Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 03 2023 15:54:06 FOUS30 KWBC 031554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Thu Aug 03 2023 Day 1 Valid 12Z Thu Aug 03 2023 - 12Z Fri Aug 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID MISSISSIPPI, LOWER OHIO, AND TENNESSEE VALLEYS TO THE SOUTHERN APPALACHIANS, AS WELL AS THE NORTH/CENTRAL PLAINS INTO EASTERN PORTIONS OF THE INTERMOUNTAIN WEST... ....16Z Update... Minimal adjustments were made with the midday update, owing to considerable uncertainty all across the country. The uncertainty isn't necessarily that flash flooding will occur or when, but where. Thus, the main changes were to trim the risk areas into central NC/SC downstream of the Appalachians, as a number of models indicating convection this morning were wrong as it's been mostly stratiform rain so far. A Marginal Risk area was added to the East Coast of south FL today for slow moving and likely heavy rain producing storms likely due to the very high PWATs well over 2 inches. Elsewhere, only very minor changes were made. There are a number of areas of concern to discuss across the country today, despite the few changes ultimately made: =46rom southeast MO through northwest TN, training convection ongoing is finally exiting MO, and is moving at a good clip across west-central TN. While flash flood warnings are out for this area, the typical diurnal cycle appears to be working and the entire complex is weakening. Nonetheless multiple inches of rain over a small corridor of west-central TN remain likely over the next several hours as the entire MCS continues moving southeast. The big concern comes later tonight, as multiple pieces of guidance suggest a renewed round of training convection in very high PWATs redeveloping somewhere in this area from central MO through northern AL. So, while confidence is increasing that training thunderstorms will develop and these too will follow the same diurnal cycle starting in the middle of the night and persisting through as late as midday, there is next to zero confidence on where this likely narrow axis of heavy rain will set up. While there's a decent chance it will overlap the areas hard hit this morning, there's greater chance that there is an offset. Any offset would significantly reduce the flooding potential. Thus, the area remains in a high-end Slight Risk but an upgrade may be needed for the overnight period in parts of this area as the details on where the storms will form get hashed out in the guidance. The other area of big concern is well out west across the ID/UT/WY area. PWATS over 1 inch (well over 1 inch in some cases) are combining with a well-defined meso-low evident over far eastern UT, with precipitation wrapping around the north, west, and south sides of the low as of this morning. The low is a very slow-mover, so with daytime heating expect much more coverage of heavy rain producing convection, with training most likely to the west of the center, but convection possible all around the low. This area has been hard-hit in the past couple days, and the threat of steady convection which will likely persist well into the overnight period. This area is considered a higher-end Slight risk. No major changes were needed for the Slights across the Plains, with expected squall line convection developing late this afternoon, with training showers and storms likely following behind the main line, primarily along or on the KS side of the KS/NE border. Wegman ....Previous Discussion... ....Mid Mississippi Valley, Lower Ohio Valley, Tennessee Valley, Southern Appalachians... The setup remains favorable for heavy rainfall and flash flooding both Thursday morning and again late Thursday night into Friday morning as shortwave energy moves through the region within the northwest flow regime. The MCS and attendant MCV track across southeast MO and southern IL early this morning support a westward shift in the back edge of both the Slight and Marginal Risk areas across the western TN and mid MS Valleys. Elevated MUCAPEs between 1000-2000+ J/Kg along with PWs aoa 2.25" within the convective environment (MCS) will support peak hourly rainfall rates of 2+ inches through 13-15Z, as the southwesterly LLJ (30-35 kts at 850mb) veers more westerly towards 12Z, becoming more parallel to the mean 850-300 mb flow and thus favoring continued upwind propagation and cell training for a while longer. Initial MCS remnants/MCV will track into the TN Valley and eventually the southern Appalachians. Surge of PWs between 2-2.25+ inches along with anomalous 850-700 mb moisture flux (3 to 4+ standard deviations above normal for early August) will make for optimal warm rain processes, even as the deep-layer CAPEs struggle to reach 1000 J/Kg farther east across the southern Appalachians. CSU GEFS-FV3 based first guess fields support a Slight Risk into western NC/Upstate SC and across northern-central GA. CAMs show the next round of convection developing along or just north of the surface stationary boundary near the KS-MO border late this evening, then evolve into another MCS overnight into Friday morning across south-central and southeast MO, far southern IL, and western KY-TN. Still a bit more spread in the QPF guidance than one would like to see at the tail end of the day 1 forecast period, however it is likely that some areas will experience repeating rounds of convection early and late in the D1 period, which could bring several inches of rainfall for the 24-hr period (2-4"+) and given the forcing, highly anomalous moisture, and favorable instability setup, intense rain rates between 1-2"/hr will be possible. ....Eastern portions of the Great Basin into Wyoming and parts of the Central Plains... Shortwave energy lifting overtop dampening upper ridge combined with favorable right entrance jet dynamics will support a broad area of forcing for ascent from eastern portions of the Great Basin across much of WY and into the Plains. Moisture anomalies will remain above normal, nearly 2-2.5 standard deviations above normal, particularly across northern UT-eastern ID into northern WY and western SD. Convection is expected to develop initially over portions of the Great Basin before moving eastward through WY/MT and pushing into the Plains later in the period where it'll interact with the greater instability/moisture. Localized rain rates 0.5-1"/hr will be possible across ID while further east rain rates up to 1.5-2.0"/hr will be possible across portions of eastern WY and western SD. 00Z deterministic and ensemble guidance point toward localized 2"+ totals possible in that Slight Risk. Further south into northeast CO, northwest Kansas, and southern Nebraska, localized rain rates and totals may be just as intense. This along with the wet antecedent conditions may produce more scattered instances of flash flooding, and as such the inherited Slight Risk was maintained.=20 ....Interior Northeast... Broad forcing for ascent associated with an approaching shortwave energy embedded within the larger troughing will overspread the region while at the surface, a cold front begins to advance from the west. Ahead of that, southerly flow will bring modest moisture characterized by precipitable water values between 1-1.25". With daytime heating producing favorable MLCAPE values, scattered to numerous showers and thunderstorms are expected which could produce localized intense rain rates in excess of 1"/hr and may lead to isolated instances of flash flooding across portions of the interior Northeast. Hurley/Taylor Day 2 Valid 12Z Fri Aug 04 2023 - 12Z Sat Aug 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN HIGH PLAINS AS WELL AS MUCH OF NEW ENGLAND... ....Northern Rockies to Central Plains... Fairly potent shortwave energy lifting through the Great Basin is expected to become closed off at 700-500 mb across eastern Montana and the western Dakotas by early Saturday. This will wrap very anomalous moisture into the region, especially across southeast Montana and far northeastern WY into central-western South Dakota where PWs of 1.3 to 1.5"+ are expected, which is over 3 standard deviations above normal. As low pressure develops and moves from WY to SD, a very favorable frontogenetical band of precipitation is expected which could produce a longer duration moderate to heavy rainfall event. The signal for 1.5 to 3"+ looks pretty strong, especially with hints of coupled upper jet streaks, such that the Slight Risk was maintained across parts of the northern High Plains (south-central and southeast MT into north-central and northeast WY) into southwest ND, western and central SD, and northern portions of NE. Per the QPF spread in the guidance, there continues to be some uncertainty in how quickly convection progresses southeast into the Plains but the Marginal Risk was generally maintained through eastern Nebraska and western Iowa. ....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 a southward shift in the Marginal Risk area, based on the model consensus of a noticeable southward trend in the location of the surface boundary and thus the deep-layer instability/moisture pooling ahead of the fronts. ....Northeast... Longwave troughing becomes increasingly neutrally tilted during the period with the trough axis centered through Upstate New York late Friday afternoon and evening. This will allow for a longer period of southerly flow to bring anomalous moisture northward across the Northeast, particularly across the coastal regions of southern New England through coastal Maine with the latest guidance pointing toward a narrow axis of 1.5"+ PW setting up. Depending on the timing of the advancing cold front, enough destabilization will be possible during the afternoon/evening to produce scattered to numerous showers and thunderstorms across the region before quickly moving east (00Z ECMWF shows mixed-layer CAPEs between 1000-1500 J/KG). The modest moisture and instability and favorable forcing will support localized intense rain rates (1-1.5"/hr) with some localized totals near 2" possible. The broad Marginal Risk was maintained, but so too was the Slight Risk across eastern NY into much of New England given the combination of higher exceedance probabilities (e.g. more widespread HREF probs aoa 50% of QPF>2" in the 12hr period ending 00Z Sat), along with the wet antecedent soils. Hurley Day 3 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... Aforementioned vigorous mid/upper level shortwave, along with the eastward progression of coupled upper level jet streaks (right-entrance region of the jet streak near the U.S.-Canadian border and left-exit region of the upper jet axis nudging into the central High Plains) will maintain a favorable synoptic setup for more organized areas of heavy rainfall. This 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 (and heavy rainfall footprint) sliding farther south of the current ensemble mean of global guidance, and as such have altered the Slight Risk accordingly -- not only across the Dakotas (mainly SD), but also downstream into southern MN, northeast NE, and northern IA. The UFVS-verified version of the CSU first-guess field supports this shift with the Slight Risk area, as the 00Z non-CAM guidance already hints at areas of 2-4"+ max totals. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9AWXssXJXz44F9Kd2Bul5wDBjd7gdFRoPPOCbjyo8NN3= YC6qBB_e6Q9ZCP7AsHiSHHRNw0ocMDDUcT7wKCevhLx7VVI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9AWXssXJXz44F9Kd2Bul5wDBjd7gdFRoPPOCbjyo8NN3= YC6qBB_e6Q9ZCP7AsHiSHHRNw0ocMDDUcT7wKCevlJOEkvc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9AWXssXJXz44F9Kd2Bul5wDBjd7gdFRoPPOCbjyo8NN3= YC6qBB_e6Q9ZCP7AsHiSHHRNw0ocMDDUcT7wKCevFnmydsI$=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) .