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FOUS30 KWBC 232032
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
431 PM EDT Tue Jul 23 2019

Day 1
Valid 1934Z Tue Jul 23 2019 - 12Z Wed Jul 24 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS WESTERN
MONTANA, SOUTH-CENTRAL ARIZONA, THE SOUTHERN MID ATLANTIC,
SOUTHEAST, & CENTRAL GULF COASTAL REGION...

...Southern Mid-Atlantic, Southeast, Central Gulf Coast...
The strong surface frontal boundary will continue to press
southeast Tuesday through the Southeast and southward through the
Lower Mississippi Valley.  The synoptic set up remains very
favorable for widespread heavy precipitation totals ahead of this
boundary.  Across these areas...very favorable right entrance
region jet dynamics will accentuate lift in the axis of pw values
in the range of 1.75-2"+ supporting the likelihood of widespread
heavy precipitation totals.  The 00z guidance showed a well
defined convective squall line across these regions with hourly
rainfall rates of 2-3" possible initially which drops to 1-2" as
the line progresses southeastward -- a solution which appears to
be coming to pass.  Local amounts of 3-5" are seen in the 12z
mesoscale guidance.   The marginal and slight risk areas remain
unchanged from the early morning issuance.  In far south Texas,
outflow boundaries sent from the western Gulf of Mexico could lead
to localized heavy totals of 1-2" are possible. The marginal risk
area was also maintained over southeast New England for Tuesday
morning when a period of heavy rains with rates of .50-1"+ per
hour are possible.

...Southern California...Southwest into the Southern Rockies...
A closed upper high is forecast to remain in place in the vicinity
of the Four Corners on Tuesday.  This will favor the persistence
of above average monsoonal pw values...1 to 2+ standard deviations
above the mean...stretching on the south and southwest sides of
the closed high across the Southwest into Southern California.
With clearing skies, scattered monsoonal convection likely with
isolated heavy totals and isolated runoff issues possible, even
into the valleys of southern AZ due to easterly 700 hPa flow in
the wake of a mid-level disturbance.  Href probabilities of 1"+
amounts highlight the areas from the Peninsular Range of Southern
California...through the Mogollon Rim of Arizona into southern New
Mexico and far Southwest Texas.  The 1"+ probabilities also
highlight the Southern Rockies from north central New Mexico into
south central Colorado.  Coordination with PSR/Phoenix AZ and
TWC/Tucson AZ forecast offices led to the introduction of a slight
risk in south-central AZ.  Short term radar trends suggested that
a northwestward expansion of the Marginal Risk area was
needed...especially given the presence of an MCV over far western
AZ.  Refer to WPC Mesoscale Precipitation Discussion number 667
for details. -Bann

...Northern Great Basin into the Northern Rockies...
A slight risk area in western MT lies within the broader marginal
risk introduced earlier this morning.  An upper trough progresses
inland across these areas Tuesday night/early Wednesday morning,
which could increase effective bulk shear and organize convection.
 ML CAPE is already on the rise, and it's possible convective
initiation would be quite early, close to 18z around the time a
weak upslope flow into western MT is expected.  Model qpfs are not
too impressive across these areas...but scattered convection
likely in the region of well defined upper difluence expected
ahead of these height falls over the Northern Great Basin into the
Northern Rockies.  With pw values rising to 1.5 to 2.5 standard
deviations above the mean Tuesday night/early Wednesday -- into
the 0.75-1.25" range -- isolated totals of 1-2" possible, which
could occur in one hour given the ample moisture available.

Roth/Oravec


Day 2
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 FOUR CORNERS REGION...

Update...The monsoonal moisture streaming into the Southwest will
generate scattered convection across the Four Corner region,
favoring areas of higher terrain. Based on some of the model
trends, increased QPF across portions of Arizona, as well as
southern California. These adjustments largely occurred within the
current area of Marginal Risk fore excessive rainfall, therefore
very little adjustments were made to the outlook itself.

The best moisture convergence, lift and instability across the
southeastern U.S. is forecast to be over northern Florida/southern
Georgia along the stalled frontal boundary. The QPF lowered about
0.25 inch between Tallahassee and Tampa front he previous
issuance, however areal averages remain 1 to 2+ inches across
northern Florida. The southern bound of the Marginal Risk was
expanded southward toward Tampa Bay to compliment the new QPF.

...Northern FL/Southeastern GA...
A cold front will become stalled across the region and off the
southeast coast by 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 are
expected across this region Wednesday afternoon. Within the upper
levels, the jet streak to the north will slowly sink south
allowing the right entrance region to become positioned over
southern GA and 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. This will likely result in
shower/thunderstorm activity training across portions of the
northeast Gulf Coast region along the FL coast between Tampa and
Tallahassee. Given this, there is the potential for isolated flash
flooding with high flash flood guidance taken into account.
Therefore, the Marginal Risk was maintained.

...Four Corners...
Mid/low-level monsoonal flow around the persistent Four Corners
high will provide moisture with instability and upslope flow to
promote convective initiation along higher terrain of the Four
Corners states Wednesday afternoon/evening. Continued light mean
flow will mean slow storm motions and the potential for the
activity to spread off the terrain into the plains. Therefore, the
Marginal Risk was maintained across portions of AZ and the Slot
Canyon region of southern UT. Southerly flow up the eastern sides
of the CO/NM Rockies will again provide moisture to storms firing
over the high terrain, so the Marginal Risk remains centered on
the San Juan mountains.


Pagano/Campbell

Day 3
Valid 12Z Thu Jul 25 2019 - 12Z Fri Jul 26 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF FLORIDA AND GEORGIA AS WELL AS THE FOUR CORNERS REGION...

Update...The most notable change to the QPF is in the Gulf
coast/Southeast region. The trailing end of the stationary front
is closer to the Gulf Coast, which in turn kept the QPF northward
near central Gulf Coast than the previous forecast. A few models
show an area of max QPF of 1 to 3+ inches near/south of New
Orleans, which resulted in the updated QPF of 0.50 to 1 inch near
the Mississippi Delta with an approximately 1.5 inch max offshore.
The middle section of the frontal boundary and associated QPF will
be a bit further offshore thus the back edge of the QPF axis will
hug the coast.

The trough tracking across the divide will provide additional lift
for the convection firing along the Central and Southern Rockies;
increasing the precipitation along this area approximately 0.10 to
0.50 in the highest terrain. The Marginal Risk had minimal tweaks
to account for the QPF adjustment near the outer bounds.


...Northern FL/Southeastern GA...
Rinse and repeat from Day 2 (Wednesday/Wednesday Night) with
respect to the stationary boundary which remains draped across
northern Florida (through Thursday night) which is the continued
focus for shower/thunderstorm activity.  Given this pattern has
shown little movement from the previous 24 hours, any
precipitation that did fall on Day 2 may allow soils to become
slightly more sensitive to additional rainfall.  However, these
sandy soils can take quite a bit of rain with soil recovery quick
to occur.  Regardless, with a continual feed of high precipitable
water across this region, lingering instability and mid-level
impulses moving atop the surface front, expect a couple additional
rounds of precipitation across this region. Areal average
precipitation within this region will range from around 0.75 to
over 1.25 inches. Therefore, decided to introduce a Marginal Risk
in basically the same location as seen on Day 2.

...Four Corners...
Mid/low-level monsoonal flow will persist across the Four Corners
region Thursday afternoon into the overnight as the mid-level
high/ridging provides moisture and impulses which when combined
with instability and upslope flow will promote convective
initiation along higher terrain. Continued light mean flow will
mean slow storm motions and the potential for the activity to
spread off the terrain into the plains. A slight difference with
respect to the synoptic pattern as observed on Day 2, a trough
crossing the northern Rockies will promote additional lift,
especially across the higher terrain in CO/NM.  Therefore expect
higher QPF totals and thus introduced a large Marginal Risk area
across this region. In addition have a Marginal Risk across a
portion of northern AZ into the higher terrain of south UT.

Pagano/Campbell


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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