Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 26 2023 08:48:59 FOUS30 KWBC 260848 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 447 AM EDT Wed Jul 26 2023 Day 1 Valid 12Z Wed Jul 26 2023 - 12Z Thu Jul 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE LOWER PENINSULA OF MICHIGAN AND SURROUNDING PORTIONS OF INDIANA AND OHIO, AS WELL AS FOR SOUTHEAST WISCONSIN AND INTO FAR NORTHEASTERN ILLINOIS... ....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 OVER VERMONT, AS WELL AS WESTERN PORTIONS OF NEW HAMPSHIRE AND MASSACHUSETTS, AND NORTHERN PORTIONS OF CONNECTICUT... ....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 Valid 12Z Fri Jul 28 2023 - 12Z Sat Jul 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST, GREAT LAKES, OHIO VALLEY, NORTHEAST, FOUR CORNERS, AND SOUTHWEST FLORIDA... ....Upper Midwest, Great Lakes, Ohio Valley, into the Northeast... A localized flash flood threat (at a minimum) looks to continue across much of the northeastern tier of the CONUS through Friday into early Thursday, as a low-amplitude trough is progged to traverse rapidly east-southeastward from the Upper Midwest to the Mid-Atlantic, likely organizing into clusters of convection (or perhaps even an MCS or two). What locations are ultimately directly impacted remains rather uncertain, given the dependence on mesoscale details that cannot be known at this time. Currently guidance is most well-clustered over portions of the Great Lakes (primarily from near Chicago eastward to near Detroit/Cleveland), as a blend of guidance suggests localized totals of 1-2" across this area. In reality, there is likely to be a strip of localized totals that are higher (especially if more organized convection into an MCS or two is involved), and the details will need to be worked out by the addition of convective allowing guidance (as we get closer to the event). That said, it is worth noting that if this same coordinator remains favored as we get closer, a very similar region is to be impacted by convection on Day 1.. so this alone may necessitate an upgrade to Slight risk in future updates (should the expected precipitation occur over a similar area as Day 1). Northwest flow aloft and the position along the northern periphery of a ridge certainly will favor this area meteorologically (with all 3 ingredients of moisture, lift, and instability expected to be in place). ....Four Corners... Very similar to prior days (and across almost the exact same areas of NM/CO/AZ on Day 2), isolated to widely scattered convection will be favored beneath the core of a dominate monsoon ridge. This convection will likely be capable of rainfall rates of up to 1"/hr (with localized sub-hourly accumulations of 0.5-1").=20 ....Southwest Florida... The aforementioned TUTT and associated highly anomalous tropical moisture begins to shift west by Friday, as veering and weakening low-level flow likely brings decreasing convective coverage to the Florida Keys. Portions of mainland southwest FL look to be favored for additional rainfall, likely driven primarily by diurnal convection in association with the sea breeze circulation. Rainfall totals of 1-3" in a short period could cause localized rapid-onset flooding issues, particularly over poor drainage metro areas (and possibly worsened by prior days rainfall). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4LxIKMgNI2Qajhia7uPIpzrkXyLkebhPwnM4Y0S543H3= UwfygJjdBw-VtRow53Mpbkz2fNQQqhFSebz2rfvFMGhfwNA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4LxIKMgNI2Qajhia7uPIpzrkXyLkebhPwnM4Y0S543H3= UwfygJjdBw-VtRow53Mpbkz2fNQQqhFSebz2rfvFdLNVX2c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4LxIKMgNI2Qajhia7uPIpzrkXyLkebhPwnM4Y0S543H3= UwfygJjdBw-VtRow53Mpbkz2fNQQqhFSebz2rfvFpeZpyMM$=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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