plains during
Day 2.

The greater threat is across portions of KS into OK, where strong
instability and deep moisture support convective clusters or an
MCS along a frontal boundary extending from the northern TX
Panhandle into southeast KS. Short wave energy ejected from a
strengthening long wave trough across the western High Plains
provides synoptic scale lift across the frontal boundary, mainly
after 25/00z. Ahead of the short wave, there is a multi model
signal for greater than 3000 J/KG of MLCAPE stretching across KS
into OK and AR during this time, which becomes elevated toward
25/06z. Convection forming in the presence of strong instability
is likely to be outflow driven initially.

However, as the synoptic scale increases, and the low level jet
strengthens (to 35/45 knots across western OK and western KS), the
convection is expected to begin upscale growth, probably forming
an MCS over south central KS, which tracks into northeast OK. The
low level jet injects 1.75 inch precipitable water air (which is
about two standard deviations above the mean) into the system,
which could support hourly rainfall rates near 2.00 inches. This
is most likely on the southwest flank of the MCS, where the best
threat for training occurs. There is a multi model signal for
2.00/3.00 inches of rainfall in this area. Three hour flash flood
guidance values are as low as 1.50 inches across KS/OK, and the
MCS could pose a threat where the guidance values are this low.
Based on the above, a Slight Risk was placed over portions of
KS/OK for Day 2.

...Central Gulf Coast...

20Z update... The latest guidance continues to keep most of the
highest QPF over the Gulf, however the EC and NAM solutions
indicate some higher amounts may edge into the periphery of the
Houston/Galveston metro area. The Slight Risk was extended west
into portions of Jefferson and Chambers counties to reflect the
new QPF.

Previous Discussion... The southern end of a long wave trough
extending from the western High Plains interacts with tropical
moisture and instability to feed convection that produces
excessive rainfall across southern portions of LA during Day 2.
There was good model agreement with the overall synoptic setup. so
the WPC QPF and Excessive Rainfall Outlook were based on a multi
model blend (including the 00z WRF ARW and 00z NAM CONUS Nest).

A deep plume of tropical moisture (with precipitable water values
near 2.25 inches, which is between two and three standard
deviations above the mean) resides along the Central Gulf Coast
during Day 2. The best instability in the moisture plume (with
MLCAPE values between 1500/2000 J/KG) is expected to be in about
the same location, with the greatest CAPE values along and south
of Interstate 10. The combination of instability and moisture
should fuel mainly diurnal convection, with the best concentration
near the Gulf Coast.

As lift associated with the southern end of the long wave trough
across the Gulf Coast, it could allow the convection to become
somewhat better organized. There is a model signal for surface
development along the coast (there is not a consensus location,
though the high resolution guidance favors western LA) along the
LA coast. If this occurs, the low level convergence to the east of
the surface development could favor banded convection moving
ashore along the immediate
coast. While the regional/global guidance is not particularly
heavy with rainfall amounts where, the 00z WRF ARW indicated the
potential for local 6.00 inches of rainfall over south central LA
(close to where banded convection could come ashore).

The potential for training with banded storms over portions of the
coast that have wet soils poses an enhanced flash flood threat,
especially over southwest and south central coastal LA. Based on
this, and after collaborating with WFOs LCH/LIX, a Slight Risk was
placed along the LA coast, mainly along and south of Interstate
10, for Day 2.

...Mid Atlantic/Southeast...

20Z update... Model guidance continues to show the Georgia to
North Carolina corridor as the most favorable area for heavy to
locally excessive rainfall given the deep moisture and instability
drawn into the frontal boundary. Several locations have areal
averages of 1/1.25" with very localized maximums of 3 to 6 inches
possible. Little to no changes were made to the Marginal Risk
area. As a side note, an area of low pressure near/offshore the
coast of Florida is progged by the National Hurricane Center to
become better organized, possibly developing tropical
characteristics) tonight/Day 1 period. This will help transport
additional deep moisture near the coast of Florida/Southeast.
Although the bulk of the heavy rain will likely stay offshore,
portions of the Florida/Georgia Coast may get locally
heavy/excessive rainfall.

Previous Discussion... A frontal boundary extending from the NC/SC
border into northern GA becomes the focus for deep moisture and
instability that feeds convection capable of heavy to locally
excessive rainfall over the area during Day 2.

Deep moisture (with precipitable water values near 2.25 inches,
which is between two and three standard deviations above the mean)
pools along and south of the frontal boundary extending from the
southern Mid Atlantic into the interior Southeast. Mainly
diurnally driven instability (with MLCAPE values of 1500/2500
J/KG) should be sufficient to support scattered to broken
convection extending from NC/SC into northern and central GA.
Forcing is weak over the region, with the best mid level support
over the Northeast states, so storm motions are expected to be
less than 10 knots along the entire stretch of the front.

The deep moisture could support hourly rainfall rates near 2.00
inches, especially were cell mergers or short term training
occurs. There is a multi model signal for local 3.00 inch rainfall
amounts. Three hour flash flood guidance values are generally
above this value, so the flash flood threat could be limited by
relatively dry antecedent conditions. Based on this, a Marginal
Risk was extended along the front for Day 2. If later model runs
support higher rainfall amounts, a Slight Risk could be needed in
subsequent forecasts.


Day 3

The Day 3 outlook will be updated by 2030Z.

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


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