FOUS30 KWBC 020827
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Fri Aug 2 2024

Day 1
Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST, OHIO VALLEY, AND NORTHEAST, ALONG WITH THE EASTERN 
TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS...

...Eastern U.S....
Slowly advancing upper trough and surface front will focus broad-
scale ascent for scattered-numerous convective elements across 
much of the eastern CONUS. Two areas of particular concern include
a W-E corridor from SE MI and the OH Valley eastward to NJ and 
southern NY, where the deep-layer forcing (DPVA) ahead of the
opening mid-level low will be a bit more robust. The combination of
favorable deep-layer instability, especially during peak heating
hours (MLCAPEs 1000-2000+ J/Kg) along with TPW values of 1.75-2.00+
inches will make hourly rainfall rates of 1-2+ inches likely
underneath the strongest cells, with potentially higher hourly rates
(2-3" farther east toward eastern PA, NJ, and southern NY given 
the closer proximity to the max TPW and low-level moisture 
transport axes.

Farther south, the guidance is also favoring eastern portions of
the TN Valley into the southern Appalachians of northern GA-western
NC for more widespread convection and an enhanced risk of flash 
flooding (i.e. Slight). This given the upslope low-level component
along with a more vigorous vort lobe traversing the area.
Elsewhere, any flash flooding is expected to be more
isolated/localized, thus the Marginal Risk areas. 

...West...
With the upper level ridge continuing to build across the
Intermountain region and monsoonal moisture lingering near the
International Border (precipitable water values up to 1.75", 
another round of late day/evening showers and thunderstorms is 
expected. Continuity was generally maintained, though we did expand
the Marginal Risk across the Sierra Nevada, Transverse Ranges, and
adjacent foothills. Hourly rain totals to 2" should be possible. 
Locally heavy rainfall would be most problematic in any area burn 
scars and dry washes/arroyos.

Hurley


Day 2
Valid 12Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTH FLORIDA ALONG WITH PARTS OF THE MID ATLANTIC AND SOUTHEAST 
COASTAL PLAIN...

...Eastern U.S...
A generally wet and unsettled pattern is likely, with ample
moisture (precipitable water values up to 2" for most, 2.25-2.50
across FL along/ahead of the tropical disturbance) and instability
in place as a slow moving upper trough and surface/frontal systems
all support numerous concentrated areas of convection. 

Across FL, per collaboration we have hoisted a Slight Risk across 
parts of South FL given the uptick in QPF ahead of the tropical
disturbance. While most of the models and ensemble means keep the 
max areal-average totals off the SW FL coast (3-5+ inches), TPWs
reaching 2.4-2.6 inches along with the possibility of more 
destabilization east of the tropical distrubance's center will make
for the potential of very intense rainfall rates (2.5-3.0+ within
an hour) underneath the more isolated stronger cells. 

Farther north into the Mid Atlantic, inflow at 850 hPa of 25 kts 
implies some level of convective organization is expected. Hourly 
rain totals up to 2.5" should be possible here, and local amounts 
in the 2.5-5" range are advertised by the guidance, which could 
easily fall within an hour or two. The guidance has come in better
alignment in advertising an axis of heavier rainfall across western
portions of the coastal plain and eastern Piedmont over the
Carolinas into parts of SE VA. This given the infusion of deep-layer
moisture surging north of the tropical disturbance toward the mid-
upper level trough and surface front. Per collaboration, have
included a Slight across this region as well. 

Elsewhere to the north (north of Richmond VA), there remains 
uncertainty in placement of the higher amounts, and a decent 
section of the Washington D.C./Philadelphia megalopolis has 
descended into drought, raising flash flood guidance values fairly 
highly. Therefore still believe the Marginal Risk continues best
over these areas for the time being. Urban areas are at the 
highest risk for flash flooding in this region.

...Southwest...
Monsoonal convection over the Southwest (precipitable water values
up to 1.75" near the International Border) continues to support a
Marginal Risk area. Locally heavy rainfall would be most problematic
in any area burn scars and dry washes/arroyos. 

...Sierra Nevada into Oregon Cascades, Blue Mountains, and parts 
of the Northern Rockies to western portions of the Great Basin...

Anomalous moisture (TPW anomalies topping 3-3.5 standard deviations
above normal per the GEFS) will advect northward through the West
on the backside of the upper ridge and ahead of the shortwave
trough off the PAC NW coast. The guidance gets TPW values up to
1-1.25", which would be most anomalous in terrain. Given these
moisture profiles, along with the diurnal heating (MLCAPEs peaking
at least ~500 J/Km), spotty hourly or sub-hourly rainfall rates aoa
1" will make for localized runoff issues, especially over burn
scares. 

...Upper Midwest...
The guidance is generally showing a modest signal for heavy
rainfall in and near Minnesota this period. Precipitable water
values rise about 1.25" with a warm advection pattern. This led to
an introduction of a Marginal Risk here, as flash flood guidance
values are modest after recent heavy rainfall.

Hurley


Day 3
Valid 12Z Sun Aug 04 2024 - 12Z Mon Aug 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE
FLORIDA PENINSULA AND PARTS OF THE SOUTHEAST COASTAL PLAIN...

...Eastern U.S....
The Slight Risk across FL and the Southeast was expanded a bit,
based on the growing consensus in terms of the forecast track and 
forward speed of the tropical disturbance. Areal-average QPF per
the smart blends/ensembles (including the NBM) is up to 3-5" over 
parts of the FL peninsula. This given the highly favorable 
environment (deep layer moisture, tall/skinny CAPE profiles) along
and ahead of the tropical disturbance. Per collaboration with the 
WFOs, have expanded the Slight Risk area, but for now given  
uncertainty by Day 3 in terms of the eventual track/speed/strength 
of the potential tropical system, have held off in hoisting a 
Moderate Risk.

...Montana into the Northern Plains and Upper Mississippi 
Valley... Aforementioned axis of anomalous TPW out West during Day
2 will ultimately get absorbed in the upper level jet streak and 
mid- level vort lobe that will cross MT and the Dakotas on Day 3. 
Dynamical forcing along with the thermodynamical profile will favor
more widespread showers and thunderstorms, though the shear 
profiles will certainly favor more progressive/forward-propagating 
cell motions and thus make for more of an isolated flash flood 
threat. Areas across northwest MT, including Glacier NP, would be 
the most susceptible given the lower FFG (1" or less within 3 hours
for most areas). An upgrade to Slight may be needed over this 
region in later outlooks, especially if we start seeing a consensus
of heavier QPF from the models. 

Hurley
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