Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 06 2024 20:28:26 FOUS30 KWBC 062028 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 PM EDT Tue Aug 6 2024 Day 1 Valid 16Z Tue Aug 06 2024 - 12Z Wed Aug 07 2024 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF GEORGIA, SOUTH CAROLINA AND NORTH CAROLINA... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY ACROSS FAR EAST GEORGIA, EASTERN SOUTH CAROLINA, AND SOUTHWEST NORTH CAROLINA... ....16Z Update... Modest changes to the High Risk and lower risk categories across the Southeast given the latest trends in radar and short term guidance. More on this setup in the "Southeast" sub-heading below. The Mid Atlantic and Northeast was generally maintained outside a few adjustments which can also be found within the respective sub- heading.... The MRGL/SLGT risk areas out west were generally maintained with a persistent signal in isolated to scattered heavy rainfall due to relevant mid-level shortwave progression(s) into the respective areas, as well as sufficient deep layer moisture presence located within the axis of greatest ascent. Progressive convective signals across MT into the Dakotas should limit some of the higher end potential for flash flood concerns, however the environment will be conducive for locally impactful flooding, especially in-of complex terrain and/or urban settings where runoff potential is greatest. Neighborhood probabilities for 2" are not very robust with marginal signals located from the Western Dakotas, across into Western MT. A general 10-20% for the 2" threshold is forecast lending credence to the MRGL risk with some borderline SLGT risk potential over Western and Central MT. Across AZ, the SLGT risk was unchanged given the recent probability fields for >2" of precip located over Southern AZ. There was some thought on the expansion furthers north to include the Central and Western Mogollon Rim, but the best instability, forcing, and moisture anomalies were aligned better over Pima/Santa Cruz counties where lower end probabilities for 1-2"/hr rates encompassed the area later this afternoon and evening. The 3hr FFG exceedance probabilities were also running between 15-20% over much of the outlined SLGT risk area as well which signified the maintenance of the SLGT risk with little deviation. ....Southeast... TS Debby continues to churn over the Southeast coastal plain with the latest center of circulation positioned directly over Savannah, GA, drifting eastward as of the latest National Hurricane Center (NHC) update. The trend will be a continued onshore component within the northeastern quadrant of the storm leading to a persistent heavy rain threat across much of the Low Country of SC up through Southeastern NC as the low meanders east over the next 24 hrs. Several inches of rain have already fallen across Southeast GA through Southeast SC with general axis of 4-8" with locally 10+" in the most persistent feeder bands in the Northeast quad of the circulation. The rest of the period will be a relative "Deja vu" in a sense as Debby will exhibit limited movement away from the SC/GA coast leading to the same areas getting impacted over the course of the forecast. There is some drier air on the far southwestern side of the broad cyclonic rotation that Debby is ingesting based on the latest mid-level water vapor satellite. This has correlated to some weakening within the convective field located under the low center and areas to the south and southwest. Enough mid-level ascent within the tropical moisture surrounding Debby will induce a tight convective gradient with persistent heavy rainfall rates across the western side of the storm. The recent CAMs, associated HREF, and ensemble bias corrected QPF footprint all reflect the current and expected evolution of the precip field surrounding the western half of the storm leading to some minor adjustments in the risk areas focused across GA and far Southeast SC. A High Risk remains in place across much of Eastern SC up into Southwestern NC with the Wilmington area now situated within the High as of this update. This was coordinated with the local Wilmington WFO and aligns with the robust neighborhood probabilities for both the >5" and >8" potential on the latest HREF. The High risk was removed for Savannah and nearby Hilton Head island, but maintained within the MDT due to some potential overnight for a heavier band or two to pivot in from the northwest on the backside of the flow as Debby drifts eastward. This area was within a minor "minimum" within the probability fields for the higher end QPF, thus was sufficient for a targeted downgrade with the High Risk still close by to the northwest. ....Mid-Atlantic/southern portions of the Northeast... The MDT across the Mid Atlantic and Northeast was relatively maintained with the primary focus across Central and Eastern PA into the northern half of NJ and the Lower Hudson through NYC/LI. Did extend the MDT a bit further west to account for the stronger signal for multiple waves of convection through the course of the afternoon and evening within an improving moist environment, along with the area lying along and south of the frontal boundary placing it within the right side of the north-south instability gradient. A strengthening jet streak over the Northeastern U.S with modest mid-level jet developing occurring to the south will impose a corridor of jet coupling within the confines of Eastern PA through NNJ and NYC/LI. Latest HREF blended mean was steady with a large swath of 2-3" located over Eastern PA through the NYC metro with the heaviest core located over NNJ, just west of NYC proper. Neighborhood probabilities for >3" are running between 50-80% across much of Northeastern PA into NNJ with even some >90% probs located west of the NYC metro. This correlated with the bullseye for >5" hovering between 60-70% probability in the same location, but a drop off once away from the area. The >8" signals were officially removed across the Mid Atlantic compared to the previous HREF iteration leading an upper threshold for totals confined to 5-8" with most deterministic capping the threat at 6-7". Rounding out the probabilities, HREF EAS for 1" was very high extending from Northwestern PA across the central portion of the state over into NYC/LI. The signal was sufficient to extend the SLGT risk up towards Lake Erie, encompassing areas along and north of I-80 until you reach the NY/PA state line. EAS for 2" remained fairly high across the MDT risk area with an areal extent of 25-50% within the highest risk forecast. The 3" EAS was non-zero but considerably lower leading to less confidence in a more widespread, high impact event that would garner a high risk forecast. The threat remains well into the MDT risk threshold with a capped upper end potential due to some progressiveness of the precip overall. Kleebauer ....Previous Discussion... ....Southwest... The monsoonal moisture and diurnal heating will keep showers and thunderstorms developing during the afternoon and evening hours across the Southwest during this period. Convection to develop along the terrain, and then move westward into some of the lower elevations resulting in an isolated to scattered flash flood threat. The well defined shortwave currently over northern Mexico will approach southern Arizona by later Tuesday, which should bolster rainfall amounts and to some degree coverage of convection across southern Arizona. A Slight Risk remains in effect for portions of southern Arizona while a Marginal Risk remains in effect for much of the Four Corners region. ....Northern Rockies into Northern High Plains... Fast-moving thunderstorms are expected to pass over the Rockies and adjacent High Plains during this period. There is potential for training of cells over areas that have low 1-hour FFGs. Isolated instances of flash flooding may occur during this period therefore maintained the Marginal Risk area. Campbell/Chenard Day 2 Valid 12Z Wed Aug 07 2024 - 12Z Thu Aug 08 2024 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH AND NORTH CAROLINA... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY... ....20Z Update... There were modest changes reflected within the higher risks across the Southeast, as well as over the Mid Atlantic with an expansion of the SLGT risk. More information is available on each area within their respective sub-headings below.... Elsewhere, there was a small expansion of the MRGL risk across parts of the High Plains in Southwest NE and Northwest KS as there is a growing consensus for upstream convective growth around the northern periphery of the ridge with sights downstream into the High Plains off the CO/WY Front Range. The progressive nature of any convection will limit the flash flood potential to a low-end MRGL threat, however a narrow corridor of 20-30% probability for >3" exists within the latest HREF output with hourly rates generally between 1-2"/hr at peak intensity. This would satisfy a lower threshold flash flood concern, but non-zero and enough to quantify for a risk extension. This was also in alignment with the First Guess fields and targeted ML output based within the latest ECMWF AIFS and GFS Graphcast. ....Southeast... A High Risk for excessive rainfall remains in effect for Northeastern South Carolina and Southeast North Carolina as Debby continues to=20 dump widespread heavy rain along its very slow track; likely=20 resulting in continued significant rainfall and flooding over the=20 High risk area. There was only a minor deviation from the previous forecast risk areas with the southern edge of the risk areas trimmed back given the latest probabilistic trends for rainfall exceeding >2". A sharp cutoff in the heavier precip across the southern flank of Debby led to the change, but the premise within the center and northeast quadrant of the storm remained as the forecast QPF still references between 4-8+" through the period. HREF neighborhood probability for >5" is very aggressive with a large footprint of 80-90+% probs located across the entire High Risk area with the primary location situated just inland of Wilmington, NC. This area has the opportunity for a 48hr total exceeding 15" leading to the threat for a prolonged life-threatening flash flood scenario that exists through D2. Bands of heavy rain are forecast to pivot all the way north into Southern VA and as far west as Central NC, but=20 the coverage and magnitude of the heavy rain threat is much less compared to near Debby's circulation. This allowed for a maintenance of the SLGT and MDT risk aways further inland, but=20 were not sufficient for additional upgrades.=20 ....Mid Atlantic... A weak front will bisect the Mid Atlantic with multiple surface waves riding along the boundary, especially early on in the period. Area PWATs will remain elevated as moisture on the northern periphery of Debby will intersect a stationary front and align=20 west to east along said surface front. The primary period for=20 impact is likely over the course of the morning as a stronger mid- level perturbation traverses over the Central Mid Atlantic with=20 convection situated along and within the confines of the front.=20 There is a growing consensus within the CAMs for a narrow corridor=20 of heavy rainfall thanks to the convective scheme tomorrow morning=20 with a larger footprint based over the Delmarva back towards the=20 Piedmont of Central MD. Neighborhood probabilities have increased since the previous HREF issuance with decent areal extent of 50-70% probs for >3" with a 20-30% area for >5" located over much of DE into the eastern shore of MD. 2"/hr rates are also forecast within the heavier convective cores as depicted within the HREF probability fields, running a good 25-45% outline between 12-18z tomorrow across the Delmarva, correlating with the higher neighborhood probabilities mentioned before. There was enough consensus to expand the SLGT risk from previous forecast further south and west to encompass more of the area within the better probabilistic output, and matches the latest UFVS First Guess fields.=20 Kleebauer ....Previous Forecast... ....Southwest... The increased moisture flux from the Gulf of California will make its way further north/east across the Four Corners region, helping maintain enhanced rainfall during the afternoon and evening. A Marginal Risk area covers much of the Four Corners region and extends into southeast Wyoming and western Nebraska. Campbell Day 3 Valid 12Z Thu Aug 08 2024 - 12Z Fri Aug 09 2024 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH AND NORTH CAROLINA... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY... ....20Z Update... There was an adjustment in the overall risk areas across the Southeast and Mid Atlantic compared to the previous forecast. The main adjustment was a westward shift in the risk areas overall as ensemble consensus has grown for Debby's circulation to move further west with a more inland push of the heavier precip. More on this change below.... The SLGT risk across the Rockies was relatively unchanged outside some minor adjustments given the latest QPF footprint. The pattern remains steadfast and should yield a targeted focus for heavy rain across portions of the Central and Southern Rockies.=20 ....Southeast and Mid Atlantic... The latest trend with Debby's track is for it to press back inland of the Carolinas with the primary circulation a bit further west compared to the latest forecast. Ensemble trends within the QPF field followed suit with further push towards the Carolina Piedmont leading to a higher risk of heavier rain focused inland from I-77 and points east to around I-95. Current forecast is for a widespread 3-6" with local to 8" of rainfall encompassing the=20 Midland's of SC across into East-Central NC and SC, including the=20 Raleigh and Columbia metros in NC and SC, respectively. Anomalous=20 PWAT presence between 2-4 deviations above normal will situate from the Carolinas up into the Central Mid Atlantic with some moisture=20 being pulled poleward ahead of an approaching trough/cold front to=20 the east. This will allow for more heavy precipitation to make it=20 into parts of Southern and Central VA with heavy rains extending=20 back near the Southern Appalachians. The setup remains tricky=20 considering the weak steering pattern around Debby prior to the=20 D4/5 time frame as the low gets absorbed into the eventual front.=20 For now, the adjustments reflected within all risk areas are based=20 on the NBM and ensemble bias corrected QPF output which signals the shift to the west of the general circulation of Debby. The High Risk is position just east of I-77 and points east. It remains a period to monitor closely for additional adjustments which could include a further westward expansion of the High Risk, pending trends in Debby's forecasted path and associated QPF maxima.=20 Kleebauer ....Previous Discussion...=20 ....Four Corners, Central Rockies and High Plains... The Monsoonal pattern will persist through this period across the Four Corners region while increasing across portions of the Rockies and adjacent High Plains. Shortwave energy passing through the Rockies will help provide additional forcing for convection to focus along the higher terrain of Colorado and New Mexico. Model guidance suggests a concentration of cells capable of producing 1 to 2 inches with very isolated maxes greater than 3 inches. A Slight Risk was maintained for portions of central Colorado and northeast New Mexico given the increased risk for isolated to scattered flooding concerns. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8uA1iHQzG-9OZE60E7njX0h4NXc4d-n_MmFGvNARrxWu= EYCFhXz_MO1_DbQ8NLwa27ndnXi45c86cjec4MqL-q3OucE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8uA1iHQzG-9OZE60E7njX0h4NXc4d-n_MmFGvNARrxWu= EYCFhXz_MO1_DbQ8NLwa27ndnXi45c86cjec4MqLK4Q6IGo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8uA1iHQzG-9OZE60E7njX0h4NXc4d-n_MmFGvNARrxWu= EYCFhXz_MO1_DbQ8NLwa27ndnXi45c86cjec4MqLP2gjTaU$=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) .