Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 29 2023 00:55:00 FOUS30 KWBC 290054 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 854 PM EDT Sun May 28 2023 Day 1 Valid 01Z Mon May 29 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... Removed the Slight risk area along the immediate coast of North Carolina as the circulation center inches inland as of 29/00Z.=20 Maintained the Marginal risk area along its path as 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. 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 1-3" of additional rainfall is possible per the 18Z 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.=20 ....Rio Grande Valley northward through the Northern High Plains... Only minor adjustments made to the periphery of the broad Marginal Risk area based on radar imagery as of early evening. 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 continue to 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 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 typically ranged from 1-2"/hr and 1.5-2.5"/3-hr. Bann 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 Valid 12Z Tue May 30 2023 - 12Z Wed May 31 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE WEST, MIDWEST, & PLAINS... ....Northern CA northeast across ID... Mid-Upper low off the Central CA Coast to start Day 3 (12Z Tue) will slowly wobble through SoCal during the period. Favorable upper level difluence north of the compact mid-upper low, south of the digging northern stream upper trough/height falls across the Pacific NW, will generate favorable broad-scale forcing across the outlook area Tue-Tue night. PWATs meanwhile are expected to climb between 0.75-1.00", which is ~1.5 standard deviations above normal for the end of May per the GEFS and plenty for heavy rain concerns at elevation. Modest uptick in deep layer forcing and moisture, along with at least some diurnal instability (mixed layer CAPEs peaking between 500-1000 J/Kg) will allow for scattered to more numerous showers/tstms within the outlook area, a few capable of hourly or sub-hourly rainfall rates of 0.50-1.00". This will pose a localized flash flood risk, especially over any recent burn scars into northeast CA and southern OR. ....Northern & Central Plains... The models show a mid/upper level shortwave trough pivoting across eastern portions of the northern Plains and across the far northern MS Valley on Day 3. Both the GFS and ECMWF do show some low-level frontogenetical response with this feature, along with pockets of upper level divergence which appear to be convectively enhanced. At this point, the QPF signal from the guidance is rather pedestrian, showing local 2" amounts in an avenues between MN and northeast CO. This is probably due to the available PWs, which peak just over 1.25". CAPEs should rise up to 3000 J/Kg into the outlook area, which is more intriguing and lends to the idea of hourly rain totals to 1.5". Low-level inflow and effective bulk shear appear sufficient to organize convection that forms. Expanded the Marginal Risk area to capture to broad/chaotic heavy rain signal seen in the latest guidance. ....Near the MO, KS, AR, OK border junction... Some of the guidance is picking up on a possible convective complex forming in this region, forming near the apex of a minor southern stream ridge in a general col in the mid-level flow pattern. Precipitable water values rise to 1.5"+, with CAPE expected to 1000-3000 J/kg. Low-level inflow appears weak, only on the order of 10-15 kts, but the mean 850-400 hPa winds are virtually null, which hints at minimal if not chaotic storm motion and better rainfall efficiency than average. GFS-based soundings (12z) in northeast OK show that the flow above 800 hPa nearly opposes the low-level flow -- effective bulk shear should be ~25 kts which would potentially organize a complex. Such a complex would try to move southeast once cold pools become established several hours after convective initiation. Since the 12z Canadian Regional, 12z NAM, and 12z GFS all have this feature, thought it prudent to add a Marginal Risk area. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_Pb2I7muIxe1hAMdvERJIqz87DkuoyI0fwYwF3X86Sae= 3iDoK_Ozj9PfzbGpD9Yn_yjMpRrYRIzbkytFzDLqn49nfsU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_Pb2I7muIxe1hAMdvERJIqz87DkuoyI0fwYwF3X86Sae= 3iDoK_Ozj9PfzbGpD9Yn_yjMpRrYRIzbkytFzDLqIvlK6gk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_Pb2I7muIxe1hAMdvERJIqz87DkuoyI0fwYwF3X86Sae= 3iDoK_Ozj9PfzbGpD9Yn_yjMpRrYRIzbkytFzDLqK30jYlQ$=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) .