Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 26 2023 20:26:38 FOUS30 KWBC 262026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 425 PM EDT Wed Jul 26 2023 Day 1 Valid 16Z Wed Jul 26 2023 - 12Z Thu Jul 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE LOWER PENINSULA OF MICHIGAN AND SURROUNDING PORTIONS OF INDIANA AND OHIO, AS WELL AS FOR SOUTHEAST WISCONSIN AND INTO FAR NORTHEASTERN ILLINOIS... ....16Z Update... ....Michigan Area... Only a few changes were needed for the midday update as the MCS and associated bowing segments are generally behaving consistent with some of the CAMs. The 06Z guidance had 2 distinct camps of models showing the heaviest rains either from the UP to the northernmost counties of the L.P., with a second camp showing southern L.P. over to Detroit likely seeing the most rain. Now with current radar trends and the 12Z CAMs guidance in, the southern camp appears to have won out as the more correct solution. That isn't to say there won't be rain further north, as there's already convection ongoing across the U.P. and northern WI, but it is not expected to be persistent enough to produce any more than an isolated flash flooding threat. Thus, the northern L.P. was downgraded to a Marginal risk with this update. Further south however, expect more storms to develop behind the initial bowing segments moving into western MI from off Lake Michigan, which will result in training storms that are likely to generate isolated to widely scattered flash flooding. Southeastern MI, including the Detroit metro area has the highest chances of flash flooding, especially when considering the locally wet soils from recent rains from Detroit north. As the storms push east across the Ontario Peninsula and into western NY tonight, lack of diurnal support should reduce instability enough to allow for a gradual weakening trend in the storms, despite the continued strong upper level support. Thus, the flooding threat through the very early morning of Thursday should be isolated due to lower rain rates. ....Florida... The Marginal Risk area was trimmed out of the Everglades and confined to the I-95 corridor from roughly Ft. Lauderdale south, and much of the Keys. Anomalous moisture to over 2.25 inch PWATs are expected to develop over the area which will support storms capable of heavy rainfall rates, and a lack of steering flow will make any storms that form capable of very heavy rainfall rates. Confidence on where those storms will develop within the Marginal Risk area is low. ....Four Corners Region... No changes to the inherited Marginal Risk area were made. Weak monsoonal flow and moisture will have to overcome a very strong cap, which will greatly limit storm coverage. However, burn scars and slot canyon in the area are susceptible to flash flooding with relatively little rain so the Marginal risk area remains. Wegman ....Previous Discussion... ....Upper Midwest through the Ohio Valley... A potent shortwave trough is digging southeastward into MN/WI/IA early this morning, along the northeast periphery of the southwest monsoon ridge. Convection is trying to organize into a consolidated mesoscale convective system (MCS) in association with this shortwave, and there is certainly a leading bow echo that is the focus (rapidly translating southeastward across southeast MN into southwest WI, at the time of writing). However, there are multiple other clusters of convection to the southwest and northwest that are complicating things, and the 00z HREF suite is all over the place its depiction of convection through 12z (and even more so for 12z and beyond). So there is still ample uncertainty with regard to how well this developing MCS is able to hold together through the morning hours, as the low-level jet supplying moisture transport begins to weaken and veer over the Middle MS Valley. Many of the HREF members weaken (or even kill off) this MCS as it moves into southwestern WI and northern IL, but convection will likely refire by mid-day in association with the digging shortwave (and potentially enhanced by an MCV) as instability (via SB CAPE) rapidly increases to 1000-3000+ J/kg with daytime heating across northern IL and southern WI. Given this potential (with HREF 40-km neighborhood probabilities for 2" exceedance over 3-hr increasing to 15-20% after 18z), the inherited Slight risk area was expanded westward across Lake Michigan into southwest portions of WI (and far northeast IL, including northern portions of Chicago). More disorganized clusters of both surface-based and elevated convection may also be ongoing farther north and northwest (into north WI and eastern MN) through morning and mid-day, where a Marginal risk was maintained. Beyond 18z, the picture becomes less clear as the evolution will depend largely on mesoscale factors that are unknown at this time, but the HREF probability-matched mean (PMM) generally depicts the most likely scenario, which is a QPF streak that favors the southwestern flank of a developing/maturing MCS. This MCS is favored to track nearly due east (with the mean storm motion vectors tending to favor a more northeast track, whereas the upwind propagation vectors will likely favor a more east-southeast track towards the LLJ and greater instability). So depending on where this MCS sets up, any portion of the Lower Peninsula of MI could favor the highest QPF. The HRRR is the farthest south of the 00z guidance (but shifted a bit north towards the 00z HREF consensus with the 06z run), suggesting the best totals could slide southward into far norther IN/OH (where the inherited Slight risk was also expanded a bit south to include). That said, the totality of the HREF (and resulting PMM and neighborhood exceedance probabilities) would most favor southern portions of MI, which would put southeastern MI (including the Detroit metro area) into the greatest risk of flash flooding (given relatively low 3-hr FFGs of 1.5-2.5"). It is here that the HREF PMM is maximized (depicting a streak of 2-4" totals) with neighborhood exceedance probabilities for 3" and 5" thresholds of 25-35% and 5-15%, respectively. These totals seem plausible (despite the relatively fast motion of the MCS), as backbuilding and training would be favored on the trailing southwest flank of the MCS. Should the HRRR solution verify, the aforementioned axis of greatest probabilities would shift south (which could bring a more direct threat to the Toledo and Cleveland metro areas). ....Four Corners... The monsoon ridge will remain amplified near the Four Corners today, but increased elongation from SW to NE continues to be noted in the mass fields, 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 rapidly eastward in the northern stream (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 near normal, but MUCAPE may reach as high as 1000 J/kg beneath the ridge (per the HREF), especially since convective debris cloudiness looks to be minimal today. 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). Overall the flash flood risk looks to remain 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 today, as tropical moisture with PWs of 2.1"+ (above the 90th percentile) in association with a Tropical Upper Tropospheric Trough (TUTT) moving into the region (coinciding with 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 high localized short-term and longer-term rainfall totals (with the 00z HREF indicating 3"/3-hr and 5"/24-hr neighborhood exceedance probabilities as high as 10-20%). Churchill Day 2 Valid 12Z Thu Jul 27 2023 - 12Z Fri Jul 28 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NEW ENGLAND AND THE URBAN CORRIDOR OF SOUTHEAST FLORIDA... ....2030Z Update... ....Northeast... The inherited Slight Risk area was expanded to include much of NH and southern ME with this update. Antecedent conditions remain highly favorable for redevelopment of flash flooding Thursday afternoon and into the overnight across this region. An MCS will restrengthen on the leading edge of a strong shortwave that will be translating eastward across New England. A very moist air mass will advect into the region ahead of the shortwave that will potentially raise PWATs to over 2 inches. That is over 3 sigma above normal for northern New England. Thus, while the storms thankfully will be fast moving, 5-10 kt Corfidi vectors out of the west will favor backbuilding storms, which will maintain the MCS. The fast moving storms will certainly keep the flash flooding threat from becoming much worse, given the plentiful atmospheric moisture available. However, because of that same abundant moisture, the storms will be capable of over 2+ inch per hour rainfall rates. Because streamflows remain very high due to previous rainfall, expect flash flooding to develop rather quickly after initiation of the heaviest rain should the storms move over particularly sensitive areas. The surrounding Marginal Risk into the Mid-Atlantic was left unchanged with this update, as the threat for more widely scattered storms that will be largely detached from the primary forcing going over New England will likely still have enough energy to cause scattered shower and thunderstorm development down to the DC area. The expansion of the Slight was coordinated with the GYX/Gray, ME and CAR/Caribou, ME forecast offices. ....South Florida... An easterly wave is slowly pushing westward across the Bahamas towards south Florida this afternoon. This morning's storms ahead of the wave have largely dissipated, but storms capable of producing multiple inches of rain per hour are just offshore. Sometime overnight tonight the storms are likely to begin to move ashore into the I-95 corridor of south Florida. There is some disagreement among the CAMs as to when those storms will begin to move into the area...however recent rainfall both from previous days and this morning have left water tables above normal in the area, meaning less rainfall is needed in order for flooding to start. In addition to the antecedent soil conditions, the environmental conditions are both favorable and concerning for additional flash flooding development. PWATs are about as high as they can possibly be outside of a tropical cyclone, forecast to eclipse above 2.25 inches, perhaps approaching 2.5 inches. This is as much as 4 sigma above normal for this time of year in this area, which is a truly remarkable statistic. Further, since the easterly wave is a slow-mover, so too is the associated thunderstorm activity, as evident by the present depictions by the Miami radar. Once these storms move ashore, they will be able to take advantage of those remarkably high PWATs to translate that to very efficient warm rainfall processes. Rainfall rates in the strongest storms may be as high as 3 inches per hour. There remains some uncertainty in the CAMs as to how much coverage of rain there will be, but there is some potential for additional upgrades in this area when the coverage of storms becomes more clear. With the heaviest rain likely holding until well into the overnight hours, the greatest potential flooding impacts will be in the Day 2/Thursday morning period. In coordination with MFL/Miami, FL forecast office, the Slight Risk upgrade was made with this afternoon's update. ....Upper Midwest and Great Lakes... No big changes were made to the Marginal Risk area across this region, as the signal for heavy rains over this area is diffuse at best. Further, overall antecedent conditions are around normal, leaning drier than normal, so the soils should be able to handle most of the rain that falls in this area, and any flooding threat remains isolated. ....Southwest... No big changes were made out in the Southwest with the ongoing monsoonal moisture. There should be storms firing Thursday afternoon into eastern UT and western CO, so the Marginal was expanded northward into those areas, but again due to a rather strong ridge in place across the area, the storms should remain isolated to widely scattered, which will keep any flash flooding threat confined to those areas with the most persistent storms or where the storms form over areas that are more sensitive to flooding, such as burn scars and slot canyons. Wegman ....Northeast/New England into the Mid-Atlantic... A potent shortwave trough (and whatever is left of an associated MCS/MCV) looks to rapidly translate eastward into the region from the eastern Great Lakes early Thursday, which will drive convective development within an anomalously moist (PWs approaching 2.0", well above the 90th percentile) and increasingly unstable (SB CAPE rising to 1000-3000 J/kg) air mass. While significant uncertainty with regard to the ultimate organization and coverage/placement of convection and resulting QPF remains, concerns have increased enough to prompt to introduction of a Slight risk area across all of VT and portions of western NH/MA and northern CT (in coordination with the local WFOs and largely owing to wet antecedent conditions and still highly elevated streamflows). A much broader Marginal risk was maintained and expanded significantly southward into the Mid-Atlantic, where CAM guidance is in pretty good agreement regarding at least isolated to scattered convective coverage (with 00z HREF neighborhood exceedance probabilities through 00z Friday of 20-40% and 15-25% for 2" and 3" thresholds, respectively). The Slight risk area with continue to be refined (with at least a good chance of meaningful expansion) with the addition of more CAM guidance on the 12z HREF cycle. ....Upper Midwest and Great Lakes... Available CAMs continue to suggest the potential for more scattered areas of convection in association with a shortwave trough (possibly staying too far north in Canada) in the northern stream, as well as shortwave impulses rounding the southwest ridge (in what can be considered the southern stream). While organized convection is looking a bit less likely due to a lack of amplitude with the primary northern stream shortwave trough, disorganized areas of convection may be able to become organized (and would likely favor portions of south and eastern portions of MN and northern portions of WI). Maintained a Marginal risk across the areas most likely to realize isolated to scattered coverage of convection (with PWs of 1.4-1.8", MU CAPE of 1000-3000 J/kg, and 30-40 kts of deep layer shear expected to support heavy rainfall and help organize any convection). ....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 area looking to shrink to include much of NM, southern CO, and eastern AZ). The diurnally driven convection will continue to support rainfall rates of up to 1"/hr (with localized sub-hourly accumulations of 0.5-1"). More vulnerable terrain (burn scars and dry washes) will be most at risk for any localized flash flooding.=20 ....South Florida and Florida Keys... Highly anomalous tropical moisture with increased convective coverage across the region (in association with a lingering TUTT) will continue (and spread northward a bit) into Thursday. Similar to Day 1, 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, but even through 00z Friday the HREF neighborhood exceedance probabilities suggest meaningful odds (15-25%) of localized 5" exceedance (over 12-hr). Churchill Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_LQDHQy9Lh6lcORFyST9VSEo7LMQ5r0gt8FS1oK1J_UX= GIXWkHb4oFgvP8XYy2X8vuFDkEE2Kgm2YBn51gU5W_oHMmI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_LQDHQy9Lh6lcORFyST9VSEo7LMQ5r0gt8FS1oK1J_UX= GIXWkHb4oFgvP8XYy2X8vuFDkEE2Kgm2YBn51gU5Jl8aI50$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_LQDHQy9Lh6lcORFyST9VSEo7LMQ5r0gt8FS1oK1J_UX= GIXWkHb4oFgvP8XYy2X8vuFDkEE2Kgm2YBn51gU5yRd1t_A$=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) .