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FOUS30 KWBC 021616
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1216 PM EDT Fri Aug 02 2019

Day 1
Valid 16Z Fri Aug 02 2019 - 12Z Sat Aug 03 2019

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

16z Update: The Moderate Risk across portions of south central KS
into northeast OK still looks good. Expect another round of
overnight elevated training north to south convection. The
environmental ingredients remain favorable for a narrow axis of
significant rainfall, similar to what has occurred the past few
nights over eastern KS. Some uncertainty remains on the exact
axis. Models did not handle last nights activity over KS all that
well, which does not raise confidence for tonights event. Given
the presence of a wave and pronounced and persistent 850mb
moisture convergence axis...tend to think that tonights activity
will be more organized than some of the high res models are
depicting. Also, based on the past few nights evolution, it seems
plausible that the heaviest axis ends up a bit west of our
expectations, from south central KS into northern OK. For that
reason, have opted to expand the Slight Risk to account for the
uncertainty. Not much change to the Moderate was made at this
time, as it still seems to represent the most probable location at
this point...although will need to monitor, as possible it will
have to be adjusted a bit westward with later updates.

Added a small Slight risk over portions of south central SD. A
wave over this region is resulting in an expansion of slow moving
convection. These cells have been very efficient rainfall
producers, and with their slow movement, and an increasing threat
of mergers given the expanded coverage, some flash flood threat
likely exists. This threat should persist into mid
afternoon...with some signs we could see a weakening of activity
by later this afternoon into the evening hours as instability
erodes.

Expanded the Marginal risk northward into PA to account for the
expectation of expanding convective coverage through the afternoon
hours. Activity should generally stay scattered in nature, however
some merging of slow moving cells could result in a quick 1-2" of
rain in spots...resulting in a localized flash flood threat. The
Slight risk over portions of southwestern VA and neighboring WV/NC
seems pretty borderline. Activity should stay disorganized, and
think the flash flood threat will probably remain quite localized.
However, with cells forming on the terrain and not moving off this
forcing mechanism all that quickly, a quick 1-3" of will be
possible, which should still pose a localized flash flood risk
near and just off the terrain...which is also where FFG is
generally lowest.

Chenard


...Previous Discussion...

...Southeast Nebraska...east central Kansas...northeast Oklahoma...
There is a strong model signal for the upcoming day 1 time period
for a narrow axis of very heavy to excessive rainfall amounts
across eastern portions of the Central to Southern
Plains...stretching from southeast Nebraska...across eastern
Kansas and into northeast Oklahoma.  Early morning convection is
firing again to the northeast of the stationary frontal boundary
across portions of the Central to Southern Plains in the axis of
pw values 2+ standard deviations above the mean.  Hi res model
consensus is for this activity to train in a northwest to
southeasterly direction to the northeast of the stationary frontal
boundary across east central Kansas Friday...possibly weakening
late Friday afternoon before convection re-intensifies Friday
night into early Saturday across southeast Kansas into northeast
Oklahoma.  Training of cells in a northwest to southeast direction
will be a strong possibility...with hourly totals of 1-2"+ and
totals 4-6"+ in areas of training.  HREF neighborhood
probabilities show 60-90%+ for 2"+ amounts...40-60%+ for 3"+
amounts and 20-35% for 5"+ amounts in the moderate risk axis.  The
axis of the slight and moderate risk areas were adjusted westward
from the previous issuance by approximately 20-40 nm.  The axis of
the highest Excessive Rainfall probability is west of the observed
axis of heavy precip over the past 24 hours with the model
consensus suggesting less overlap with the next round of heavy
rains and the previous 24 hour heavy rain axis.

...Portions of the Southwest into the Southern Rockies...
Widespread scattered monsoonal convection likely on the western
peripheries of the Southern High Plains/Southern Rockies upper
high center from portions of the Southwest into the Southern
Rockies.  PW values will remain 1 to 2 standard deviations above
the mean this period across these areas...supporting potential for
locally heavy rains and isolated runoff issues.  The previous
marginal risk area over the Southern Rockies for this period was
expanded into the Southwest to cover the higher neighborhood
probabilities from the HREF for 1 and 2" 24 hour totals.  A well
defined area of 50-80% probabilities of 1"+ totals and area of
10-30% probabilities of 2"+ totals are encompassed by the marginal
risk area.

...Florida Peninsula into the coastal Southeast...
The upper trof over the Central Gulf of Mexico will continue to
transport tropical moisture across the Florida Peninsula into the
coastal Southeast day 1.  PW anomalies 2 to 3 standard deviations
above the mean expected to persist across these areas.  Mid to
upper level circulations on the eastern side of the Gulf of Mexico
trof will accentuate lift in this hi pw axis...supporting
potential widespread area of heavy rain.  Confidence is not great
with qpf details...but given the favorable large scale
pattern...there will be potential for isolated runoff
issues...especially in urbanized areas.  The previous marginal
risk area over the Florida Peninsula was expanded northeastward
into Northeast Florida and the coastal Southeast to cover the
model spread of heavy qpf solutions across these areas.

...Southern to Central Appalachians into the Mid-Atlantic...
There were only some minor changes made to the previous marginal
risk area stretching across portions of the Southern to Central
Appalachians into the Mid-Atlantic.  A widespread region of
scattered convection again likely in the vicinity and north of the
stationary frontal boundary stretching from the Southern
Appalachians into the Southern Mid-Atlantic.  HREF neighborhood
probabilities for 1 and 2"+ amounts are high across these region.
3"+ probabilities are focused across northeast NC into Southeast
VA...but with high ffg values here the risk was kept at marginal.
A slight risk area was added from far northeast TN into far
southwest WV and Southwest VA where ffg values are relatively
lower.  Slow moving cells here Friday afternoon may produce the
best chance of runoff issues across this region.  The hi res
neighborhood exceedance probabilities for 1 and 3 hour ffg values
are well defined in the nam conest 40-70%+ and hi res arw  20-40%
across these areas.

Oravec





Day 2
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...

Still expecting convection to be lingering into the early part of
the Day 2 period across portions of the Southern Plains and the
Lower Mississippi Valley as shortwave energy from the Day 1 period
sags southward.  There should be plenty of moisture
available...with the GFS and ECMWF both depicting Precipitable
Water values in the 2 to 2.25 inch range...with MUCAPES of 1500 to
2500 J/kg.  Overall, the model agreement is good with the
placement and amounts...although the NAM appeared to be suffering
from convective feedback that manifest itself as a much-larger
than consensus QPF and it produced a mid-level low/vort center
much stronger than the other models.  Convergence and dynamics
weaken and CAPES diminish Saturday night/Sunday morning with
models showing a corresponding drop in rainfall amounts/rainfall
rates.

...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 and Southern Great Basin on Saturday afternoon
into the overnight. Areal average precipitation will be around
half an inch, although localized rainfall amounts of 1.00 to 1.25
inches will be possible in the upslope region of the complex
terrain.  Still not confident enough to hoist a Slight Risk.  As
it stands, thinking is that the burn scars areas have the greatest
potential for flooding.

...Central/Southern Appalachians...

A weakening frontal boundary from the southern Mid-Atlantic into
the Tennessee Valley will still be in place on Day 2. 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 and the Precipitable
Water values should generally range from 1.7 to 2.0
inches...moderate to localized heavy rainfall amounts are
possible.  Parts of the Southern Appalachians and adjacent areas
of the Tennessee Valley have received heavy rainfall over the past
few days...so opted to make few changes to the on-going Marginal
Risk area.

...Florida...

An area of disturbed weather/surface trough with its associated
area of rain should generally remain off the Florida
coast...although the atmosphere across the peninsula has deep
moisture and instability that can be released to produce locally
heavy rainfall rates and amounts as the dynamics move across the
peninsula.  Flash Flood Guidance is high...but there could still
be instances of flooding or problems resulting from runoff or
ponding in areas of poor drainage.

Bann


Day 3
Valid 12Z Sun Aug 04 2019 - 12Z Mon Aug 05 2019

...MARGINAL RISK OF FLASH FLOODING OVER PORTIONS OF THE SOUTHERN
ROCKIES AND ACROSS THE SOUTHERN APPALACHIANS...

...Southwest U.S....
Monsoonal moisture will still be present over parts of the
Southwest U.S. on Sunday...although the global models were showing
a drying trend over parts of the Desert Southwest where
Precipitable Water Values will start the period above 1.5 inches
but decrease to under 1.25 inches (and in some cases drop under
1.00 inches).  At the same time, PW values will be generally be
steady farther north in portions of Utah and Colorado.  Thus,
think the afternoon and evening convection will be spreading a bit
northward and eastward compared with QPF for Saturday.  Highest
areal average amounts should be under half an inch...although
local amounts approaching 1 inch are possible in areas of the most
active convection, in regions of upslope flow in complex terrain,
and over burn scars.

...Southern Appalachians...
WPC QPF showed scattered convection on Day 3 with the highest
amounts along and near the spine of the Southern Appalachians and
adjacent portions of the Southeast U.S.. With no real change in
airmass...and with lingering mid-level dynamics...over some
locations where Flash Flood Guidance had been lowered by moderate
to heavy rainfall amounts recently have opted to keep a Marginal
Risk in place from Sunday into early Monday. With light flow in
the low levels and PW values gradually climbining to values at or
slightly above 2.0 over parts of GA and the Carolinas, thinking is
that cells capable of producing locally intense rainfall rates
will move little before dissipating.

Bann



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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