Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 25 2023 20:19:20 FOUS30 KWBC 252019 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 PM EDT Tue Jul 25 2023 Day 1 Valid 16Z Tue Jul 25 2023 - 12Z Wed Jul 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC... ....16Z Update... ....Mid-Atlantic... No big changes were made to the ERO for today/Tuesday. The Slight Risk area was expanded a bit towards the north and east, to include all of the Jersey Shore and all of Delaware, but was trimmed a bit on the southern end to remove portions of central VA and southern MD, which are both more difficult to flash flood and less signal for heavy rainfall rates there, but also expanded to include all of the NYC metro out to central Long Island and far southern NY to include Rockland and Westchester Counties. In addition to the rainfall, CAMs guidance has been in good agreement that the storms will impact Baltimore through NYC around the evening rush, which will maximize potential impacts. These changes were coordinated with OKX/Upton, NY, PHI/Mt. Holly, NJ, and LWX/Sterling, VA forecast offices. Elsewhere, there is considerably more uncertainty across northern VA and also into southern New England as to how much coverage of storms there will be. Thus, the inherited Marginal Risk remains unchanged. Of course there's potential for localized flash flooding, but since the greatest coverage will be in the Slight Risk area, the potential for flash flooding due to less coverage of storms is lower in the surrounding Marginal Risk area. The 12Z HREF guidance is very similar to the previous 06Z guidance, so no changes were needed on account of the 12Z guidance. ....Northern Plains/Midwest... Very few changes needed to the inherited Marginal Risk area. There remains some potential for storms to move across this region, highlighting far eastern SD and much of southwestern MN. However, there remains considerable uncertainty as to the coverage and intensity of those storms, with decent agreement that the storms will be fast-moving. As mentioned below instability and PWATs will be quite high in this region, which should allow the storms that form to become intense and will be capable of high rainfall rates. Due to the aforementioned uncertainty, rather rural area with lots of agriculture, and dry antecedent conditions, the Marginal Risk was determined sufficient with this update. ....Four Corners... No major changes to the low-end Marginal Risk across this region. Isolated to widely scattered convection still looks probable this afternoon, but dry atmospheric conditions and a limited moisture supply should greatly limit the ability of any of the storms to produce flash flooding.=20 Wegman ....Previous Discussion... ....Mid-Atlantic/Northeast... A longwave trough will persist across the eastern CONUS today, extending from a closed-low near the Hudson Bay. Within this trough, an embedded shortwave will advect eastward while amplifying, moving across the coastal Mid-Atlantic and into New England during peak afternoon heating. This ascent will work upon a favorable thermodynamic environment characterized by PWs reaching above 1.5" collocated with MUCAPE eclipsing 1500 J/kg. The result of this will be rapid development of thunderstorms during the afternoon/evening, with cells organizing into clusters through 25-35 kts of effective bulk shear. Storm motions of these cells are likely to remain progressive to the east/northeast, but some short duration training is possible. Rain rates within convection will likely exceed 1"/hr, and may reach 2"/hr as shown by HREF probabilities and the HRRR sub-hourly fields. This will likely result in bands of 1-3" of rainfall with locally heavier totals possible (as reflected by 00z HREF 40-km neighborhood exceedance probabilities for the 3" threshold as high as 25-35%). The greatest risk area for this heavier rainfall is generally along the I-95 corridor from around Fredericksburg, VA to just west of New York City (with a modest northward expansion of the Slight risk for this cycle, incorporating more of northern NJ and edging into portions of southern NY and NYC). Confidence has increased a bit with this cycle, and the Slight risk continues to overlap where the highest exceedance probabilities coincide with the lowest FFGs and saturated soils is across this same region. Scattered instances of flash flooding are possible, especially where these intense rates track across urban areas, with a more isolated potential existing elsewhere across the Northeast. ....Northern Plains/Midwest... A Marginal risk has been introduced across portions of southeastern ND, eastern SD, southwestern MN, far northeastern NE, and northern IA for this cycle, as the 00z HREF has become increasingly focused on an area of convection developing later this evening and into tonight. This convection will largely be driven by a shortwave trough within the northern stream, traversing the northern periphery of the very dominant Southwest ridge. As the trough rounds the ridge and digs southeastward this evening, thunderstorms are expected to rapidly initiate and organize amidst impressive deep layer shear of 30-40 kts. Instability will likely range from 2000-3000+ J/kg (MU CAPE) with PWs progged to increase to 1.5-1.8" (between the 90th percentile and max moving average, per ABR sounding climatology) as the low-level (850 mb) jet strengthens to 15-25 kts. This will result in strong to severe storms that will be capable of dropping localized totals of 2-4" over a rather short period (with 00z HREF neighborhood exceedance probabilities of 3"/3-hr as high as 10-20%). Convection is expected to remain rather progressive, limiting training potential (and likely keeping any instances of flash flooding localized). ....Southwest/Four Corners... Mid-level monsoon ridge will remain entrenched across the Four Corners/Southwest Tuesday with only slight reduction in amplitude as weak shortwaves traverse the periphery and a more amplified shortwave trough digs out of the Pacific Northwest. This will produce slightly greater deep layer ascent, which when combined with 500-750 J/kg MUCAPE should result in scattered showers and isolated thunderstorms Tuesday. The PW plume from Monday is progged to weaken D2 noted by NAEFS ensemble tables showing just near normal PWs, but this will still be sufficient to produce rainfall rates of 0.5"/hr, with local rates as high as 1"/hr as progged by HREF neighborhood probabilities. The guidance has backed off slightly on total coverage and rainfall amounts, but the high-res simulated reflectivity still suggests storms will rotate SW to NE at around 15 kts, with some multi-cell organization possible within effective bulk shear of 15-25 kts. The flash flood threat appears modest overall, but if any of these more intense rain rates can occur atop sensitive features like burn scars, washes, or urban areas, isolated instances of flash flooding could be the result. Churchill/Weiss Day 2 Valid 12Z Wed Jul 26 2023 - 12Z Thu Jul 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF LOWER MICHIGAN... ....2030Z Update... ....Michigan... A potent MCS being forced by a strengthening shortwave moving east across the northern tier of states is likely to develop across Lower Michigan Wednesday afternoon. The combination of peak daytime heating and a 40 kt southwesterly jet advecting an over 2 inch PWAT airmass into Lower Michigan will set the stage for numerous areas of 2 inches or more rainfall amounts to occur as a result of the MCS, particularly in southeastern Lower Michigan, including the Detroit area. PWATs above 2 inches is more than 2.5 sigma above normal for this time of year. Antecedent conditions are a bit wetter than normal, though not extremely so. This should allow for a somewhat enhanced susceptibility to flash flooding across the Slight Risk area, especially into Detroit where urbanization will further lower FFGs. Neighborhood probabilities remain around 60% for 3 inches of rain through 12Z Thu and up to 20% for 5 inches. There remains considerable but understandable variability in exactly where those higher rainfall amounts will occur as the CAMs try to resolve the convective evolution of the MCS. Nonetheless given the excellent agreement that an MCS will cross the area Wednesday afternoon and the overall environmental setup with instability, high PWATs and a strong LLJ, confidence that flash flooding will occur has increased. The Slight Risk upgrade was coordinated with DTX/Detroit, MI and APX/Gaylord, MI forecast offices. ....Rest of the U.S.... Elsewhere, much of southwest FL was removed from the Marginal due to both dry antecedent soil moisture conditions and the Everglades being rather flood-proof, and so the Marginal risk was limited to the I-95 corridor from West Palm Beach south, including the Keys. Across the Southwest, the guidance has backed off significantly on convective coverage across the area, likely due to dry mid-level conditions. Certainly there will still be convection around, but it should be both weaker and more widely scattered based on the most recent guidance. The Marginal Risk area was trimmed away from where the WPC consensus had no meaningful precipitation with this update. There remains considerable uncertainty in this area, however so future expansions may be needed. Wegman ....Previous Discussion... ....Upper Midwest through the Ohio Valley... Dual shortwaves moving across the region on Wednesday will advect more easterly through the period in response to flattening flow north of an expanding/elongating mid-level ridge centered near the Four Corners. The first of these is progged to move from the Arrowhead of MN early to Upstate NY late, with a secondary impulse trailing it to move across a similar area the latter half of the day. Each of these will be accompanied by a modest wave of low pressure with associated fronts to enhance ascent from west to east. While uncertainty continues into the timing and latitude of these features, each one will likely produce scattered convection through PWs that approach +2 standard deviations overlapped with MUCAPE that may surge above 3000 J/kg ahead of the fronts. The simulated reflectivity during this time indicates that convection may organize into clusters or MCSs through 30-40 kts of effective bulk shear, driving rain rates that may reach 2"/hr at times. With the mid-level flow becoming increasingly zonal, mean flow will become aligned to the warm fronts suggesting an increasing likelihood for training from west to east, with some backbuilding into the greater instability also possible. This could result in multiple rounds of heavy rain producing thunderstorms moving across the region Wednesday, and an eventual Slight risk introduction may be needed (most likely across portions of the lower peninsula of MI and possibly into portions of northern IN/OH). Neighborhood (40-km) probabilities for 3" and 5" exceedance are already as high as 30-60% and 15-25% (respectively) through 00z Thurs (via the latest 00z HREF), but closer examination of the individual members suggests a high degree of north/south spread in the QPF axis (with some members still suggesting QPF magnitudes closer to the Marginal threshold), so this Slight risk potential will be reevaluated with subsequent outlooks. ....Four Corners... The monsoon ridge will remain amplified near the Four Corners on Wednesday, but some elongation is noted in the mass fields from SW to NE which should allow higher PW anomalies to drape farther northeast than on previous days. This evolution of the mid-level ridge is due to a shortwave advecting east from the Pacific which will drive height falls from the NW, resulting in this elongation of the ridge. PW anomalies are again progged to be normal to slightly below according to the NAEFS ensemble tables, but MUCAPE may reach as high as 1000 J/kg beneath the ridge, especially since convective debris cloudiness is expected to be minimal on Wednesday. This results in still favorable thermodynamics for heavy rainfall, and as forcing increases through subtle height falls and any weak impulses rotating around the periphery of the ridge, scattered showers and thunderstorms are likely during the afternoon/evening. Rainfall rates of up to 1"/hr are probable, which despite generally fast forward motion to the E/NE could produce locally 0.5-1" of rain (and often in as little as 15-30 min) as shown by modest ECENS/GEFS probabilities. Overall the flash flood risk on Wednesday appears isolated, but a few instances are possible (especially over the most vulnerable soils or terrain features, including in slot canyons, dry washes, and burn scars). ....South Florida and Florida Keys... Storm coverage looks to increase meaningfully across the region on Wednesday, as tropical moisture with PWs of 2.0"+ (above the 90th percentile) moves into the region upon increasing east-southeasterly low-level flow (via the influence of the westward shifting Atlantic subtropical ridge). While any instances of rapid-onset flooding are anticipated to remain localized (and primarily confined to where convection is able to effectively train), this air mass will be supportive of very high short-term rainfall totals (with the 00z HREF already indicating 3"/3-hr neighborhood exceedance probabilities as high as 10-20% through 00z Thurs). Churchill/Weiss Day 3 Valid 12Z Thu Jul 27 2023 - 12Z Fri Jul 28 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST AND GREAT LAKES, THE NORTHEAST/NEW ENGLAND, THE FOUR CORNERS, AND SOUTH FLORIDA AND THE FLORIDA KEYS... ....2030Z Update... No significant changes were made to the ERO areas in various portions of the country. The one most likely to need upgrading is across New York and New England. The MCS that will impact Michigan on D2/Wed will continue to press eastward into this region on Thursday. Due to very sensitive antecedent conditions widespread 1-2 inch rainfall amounts forecasted for this region is likely to result in isolated to widely scattered flash flooding. In coordination with BTV/Burlington, VT forecast office, the option to hold off another round of models to ensure the models are handling the MCS well was made. As the guidance typically is highly variable in their handling of MCSs, this was the most prudent course of action. It's possible portions of southeast FL and the Keys may also need to eventually be upgraded to a Slight as well. This is highly dependent on how much rain is picked up over the intervening 2 days, but the signal remains strong for widespread 1-2 inch rainfall amounts, though given the abundance of moisture, locally higher amounts are a near certainty. The upper Lakes and the monsoonal flow into the Southwest were left largely unchanged with this update. The signal for storms across the Southwest appears to be the least widespread on Thursday as compared with today and Day 2. Wegman ....Previous Discussion... ....Upper Midwest and Great Lakes... Early indications suggest that another shortwave and associated frontal system will rapidly progress through the region on Thursday, possibly bringing organized convection and localized totals of 1-2"+ over a short period. Given the speed of the system within the northern stream, guidance is presenting a large amount of uncertainty with regard to both the location and intensity/organization of any mesoscale system out at this range. Opted to maintain an inherited Marginal risk across the areas most likely to realize isolated to scattered coverage of convection (as all ingredients for heavy rainfall are likely to be in place). ....Northeast/New England... A shortwave trough (and possible MCS/MCV) looks to rapidly translate eastward into the region from the Great Lakes overnight on Wednesday, possibly resulting in a streak of 1-3" totals over a relatively short period. This may eventually result in the introduction of a Slight risk, but have opted to hold the region at the inherited Marginal risk (while shifted/expanded a bit northward) for this cycle. This is primarily due to the uncertainty in the track of the anticipated mesoscale feature, but there is still some uncertainty with regard to intensity/organization of the mesoscale system as well. Given the relatively low FFG for much of Upstate NY and into VT, an eventual Slight risk is more likely than not, pending increased confidence in the strength and track of the feature. The addition of CAM guidance will provide additional clarity as the event approaches, though the appropriate ingredients (elevated instability, moisture, and lift) all look to be present. ....Four Corners... The increasingly elongated monsoon ridge will remain amplified near the Four Corners into Thursday, presenting very similar risks (i.e. highly localized risk of flash flooding) to many of the same areas as earlier in the week (with the threat seemingly decreasing a bit farther west across much of UT and east-central AZ). The diurnally driven convection will be mostly focused across east-central CO into much of NM and southeastern AZ, as rainfall rates of up to 1"/hr will remain probable (with localized sub-hourly accumulations of 0.5-1"). ....South Florida and Florida Keys... Highly anomalous tropical moisture with increased convective coverage across the region will continue (and spread northward a bit) into the Day 3 period. Similar to Wednesday, any instances of rapid-onset flooding will likely remain confined to localized areas where highly efficient rainfall (with rates up to 2"/hr) is able to train/repeat. The addition of high-resolution guidance will provide more clarity in future outlooks. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7_xdwgs8y0ZwR0qABAxur0DMbvrcrWPUCa3f6vqKUI_r= lXMs5bLwM7ApkU5nVTHux8AqKui43t_cCAqcu4xdq3e1RJ4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7_xdwgs8y0ZwR0qABAxur0DMbvrcrWPUCa3f6vqKUI_r= lXMs5bLwM7ApkU5nVTHux8AqKui43t_cCAqcu4xdWxiEtmI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7_xdwgs8y0ZwR0qABAxur0DMbvrcrWPUCa3f6vqKUI_r= lXMs5bLwM7ApkU5nVTHux8AqKui43t_cCAqcu4xdsxOxifA$=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) .