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FOUS30 KWBC 221943
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
342 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/Hayes



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 south and east Tuesday shifting
off the NC coast by early Wednesday as a longwave trough pivots
through the OH/TN Valley.  Moisture and instability will pool
along and just south/east of the front allowing for pre-frontal
convection to likely develop Tuesday afternoon/evening across
eastern portions of VA/Carolinas and Georgia. There is strong
evidence for heavy rainfall as seen by the multi-model signal for
high QPF totals, especially across the sandhills/coastal plain of
VA/NC/SC.  The timing of the front will certainly play a role in
terms of convective initiation and the evolution through late
afternoon/evening, thought models have come into fairly good
agreement.  All indications are that the precipitable water values
will climb above 2 inches and with pockets of MUCAPE values around
3500 J/kg, the ingredients point to storms being heavy rainmakers.
Hourly rain rates may exceed 1.5 inches. Mid-level energy moving
atop the surface front combined with the right entrance region of
the upper level 125kt jet will only act to enhance lift across
this region.  Given this, fully expect a round or two of
convection with the potential for training as noted by the mean
flow becoming better aligned with the corfidi vectors through the
afternoon/evening.

Areal average precipitation of 1.5-2.5+ inches with locally higher
values can be expected. While there is strong multi-model signal
for a large footprint of heavy precipitation, with fairly high FFG
values across this region, feel this will be a mainly urbanize
flash flooding threat. Therefore, trimmed the Slight Risk to
account for the best potential for localized flash flooding.

While there was previously an effort made to maintain continuity
between Day 1 and Day 2 Slight Risk areas, there appears to be a
region during the overnight Monday night into Tuesday morning
where convection will diminish.  Stronger and more vigorous
convection will redevelop Tuesday afternoon where the Slight Risk
then comes into play.

...Southwest/Southern Rockies...

With the cold front well south of the area and high pressure
continuing to build in from the north, easterly flow will enhance
upslope convection across the higher terrain.  Low to mid-level
moisture will still be present as noted by precipitable water
values of 1-1.25. Instability will climb to over 1500 J/kg MUCAPE.
 And with slow storm motion just after convective initiation, this
will exacerbate the potential for flash flooding across this
region, especially if heavy rainfall falls over residual burn
scars.  Therefore a Marginal Risk was maintained across portions
of the higher terrain in AZ/NM/CO.

Pagano



Day 3
Valid 12Z Wed Jul 24 2019 - 12Z Thu Jul 25 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF FLORIDA AND GEORGIA AS WELL AS THE SOUTHERN ROCKIES...

...Northern FL/Southeastern GA...

A surface front will become stalled across the region and off the
southeast coast early Wednesday. With a decent moisture and
instability gradient present combined with mid-level impulses
riding atop the aforementioned surface boundary, convection will
likely develop with efficient rain makers.  Precipitable water
values of 2+ inches will be transported by 15-25 knot
southwesterly winds.  Marginal MUCAPE values of 1000+ J/kg is
noted across this region Wednesday afternoon.  Within the upper
levels, the jet streak will slowly sink south allowing the right
entrance region to become positioned over southern GA into
northern FL, promoting additional lift.  Mean wind around 20 knots
from the southwest will become aligned with corfidi vectors in
terms of both speed and magnitude for a period of time Wednesday
afternoon/evening which could result in convective training.  As a
result, isolated flash flooding is possible, with high flash flood
guidance taken into account.  Therefore, only a Marginal Risk was
introduced.

...Southern Rockies...

Mid/low-level moisture, easterly upslope flow and marginal
instability will continue to promote convective initiation along
the higher terrain of CO/NM Wednesday afternoon/evening.  With mid
level high pressure anchored over the four-corners region and weak
impulses rounding the ridge, anticipate this will promote
additional lift across this region.  Convection developing along
the slopes may be slow to move off the terrain, but looks as if
there may be a bit more storm motion than seen in previous days.
Regardless, heavy rainfall is possible with some of these storms
which if over residual burn scars could result in localized
flooding.  Therefore, a Marginal Risk was added for this region.


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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