Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 04 2023 19:39:37 FOUS30 KWBC 041939 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 338 PM EDT Fri Aug 04 2023 Day 1 Valid 1929Z Fri Aug 04 2023 - 12Z Sat Aug 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR THE WESTERN DAKOTAS... ....Northern High Plains... Indicators for a potentially very significant rainfall continue to sound across northwestern SD primarily, but also into southwestern ND. 100 year ARI values (which represent total rainfall between 4 and 5 inches of rain) have spiked over 60% for a small portion of northwestern SD, with a 50% area into far southern ND. This is now considered a high-end Moderate Risk area. The broader Moderate Risk has been expanded north and east to include more of the steady but still stratiform rainfall ongoing across west-central ND, with the Slight Risk brought nearly to the Canadian border, as the steady rain is likely to amount to 1-3 inches, but will be falling over a longer period of time. This will still be a highly unusual amount of rain, but for northwestern ND may fall just slow enough that the river systems can mostly handle it. Such is not expected to be the case further south, where instability both from daytime heating, but far more significantly from the advection of high theta-e air continuously into the vertically stacked and intensifying low over SD will likely make for much higher rainfall rates in thunderstorms, which are expected to largely overwhelm the low FFG values between 1 and 1.5 inches in an hour over much of the Moderate Risk area as storms capable of producing 2 inch/hour rainfall rates are possible this afternoon and evening. ....Intermountain West... The Slight and Marginal Risks were expanded to include more of OR and ID, where there's good signal for 1-3 inches of rain on persistent easterly upslope flow, and the Marginal into OR where light to moderate rain has been ongoing all morning and convection is expected to continue developing this afternoon. ....Mid-Mississippi Valley... The Slight Risk across northern and eastern MO is mostly in place for an expected MCS that will develop this afternoon -- with the initial thunderstorms appearing to form currently -- and evening and move over the area, with a leading squall line followed by training convection. This area has been quite hard hit in recent days, and more rainfall is occurring presently, though fortunately is largely moving at a fast pace. This area will be monitored into the evening for a potential Moderate Risk upgrade as well. ....Northeast... The Slight Risk over the area matches messaging with the ongoing Flood Watches across portions of northeast NY and all of VT, NH, and southern ME. Training storms are beginning to develop with an MCV moving into ME and new convection forming presently across NY, which will move across the region this afternoon. After heavier than expected flooding in VT last night, the Slight Risk looks good for this region with no significant changes planned. ....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. ....Southeast FL... No changes were made to the Marginal Risk as slow-moving convection with heavy rainfall, which has mainly left the area this afternoon, may return during the overnight hours which may result in additional areas of localized flash flooding and ponding on roads. Roth 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. ....Tennessee Valley... Locally heavy rainfall may occur in proximity to a quasi-stationary front during the afternoon and evening.=20 Spaghetti plots from the GEFS show at least some threat of 2+ inch rainfall amounts where both the ECMWF and the GFS started to pool moisture along the front. Saw little reason to go any more than a Marginal at this point. ....Northern Rockies... Introduced a Marginal risk area in parts of the Northern Rockies where the operational model/ensemble consensus was for 0.5 to 0.75+ inch amounts. With steepening lapse rates combined with low level upslope flow, the thinking is that enough of the rainfall amounts could be delivered quickly enough to cause isolated problems,=20 Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5qSSaY8OPixSRo7QPwh9Za9jr-DqCRpVXpAhrqBSbvCr= N_DNITnp-_KL67ol1lxvAZXxw7W_UXgjbVD1j24rgI67U5o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5qSSaY8OPixSRo7QPwh9Za9jr-DqCRpVXpAhrqBSbvCr= N_DNITnp-_KL67ol1lxvAZXxw7W_UXgjbVD1j24rMa1wsX0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5qSSaY8OPixSRo7QPwh9Za9jr-DqCRpVXpAhrqBSbvCr= N_DNITnp-_KL67ol1lxvAZXxw7W_UXgjbVD1j24rBL4CJ3I$=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. 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