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FOUS30 KWBC 222037
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
437 PM EDT Mon Jul 22 2019

Day 1
Valid 1942Z Mon Jul 22 2019 - 12Z Tue Jul 23 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL APPALACHIANS INTO THE MID ATLANTIC...

...Lower Mississippi Valley, through the Tennessee Valley and
Central Appalachians, into portions of the Mid Atlantic and
Northeast...
The expansive band of Slight to Moderate Risk was generally
maintained with minimal changes on this forecast update,
stretching from Arkansas to southern New England. Among the minor
changes were to extend the length of the Moderate Risk on both
ends: (1) along the Kentucky and Tennessee border to account for
an alignment of ongoing convection in a zone of low-level
confluence and on the northern periphery of a pool of stronger
instability; and (2) from the NYC metro area into southwest
Connecticut as hi-res models have a fairly robust signal for
heavier QPF (2+ inches) into Connecticut. The Slight Risk area was
drawn further northwest in parts of southeast Ohio and southwest
Pennsylvania to account for the position of developing convection
along the front in those areas. The overall forecast reasoning
remains similar, with a ribbon of precipitable water values near
or above the 95th percentile along a gradually advancing front.
HREF probabilities indicate a high likelihood of pockets of at
least 2-3 inches of rainfall over most of the Slight and Moderate
Risk regions, and individual members show widespread rain rates of
1-2 in/hr with isolated maxima approaching 3 in/hr. This seems
reasonable based on the combination of moderate instability and 2+
inch PW values.

Several areas of greater concern exist. First, from eastern
Pennsylvania into the NYC metro area hi-res models generally have
a stronger signal for rain rates exceeding 2 in/hr. The nose of
stronger low-level moisture transport will be focused in this area
as well, and GOES-16 visible satellite imagery indicates limited
cloud cover as of 16Z, so considerable instability should be able
to build (supporting higher rain rates). The second area of
concern is near the Kentucky-Tennessee border, where convection is
becoming aligned with the deep layer mean flow already, and the
inflow region over Tennessee is strongly unstable with SBCAPE
around 3000 j/kg at 16Z. Once again, the stronger instability may
support a concentrated region where higher (2+ in/hr) rain rates
are more likely. Finally, there is concern from far eastern
Kentucky into West Virginia and far southwest Pennsylvania. A
stronger westerly component to the low-level flow may support a
greater concentration of convection in the upslope regions of the
Appalachians, and these areas of terrain can be more vulnerable to
flash flooding. Furthermore, GPS observations just upstream in
south-central Ohio show PW values near or in excess of 2 inches,
which indicates significant, deep moisture impinging on the
Appalachians. Therefore, flash flooding could also be more
prevalent in these areas.

...Portions of the Southwest into the Southern High Plains...
20z update: A Slight Risk was placed over a small portion of south
central AZ, mainly for the evening hours. There is a multi model
signal for convection developing over nearby Mexico to track over
south central AZ after 23/00z. The convection follows a monsoonal
moisture plume (with precipitable water values nearing 1.75
inches, which is near two standard deviations above the mean) on
the broad southeast flow under the mid level ridge in place over
the Southwest.

Being just east of the mid level ridge position, slow northwest
cell motions are expected (generally under 10 knots) into south
central AZ. The slow cell motions could foster an environment for
cell mergers and short term training, which in turn could result
in hourly rainfall rates approaching 1.50 inches (which is
supported by members of the 12z model suite, as well as the last
few runs of the HRRR). This is expected to result in an enhanced
flash flood threat here, so after collaboration with WFO TWC, a
Slight Risk was placed over south central AZ. The threat could
extend into the early overnight hours, as the most recent RAP
indicated MUCAPE values of 2000/3000 J/KG persisting through
23/08z.

Isolated to scattered monsoonal convection is expected across
portions of the Southwest, with some overlap into the southern
High Plains of far eastern New Mexico and west Texas. Three
primary updates were made to the outlook in this region: (1) a
Slight Risk was added in northwest New Mexico to align with a
Flash Flood Watch from the local NWS office and a region of higher
QPF in the HREF blended mean; (2) a Slight Risk was added in
southeast New Mexico into the Permian Basin region of far west
Texas; and (3) the Marginal Risk was extended westward in southern
Arizona into the Phoenix metro area based on recent HRRR and
U-Arizona WRF runs. A MCV was evident on radar and satellite in
eastern New Mexico, dropping to the south. This may focus an area
of convection in southeast New Mexico later today, and hi-res
model QPFs were higher in this concentrated region as well.

Lamers/Oravec



Day 2
Valid 12Z Tue Jul 23 2019 - 12Z Wed Jul 24 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST...

...Southeast Virginia/Carolinas/Georgia...
A cold front will continue to move southeast Tuesday across the
southeastern CONUS before stalling over northern FL Tuesday night.
A low level wave looks to develop over the Carolinas through the
day and provide a focus for thunderstorm development. Moisture and
instability ahead of the front are above normal given the recent
heat. Prefrontal activity has been occurring ahead of this cold
front which will continue Tuesday over eastern VA/NC/SC/GA. Shrank
the Slight Risk area to be around and north of this surface low
track with less in GA. Most models indicate weakening activity
Tuesday evening over southern GA as the front gets strung out, but
a Marginal Risk was maintained there and west to the central Gulf
Coast given the ample moisture despite high FFG and areas of
drought.

...Southern New England...
A wave along the cold front moving off the northeastern seaboard
Tuesday allows a plume of 2 inch PW air to cross southern New
England. This high moisture, along with some instability should
allow some prolonged heavy rain around the north side of the low.
Therefore, a Marginal Risk was introduced for roughly Boston and
south/east.

...Four Corners States...
The ridge centered over the Four Corners brings monsoonal moisture
up from the eastern Pacific and Sea of Cortez across AZ. Air with
a positive 2 to 2.5 standard deviation moisture anomaly lifted
over the AZ terrain with instability and a low mean wind warrants
a flash flood threat. Expanded the Marginal Risk to include more
upslope portions of the Mogollon Rim. A Slight Risk may be
warranted for portions of this area given the continued monsoonal
flow.

Southerly flow up the eastern slopes of the NM/CO Rockies brings
one inch PW with instability over the terrain will set off
afternoon thunderstorms centered on the San Juans of CO. Light
northerly mean flow, counter to the low level flow will make for
particularly slow storm motions, so the Marginal Risk was
maintained.

Jackson



Day 3

The Day 3 outlook will be updated by 2030Z.


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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