Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 04 2023 08:24:48 FOUS30 KWBC 040824 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 424 AM EDT Fri Aug 04 2023 Day 1 Valid 12Z Fri Aug 04 2023 - 12Z Sat Aug 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN HIGH PLAINS... ....Portions of the Northern High Plains... Maintained the Moderate risk area for portions of the northern High Plains with anomalous moisture and persistent upper level energy helping to focus and sustain convection. The global models, high resolution CAMs...ensembles and the NBM all point to potential for 1 to 3 inches across parts of Montana into the western Dakotas on Friday and Friday night. A closed mid and upper level low located he right rear entrance region of an upper level jet over Canada and the nose of a 90 to 110 kt upper level jet moving on-shore and progressing across California/Nevada during the day and into the Great Basin this evening will tap into a moisture atmosphere for a numerous showers and thunderstorms capable of producing intense rainfall rates and potentially excessive rainfall amounts. The ECMWF ensemble mean shows Precipitable Water values more than 3 standardized anomalies greater than climatology for this time of year in southeast MT and southwest ND by 05/00Z...placing it in the top few climatological percentiles...and the GEFS paints a similar picture with only slightly less magnitude. Latest 40 km neighborhood probabilities still showed values at or somewhat above 40 pct for the 5 inch threshold and nearly 15 pct chance of exceedance of 8 inches. The expectation is that convection with the most intense rainfall rate will develop during peak heating across the Moderate Risk area, but will continue into Thursday night with somewhat diminished rainfall rates as the area builds towards the better instability farther south.as more stratiform rain that very slowly sags southward towards the Black Hills. This pattern will favor more widespread flash flooding since the rainfall is expected to persist much longer and over a larger area than previous days. ....Northeast... Reintroduced a small portion of the Slight Risk area over hydrologically sensitive portions of New England...in part given the expected on-going rainfall early. Satellite and radar imagery showed the convection to be fast moving which should help mitigate many problems. However, the latest CAMs show a subtle increase in the probabilities of rainfall exceeding flash flood guidance in the short term...perhaps tied to steepening low level lapse rates which lead to somewhat enhanced rainfall rates. Kept the Slight Risk fairly targeted thinking that much of the area has been drier than normal and more likely to handle the expected rainfall amounts. ,,,Mid Mississippi Valley... Introduced a Slight Risk in portions of the Mississippi Valley where on-going convection may still be overlapping with the lowest flash flood guidance. ....Southeast... Anomalous moisture (PWs > 2") combined with favorable forcing for ascent associated with shortwave energy moving through the region will keep an active/unsettled pattern in place and the threat of heavy rainfall and flash flooding. The notable difference however from yesterday's Day 2 ERO was an uptick in the rainfall along the NC/VA coastline as a weak shortwave approaches from the west later today. Extended the Marginal risk area to cover the potential that some flooding or run-off problems occur. On the other hand...flash flood guidance is very high which should help mitigate any concerns. Bann Day 2 Valid 12Z Sat Aug 05 2023 - 12Z Sun Aug 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS INTO PARTS OF THE UPPER MIDWEST... ....Northern Rockies and Plains into the Midwest... A vigorous mid/upper level shortwave, along with the eastward progression of coupled upper level jet streaks will maintain a favorable synoptic setup for additional organized areas of heavy rainfall. This occurs as the 850-700 mb moisture flux anomalies remain 3 to 4 standard deviations above normal. Much stronger deep-layer instability south of the best frontogenetical forcing will likely result in organized convection. The anticipated heavy rainfall being a bit farther south of the current ensemble mean of global guidance has already been accounted for and still looks reasonable. The UFVS-verified version of the CSU first-guess field was less expansive with its Slight risk area than WPCs...apparently as a result of the spread in placement of the maximum QPF. But inspection of the spaghetti plots for 2 and 3 inch contours suggests a broader Slight was warranted. Bann Day 3 Valid 12Z Sun Aug 06 2023 - 12Z Mon Aug 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT LAKES... The potential for heavy rainfall will continue to spread eastward from the Upper Midwest on Saturday into the Great Lakes region on Sunday/Sunday night. Growing spread in the guidance makes zeroing in on a specific area...with the SREF being considerably slower than the GEFS (the difference spanning from Wisconsin and far eastern Minnesota across southern Lower Michigan. There was agreement between the various ensembles that 2+ inch amounts are possible. A shift in the 04/00Z ECMWF did give somewhat more support to the GEFS QPF placement over eastern Wisconsin into western Michigan. Opted to place a broad and rather unfocused Slight Risk over the western Great Lakes which was aligned with northeast to southwest fgen associated with a deepening surface low. The expectation is that that additional shifts and modifications will be needed as the guidance becomes clearer. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5-q93avBkqp6S8XvsZpWtfl0fo-dEqbtUWS-Trq26tWt= hcdIzX0FcGu_5Tg3dhsSy-AbDoWZHJ-4wEjOtD1OQcaC6WM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5-q93avBkqp6S8XvsZpWtfl0fo-dEqbtUWS-Trq26tWt= hcdIzX0FcGu_5Tg3dhsSy-AbDoWZHJ-4wEjOtD1OsKyFkR0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5-q93avBkqp6S8XvsZpWtfl0fo-dEqbtUWS-Trq26tWt= hcdIzX0FcGu_5Tg3dhsSy-AbDoWZHJ-4wEjOtD1Of3NUhew$=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) .