Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
558 PM EDT Wed Aug 15 2018

Day 1
Valid 2156Z Wed Aug 15 2018 - 12Z Thu Aug 16 2018

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER WESTERN
ARKANSAS AND EASTERN OKLAHOMA...

2200Z Update...

...Southwest U.S....
The Marginal Risk area for the Southwest U.S. has been expanded
north to include southern UT, a fairly large area of central and
southern NV and also localized areas of the Sierra-Nevada and far
the higher terrain of southern CA to capture the latest satellite
and radar trends. Strong diurnal heating is resulting in areas of
1500+ MLCAPE values across especially areas of eastern NV,
southern UT and most of AZ, with values of as much as 500 to 1000
j/kg seen elsewhere across areas of southern CA. While surface
dewpoint depressions are rather large and as much as 45+ degrees,
the PWATS across the region are running 1 to 2+ standard
deviations above normal with a more concentrated degree of
moisture seen in the 500/700 mb layer. The latest GOES-16 visible
satellite imagery shows numerous slow-moving showers and
thunderstorms developing and expanding a bit in coverage over the
last 1 to 2 hours. This is tending to moisten up the low levels
enough, that the stronger convective cores are becoming more
efficient in producing heavier rainfall rates. The convection will
continue to be scattered to broken in coverage going through 00Z,
and will be capable of producing heavy rainfall rates that will
locally exceed 1 inch/hr. As a result, some isolated flash
flooding will be possible, with an emphasis on burn scars, dry
washes, and slot canyons.

...Lower Mississippi / Lower Missouri / Lower Ohio Valleys...
An upper trough will lift slowly northeastward from the Plains to
the Midwest states. A corridor of stronger deep layer flow around
the southern and eastern periphery of the trough will coincide
with a plume of sub-tropical moisture / PW values about 1.5 to 2.0
standard deviations from climatology / and sufficient lapse rates
to support moderate CAPE values and associated heavy rainfall. In
recent days throughout the country the areas of organized rainfall
have been largely tied to the synoptic forcing, as instability is
limited outside of these areas. The hi-res models have done a good
job of depicting the local concentrations of heaviest rain.

Will maintain a small Moderate Risk across portions of far eastern
OK into western AR. Continue to see training convection, with
locally heavy rates across this area. May continue to see some new
development here until the shortwave clears. Given the wet
antecedent conditions, the continuation of the Moderate Risk is
warranted until the wave clears the area allowing drier air aloft
to move in.

Diurnally driven convection will continue to stream northeastward
across the broad Slight risk area stretching from AR into IL/IN.
Locally intense rates will pose at least a localized flash flood
risk into the evening hours across this axis. Did shrink the
northern extent of the Slight Risk based on recent observational
and 12z ARW/ARW2/HRRR trends. Still concern for some localized
more extreme rainfall amounts occurring within the Slight Risk
area overnight. A more focused training scenario could evolve
overnight as we increase moisture transport into the confluent low
level flow. Thus possible that an additional Moderate Risk area
may need to be considered. With a preference towards the 12z
ARW/ARW2 and recent HRRR runs, the most probable location for
these localized 5"+ amounts is somewhere from northern AR,
northeast along the OH River into far southeast MO, far southern
IL, and far western TN/KY. However confidence in both the
magnitude and exact placement is too low at this time to warrant
an upgrade. With that said, this system does have a history of
producing localized small scale extreme rainfall amounts, and
another occurrence of that overnight is certainly possible over
the aforementioned areas...so will continue to closely monitor.

Scattered convection with locally heavy rates is also occurring
near another shortwave moving across IA/IL. This activity should
be less organized and more pulse in nature. Thus an organized
flash flood threat is less likely...although slow cell motions and
PWs of 1.75" does suggest that a localized flash flood threat
exists. Thus will maintain a Marginal Risk across portions of
IA/IL/WI.

...Southern Plains / Rockies...
A northwest flow event with residual post-frontal upslope moisture
will support scattered to numerous thunderstorms from eastern
Colorado and southwest Nebraska down through northern New Mexico.
A heavier concentration of cells, including some mergers and
upscale growth capable of producing heavier short term rain rates
is forecast in the corridor of stronger deep layer flow leading
out near the Raton Mesa down to the OK/TX panhandles. Would appear
like most of this activity will remain progressive enough to limit
the flash flood threat, but given a continued signal amongst the
high res guidance for locally higher amounts, think maintaining
the continuity of a Slight is the best course of action for now.

...Central Plains...
A tight and slow moving vort max is cause for some concern over
South Dakota and northern Nebraska today. Locally intense
convection continues to move across the SD/NE border and will thus
maintain a small Slight Risk. To the south of the Slight you run
into the Sand Hills, where it is unlikely this event would exceed
the high FFG. While to the north, convection should be more
isolated and less organized in nature...although locally heavy
amounts and some flash flood risk probably does extend into
eastern SD and portion of MN/IA warranting the Marginal Risk.

...Northeast / Mid Atlantic...
As troughing lifts out of New England, the next upstream system
will approach from Canada, focusing a corridor of moderate
instability from northern NY to northern New England this
afternoon. Although deep moisture will move offshore, sufficient
seasonable column moisture remains, and the more robust convection
may produce a quick 1 to 2 inches of rain, especially wherever
terrain influences cause some slow-downs or redevelopment.

Meanwhile, the threat of rain has not been entirely removed from
the severely impacted areas in eastern Pennsylvania and southern
New York. Scattered diurnal convection will produce some brief
moderate to heavy downpours. With ongoing flooding and FFG having
become near meaningless at this point (i.e., less than a quarter
inch needed to produce new impacts), we included a Marginal Risk
here.

Orrison/Chenard/Burke


Day 2
Valid 12Z Thu Aug 16 2018 - 12Z Fri Aug 17 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE MIDWEST AND
MID-SOUTH...

...Eastern Midwest to the Ohio Valley...
Timing of a weak remnant low across the lower Michigan peninsula
is a bit faster in 12Z consensus of the GFS/ECMWF/UKMET.
Precipitable water values around 2 inches ahead of the low are 2
to 2.5 sigma above climatology. This tropical surge will spread
abundant rainfall along and ahead of the advancing cold front with
the heaviest amounts focusing over the Lower Great Lakes where
stronger height falls should overspread the region. The Slight
Risk was spread south along the frontal zone over IN/OH to
Cincinnati and KY.

...Central to Southern Great Plains to the Mid-South...
An upper low opens over the northern plains Thursday with
associated locally heavy rainfall spreading from SD to IA where a
Marginal Risk remains warranted. Farther south, featured best in
the 12Z GFS shifts east across OK Thursday. As the impulse
encounters an increasingly moist and buoyant atmosphere
downstream, some convective complex should emerge by the evening
hours. Given wet antecedent conditions in eastern OK, a Slight
Risk was raised there into AR. Should the north and south
complexes merge, it would be near northern AR which also has wet
conditions currently. Therefore, the Slight Risk spans northern AR
into western TN.

...Arizona into western New Mexico...
High precipitable water air continues to lift north out of the
Gulf of California/Sea of Cortez in a monsoonal pattern. Diurnal
heating over the region should force convection over the higher
terrain of Arizona/New Mexico initially although the weak 700-mb
winds suggest the activity may struggle to push off the mountains
into the lower elevations. Little adjustment was again necessary
to the Marginal Risk area.

Jackson

Day 3
Valid 12Z Fri Aug 17 2018 - 12Z Sat Aug 18 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE SOUTHERN
HIGH PLAINS...LOWER OHIO VALLEY...AND MAINE...

...New England into the Mid-Atlantic...
A surface wave advances from upstate New York toward Upper New
England, pumping 1.75 to 2 inch precipitable water values
northward along a tropical moisture plume. 850-mb moisture flux
anomalies begin pushing the 3 to 4 sigma above average range. The
12Z consensus of the preferred GFS/ECMWF/UKMET focus the best QPF
signal across northern Maine. A Slight Risk was maintained over
this region while a Marginal Risk area was maintained southwest
across the Mid-Atlantic. While QPF numbers are markedly lower,
recent weeks have been incredibly wet suggesting at least isolated
flash flood threats there.

...Ozarks east to the Central Appalachians...
The signal for continued heavy rainfall from the Ozarks to the
Mid-South remains as a frontal zone stalls across the region.
Forecast soundings show quasi-unidirectional flow setting up with
anomalous moisture pooling along it. A max QPF axis generally sits
along and just south of I-64 across sections of the Lower Ohio
Valley where the Slight Risk remains. Given heavy rainfall
expected during preceding outlooks, would anticipate flash flood
guidance values to drop leading into the Day 3 period.

...New Mexico to Texas and Oklahoma Panhandles and southeastern
Colorado...
Forcing shifts eastward from AZ to the southern Rockies Friday as
the high retrogrades west. Expect diurnally forced convection to
first take shape across elevated heating sources before convective
outflows take the activity toward the surrounding lower
elevations. 700-mb winds remain on the weaker side, though east
onto the plains the low level jet gets organized. Given
consistency for 1-3 inches in the 12Z consensus and expected wet
conditions tonight, a Slight Risk was raised from southeastern CO
into the TX Panhandle.

Jackson





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