Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 07 2024 00:54:10 FOUS30 KWBC 070053 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 853 PM EDT Fri Sep 6 2024 Day 1 Valid 01Z Sat Sep 07 2024 - 12Z Sat Sep 07 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTHEASTERN LOUISIANA... ....01Z Update... ....Gulf Coast & Southeast... Maintained the Moderate Risk over southeast LA given the persistent, convergent activity extending east of an mesolow from Lake=20 Pontchartrain through the MS Gulf Coast and Mobile Bay which could deviate/spread over New Orleans or other urban locales with impervious surfaces and cause further flash flooding overnight. The deep/warm/moist/tropical environment with 2.5" PW and some=20 instability south from I-10 with a frontal boundary near the shore and increasing divergence aloft from a SWly jet streak developing over northern MS/AL is conducive to localized copious rainfall that is difficult to forecast. Therefore the only changes were to tighten the outlook areas toward the coast. Most of the Slight Risk over the Southeast was dropped with an extension to the FL Panhandle and now a separate area over the GA=20 coast where a low offshore and an approaching mesolow over southern GA may yet allow some heavy rainfall near Savannah (something the=20 HRRR has been advertising for several runs). The warm sector south of the frontal zone is over the FL Peninsula with some continued inland heavy activity that may yet cause some localized concerns in any sensitive areas. ....Upper Ohio Valley... Lingering instability of 500-1000 J/kg and PW of 1.5" has allowed evening activity with localized rates over 1"/hr to persist over=20 southern OH and northern KY. Deep layer WSW flow of 30kt is=20 allowing the activity to move well, but the motion is fairly=20 parallel to the approaching cold front allowing for some repeating=20 segments. As this activity shifts into WV, the FFG quickly=20 decreases, bringing a Marginal excessive rain threat for the rest of the evening. The instability gradient over WV should cause activity to wane before reaching the crest of the central Appalachians/the Allegheny Front, so the Marginal Risk stops short of that feature. ....New Mexico... Post-fropa activity has waned over southern NM, so the Marginal was removed. Jackson Day 2 Valid 12Z Sat Sep 07 2024 - 12Z Sun Sep 08 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... ....20Z Update Summary... The SLGT risk across the Central Gulf Coast was expanded back further west to adjust to the latest trends within CAMs on lingering heavy convection along the slow moving frontal boundary Saturday morning. Additional MRGL risks were added to portions of the Western U.S, including the Desert Southwest and parts of the Southern Rockies between the NM/CO border. Details on the additional MRGL's included in the, "Southwest" and "Southern Rockies" sub- headings below. Kleebauer ....Southeast... 20Z Update: The previous forecast SLGT risk was generally maintained with a small adjustment back westward to include the Southeastern Parishes of LA and along the immediate AL/MS coasts. The trend is for the western portion of the quasi-stationary front to move a bit slower than previous forecast as of the latest 12z HREF output of mean dew point temps. The change would allow for a lingering heavy rainfall potential within the Parishes south of Lake Pontchartrain before finally vacating the area by the afternoon hours. The heavy rainfall threat is still aligned most favorable within the confines of the Florida Panhandle where the strongest anomalous PWAT signal remains during the morning and early afternoon hours prior to the heavy rain threat moving off into the adjacent coastal waters. 12z HREF neighborhood probabilities for >3" remain elevated (40-60%) along the above areas, but there is a precipitous drop off in the probabilities for >5", but still a low- end potential for higher impacts within the Emerald Coast. This is coincident on the timing of the mean trough to the north swinging eastward allowing for a push of drier air advecting in from the north that will put an end of the heavy rain threat across the Gulf Coast. A further adjustment could be necessitated if the front is progged to move faster/slower in the later forecast updates, a lot contingent on how the CAMs handle the dry air press from the north, and any convective outflows that can propel the boundary off the coast faster than modeled. This will be something to monitor closely, although the threat for a higher risk category is low, so the SLGT risk will likely be the highest category forecast. Kleebauer ...Previous Discussion... 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. Wegman ....Southwest... A prominent mid-level ridge axis will remain parked over the Great Basin with the clockwise flow controlling the steering pattern over the Southwestern U.S. A modest PWAT advection regime will unfold across the interior Southwest through parts of Southern CA, Southern NV, and Western AZ creating a better threat of scattered convective pulses in-of the Deserts and focused terrain (Mogollon Rim and Peninsular Range). Areal averaged SBCAPE forecast calls for values between 750-1500 J/kg within the confines of the above locations, within the +1 deviation moisture anomaly anticipated by the afternoon hours Saturday. Convective pattern will likely yield some locally enhanced rainfall rates that would exacerbate the flash flooding threat, especially over the more prone locations in the terrain and flashier dry creek beds and arroyos found in those locations. A MRGL risk was added within the corridor of expected convection with a low-end (5-10%) threat of flash flooding within any stronger convective cores. ....Southern Rockies... The same ridge engulfing the Southwestern U.S will lie right overhead of the Southern Rockies as the eastern flank of the high will settle along the terrain of the Rockies. Sufficient diurnal destabilization within the confines of the Southern Rockies will lead to an isolated to widely scattered shower/storm threat with a few cells capable of dropping upwards of 1-1.5" of rainfall in a few hours time span. This threat is low-end potential for a risk, however the area depicted within the CAMs for "highest" potential is littered within remnant burn scars, and within a location that has seen a decent amount of rainfall in the past week to put the 0-40m soil moisture anomalies up around the 90-98th percentile. This adds enough to the threat to allow for an additional targeted MRGL risk to account for the convective expectations overlapping the increasingly sensitive area of the Southern Rockies. Kleebauer 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... ....20Z Update... A minor adjustment was made to the MRGL risk area encompassing the Gulf Coast with a shaving of the western edge of the risk area with the threat now more confined to Florida and the Southeast coast. This was in conjunction to the latest ensemble bias corrected QPF forecast and NBM mean QPF output situated a little further south with the moisture delineation and the frontal boundary back west a bit further off the LA/MS/AL coasts. QPF is pretty mundane currently across the Southeast, so there's a chance the risk area fades entirely. Did not want to remove given the time frame still being a few days away from impact, as well as a lack of CAMs support that could sway the setup one way or another. Across Deep South TX, the pattern is still leaning towards a more active setup with deeper moisture advecting northward into the lower RGV with convection pivoting around the northern flank of a disturbance nestled over the Western Gulf. The GFS is the most bullish on the threat with other globals less privy to the setup. The recent ML output from the Graphcast and AIFS models seem to deviate away from the higher end scenarios the GFS is pushing, but a better middle ground compared to the slower and further north advancement of the wave via the ECMWF. The blend was still depicting some of the heavier rains within the mean QPF field, so maintained continuity from the previous forecast, but will be a time frame to monitor for changes in the overall orientation, or even removal of the risk depending on how the synoptic evolution transpires within the coming days. Kleebauer ...Previous Discussion.. ....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!6FIHxb5SjCroBUopajKbb-1-3fEzuPDFLg1J6rNqemwn= TDczJzxEbRLPpRO9FlZMK3VOa1q2hu-_HhploQXMcq9HzjQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6FIHxb5SjCroBUopajKbb-1-3fEzuPDFLg1J6rNqemwn= TDczJzxEbRLPpRO9FlZMK3VOa1q2hu-_HhploQXMvbxKXqc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6FIHxb5SjCroBUopajKbb-1-3fEzuPDFLg1J6rNqemwn= TDczJzxEbRLPpRO9FlZMK3VOa1q2hu-_HhploQXMJ1-51pc$=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) .