Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 29 2023 08:06:13 FOUS30 KWBC 290806 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 404 AM EDT Mon May 29 2023 Day 1 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 00z HREF exceedance probabilities showing 15-25% of exceeding 3" amounts in far northern CA. This would be particularly problematic in any burn scars. ....Northern Plains... Precipitable water values of up to 1.25-1.50" 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 CAMs show 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. The best signal for excessive rainfall has consolidated a bit over the MN/ND/SD border region, but the NAM-nest is an extreme outlier that is driving the extreme HREF signal (with 5" exceedance probabilities of 10-15%). A Slight Risk upgrade may be considered with subsequent updates, should better agreement occur between the CAMs. ....Oklahoma/Kansas and North Texas... Precipitable water values of ~1.25" are expected 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 00z ARW2 and NAM-nest are most bullish with heavy rain potential (specifically into southern OK and possibly far North TX) though exact placement has shifted from run to run. Given the lack of consistent signal with the remainder of the CAMs, have held of on an introduction of a Slight Risk, though the 00z HREF 5" exceedance probabilities of 10-20% are duly noted. ....Southern Texas... There is still a more enhanced/agreeable signal in the guidance for somewhat 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. The now adjusted Slight Risk in portions of southern TX covers where the heavy rain signal in the guidance best overlaps, though amounts have decreased a bit from prior cycles (though HREF PMM suggests localized totals of at least 2-3" still possible). Any locales that receive 5"+ (with 00z HREF 5" exceedance probabilities as high as 10-20%) would be due to either warm advection bands showing 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 this morning will migrate slowly into eastern NC by the start of Day 2 (12Z Tues). 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 received appreciable rainfall yesterday, primarily from western VA northward into WV). Churchill/Roth Day 2 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 and southwestern MT/northwestern WY... Mid-Upper low off the Central CA Coast to start Day 2 (12Z Tue) will slowly wobble through SoCal during the period. Favorable upper-level diffluence 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 and Central Plains into Midwest... 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 2. 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. Now that we are getting more CAM guidance into this period, both the FV3 and NAM-nest are in favor of localized totals of at least 2-3" (and possibly higher), with the best agreement specifically across southwest KS into the OK/TX Panhandles. Available PWs look to peak just over 1.25", with SB CAPE rising to as high as 3000 J/Kg into the outlook area. Low-level inflow and effective bulk shear appear sufficient to organize convection that forms. The Marginal Risk area was expanded again to capture the broad/chaotic heavy rain signal seen in the latest guidance, but have held off on any Slight Risk upgrade to KS/OK/TX until the full HREF suite comes into the period with better agreement (with the impact of still fairly dry antecedent conditions also needing to be fully considered). ....Southeast OK into Northeast TX and Ark-La-Tex... Guidance has shifted notably south from prior cycles, suggesting that the best area for organized convection may occur closer to the Ark-La-Tex (primarily the NAM-nest, though the ARWs are showing favorable signs from prior convection as well). Precipitable water values rise to 1.5"+ with SB CAPE expected to increase 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. Effective bulk shear should be ~25 kts which should help to organize storms (and would potentially organize a complex). Confidence remains fairly low across this region (and higher FFG also casts some doubt on any organized flash flood potential). Churchill/Roth Day 3 Valid 12Z Wed May 31 2023 - 12Z Thu Jun 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE WEST INTO THE NORTHERN ROCKIES AND PLAINS, AS WELL AS ACROSS THE CENTRAL AND SOUTHERN HIGH PLAINS... ....West into the Northern Rockies and Plains... The localized risk for excessive rainfall looks to continue across portions of northern CA into northwest NV into Day 3 (12Z Weds), also extending farther northeastward into more of MT as well. This is primarily due to the aforementioned mid-upper low over SoCal beginning to lift northeastward through the Four Corners region (with another shortwave trough in the northern stream digging on its approach into the PacNW, per the GFS). While these two features may have larger implications going into Day 4, it's possible that there will already be some more organized flash flood potential taking shape late on Day 3. Will hold off on any introduction of a Slight Risk area until there is better convective-allowing guidance, but the ingredients (PWATs up to an inch, ML CAPE rising to 500-1000 J/kg, and lift/shear from the upper features) look to be there. ....Central and Southern High Plains... After a bit of a lull in activity on Tuesday, the return of convection to the High Plains looks likely by Day 3 (12z Weds). This will largely be due to the same aforementioned shortwave moving into the Four Corners region, which should direct the favorable left-exit region of a ~100 kt jet streak over the confluence of the Central/Southern High Plains. PWATs pushing 1.25" and CAPE of 1000-2500 J/kg should easily support convection capable of heavy rainfall, and the upper-level support (and resulting shear) may help to organize convection into a complex as well. Since any introduction of a Slight Risk would be highly dependent on a better spatial signal (with CAM support) and possible wet antecedent conditions from previous days (given that long-term soils are still rather dry broadly), will continue to hold off for now (while expanding the inherited Marginal Risk fairly significantly). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4CcbxAYeA706usC7CiEbpmHrTR4OkcLmvVp9pz6eFvfj= cWZzfxm1dkP7Z-QQAKWl6UpfaLFqIH82ffx2tWZiOX3SznE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4CcbxAYeA706usC7CiEbpmHrTR4OkcLmvVp9pz6eFvfj= cWZzfxm1dkP7Z-QQAKWl6UpfaLFqIH82ffx2tWZiKJ3OvdM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4CcbxAYeA706usC7CiEbpmHrTR4OkcLmvVp9pz6eFvfj= cWZzfxm1dkP7Z-QQAKWl6UpfaLFqIH82ffx2tWZiu907MeE$=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) .