FOUS30 KWBC 010710
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Thu Aug 1 2024

Day 1
Valid 12Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST AND OHIO VALLEY...

Midwest/Great Lakes/Southern Appalachians...
The suite of numerical guidance continued to support a Slight Risk area
over portions of the Ohio Valley from Thursday into Thursday night as
fairly impressive forcing approaches the region. Low-level moisture
should be place (precipitable water values up to 2" as the closed
mid- level low drops in from the north...helping to focus moisture
flux convergence along a quasi- stationary front in the area at 
the same time that divergence aloft increases in response to the 
presence of an upper level speed maximum. The low-level inflow and
effective bulk shear should be high enough in magnitude (25-30 kts) 
to lead to convective organization. Hourly rain totals to 2.5", and
local amounts to 5", are possible.  The ingredients still suggest 
a Slight risk area is warranted. Changes to continuity were 
minimal.


...Southwest into southern California...
The upper ridge over the Southern Plains is forecast to build to
the west and northwest across the Rockies, Great Basin and
Southwest. An easterly wave under its base moves south of CA, helping
push the axis of above average precipitable water values (as high 
as 1.75-2" farther west into southern to central California and 
southern Nevada. This will foster an environment in which scattered
diurnal convection will be expanding into portions of central and 
southern California/southern Nevada. Hourly rain totals to 2.5" are
possible, given the above. This would be problematic in area burn 
scars and dry washes/arroyos.

Roth/Bann


Day 2
Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND AROUND
PORTIONS OF CENTRAL & EASTERN KENTUCKY...

....Ohio & Tennessee Valleys...
The upper pattern agrees on the approaching trough and focused 
ascent over the Ohio Valley into the Mid Atlantic. Precipitable 
water values up to 2" are advertised with this system. The low-
level inflow and effective bulk shear should be high enough in 
magnitude (25-30 kts) to lead to convective organization. The 
available ingredients suggest hourly rain totals to 2.5" and local 
amounts to 5" would be possible. However, there is much spread in 
scattered convective signals underneath the mean trough. The 
environment is conducive for locally heavy rains anywhere from the 
Central Midwest through the Central and Northern Mid Atlantic with 
the primary likely falling under the center of the upper 
circulation and mean trough. This pinpoints areas like Ohio and 
Western PA down into WV/KY as the best chance for convective 
coverage capable of more flash flooding capabilities.  Much is 
contingent on the pattern evolution and with time leading into the 
setup, but the mass fields along with the GFS/NAM suggest that 
portions of central and eastern KY should be most favored; added a 
Slight Risk in this area to address that concern which was
coordinated with the JKL/Jackson KY and LMK/Louisville KY forecast
offices. The previous Marginal Risk east of the Mississippi was 
expanded slightly, but remains fairly close to continuity. 

...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.
Hourly rain totals to 2" should be possible.  Locally heavy 
rainfall would be most problematic in any area burn scars and dry
washes/arroyos.

Roth/Kleebauer/Bann


Day 3
Valid 12Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
SOUTHWEST, OREGON, MINNESOTA, FLORIDA, & IN AND NEAR THE MID-
ATLANTIC STATES...

...In and near the Mid-Atlantic States...
A generally wet and unsettled pattern is likely with ample 
moisture (precipitable water values up to 2" and instability in 
place as a slow moving upper trough and surface/frontal systems all
support areas of showers and thunderstorms. 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. As there remains
uncertainty in placement of the higher amounts, and a decent
section of the Richmond/Washington D.C./Philadelphia megalopolis 
has descended into drought, raising flash flood guidance values 
fairly highly, believe the Marginal Risk continues best 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. Expanded the area
westward to account for the signal seen in the 00z Canadian
Regional which implies a risk as far west of the Peninsular Ranges
of CA.

...Florida Peninsula...
Moisture and instability increase as a tropical disturbance moves
in the vicinity of the FL Peninsula. Precipitable water values rise
to 2.25"+, and plenty of instability resides offshore over the warm
waters. There has been a westward shift in the guidance over the 
past 24 hours more into the Gulf of Mexico due to its slower than 
anticipated (in model land) development, and the best indication as
to the center of any low- to mid-level vorticity has been near
Puerto Rico.  With the potential for 3" an hour totals and a 
possibly better organized system to enhance rainfall potential, 
went ahead and broadened the existing Marginal Risk area to 
encompass the FL Peninsula for the time being to account for the 
uncertainty. 

...Minnesota...
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.

...Oregon... 
Moisture advecting northward through the West gets drawn into the 
leading edge of an upper level trough between southwest OR and 
northeast WA. Both the GFS and NAM get the precipitable water 
values up to 1-1.5", which would be most anomalous in terrain. 
There is some reflection of the trough in the 850-700 hPa wind 
pattern (the NAM appears more bullish on the low-level convergence 
prospects). Considering the weakness of the upper level feature, 
it shouldn't preclude daytime heating/diurnal insolation, so there
should be decent instability as well. The concern is highest in 
the terrain, particularly near any burn scars. This led to a new 
Marginal Risk area across portions of OR, which have the best QPF 
signal, albeit not that high. Hourly rain totals to 2" are 
considered possible.

Roth
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