Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 23 2023 20:27:49 FOUS30 KWBC 232026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 325 PM EST Sat Dec 23 2023 Day 1 Valid 16Z Sat Dec 23 2023 - 12Z Sun Dec 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER AREAS ALONG THE MIDDLE AND UPPER TEXAS COAST, AND ACROSS PORTIONS OF NORTH TEXAS INTO SOUTHERN OKLAHOMA... ....16Z Update... ....North-Central Texas and Oklahoma... All is mostly on track with this afternoon through overnight's rainfall event across portions of Texas. A squall line is expected to develop over or near the Texas Panhandle region, and expand south and east with time as it tracks across TX and OK. Storms that will make up the line itself are currently over central NM. A fast, eastward moving upper level shortwave is approaching the storms, which will give them a significant boost in speed starting over the next couple hours and continuing right through the overnight. While rainfall rates with the line may approach 1.5 inches per hour, the line is likely to be moving at a fast enough speed that flash flooding due to it is unlikely, unless it happens to hit an area previously hard-hit by other storms. Those preceding storms may be what allow for flash flooding to develop, as they track more slowly towards the north ahead of the line. Portions of north-central TX, including the DFW Metroplex have seen a bit of rain recently, making their soils wetter than normal. So a quick 1 to 3 inches of rainfall should be enough to cause at least isolated flash flooding concerns. There is some disagreement on how widespread and robust the preceding convection will be, as we are at the annual solar minimum and instability levels will be meager (around 500 J/kg MUCAPE after 06Z). This may play a primary role in how much total rainfall and flash flooding there will be. The Slight area was trimmed a hair on the western side from the inherited forecast, as models disagree on how long it takes the squall line to fully mature...meaning it may take until the line is further east to maximize the flash flooding threat. ....Texas Gulf Coast... Few changes needed here as well. Storms are expected to develop over or near Corpus Christi this afternoon, tracking northeast up the coast into the overnight. By then, additional light rain will congeal with any storms, resulting in an overall slowing of the movement of the broader precipitation shield near the Houston-Galveston area. Thus, the heaviest rainfall and the greatest likelihood of flash flooding is expected near Houston. Following latest HREF guidance trends, the storms will have more diurnal heating benefit while they're closer to Corpus Christi, so the Slight was expanded a bit towards the west. Wegman ....Previous Discussion... Longwave trough, with both northern and southern stream shortwaves, will gradually push east of the Rockies and into the High Plains late Sat and Sat night. Large-scale, robust deep-layer ascent will quickly develop downstream across the lower Central-Southern Plains and the western Gulf Coast, maximized within the ERO outlook areas within both the left-exit region of the upper level subtropical jet streak traversing northern Mexico, and the right exit region of the N-S oriented upper jet on the lee side of the upper trough. By Sat night, the thermodynamic profile quickly becomes conducive for heavy rainfall as the uptick in low-level southerly flow (30-40+ kts at 850 mb) draws in more favorable moisture/theta-e air off the western Gulf. The latest SREF and GEFS show moisture and moisture transport most anomalous farther north across the lower Central Plains (both PW and 850-700 mb moisture flux standard deviations between +3 and +4), however there is a trade-off as the degree of deep layer instability across northern OK into KS will be more limited (as then will be the short term rainfall rates). The Slight Risk from yesterday's Day 2 ERO -- from the central Red River Valley south-southwest through the DFW Metroplex and into central TX -- was unchanged based on the latest guidance. Still expect the more diffuse, widespread pre-frontal rainfall (largely stratiform) to ultimately give way to a quasi-linear, convective segments after 02-04Z. By that time, with the uptick in the LLJ and better alignment with the low-level flow with the 850-300 mb mean wind, expect an increase in upwind propagation and slower eastward progression of the convective segments, which would allow for a better chance of cell training. This along with the favorable thermodynamic profile (PWs 1.25-1.5" and MUCAPEs 500-1000+ J/Kg), hourly rainfall rates of at least 1-1.5" will become more likely within the strongest cores, possibly higher. Another Slight Risk was added along northern portions of the Middle TX Coast through the Upper TX Coast, including the Houston-Galveston Metros. Again, the LLJ and thus moisture flux anomalies over this region are not as anomalous compared to areas farther north; however, available deep-layer instability along the Gulf Coast will be more robust (1000-2000 J/Kg), as will the 1-3 hourly rainfall rates per the higher HREF probabilities. 00Z HREF probabilities of at least 3" are >50% over a large portion of this corridor, with 20-40% probs of at least 5". As such, localized totals between 3 and 5+ inches can be expected, as advertised by the 00Z CAMs. Hurley =20 Day 2 Valid 12Z Sun Dec 24 2023 - 12Z Mon Dec 25 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE WESTERN OZARKS THROUGH THE KANSAS CITY METRO AND FOR PORTIONS OF THE CENTRAL GULF COAST... ....2030Z Update... A strong shortwave rotating around the base of a trough will move east across Texas and then turn north into Missouri by early Christmas (Monday) morning. The trough will close off into an upper level low, which will help turn the shortwave northward. The result will be continued eastward progress of the ongoing squall line and prefrontal convection from east Texas east through the Florida Panhandle. Greater instability, moisture (PWATs potentially exceeding 1.5 inches), will support storms ahead of the main line. Flash flooding here is most likely in the included urban areas, such as New Orleans, Gulfport, Mobile, and Pensacola. For the northern Slight, with the forcing turning northward, the area along the KS/MO/OK/AR border covers a much longer time period of somewhat lighter intensity precipitation...though thunderstorms may impact the area Sunday morning. Flash flooding in this area is most likely in the urban areas included in the Slight, such as Kansas City, St. Joseph, Joplin, and Fayetteville/Bentonville. The Slight risk area was downgraded between the 2 remaining slights for portions of east Texas, much of Louisiana, and portions of Arkansas. Much higher FFGs in this area, the main forcing largely missing this area to the north, and much of the Gulf instability remaining south closer to the coast were the primary factors resulting in a downgrade for this area.=20 Wegman ....Previous Discussion... Aforementioned mid/upper trough is expected to close off at 500 mb by 12Z Christmas Day across the Plains. Strong southern stream enhancement (130-150 kt upper jet streak moving across South TX) will boost the deep-layer ascent and south-north ageostrophic low-level moisture/theta-e transport into eastern OK-TX as well as much of AR-LA and the MS-AL Gulf Coast regions. PWs >1.5" begin to creep into the Upper TX Coast region early in the period, then across southern LA and coastal MS later Sun-Sun night. The anomalous low-level transport, aligned low-mid level flow with an anomalous LLJ (favoring increased upwind propagation/backbuilding and thus risk for cell training), and at least modest deep-layer instability (at least 500-1000 J/Kg) will lead to a favorable thermodynamic and kinematic profile for heavy rainfall, especially given the active, robust southern stream which is indicative of the current strong warm-phase ENSO pattern. As such, modifications to both the Slight and Marginal Risks areas from yesterday's Day 3 ERO were relatively minor based on the latest spatial and temporal trends from the 00Z models. Hurley Day 3 Valid 12Z Mon Dec 25 2023 - 12Z Tue Dec 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... ....2030Z Update... In coordination with TAE/Tallahassee, FL and TBW/Tampa, FL forecast offices, the Slight was downgraded to a Marginal with this update. Much of the guidance has 3 areas of heavy rain around the Southeast. Near the FL Panhandle, most of the guidance keeps the storms offshore, and quickly weakens any convection to a steady rain as it moves ashore. The lack of forcing and instability along with high FFGs along the coast should preclude all but an isolated instance or 2 of flash flooding where the heaviest rain persists the longest, so the Slight was downgraded. The second area of convection is offshore of SC, and is unlikely in any of the guidance to move ashore, as much of the rainfall should push east. The third area of heavy rain will be in the southern Appalachians, on the south/east facing slopes of GA/SC/NC. While forecast rainfall of 2 to 3 inches is likely, the combination of very dry soils, and a lack of instability largely causing much of the upslope enhancement to be weak, think those 2 to 3 inches of rain will fall over much of the day into the overnight, and will be too light to cause flash flooding. Thus, the general Marginal Risk area remains for much of the Southeast. Wegman ....Previous Discussion... Closed mid-upper low will move very little during day 3 (12Z Christmas-12Z Tue), as upper level difluence increases over the southeastern U.S. The aforementioned subtropical jet will remain quite robust, with 250mb winds between 150-170kts moving into the western and central Gulf Coast regions Monday night. A rather quick uptick in deep-layer ascent and low-level theta-e/moisture transport from the GOMEX is expected, although per the latest GEFS and NAEFS, moisture flux and IVT anomalies start off near +1.5 to +2 early in the period across the FL Panhandle, then decrease thereafter. Also limiting the potential for a Slight or better risk over much of the Southeast region will be the faster progression of the leading cold front, thus increasingly transient quasi-linear convective segments over most areas. One exception may be along the FL Gulf Coast between the eastern Panhandle and Nature Coast, including the FL Big Bend. It is within this area where the convective segments originating over the thermodynamically-rich eastern Gulf waters may start to train Monday night with the uptick in elevated low-level flow, as hinted with the 00Z ECMWF. For now, based on the thermodynamic and kinematic trends, we have slightly reduced the areal extent of the Slight Risk inherited from yesterday's Day 4 ERO. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-N1UV8ZqpJe4UwM88te6eVyKnpusmy9bTaYKLUQh2q0o= XgyjPwdAVlvrsJEpk9AfUKENsWue-IrwO69TVT6KLdW4G3o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-N1UV8ZqpJe4UwM88te6eVyKnpusmy9bTaYKLUQh2q0o= XgyjPwdAVlvrsJEpk9AfUKENsWue-IrwO69TVT6KdjMywqg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-N1UV8ZqpJe4UwM88te6eVyKnpusmy9bTaYKLUQh2q0o= XgyjPwdAVlvrsJEpk9AfUKENsWue-IrwO69TVT6Kc3AMxEU$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .