Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 27 2023 00:52:11 FOUS30 KWBC 270052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Wed Jul 26 2023 Day 1 Valid 01Z Thu Jul 27 2023 - 12Z Thu Jul 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH FLORIDA & THE CENTRAL GREAT LAKES AREA... ....South Florida and Florida Keys... A disturbed weather area appears to have some organization as it moves towards FL ahead of a substantial upper level low retrograding through the Bermuda Triangle and southwest of an MCV apparent in recent water vapor imagery. There is expected to be an uptick in heavy rainfall Wednesday night into Thursday morning as the disturbed weather moves into the region. Precipitable water values (2.25"+) and available instability (pockets 2500+ J/kg of ML CAPE moving in from the warmer than average Gulf Stream) are more than enough for heavy rain issues. Since it's a warm core scenario, the low-level inflow is roughly 50% higher than the mean 850-400 hPa wind. Effective bulk shear of 25+ kts lies just offshore FL Wednesday evening. Conceptually, this is a troubling scenario as it implies highly efficient heavy rainfall where convective banding can set up. While any instances of rapid-onset flooding are anticipated to remain localized, this air mass will be supportive of high localized short-term and longer-term rainfall totals, up to 3" in an hour and local amounts to 6" where banding can sit for a couple hours. A Slight Risk area remains for urban South FL as the area is more sensitive than usual to heavy rainfall, per coordination led by the MFL/Miami FL forecast office Wednesday afternoon. ....Upper Midwest through the Ohio Valley... Convection continues to edge along through the southern Mitt of MI, with progressive convection fading as it moves into western NY and an MCV moving into southern Ontario potentially leading to a new convective round across portions of northern NY late Wednesday night into early Thursday morning. The southern MI convection should enter western NY early Thursday morning, leading to a second round of heavy rainfall Used radar reflectivity along with the 18z HREF to reduce the area of the Marginal and Slight Risks in this region. ....Four Corners... The monsoon ridge remains near the Four Corners, but higher PW anomalies drape farther northeast than on previous days. PW anomalies are again progged to be near normal, as this is the monsoon time of the year, but MUCAPE of ~1000 J/kg has resulted in pockets of heavy rainfall. 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). The area was reconfigured based on radar reflectivity trends and the 18z HREF probabilities of 0.5"+ in an hour. Convection out on the High Plains of CO, southwest NE, and western KS hasn't been as mobile, and some of it could merge to lead to hourly rain totals of 1-2" with locally higher amounts, so the Marginal Risk was extended slightly farther eastward. Overall, believe this convection should be fading by 06z. Roth 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 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 SOUTH FLORIDA... ....2030Z Update... No major changes were made to any of the Marginal Risk areas across the country for Friday. In south Florida, with the easterly wave now out into the Gulf, the concern is any of the more typical afternoon thunderstorms, generally forced by the sea breeze will move over areas newly sensitive from rains from the easterly wave on Day 2/Thursday. The Marginal Risk was expanded to include southeast Florida for this reason. Any storms that form Friday will be isolated to widely scattered, but given the likelihood that one of those storms may move over an area with ongoing/recent flooding, think the threat for flash flooding is present into the urban east coast corridor, even though a large majority of the day should be dry in this region. In the Upper Midwest, another MCS may develop along a sagging cold front moving across the same areas being hit by the MCS moving through today. However, on Friday the focus of the heaviest rain will be a bit further west across WI. Due to generally average soil moisture conditions and unremarkable rainfall totals from today, there should be sufficient recovery time across the area for the soils and rivers to drain. Further, with significant uncertainty as to how the MCS will evolve, this area will get additional monitoring but the Marginal Risk is sufficient for the threat to the area. Across the Southwest, the storms will be relegated mostly to AZ and NM not not impact areas further north as they have today and on Thursday, but there may be a bit more coverage of storms, particularly in southeast AZ. Nonetheless, with a weak signal for storms, the Marginal Risk remains in place. Wegman ....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 Saturday, 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!-eTGKhdgf0NNBHzT1mhSdrEKfVe6y2-TFGbSktx9DjRP= 4yPmv415STZKz8Hktn1LyATEfmctFZI0I-UfrXXHfSnnRqg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-eTGKhdgf0NNBHzT1mhSdrEKfVe6y2-TFGbSktx9DjRP= 4yPmv415STZKz8Hktn1LyATEfmctFZI0I-UfrXXHLx8Vw7o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-eTGKhdgf0NNBHzT1mhSdrEKfVe6y2-TFGbSktx9DjRP= 4yPmv415STZKz8Hktn1LyATEfmctFZI0I-UfrXXHH80d_QY$=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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