FOUS30 KWBC 290823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Thu Aug 29 2024

Day 1
Valid 12Z Thu Aug 29 2024 - 12Z Fri Aug 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST AND THE MID ATLANTIC AS WELL AS ALONG THE GULF COAST...

Lower Missouri Valley to the Upper Midwest...

Height falls associated with an upper trough and surface cold 
front will continue eastward today and tonight as a deep mid- 
upper low lifts north into western Ontario by 12Z Fri. Given the 
progression of synoptic features, the bulk of the deep- layer 
forcing and more anomalous low-level inflow/moisture flux will 
pivot across the Upper MS Valley and southern Canada.  HREF
neighborhood probabilities for greater than 2 inch and greater than
3 inch amounts remain fairly aggressive (40-70%) within the
previously issued Slight risk area with the core of the potential 
being located over parts of Iowa and southern Minnesota. As 
mentioned before...this area has been dry as of late, so some of 
the precip will be beneficial, but anomalous moisture and strong 
mid- level ascent could generate rainfall rates capable of 
flooding, especially within any urban footprint.

Mid Atlantic Region...

Maintained the Slight risk area across the VA Piedmont down into 
portions of Southeast VA and as far north as the WV/MD/PA line with
only minor modifications. There will be a fairly well-depicted 
axis of low- level instability and anticipated surface convergence
in proximity to a backdoor front settling in to the area.  A core 
of elevated theta-E will align with the front from northwest to 
southeast during the afternoon hours while a jet streak to the 
northeast catches the Central Mid Atlantic just within the right- 
entrance region of the jet leading to appreciable upper forcing 
during peak destabilization. Convective activity is expected to initiate
across the Blue Ridge and adjacent Appalachian Front then drift 
eastward. various CAMs and global guidance. MLCAPE between 
2500-3500 J/kg will be common within the vicinity of the front, so 
ample surface based instability will be present to maximize 
potential of any thunderstorm development. 

Louisiana and Middle-Upper Texas Coast...

Models show a subtle mid-upper level shortwave lifting gradually 
northward toward the central- western LA Coast today and tonight. 
Subtle upper- level forcing along with persistent (albeit not 
overly robust) low- level moisture transport from the Gulf of
Mexico will result in a localized flash food risk, especially 
coinciding with peak diurnal heating . Precipitable water values 
increase to 2.3-2.4 inches along with MLCAPEs between 1000-2000 
J/Kg and WBZ heights climbing to aoa 15,000 feet will establish an
optimal thermodynamic profile for heavy rainfall as shown in the 
NAM and ARW guidance. This should result in rainfall rates of 2-3 
inch on a sub-hourly time scale will be possible underneath the 
strongest cores. Given that the overall forecast reasoning has not 
changed and the latest and the latest guidance continues to show 3 
to 5+ inch amounts hugging the coastline...saw little reason to 
make more than minor nudges to the Slight risk and Marginal risk 
areas.

Bann


Day 2
Valid 12Z Fri Aug 30 2024 - 12Z Sat Aug 31 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE IMMEDIATE
GULF COASTAL AREAS OF SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA ...

Gulf Coast...

There is a Slight Risk of excessive rainfall along the immediate
Gulf coast region of southeast Texas into a small portion of 
adjacent Louisiana...with a Marginal risk area extending as far 
north as the upper Great Lakes in response to gulf moisture 
streaming northward ahead of a well-defined cold front advancing 
eastward. In addition...a well defined circulation that has 
persisted a couple of days over the Gulf of Mexico and Texas will 
meander eastward into Arkansas. While its impact will be waning as 
the system fills...it still could allow for additional convective 
flare-ups along and near the Gulf of Mexico. Precipitable water 
values of 2 to 2.25 inches are widespread from eastern Texas into 
Louisiana and Arkansas with an axis then extending north and east 
ahead of the front being some 2 to 2.5 standard anomalies above 
climatology being an indication of the moisture-laden atmosphere 
still in place. Discrepancies linger in the guidance on placement 
and amounts in where the deepest moisture gets funneled...with the
above- mentioned upper level feature playing a key role. Between 
the NBM still showing 2 to 4+ inch amounts along the southeast 
Texas coast on Day 2 alone and a more than 6 inches for a 2-day 
total in proximity to the 28/12Z guidance and already covered by 
yesterday's Day 3 outlook...saw little reason to make any changes 
to the Slight Risk area.

Great Lakes to Southern Rockies/Southern High Plains...

Convection will continue to be driven by deep gulf moisture that is
initially drawn northward and then northeastward towards the Great
Lakes ahead of a well defined cold front pushing southward and 
eastward. By 00Z Saturday...moisture anomalies of 2 standardized
anomalies will be in place. The overall upper forcing still does
not appear to be terribly robust but the diurnal instability
supports at least the potential for locally excessive rainfall.

Bann


Day 3
Valid 12Z Sat Aug 31 2024 - 12Z Sun Sep 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG A SMALL
PORTION OF THE UPPER TEXAS COAST AND ADJACENT COASTAL LOUISIANA...

There is a Slight risk of excessive rainfall once again over
portions of the Upper Texas coast and adjacent portions of 
Louisiana with a broad Marginal risk are extending northward and 
eastward as gulf moisture streams northward ahead of...and 
eventually intersects with...a cold front. The 29/00Z suite of 
guidance focuses highest rainfall amounts from Saturday into early 
Sunday over portions of the Upper Texas coast and nearby Louisiana 
where a moisture plume with precipitable water values in excess of 
2.25 inches should be positioned at the start of the period (a 
nearly 2.5 standardized anomaly greater than climatology for this 
time of year and above the 90th climatological percentile along the
gulf coastline shown by both the ECENS and GEFS). Two inch 
precipitable water values are forecast to extend from eastern Texas
northeastward in a corridor to far southern Indiana. During the 
ensuing 24 hours...the northern portion of the front reaches 
portions of New England and the Mid-Atlantic region but the 
southern portion of the front makes little progress into Texas and 
Arkansas. Expectations are that downpours will be possible along 
and ahead of the front. Given the amount of moisture and weaker 
steering flow, though, the southern portion of the Marginal area 
has a better chance for excessive rainfall while storms to the 
north should be more progressive leading to a nominally low/more 
isolated excessive rainfall risk. The question remains whether or 
ot the stronger dynamics farther north will enhance amounts/rates. 
Will leave as a Marginal risk area for the time being in deference 
to the NBM amounts and guidance from the CSU ML EROs.

Bann
--- SBBSecho 3.20-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)