Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 29 2024 15:39:53 FOUS30 KWBC 291539 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1139 AM EDT Thu Aug 29 2024 Day 1 Valid 16Z Thu Aug 29 2024 - 12Z Fri Aug 30 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF=20 THE UPPER MIDWEST AND THE MID ATLANTIC AS WELL AS ALONG PORTIONS OF THE UPPER TEXAS GULF COAST... ....16Z Update... ....Mid-Atlantic... The primary changes to the ERO for this period were focused across the Mid-Atlantic. The guidance has expanded northward as far as where the strongest storms capable of producing flash flooding are expected to develop later this afternoon through this evening. In coordination with CTP/State College, PA and LWX/Sterling, VA forecast offices, the Slight Risk across the Virginia Piedmont was expanded well to the north to include much of central Pennsylvania today, as well as nudged eastward to include the Washington DC area.=20 Guidance suggests a small but potent shortwave in the upper Midwest will race southeastward into the northern Mid-Atlantic tonight. Combined with lift from the right entrance region of the jet as well as PWATs above 1.5 inches, there will be ample forcing for thunderstorms across Pennsylvania this afternoon and evening, while the greatest moisture stays south across the Delmarva. This discontinuity should help to prevent more widespread impacts from flooding. ....Gulf Coast... No significant changes were made other than nudging the Marginal to include New Orleans and Baton Rouge. The tropical low responsible for the rain will continue to send slow-moving bands of heavy rain into the Texas and Louisiana Gulf Coasts into tonight, with the heaviest rain expected after midnight local. The Slight for the Upper Texas Gulf Coast is primarily for the urban centers of Beaumont and Galveston, as there remains considerable uncertainty as to how far inland the bands will maintain themselves as they move away from their Gulf fuel source.=20 ....Midwest... The potent front across the Midwest and Plains will produce an area of fast moving but training storms across this region this afternoon and evening. The guidance has been very consistent with this feature, so no changes were made. Wegman ....Previous Discussion... 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 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_SYcRWNB8cRjgro0Q3vmA8PKNKqG-Cy4n0tS5pqIJCCX= RbGyOZRh_hc3pjYKbqDd1ycAtbwkLH6kqFibWhXvTLIP4eE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_SYcRWNB8cRjgro0Q3vmA8PKNKqG-Cy4n0tS5pqIJCCX= RbGyOZRh_hc3pjYKbqDd1ycAtbwkLH6kqFibWhXv4ktD330$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_SYcRWNB8cRjgro0Q3vmA8PKNKqG-Cy4n0tS5pqIJCCX= RbGyOZRh_hc3pjYKbqDd1ycAtbwkLH6kqFibWhXvG3CCcN4$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .