Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 06 2024 15:45:06 FOUS30 KWBC 061544 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1144 AM EDT Fri Sep 6 2024 Day 1 Valid 16Z Fri Sep 06 2024 - 12Z Sat Sep 07 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTHEASTERN LOUISIANA... ....16Z Update... The previous forecast garnered only minor adjustments within the targeted MDT and SLGT risk areas across the Southeastern U.S. Recent radar/satellite composite continues to indicate a focal of heavy rain bands in-of Southeast LA within the eastern periphery of a surface wave migrating along the quasi-stationary front bisecting the region. A plethora of cloud cover through the morning and afternoon will limit surface based destabilization, however the environment is still very much conducive for heavy rainfall prospects thanks to a strong +2-3 deviation PWAT anomaly centered over the Central Gulf coast. 12z soundings out of KLIX and KLCH indicated PWATs ~2.4" with warm cloud layer depths between 15.5-16.5k ft MSL which correlates to efficient, tropical type rain processes that would enhance local rain rates within any stronger convective cores. 12z HREF and associated CAMs are in agreement for the late-morning through the afternoon time frame as the peak for rainfall during the period with a slow shift southward of the front as the pattern evolves upstream with a secondary front beginning to press south out of the Tennessee Valley. Probability fields remain bullish for localized totals exceeding 5" (60-90%)=20 within the confines of the MDT risk area Scattered heavy cores are=20 plausible to the north of the surface front thanks to remnant=20 elevated moisture and better destabilization during the afternoon=20 hrs. A focused pulse of heavy rain initiated across Central MS this morning thanks to a weak shortwave pulse riding along the southern periphery of the mid- level trough swinging through the Deep=20 South. A SLGT risk was added a bit further north to account for the weak disturbance which could trigger more convection downstream in AL/GA prompting a touch better focus of heavy rain and flash=20 flooding considering the moist environment in place.=20 Not much has changed in the other areas of interest within FL and coastal CA/SC as the stationary front remains draped over Northern FL and the coastal GA region. A weak surface low is progged to=20 develop across Northeast FL and migrate slowly to the northeast as=20 it rides the front. A focused point of surface convergence within=20 the western flank of the flow is pinpointing a secondary heavy rain axis near Savannah later this afternoon. Some guidance has this a=20 bit further south, but the prospects for a narrow corridor of heavy rain is in the cards during peak diurnal instability, so the SLGT=20 within that area was warranted.=20 Lastly, the isolated convective threat across the Sacramento Mountains is still anticipated with the targeted MRGL positioned over Ruidoso where the active burn scar creates a micro-focus for flash flooding concerns within the town and surrounding terrain. Considering the convective prospects within the latest CAMs, there was no reason to remove the risk area and will maintain continuity.=20 Kleebauer ...Previous Discussion..=20 ....Gulf Coast & Southeast... A very slow moving tropical low will track east along a stationary front draped along the Gulf Coast today. Near record amounts of atmospheric moisture in place across the Gulf will be drawn northward into the front ahead of the low. PWATs currently are around 2.6 inches and will not change much through the day. This would be over 3 sigma above normal for this time of year...which for southeastern Louisiana is no small feat. The southerly flow ahead of the low will keep a steady supply of deep Gulf moisture tracking into the Gulf Coast primarily...but will spread east across portions of the Southeast in the Slight Risk. The big question about heavy rain will be how much instability will be available for any showers and thunderstorms. Since it's currently raining over much of Louisiana and will continue into tonight, any instability will need to be advected in to the Gulf. That said, given the near record levels of moisture available, it will not take much to result in 2+ inch per hour rates with the strongest convection. The area's soils are quite water-logged from multiple days of rains, so today's rainfall should fully convert to runoff...greatly increasing the flooding threat. In coordination with LIX/Slidell, LA forecast office, the Moderate Risk area was left unchanged with this update, with the greatest threat for the greater New Orleans and Baton Rouge areas due to lower FFGs. The Slight was nudged a row of counties north, and removed from most of northeast Florida. Due to recent heavy rains across much of the Peninsula however, the Marginal risk was expanded to include the Orlando, Tampa, and Ft. Myers metros. ....New Mexico... A small low-confidence Marginal Risk area was added for the Sacramento Mountains of south-central New Mexico with this update. Afternoon showers and thunderstorms may form right over the very flood prone burn scars near Ruidoso, which could cause local flash flooding once again. However, it would require just that scenario to unfold for flooding to occur which is low confidence since the storm would have to form right over the correct spot, a microscale challenge that's nearly impossible to know for sure well in advance. The pattern supports one more day of area showers and storms, prompting the localized upgrade. Wegman Day 2 Valid 12Z Sat Sep 07 2024 - 12Z Sun Sep 08 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE EMERALD COAST OF THE FLORIDA PANHANDLE... ....Southeast... The stationary front from Friday will drift southeastward across the Southeast on Saturday. For the Central Gulf Coast, it would probably be more accurate to call it a cold front, as the front will make steady progress across southern Louisiana and into the northwestern Gulf. Meanwhile across the Southeast, the front will stall out over the Florida Panhandle, southern Georgia and up the Carolina coast. It's in this region where training storms may cause localized flash flooding. Moisture will not be a problem at all as PWATs will be from 2 inches in the Carolinas to nearly 2.5 inches in the Florida Panhandle. Thus, just as on Friday, the big limiting factor for heavy rainfall will be instability, as very slow moving fronts tend to be very lacking in that department due to extensive cloud cover. In coordination with TAE/Tallahassee, FL forecast office, a Slight Risk area was introduced with this update. Storms are likely to be robust rain makers out over the northern Gulf on Saturday. With very weak steering flow and limited instability inland however, the area with greatest likelihood for the storms to impact land will be around the Emerald Coast of the Florida Panhandle. The most likely hazard area was highlighted in the Slight. The expectation is that any storms that move into the Emerald Coast will greatly struggle to maintain the heavy rainfall rates more than a few miles inland from the coast, so the Slight is only for the immediate coast. The Marginal Risk area was expanded several rows of counties north with this update, while in the Carolinas it was more like a couple states. The primary driving factor for these changes is the slowing of the front in recent model runs. Thus, the rain may persist for a bit longer than expected into Saturday morning on the northern edge of the Marginal Risk area. Meanwhile Saturday will be another day with potential strong thunderstorms capable of heavy rains across the northern Florida Peninsula. This hard-hit area is likely to see multiple days of afternoon thunderstorms capable of isolated flash flooding. ....Northeast... The inherited Marginal Risk area was removed in coordination with BTV/Burlington, VT and ALY/Albany, NY forecast offices. Fast-moving storms with this fast-moving front in this part of the country, very limited/no instability, and somewhat dry weather in recent days should all preclude any flash flooding in this area. Wegman Day 3 Valid 12Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GULF COAST... ....Louisiana to the Atlantic Coast... The stalled out front will weaken in place through the day on Sunday. This once again should prevent most areas from getting prodigious amounts of rain, despite abundant atmospheric moisture. Further, there is increasing disagreement in the guidance as to where the axis of heaviest rainfall associated with the front will be on Sunday. Guidance trends have been towards the northwest, resulting in a large expansion of the Marginal in that direction due to the aforementioned uncertainty. It's likely given how wet this entire area of the country has been that an area of Slight Risk impacts will occur somewhere within the Marginal risk, but where that will be is highly uncertain right now. Thus, it's likely a Slight Risk upgrade will be needed with increased confidence. ....Deep South Texas... A Marginal Risk area was added to the Brownsville to South Padre Island area of Deep South Texas with this update. A tropical wave will have moved through the Caribbean and into the western Gulf by Sunday. Associated heavy rainfall may impact the immediate Deep South Texas coast. Recent heavy rainfall has lowered FFGs here which may not fully recover by the arrival of this heavy rainfall due to potential additional shower and thunderstorm activity both today and Saturday prior to this period. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4LwgVCXmD7AY6dFln4x_sRvmw4VEyVE-K8vqHUdjI4-K= cpHJr-_W9B8h3NKFI6fQtOBuMStEE5j087xCnCq5JzieRwE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4LwgVCXmD7AY6dFln4x_sRvmw4VEyVE-K8vqHUdjI4-K= cpHJr-_W9B8h3NKFI6fQtOBuMStEE5j087xCnCq5xFLPIus$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4LwgVCXmD7AY6dFln4x_sRvmw4VEyVE-K8vqHUdjI4-K= cpHJr-_W9B8h3NKFI6fQtOBuMStEE5j087xCnCq5E5t6P24$=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) .