FOUS30 KWBC 300802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Tue Jul 30 2024

Day 1
Valid 12Z Tue Jul 30 2024 - 12Z Wed Jul 31 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE MID MISSISSIPPI VALLEY, LOWER OHIO VALLEY, UPPER TENNESSEE
VALLEY INTO THE SOUTHERN APPALACHIANS...

...Mid Mississippi Valley, Lower Ohio Valley, Upper Tennessee
Valley and Southern Appalachians...

Another day of active convection likely in the broad west 
northwesterly mid to upper level flow from the Mid to Upper 
Mississippi Valley, into the Ohio Valley, Tennessee Valley and 
Central to Southern Appalachians. Shortwaves in this flow will be 
pushing through an area of above average PW values, 1 to 1.5+ 
standard deviations above the mean. While there is a lot of model 
qpf detail spread, there is consensus for a northwest to southeast 
oriented axis of potentially heavy rains across these areas. The 
two previous slight risk areas were consolidated into a more 
elongated slight risk area that corresponds well with the latest 
HREF mean and the higher HREF neighborhood probabilities for 1 and 
2"+ amounts and the axis of HREF EAS probabilities for 1"+ amounts.
There has been some overlap in the slight risk area of precip over
the past 48 hours and potential for additional heavy rains in this
axis. 

...Central Appalachians, Mid-Atlantic into NY State...
Much above average PW values, 1 to 2.5+ standard deviations above 
the mean also expected to push northeast from the Central 
Appalachians, across the Mid-Atlantic and into NY State. There is 
a lot of qpf spread in the latest model suite, but consensus for 
potential for widespread moderate to locally heavy totals. HREF 
neighborhood probabilities are high for 1"+ amounts, but drop off
significantly for 2"+ amounts. The HREF EAS for 2"+ amounts is 
showing probabilities of less than 5%, illustrating poor overlap in
HREF heavy rain areas. A marginal risk is depicted across these 
areas, with the previous issuance slight risk along the northeast 
PA/Southern Tier of NY removed.

..Northern Rockies...

Only some small changes made to the marginal risk area across 
portions of the Northern Rockies from northern Idaho into southwest
Montana. Latest guidance continues to show potential for an 
organized area of moderate to locally heavy precip ahead of a fast 
moving vort pushing east from the Pac NW into the Northern Rockies 
in the 1200 UTC Tue to 0000 UTC Wed period. The marginal risk
corresponds to where the HREF neighborhood probabilities show
spotty high values in the 1200 UTC Tue to 0000 UTC Wed period.
The overall quick movement of the vort should keep precip amounts
from being very heavy, but anomalous PW values 2 to 2.5 standard
deviations above the mean will support some short term rainfall
rates of 1 to 1.5" per hour, supporting potential for isolated
runoff issues in areas of steep terrain and over burn scars.

...Southwest...
No changes expected to the large scale flow with the upper ridge
remaining centered over the Southern Plains. This will again
support shortwaves rotating northward on the western side of this
ridge into the Southwest, supporting scattered convection,
localized heavy rains and isolated runoff issues.

...Southwest Florida...
No significant changes to the marginal risk area over Southwest
Florida. Above average PW values expected to persist across the
southern portion of the FL Peninsula day 1, along and south of the
east to west trof axis stretching from the Atlantic into South
Florida. The marginal risk continues to fit in well with the high
HREF neighborhood probabilities for 1, 2 and 3"+ totals. This 
heavy rain potential will support isolated runoff issues, 
especially in more urbanize regions.

Oravec


Day 2
Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY INTO THE MID WEST AND OVER PORTIONS OF
NORTHERN NEW ENGLAND...

...Upper Mississippi Valley into the Midwest...

The progressive northern stream flow will continue into day 2 with
the next strong area of height falls expected to push eastward 
across the Northern Plains and into the Upper Mississippi Valley. 
There is potential for additional organized convection/heavy 
rainfall to push eastward with these height falls across eastern 
portions of the Northern Plains and into the Upper Mississippi 
Valley as low level southerly flow strengthens into the west to
east oriented frontal boundary expected to stretch from the NE/SD
border, east across northern IA into northern IL. 850-700 mb moisture
flux anomalies into this front expected to rise to 2 to 3 standard
deviations above the mean in the 0000 to 1200 UTC Thursday period.
There are still a lot of model qpf spread with some typical 
latitude differences with the max qpf axis. The day 2 slight risk 
area was drawn to favor the more southern solutions of the NAM, 
NAMNEST and CMC. 

...Eastern New York State into western New England...
The strong height falls moving out of the Central Appalachians
into the Northern Mid-Atlantic day 1, will progress across eastern
NY State into New England day 2. There is a typical amount of qpf 
spread for the day 2 period, but general consensus for potential 
of moderate to locally heavy rainfall amounts in a region of much 
above average PW values ahead of these height falls. A small slight
risk area was added along the VT/NH border where very heavy
rainfall amounts have occurred early Tuesday morning and where
several models show potential for additional heavy rains during day
2. Otherwise,a broad marginal risk area was maintained from the 
previous issuance to capture the heavy rainfall spread in the 
models across this area day 2.

...Southwest...
Similar to the day 1 period, the mid to upper level ridge will 
remain centered over the Southern Plains. Scattered convection 
again likely over portions of the Southwest as shortwave energy 
again rotates northward on the western side of this ridge and into 
the Southwest. Localized heavy rains and isolated runoff issues 
again possible for southeast Arizona into southwest to central New 
Mexico.

Oravec


Day 3
Valid 12Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE UPPER MISSISSIPPI VALLEY, ACROSS THE LOWER GREAT LAKES, OHIO 
VALLEY INTO THE CENTRAL APPALACHIANS AND ACROSS PORTIONS OF THE 
SOUTHWEST AND SOUTHERN CALIFORNIA...

...Upper Mississippi Valley, Lower Lakes, Ohio Valley... 
The strong height falls pushing through the Northern Plains into 
the Upper Mississippi Valley day 2 will sink more to the southeast 
into the Mid Mississippi Valley/Mid West region day 3. Additional 
heavy rains likely ahead of these height falls and the associated 
surface low in a region of above average PW values and anomalous 
850-700 mb moisture flux. There are some large model qpf 
difference's with respect to the axis of the heaviest rains. The 
marginal risk was drawn to encompass this range form the northern 
solutions of the UKMET/GEM from southern WI into the southern L.P. 
of MI, to the southern solution of the NAM and EC from southern IL,
southern IN, southern OH, into northern KY. Given the spread and 
low confidence at the moment with which qpf axes is the best, the 
risk level was kept at marginal. 

...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 during day 3. This will help to push the axis of above 
average PW values farther west into southern to central California 
and southern Nevada. This will support an expanding area of 
scattered diurnal convection into portions of central and southern 
CA/southern NV from days 1 and 2. The marginal risk area was also 
expanded westward from the previous issuance to include western AZ,
southern NV into southern to south central CA.


Oravec
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