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FOUS30 KWBC 220819
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Mon Jul 22 2019

Day 1
Valid 12Z 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...Tennessee Valley...Central
Appalachians into the Mid Atlantic...
An expansive slight and moderate risk area was maintained this
period along and ahead of the surface cold front forecast to push
east and southeast from the Mid Mississippi Valley/Ohio Valley
into the Lower Mississippi Valley...Central Appalachians and Mid
Atlantic.   There is a strong model signal for the potential for
widespread heavy precipitation along and ahead of this front where
pw values are forecast to remain 1.5 to2+ standard deviations
above the mean.  The associated amplifying mid to upper level trof
will also provide favorable right entrance region jet dynamics
along the boundary during the first half of the upcoming day 1
time period.  The primary change from the previous outlook was to
expand the moderate risk area that was primarily over the Mid
Atlantic farther to the southwest across WV to the eastern
KY/Southwest VA/northeast TN area.  This was to cover where the
model consensus for heavy totals coincided with lower ffg values.
These areas are also where the higher href neighborhood
probabilities for 2 and 3"+ amounts are...80 to 90%+ for 2" and 50
to 80% for 3".

...Portions of the Southwest into the Southern High Plains,,
Additional widespread scattered convection likely across the
Southern High Plains into portions of the Southwest this period.
Post frontal low level easterly flow will keep pw values well
above average....1.5 to 2+ standard deviations above the mean from
southeast Arizona...across much of New Mexico and into portions of
Southwest Texas.  While model guidance is showing a lot of
variability model to model with qpf details...there is a signal
for heavy to locally excessive amounts.  Neighborhood
probabilities from the href show a large region of high
probabilities of 1"+ amounts across the above mentioned
regions...with much of this region having probabilities over 50%.
For higher precip thresholds...the areal extent of higher
probabilities decreases significantly. However...there are still
areas of 50%+ probabilities of 2"+ totals and a few 20-45% for 3"+
amounts.  Confidence is low with the placement of the probability
maxima...but there is a signal for potentially very heavy amounts.
 The marginal risk area was expanded from the previous issuance to
cover the above mentioned probabilities for 1, 2 and 3"+ amounts.

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 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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