Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 02 2023 15:54:48 FOUS30 KWBC 021554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Wed Aug 02 2023 Day 1 Valid 16Z Wed Aug 02 2023 - 12Z Thu Aug 03 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID MISSISSIPPI VALLEY... ....16Z Update... ....Mid Mississippi to Lower Ohio Valleys... Very few changes needed, pending a more thorough analysis of the 12Z data. The general consensus in the guidance is for tonight's round of training thunderstorms to likely go over or very near St. Louis as they track southeast from southern IA into western KY. The Moderate Risk area was expanded northeast about 10-20 miles to account for the various differences in the CAMs guidance. For now it also appears that the heaviest of the rain will stay north and east of the hardest hit parts of extreme western KY hit with devastating floods from a similar weather pattern a couple weeks ago. That said, this area may still pick up a few inches of rain, which may cause more localized flash flooding, so the Slight Risk remains in effect. The current activity stretching from eastern NE to around St. Louis with a separate area over western KY is all generally dissipating, though a few smaller elements embedded within the broader activity remains capable of heavy rain. With additional daytime heating disrupting the low level jet (LLJ) more, the overall weakening trend should continue, though widely scattered storms are possible through the afternoon, driven by the daytime heating and abundant moisture. The second round tonight is likely to begin over or near St. Louis late this evening and persist through the overnight. The general axis of heaviest rain will stretch close to where the current activity is ongoing. This overlap could exacerbate flooding concerns along the Mississippi River through St. Louis and into southern IL. ....Great Basin to North/Central Rockies and High Plains... Widespread showers and thunderstorms are ongoing all along the NV/AZ border through southern NV with a separate area across southeast ID this morning. The storms over southern NV are associated with a strong meso-low that is taking advantage of anomalously high PWATs. Salt Lake's 12Z sounding this morning featuring PWATs of 1.35 inches ranks in the top 50 soundings on record there. The meso-low will track NNE through the day, keeping heavy rainfall ongoing to the west of the track over eastern NV with more scattered storms likely across western UT. Due to the anomalously high moisture in place across the Great Basin, and rainfall rates locally exceeding 1 inch per hour already, the Slight Risk was expanded south and west in coordination with SLC/Salt Lake City, UT; LKN/Elko, NV; VEF/Las Vegas, NV; and SLC/Salt Lake City, UT forecast offices. ....NC/SC/GA Junction Region... Widely scattered but training showers and a few storms have already formed in this area along the same front causing the activity northwest into MO/IL. While the impacts will be much lesser here, think with the return of the LLJ tonight that the training storms will persist enough to cause local rainfall totals of 1-3 inches in the terrain of the area. Thus, a Marginal Risk area was introduced with this update. ....Southwest FL... HREF probabilities of over 50% just west of Lake Okeechobee for afternoon rainfall totals locally exceeding 5 inches with higher probabilities more widespread up through Tampa and Ft. Myers prompted the introduction of a Marginal Risk for this area. While the probabilities extend further southeast, think the heaviest rainfall should stay inland of the Miami/Ft Lauderdale area over the Everglades. Like most afternoons, the convection today is not expected to be very organized, but the prevalence of atmospheric moisture and slow-movement will once again raise a few isolated flash flooding concerns. Wegman ....Previous Discussion... ....Mid Mississippi to Lower Ohio Valleys... A significant flash flooding event is expected across portions of Mid Mississippi Valley this morning and then again late tonight into Thursday morning, impacting much of north-central to east-central Missouri with a narrow but potentialy extreme amount of rainfall in short period of time. This includes the greater St. Louis metro area, particularly for tonight/early Thursday morning. Early this morning, convection has continued to blossom and expand in coverage from southern Iowa through central Missouri at the nose of the strengthening low level jet. The latest RAP analysis showed upwards of 40 kts overrunning a frontal boundary draped across the region. This coincides with favorable forcing for ascent from shortwave energy rounding the periphery of the strong upper ridge axis. Finally, moisture anomalies are quite high this morning, where the blended TPW product shows values exceeding 2", and likely to increase further. The current convection will slowly drift south/southeast through central/east-central Missouri through mid to late morning. High warm cloud depths and an axis of 2000-2500 J/kg MUCAPE this morning will support higher end rain rates exceeding 2-3"/hr (localized 3"+ hourly totals possible) and with backbuilding/training convection, several hours of these intense rain rates in a narrow/localized swath is likely to produce a swath of extreme rainfall totaling 4-8"+ (localized 8-10") through the morning, which is supported by the 00Z HREF showing 50-60 percent probabilties for 5"+. This activity will begin to wane after 15Z (most intense rain rates through 15Z) but may not completely subside until early afternoon. Then later tonight into Thursday morning, another round of locally significant heavy rainfall is expected to develop across eastern Missouri into southern Illinois. Continued anomalous moisture, another surging low level jet, favorable instability, and high warm cloud depths all point toward intense rain rates (2-3"/hr) and localized higher end rain totals. The axis of heaviest rain may fall just east/southeast of this morning's heavy rainfall footprint, but very close overlap and also in and around the St. Louis metro. The 00Z HREF probs for 5"+ again reach 50-60 percent for tonight/early Thursday morning, maximized just west of St. Louis which shows even a near 20 percent chance for 8" totals. The 00Z CAMs generally all point to an higher end Moderate Risk flash flood event event and with the proximity to the STL metro, higher end flash flooding will be possible due to the intensity of rainfall for several hours over a more urbanzed location. ....Great Basin to North/Central Rockies and High Plains... Widespread showers and thunderstorms are expected Wednesday into Wednesday night aided by shortwave energy and right entrance dynamics with a strengthening jet streak over the northern Rockies. Moisture will be very anomalous, upwards of 1-1.25" of precipitable water and nearly 3 standard deviations above normal, based on the latest model guidance and with the daytime heating and terrain influences, localized intense rain rates between 0.5-1"/hr will be possible. The 00Z guidance continues to highlight portions of eastern Idaho, western Wyoming, and northern Utah with the greater probabilities of seeing the higher rainfall totals (localized 1-2") as well as portions of far northern Colorado and southeast Wyoming where Slight Risks were maintained from the previous forecast cycle. Taylor Day 2 Valid 12Z Thu Aug 03 2023 - 12Z Fri Aug 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER OHIO VALLEY TO SOUTHERN APPALACHIANS AND NORTH/CENTRAL HIGH PLAINS... ....Lower Ohio Valley, Tennessee Valley into 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. At the surface, an area of low pressure slide southeastward, advancing a warm front into the region. Strong southwesterly low level flow will bring a surge of moisture into the region, characterized by precipitable water values increasing from around 1.5" to over 2" from southern Kentucky through Tennessee and western NC. A complex of storms is expected to be moving through portions of southern Kentucky and middle Tennessee early in the period followed by another round of convection igniting late in evening and overnight into Friday across middle/eastern Tennessee into western North Carolina. The repeating rounds 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. There remains still some spatial uncertainty in where the axis of heaviest rainfall falls (global models further north/northeast compared to the southern CAMs) and the axis of heavy rainfall is likely to be rather narrow but could be locally significant, especially for the more susceptible locations and complex terrain areas. ....Great Basin to Northern High Plains... Shortwave energy lifting through the region combined with favorable right entrance jet dynamics will support a broad area of forcing for ascent from portions of the Great Basin to the Northern High Plains. Moisture anomalies will remain above normal, nearly 2-2.5 standard deviations above normal, particularly across portions of northeast WY, southeast MT 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"/hr will be possible across portions of northeast WY, southeast MT, 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, while rain rates and totals may not be as intense, the much wetter antecedent conditions combined isolated/scattered convection producing upwards of 1"/hr rain rates and localized 1-2" totals may produce more scattered instances of flash flooding and 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. Taylor Day 3 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... ....Northern Rockies to Central Plains... Fairly potent shortwave energy lifting through the Great Basin is expected to become closed off at 500 mb across eastern Montana and the western Dakotas. This will wrap very anomalous moisture into the region, especially across southeast Montana into western South Dakota where PWs > 1.5" are expected, which is over 3 standard deviations above normal. As low pressure develops and moves from WY to the Dakotas, a very favorable frontogenetical band of precipitation is expected which could produce a longer duration moderate to heavy rainfall event. The signal for 1-2"+ looks pretty strong such that the Slight Risk was maintained across western SD but also expanded into southeast MT. There remains 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. There remains uncertainties in the placement and evolution of the convection during the period but the environmental ingredients from the 00Z guidance all point toward some threat of flash flooding so the Marginal Risk was broadly placed from the Southern Appalachians toward the coastal areas of North Carolina and South Carolina. ....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. 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. A Marginal Risk was maintained but as mesoscale details become clearer, an upgrade could be needed across portions of southern New England where antecedent conditions are much wetter (14-day rainfall departures are running 200-400 percent of normal). Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5dnqtgNo6Y9amKg0zfEeSz7CukCHGJlbw2iDGGAh03hj= TqqcxD-cVYRyEkvhoQxOWoJIgtjaRCBDfSAOZJWjVvpmAyo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5dnqtgNo6Y9amKg0zfEeSz7CukCHGJlbw2iDGGAh03hj= TqqcxD-cVYRyEkvhoQxOWoJIgtjaRCBDfSAOZJWjlf-Y1Dg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5dnqtgNo6Y9amKg0zfEeSz7CukCHGJlbw2iDGGAh03hj= TqqcxD-cVYRyEkvhoQxOWoJIgtjaRCBDfSAOZJWjYbdDNAI$=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) .