Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 28 2023 18:43:56 FOUS30 KWBC 281843 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 243 PM EDT Sun May 28 2023 Day 1 Valid 16Z Sun May 28 2023 - 12Z Mon May 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR PORTIONS OF VIRGINIA, NORTH CAROLINA, & TEXAS.... ....Central Appalachians into parts of the Mid Atlantic... The surface low will migrate onshore near the SC/NC border shortly as it drifts slowly northward and becomes vertically stacked. Even though the surface cyclone and 850 mb inflow will weaken substantially, periods of heavy rain producing showers are likely, especially on the western edge of the mid-level circulation from the higher elevations of western NC to nearby southwest VA, and in the very short term, in and near the Albemarle-Pamlico Peninsula where a heavy rain band is shifting into the region from the south. Other isolated instances of localized heavy rain are possible across central NC and southwest VA into far eastern KY-TN associated with low level frontogenesis and ~500 J/kg of MUCAPE, supporting a Marginal Risk. More organized convection falling atop sensitive terrain is possible across the Blue Ridge of NC and into southwest VA, where local 3-5" of additional rainfall is likely per the 12z HREF probabilities of such, due in large part to the additional upslope enhancement. Some of the terrain is inherently flashy, particularly near the VA/WV border, which could result in more scattered instances of flash flooding. Adjustments to the areas were mainly based on the new 12z HREF output. ....Rio Grande Valley northward through the Northern High Plains... A persistent and slow moving pattern driven by a trough to the west and ridge to the east will promote another day of scattered to widespread showers and thunderstorms from Texas to the Dakotas. Localized rainfall totals of 3" are possible throughout much of the Marginal Risk area, though a more elevated risk for flash flooding will be from southwest OK southward through northwest TX, the TX Big/Hill Countries, then south to the Rio Grande River including the Edwards Plateau and South TX Brush Country. This is due to the potential for additional heavy rain (more organized convection), along with the antecedent wet soils given the recent heavy rainfall. Across TX, PWATs are expected to peak between 1.75-2" along the Rio Grande River Valley while MUCAPE also increases to 2000-2500 J/kg. Shortwave approaching out of northern Mexico and potential lingering surface boundaries from previous day's MCS activity could provide focus for development and brief training of higher rain rates in this area. FFGs across the Edwards Plateau and surrounding regions are very low (1-2"/hr and 1.5-2.5"/3-hr). The Slight Risk area generally covers the spotty 20-50% probabilities of 3" seen in the 12z HREF probabilities for such. The Marginal Risk area saw some slight reconfiguration based on the 12z HREF probabilities as well. ....Southern FL... A convergence zone/baroclinic trough shifts northward back into southern FL today, luring precipitable water values back to 1.5"=20 Effective bulk shear remains 25-30 kts, so any thunderstorm activity has the potential for being more organized than usual.=20 ML CAPE is already 500-2000 J/kg, and daytime heating to raise that another 1000+ J/kg. Probabilities of 3"+ and 5"+ in and near Southeast FL are non-zero. Considering recent rains, thought it prudent to add a Marginal Risk area for eastern portions of the southern peninsula. Roth Day 2 Valid 12Z Mon May 29 2023 - 12Z Tue May 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN TEXAS... ....Near the California/Nevada Border... A low pressure system approaching from the West combined with ample moisture at elevation (PWs of 0.75"+) and CAPE that builds to 1000+ J/kg should set the stage for convection in and near portions of the Sierra Nevada, Shasta, and Siskiyou Ranges in and near northern and eastern CA. Some of these storms may spill out along a trough extending across portions of NV. There is a general signal in the guidance for local 1-2" amounts, which could occur in an hour or two, with the 12z HREF showing 15-20% probabilities of reaching 3" amounts in far northern CA. This would be particularly problematic in any burn scars. Coordination with the REV/Reno NV forecast office helped lead to the introduction of this new Marginal Risk area in order to maintain better messaging with their recently issued flood watch. ....Northern Plains... Precipitable water values of up to 1.25-1.5" with an area of decent low-level inflow ahead of an incoming mid-level shortwave should set the stage for showers and thunderstorms with heavy rainfall. The guidance shows local amounts near to above 2", which could fall within an hour or two should cell training/mergers occur. Within this region, there's a thin stripe where rainfall over the past week is 200-400% of average, aligned from roughly Bismarck east-northeast to Steele. A Marginal Risk area was added to cover the possibility of isolated flash flooding. ....Oklahoma/Kansas... Precipitable water values to ~1.25" within a moisture plume extending north from the Gulf of Mexico ahead of an upper level trough. CAPE should rise to 1500+ J/kg across portions of the Plains, adding to the heavy rain potential. The 06z-12z Canadian Regional models, and to a lesser degree the 12z NAM, are most bullish with heavy rain potential, though exact placement has shifted from run to run. While local amounts of 5-8" appear a bit dramatic at this time, the pre-existing Marginal Risk area was extended north to cover this potential. ....Central and Eastern Texas... Weak, broad mid-upper level trough with embedded MCVs will make for a favorable dynamical and thermodynamical environment for more numerous thunderstorms with embedded localized areas of more intense rainfall. The Marginal Risk aligns with the broad signal in the mesoscale guidance in terms of localized deterministic QPFs between 2-4+ inches within an environment characterized by 1.5-2" PWATs and MUCAPEs of 1000-2000 J/Kg.=20 ....Southern Texas... There is a more enhanced/agreeable signal in the guidance for more organized heavy rain in southern portions of TX where an organized convective area is expected to transit due to a progressive mid-level shortwave moving into the region from northern Mexico, with the convective system itself showing up fairly clearly in the 850 hPa wind pattern in the 06z NAM/GFS guidance. Prior MCS activity across south-central TX could influence where additional rounds of thunderstorms set up by Tuesday, but those details tend to be uncertain until the last minute. The new Slight Risk in portions of southern TX covers where the heavy rain signal in the guidance best overlaps. Any locales that receive 5"+ -- hinted at in the 12z HREF -- would be due to either warm advection bands show up in advance of any mesocyclones or backbuilding/training occurring on the southwest side/in the wake of any organized convective activity. ....Southern Mid Atlantic States... Vertically-stacked low near Upstate SC at the start of Day 2 (12Z Mon) will migrate slowly into eastern NC by the start of Day 3 (12Z Tue). Owing largely to the steepening mid level lapse rates (7-7.5 C/KM between 700-500 mb), mixed-layer CAPEs are likely to peak between 1000-1500 J/Kg during the peak heating hours Memorial Day afternoon into early evening. This along with PWs between 1.25-1.5" will support isolated hourly rainfall rates of 1.5+ inches, which could pose a localized flash flood threat, especially across areas that receive appreciable rainfall on Sunday. Roth Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4EcMUhZBTg73Ba4sQeg2PoMbcmFaQNdrGqGAT9RxToKZ= 3tQ0QaQzNZGIa4xT1TCTeV1szJMV1Ue8wxDhF_7PIpgnJ4Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4EcMUhZBTg73Ba4sQeg2PoMbcmFaQNdrGqGAT9RxToKZ= 3tQ0QaQzNZGIa4xT1TCTeV1szJMV1Ue8wxDhF_7Pyb_sqtc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4EcMUhZBTg73Ba4sQeg2PoMbcmFaQNdrGqGAT9RxToKZ= 3tQ0QaQzNZGIa4xT1TCTeV1szJMV1Ue8wxDhF_7PaIT-OVQ$=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) .