Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 23 2023 01:01:51 FOUS30 KWBC 230101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 901 PM EDT Sat Apr 22 2023 Day 1 Valid 01Z Sun Apr 23 2023 - 12Z Sun Apr 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF CENTRAL TEXAS, INCLUDING THE SAN ANTONIO AND AUSTIN METRO AREAS AND MUCH OF THE TX HILL COUNTRY... ....01z Update... Some adjustments were made to the Marginal Risk area (mainly to shrink the northern extent due to the slow southward progression of a front), but the Slight Risk remains unchanged for this update with the bulk of the precipitation still yet to come. Radar and satellite trends favor the 18z HREF PPM solution, depicting localized totals of 2-3" across the Slight Risk area. While this localized threat is expected to manifest, these totals may end up being realized across a larger area (i.e. scattered coverage) if a developing mesoscale convective system (MCS) is able to grow upstream (with very impressive deep layer shear of 50-70 kts). Instability is still plentiful (500-2000 J/kg of ML CAPE) with precipitable water values of 0.9-1.2 inches (near the 75th percentile, per FWD and DRT sounding climatology), so vigorous updrafts should remain capable of 1-2"/hr rates (with backbuilding convection to the west possibly leading to localized training as well). Churchill ....16z Update... Little changes were needed for the inherited areas across central TX, with the 12z HREF suite remaining is rather good agreement concerning the potential for short-term, localized totals of greater than 2" this afternoon. The Slight Risk area was expanded just a tad southwestward to encompass the best signal CAM signal for 2" exceedance (including more of the TX Hill Country), but also maintained the eastern edge considering the hydrologic sensitivities of the San Antonio and Austin metro areas. The Marginal Risk area was also expanded a bit farther south to encompass more of the South TX Brush Country (primarily along the Rio Grande). The meteorological reasoning below remains unchanged. Also considered the introduction of a Marginal Risk area for portions of Upstate NY southward into the Mid-Atlantic, but concluded that the combination of very dry antecedent conditions (and associated high FFGs) and highly marginal hourly rates will not necessitate one. The only place that 3-6 hourly totals look to get even close to the associated FFG is across portions of NY State (mainly from the Catskills to portions of the Hudson Valley), but any resultant flooding there should be nuisance at best (as the forecast rainfall should be mostly beneficial for the region). Churchill ....Previous Discussion... ....Central Texas... The rainfall forecast across central Texas has increased markedly this forecast cycle. The pattern remains similar, but the guidance is locking in on the potential for training thunderstorms. At the surface, southeasterly flow will increase through the day today across south Texas, turning more southerly as it advances east of the dryline across central Texas. At the northern end of this plume of moisture, a cold front will be advancing southward, drawing unseasonably cold air southward across the Plains. The collision point between these two air masses is likely to be over central Texas. A bit higher up in the atmosphere, the warm, moist air will advance much further north into northern Texas. The separation between surface and mid-level fronts opens up the possibility for separate areas of training convection to develop. Most of the guidance suggest the surface/southern front will feature stronger storms than the northern one over north central Texas. Nonetheless either or or both may act as foci for convection. Steering flow/Corfidi vectors all across Texas will be westerly at about 15 kts, which means any storms that form will move off to the east. However, the persistent southeasterly flow of Gulf moisture colliding with the front will support backbuilding storms along the front. There is good agreement that this training pattern will develop, but much less agreement on where. Thus, the inherited Marginal Risk area was expanded well to the north to cover the area where storms may develop along the mid-level front over north central Texas. Those storms that form south of the Red River may track into the Dallas-Fort Worth metro. However, the greater threat and better agreement will be along the surface front further south closer to San Antonio/Austin. The most likely area of development will be north of San Antonio and west of Austin. This area happens to be the same one that was hard hit just 24 hours ago with strong, slow-moving, training thunderstorms, which produced numerous instances of flash flooding. This area has not yet fully recovered. Thus, FFGs in this particular area are lower than they would be at any other time. The better agreement in the CAMs of this area being the focus for training thunderstorms, and coordination with the EWX/San Antonio, TX and SJT/San Angelo, TX forecast offices necessitated a local upgrade to a Slight Risk, covering the area that has both lower FFGs and much greater forecasted rainfall. This remains a low confidence forecast, and as such, changes with future updates are more likely than usual for a Day 1 forecast. ....Mid-Atlantic... A single squall line is forecast to move east across all of the Mid-Atlantic later today. The progressive nature of the storms, high FFGs, and very dry antecedent conditions are keeping this area's flash flood threat below Marginal Risk thresholds. Wegman Day 2 Valid 12Z Sun Apr 23 2023 - 12Z Mon Apr 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF SOUTH TEXAS... ....21z Update... Expanded the Marginal and Slight Risk areas across South TX a bit, based primarily on the spread of the new CAM guidance (12z HREF). Also removed the Marginal Risk area across portions of New England, as rainfall rates look to generally remain at or below 0.5"/hr (with localized totals topping out around 2" over a period of many hours). This rainfall should prove to be largely beneficial, with any nuisance flooding limited mainly limited to areas of poor drainage.=20 Churchill ....Previous Discussion... ....South Texas... As the front that was over central Texas Saturday sags south into south Texas Sunday morning, even greater moisture convergence will develop along and ahead/south of the front, drawing a truly tropical air mass north into deep south Texas. PWATs may approach 2 inches with this air mass, as the surface easterly flow increases to 20 kts. Meanwhile the flow in the mid-levels will be southerly, while the upper level flow remains westerly. This high shear, moisture-rich, and normal instability air-mass will continue to favor development of storms that are likely to continue moving eastward, while backbuilding westward. The storms are likely to focus along an axis over or just south of Corpus Christi, TX. With such abundant moisture available, the storms will have no problem locally generating 2-3 inch per hour rainfall rates, with the instability around 1,000 J/kg not high enough to support widespread hail, and instead supporting very efficient warm rainfall processes within the storms. While this area hasn't been quite as wet as areas north towards San Antonio, soil moisture is above average for this time of year from recent rains, and so the flash flooding risk, particularly in metro Corpus Christi is notable. Thus, in coordination with the CRP/Corpus Christi, TX forecast office, a targeted Slight Risk was introduced for this area, given improving agreement in the guidance that the first half of the day will feature strong and slow-moving thunderstorms. Nearby, the southward trend in the guidance resulted in lower rainfall totals forecast further north towards San Antonio by Sunday morning, so the Marginal Risk area was shrunk a little bit on the northern end of the risk area. ....New England... The front that tracks across the Mid-Atlantic on Saturday will stall out over New England on Sunday. Onshore flow will favor areas closer to the coast as having the best chance of the highest rainfall totals. As rainfall amounts are holding steady across portions of interior New England, particularly towards VT, the Marginal Risk area was trimmed towards the east, as increasing leafing vegetation should preclude any flash flooding further west. Particularly sensitive urbanized areas such as Portsmouth, NH or Portland, ME have the best chance of isolated flash flooding, particularly in poor drainage areas. Wegman Day 3 Valid 12Z Mon Apr 24 2023 - 12Z Tue Apr 25 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS... ....21z Update... Adjusted the inherited Marginal Risk area across the Central Plains, mainly to shift the area southwest to encompass more of the TX/OK Panhandle region. This is a rather marginal Marginal Risk, as the needed instability to produce localized 1"+/hr rainfall rates will likely not arrive until late in the forecast period (06-12z Tuesday). An area of relatively strong frontogenesis looks to precede the potential convection during the late night/early morning, but this rainfall earlier in the day will be primarily beneficial for the drought-stricken region (with not too much falling all at once). Churchill ....Previous Discussion... ....Central Plains... Showers and thunderstorms are likely to develop ahead of an approaching shortwave across Kansas and Oklahoma Monday and especially Monday night. Return flow off the Gulf will encroach as far north as the Marginal Risk area, bringing both moisture and some instability, especially to OK. The big question is how much organization the storms can generate, which due to that uncertainty, the area remains in a rather large Marginal Risk area until more compact areas with better chances of storms can be identified. ....New England... In coordination with GYX/Gray, ME forecast office, the inherited Marginal Risk was downgraded with this update. The stalled out front over the area will continue weakening from Sunday, with general rainfall amounts for the day barely getting to 1 inch. While many of the same areas as Sunday will see rain again on Monday, the lack of instability should mean any rainfall activity will be scattered and light, so flash flooding is highly unlikely. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5nC-nj4qOCuIyqSEKO4gdaAcBUbRdx_o3KjicbPO4Lxe= f8OUjSRI635aTOl9Ldwg3UROWY2_ghR7VB7e6GwV-x1Pq1o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5nC-nj4qOCuIyqSEKO4gdaAcBUbRdx_o3KjicbPO4Lxe= f8OUjSRI635aTOl9Ldwg3UROWY2_ghR7VB7e6GwVWgnOq30$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5nC-nj4qOCuIyqSEKO4gdaAcBUbRdx_o3KjicbPO4Lxe= f8OUjSRI635aTOl9Ldwg3UROWY2_ghR7VB7e6GwVei3Ht7A$=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) .