Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 29 2023 20:09:23 FOUS30 KWBC 292009 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Mon May 29 2023 Day 1 Valid 1930Z Mon May 29 2023 - 12Z Tue May 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN TEXAS... ....1930Z Special Excessive Rainfall Discussion... Amended the Day 1 Excessive Rainfall Outlook in response to a some bands of heavier rainfall over parts of the Central/Southern Delmarva and the Metro DC area. Some streaks of 2 or 3 inches of rainfall are possible in a 1 to 3 hour period. Refer to the WPC Mesoscale Precipitation Discussion number 378 for more details. Bann ....Previous Excessive Rainfall Discussion. ....Near the California/Nevada Border... An upper low just west of central CA will meander along the coast today between the polar and subtropical jets to the northwest and south, respectively. Ample moisture remains over the region (PWs near and over 0.75" which is +1 to +2 sigma) and CAPE should build to 1000+ J/kg with afternoon heating. This should set the stage for convection in and near portions of the Sierra Nevada, Shasta, and Siskiyou Ranges around northern and eastern CA much like yesterday. 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 exceedance probabilities showing 10-40% of exceeding 2" amounts in far northern CA. This would be particularly problematic in any burn scars and over areas that saw heavier rain in the previous 24 hours. ....Northern Plains... Precipitable water values of up to 1.25-1.50" (+1 to +1.5 sigma) 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 lies near and east of the MN/ND/SD border region, where several 12Z CAMs show a small area of repeat convection this evening into the early overnight hours along a stationary or warm front. Small areal coverage precludes a Slight Risk outline. ....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 12Z CAMs have backed off a bit in QPF intensity (likely correctly), though 12Z HREF neighborhood probabilities of at least 2"/hr lie in the 10-35% range. ....Southern Texas... A progressive mid-level shortwave moving into the region from northern Mexico/Big Bend region may act to enhance the general heavy rainfall threat later this afternoon over eastern/southeastern/southern TX. PW values over 1.5" amid a rather deep moisture sounding supports some 2-3" totals which couple creep close to FFG values (that increase toward the coast). Maintained the Slight Risk area as this region holds the highest rainfall rate potential, but this is contingent upon any more organized activity this afternoon as well as potential backbuilding/training which has lower predictability. ....Southern Mid Atlantic States... Introduced a targeted Slight Risk area where morning rainfall has persisted and resulted in flash flooding over southwestern VA. Activity here should slowly diminish this afternoon but rainfall just after 16Z start time of this update will only aggravate ongoing issues. Otherwise, the 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 this afternoon into early evening along/north of the occluded front. Coupled with PWs between 1.25-1.5", this 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 and this morning, primarily from western VA northward into WV). Trimmed the northern edge of the Marginal Risk area as the moisture transport maximum should be limited/squeezed in a narrow east-to-west axis. Fracasso/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/storms within the outlook area -- a few will be capable of hourly or sub-hourly rainfall rates of 0.50-1.00" which is around FFG values. 12Z HREF neighborhood probs max out around 35% for exceeding these FFG values, though perhaps overdone at nearby lower elevations. Nevertheless, this will pose a localized flash flood risk for yet another day, especially over any recent burn scars into northeast CA and southern OR and/or over areas that have received recent appreciable rainfall. ....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. Relatively higher probs for greater rainfall/rates exists over/near southwestern KS tomorrow evening as convection moves into the region from the west. 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 and SPC denotes a Marginal Risk area for severe storms here as well. Models have loosely focused in on this area with varying intensity of QPF (typically high FV3 and NAM nest have backed off just a bit), and this region may be upgraded to a Slight Risk in a subsequent outlook. ....Southeast OK into Northeast TX and Ark-La-Tex... With a focus farther south than in previous issuances, nudged the entire Marginal Risk area southward per recent guidance and to maintain more separation from the QPF axis to the north. Precipitable water values rise to 1.5"+ tomorrow 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), though FFG values are quite high in this area so any excessive rainfall concerns may be very isolated. Fracasso/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... 21Z Update... 12Z guidance shows no appreciable change in the overall pattern/setup, and left the two Marginal Risk areas intact. 12Z CSU Machine Learning first guess fields have shown small Slight Risk areas anywhere from the TX Panhandle northward to southwestern NE, and will likely need some clarification in the D2 time frame before honing in on this D3 outlook. Fracasso ....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!6BfwVtuDiGiPx63iZfohfTk8ncPWZ1DSvgUvPUag6EyA= 54OJQu8Ck35xu7ultr7Xh7WHEEs_m9rJ44ty6816J12vmWY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6BfwVtuDiGiPx63iZfohfTk8ncPWZ1DSvgUvPUag6EyA= 54OJQu8Ck35xu7ultr7Xh7WHEEs_m9rJ44ty6816DvD3A18$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6BfwVtuDiGiPx63iZfohfTk8ncPWZ1DSvgUvPUag6EyA= 54OJQu8Ck35xu7ultr7Xh7WHEEs_m9rJ44ty6816sJOdWrY$=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) .