Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 09 2023 20:14:48 FOUS30 KWBC 092014 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Sun Jul 09 2023 Day 1 Valid 16Z Sun Jul 09 2023 - 12Z Mon Jul 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC INTO WESTERN NEW ENGLAND... ....16Z Update... ....Mid-Atlantic to the Northeast... Very few changes to the overall look of the ERO for Day 1/Sunday across the Northeast today, but the heightened threat for numerous instances of flash flooding, especially in the Moderate Risk area continues to become more clear. The event is already underway across portions of central to northeastern PA, as was as an area of rain crossing into northern VA from eastern WV. There are numerous fronts, boundaries, old outflow boundaries, and other areas which are acting as a focus for the convection beginning to get underway. The below discussion sections are in order of geography from south to north as the event evolves northward, however the flash flooding threat generally also increases with latitude. ....Delmarva... Across the Mid-Atlantic, expect the area of rain over eastern WV to develop embedded convection as it pushes towards the DC area. This area has been hard hit with thunderstorms almost daily for the better part of a week, so the embedded convection here is likely to act in a similar fashion. It will be scattered in nature, but the abundance of moisture will allow the storms to locally produce rainfall rates to 2 inches per hour. Fortunately, the storms will be moving eastward at a fast enough speed that major impacts are not expected here. Due to the favorable antecedent conditions however, it won't take much due to the low FFGs for localized flash flooding to occur, especially in the urbanized areas around DC. Moving up towards Baltimore, northeastern MD and northern DE, storms that are likely to develop over the next 2 hours will have more time to move and train northeastward before the back edge of the boundary, noted by the area of rain over WV and far western MD moves through. This area has also been hard hit in recent days, so there is additional threat for flash flooding in this region as compared with areas south and west towards DC. ....PA/NJ/Downstate NY... Training storms have already broken out along the I-81 corridor through Scranton/Wilkes-Barre with less organized but quickly becoming numerous convection noted from the Philadelphia area northeast into northern NJ. As the upper level trough driving all of this rainfall across the Northeast pivots northeastward, the forcing lining up with the mean southwesterly flow will allow convection to continue growing along the aforementioned local scale fronts, boundaries, and instability axes. Since they will be training, portions of the area hardest hit should see a widespread 2-4 inches of rain, with localized totals as high as 8 inches in the hardest hit areas. Of course, it's very difficult to predict exactly where these boundaries set up and how able they are to sustained repeated storms moving over the area, but needless to say given the widespread urbanization of this area and inherent lower thresholds of rainfall to achieve flash flooding, that impacts are likely. 12Z HREF 100-year ARIs are up to 20-25% for the lower Hudson Valley and Fairfield County, CT. ....New England/Upstate NY... The rainfall event is, as expected, taking a little bit longer to get started as New England is further away from the forcing back across the Midwest, but when combining the expected rainfall for both today through Monday, this area is likely to be hardest hit of the entire area noted in the Moderate Risk. That said, most of the most significant impacts will be felt Monday into Monday night in this area. Nonetheless, given the very favorable antecedent conditions all across western New England, with widespread areas running 300% of normal based on 2-week AHPS output of the past 2 weeks of rainfall, even the first half of the event today is likely to cause numerous instances of flash flooding across New England and far eastern NY. Southerly flow of abundant Atlantic moisture will draw northward along with the developing convection, which will persist from this afternoon through the overnight and into the day on Monday. A surface low developing along the coast will help focus the convection as the flow gradually pivots southeastward. The surface low will be the dominant factor in the heavier rains on Monday, so more on that in this afternoon's Day 2 discussion. 12Z HREF 100 year ARIs are around 30% through 12Z tonight along the VT/NH border in the southern portions of those states, as well as into central VT. ....Eastern NC... The southern end of a potent shortwave will move across NC today into tonight, which will become a frontal boundary separating drier air over the Midwest from the tropical air mass both across the South and along the Eastern Seaboard. The convection that has developed across northern NC will continue to progress eastward, but with afternoon heating, expect scattered thunderstorms to develop out ahead of that main line over the Slight Risk area, which will lead to instances where heavy rain is prolonged when the leading storms are absorbed into the main line. This area has also been hit with daily convection in recent days, and soils in this area are also nearly saturated and are unlikely to absorb much of the heavy rainfall before the rest becomes runoff into already nearly full streams and rivers. The Slight Risk area was maintained for this issuance. ....MS Valley... A weakening MCS across central AR and northeast TX is itself fortunately becoming less of a threat as time goes on, but with a very tropical air mass out ahead of it, expect daytime heating to result in scattered to numerous thunderstorms across the Slight Risk area into MS and AL. The northern boundary of the MCS over northern AR may become a focus for slow-moving thunderstorms as well this afternoon. For northeast TX, expect an area of training showers with maybe a few embedded thunderstorms to develop overnight at the nose of an LLJ, which will keep the flash flooding threat, albeit isolated, continuing in that area into tonight. The only changes were to trim back the Slight risk area behind the MCS as OK finally gets a bit of a break from the active weather for the rest of today. OK will be ok! Wegman ....Previous Discussion... ....Mid-Atlantic to New England... Signals for heightened risk for flash flooding with isolated significant localized impacts possible across the Mid-Atlantic into Northwestern New England are coming into focus. A potent shortwave is starting to round the base of the larger scale trough across the Ohio Valley, resulting in some increased negative tilt by late afternoon. Strengthening broad southerly flow off the very moist Northwest Atlantic will increase total moisture across the Delaware/Hudson Valley into New England with 1.75 increasing to 2"+ by midday. In response a surface low and associated cold front (and inverted trof across NY) should support strong moisture flux convergence along its length by mid-morning (if not already ongoing by 09.12z) across NY, central PA to Western VA/NC.=20 Efficient rainfall production should result in localized rain-rates to 2"/hr and possible instances of flash flooding, deep layer steering will support northward moving cells supporting short-term training as the front slowly progresses eastward.=20 As the afternoon evolves, upper-level divergence increases along with heating to increase instability across the eastern US while the surface low strengthens in N VA. This will back the local flow across the Delaware Valley into the Hudson Valley, increasing moisture flux and FGEN ascent through E PA, further slowing forward progress of the convective line while cells still train/repeat to the north. By evening instances of 3-6" locally will become increasingly plausible with 00z HREF probability fields greater than 60% from DC to N VT (with spots near 90-100% for 3" and 30-60% of 5" across NE PA/N NJ to S VT. 00z Hi-Res CAMs have trended slightly eastward to align more favorably to the greatest 14-day precipitation over 300% of climatological normal axis extending from near Baltimore to Philly to N NJ and W MA, suggesting increased runoff potential for these. As the surface wave advances toward the New York Bight by early morning, increased low level flow, will advect unstable across across S NY/New England and with continued isentropic ascent through warm conveyor belt along/north of the surface low, additional efficient thunderstorms a likely to linger overnight, in proximity to SE NY/SW New England as well as into south facing terrain making a more likely scenario for flash flooding. As such, the Moderate Risk of excessive rainfall was shifted eastward slightly to account for the guidance trends. ....Eastern North Carolina... At the tail end of the cold front, the Slight Risk was expanded south and west to account for lower FFG over recently affected areas of west-central VA. Some consideration was made toward including the NC Fall Line/Piedmont region including Charlotte as the southern end of thunderstorm activity may lay out outflow boundaries for late evening/early overnight redevelopment of thunderstorms on increased orthogonal ascent of LLJ across the boundaries. The signal in guidance has increased for this thunderstorm development, but remain generally too isolated/scattered with only isolated enhanced rainfall totals to expand the Slight Risk at this time. ....Southern Plains to Central Gulf Coast... Like prior days, there is likely to be an ongoing MCS across Oklahoma at the start of the forecast period from prior night's upslope flow. Current RADAR and satellite imagery trends denote some increased in elongation of the wave/convective line from west to east with weaker isentropic ascent, though the length should maintain duration for modest rainfall crossing saturated grounds from multiple prior days' complexes, there remains a slightly faster trend toward the east. By mid-day, insolation and convergence in proximity to the MCV across LA/S AR should spur an additional round of stronger, moderately organized thunderstorms.=20 00z Hi-res guidance trends depict a more loosely organized clusters with some weak southwesterly isentropic ascent with cells moving off/away from the boundary into weaker elevated cells across LA/MS into SW AL. This is a general reduction of repeating/training environment expanding the potential for clusters than one or two broader heavy rainfall signatures/footprints. As such, the broad Slight has reduced in confidence for number of potential flooding incidents but remains generally broad enough to maintain, with a reduction of the western extent to account for the faster cell motions of the initial MCS this morning. Gallina=20 Day 2 Valid 12Z Mon Jul 10 2023 - 12Z Tue Jul 11 2023 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR THE CHAMPLAIN VALLEY AND MUCH OF NORTHERN VERMONT... ....2030Z Update... ....New England... Continued increases in both the total amount of rain expected across Vermont and northeastern New York prompted much of the 12Z guidance, particularly the 12Z HREF to indicate even higher chances of exceeding some remarkable thresholds Monday through Monday night. 24-hour rainfall totals of 3-5 inches across essentially the entire High Risk area, with local amounts approaching 12 inches for storm total rainfall over areas that have already been particularly hard hit in recent days by heavy rains prompted the upgrade. 12Z HREF probabilities indicate a 70-80% chance of exceeding 5 inches of rain for east-central Vermont. This same area has a 45% chance of exceeding 3 hour FFGs and a 60-70% chance of exceeding 6 hour FFGs. However, it's important to note that these are based on current FFGs, which are already low, and will likely come down considerably further as the event gets going across VT tonight. As far as 100 year Annual Recurrence Intervals go, expect a 60-70% chance of exceeding those values of 5-7 inches on the Vermont side of Lake Champlain. With all of these thresholds in mind, expect widespread considerable flooding impacts across much of Vermont and nearby surrounding areas, except perhaps the northeastern corner of the state. As everything continues to play out largely as expected, there are likely to be scattered Flash Flood Emergencies declared, along with mudslides and widespread flash flooding of not only small creeks and streams, but also the larger rivers. The widespread flooding is expected to be analogous to the 2013 Irene remnants event for this area, though this time no tropical storms are associated with this rainfall. Little has changed regarding the overall setup. A surface low starting out the day near the Jersey shore will slowly track northward to near Boston by Tuesday morning. This will make for a prolonged southeasterly flow of deep Atlantic moisture that will advect northeastward into the Green and White Mountains. Upper level support in the form of a rapidly strengthening negatively tilted shortwave trough...a pattern far more common in the cold season than mid-July... will enhance synoptic scale lift considerably across New England as it lifts northeastward. Further, a southwesterly 90 kt upper level jet streak is expected to form off the coast, which will support the strengthening shortwave, strengthening surface low, and lift which will wring out additional moisture. It's never a good thing when nature combines cool season upper level dynamics that are completely out of place for mid-July with far more typical heat/instability and tropical moisture into an area that already has been hard hit with heavy rainfall in recent days. Thus, localized catastrophic flooding is possible. As the low lifts northward Monday night, the High Risk area will enter the comma-head region of the cyclone, which will limit the instability and heavy convection a bit, especially the further west you go, but rain will continue right through the overnight across this region as areas to the east dry out under the dry slot. By 12Z Tuesday morning, the start of the Day 3 period, all but some lingering rain will be over, but another 1/2 to 1 inch may fall through midday Tuesday in northern VT/northeast NY. For southern New England, proximity to that aforementioned surface low will allow the heavy rain which will be ongoing at the start of the Day 2/Monday period to end from southwest to northeast as the dry slot of the low slowly tracks northward. The slow movement of the low, typical of summertime systems, will of course prolong the period of heavy rain ahead of it into New England, but the abundant moisture and flood sensitive areas of Worcester and Springfield, MA along with Providence, RI and Hartford, CT will likely cause localized flooding problems in those respective cities, so the Moderate Risk area was expanded south through all of western MA and the northern halves of CT and RI. For the Tri-State area around NYC, a nominally faster motion of the surface low over the city should end the rainfall right as this Day 2/Monday period begins Monday morning, so the Marginal Risk area was trimmed towards the Northeast and much of NYC was removed for this update. ....OK/TX Border area... A mesoscale convective system (MCS) is expected to form over the TX Panhandle Monday evening and progress eastward down the Red River overnight Monday night. While much of the activity will be in the form of a steadily moving squall line, pre-line convection likely to form will increase the threat, particularly over southern OK, for the pre-frontal convection will merge with the line, resulting in locally prolonged heavy rains over this area. Synoptically a front over much of northern OK will clash with a low-level jet (LLJ) advecting some impressively moisture-laden air into the area. In fact, PWAT values over the area will be around record territory for mid-July, which is no small feat. Further, instability will be plentiful, over 3,000 J/kg, which may support hail development, but will also support the very heavy rainfall rates expected over the area. The factors working against flash flooding will be around average soil moisture over the area, particularly on the OK side of the Red River (it's a fair bit wetter on the TX side), and fast storm motions which will diminish general time of heavy rain in any one area. In coordination with OUN/Norman, OK and FWD/Fort Worth, TX forecast offices a Slight Risk area was introduced with this update. ....Northeastern WA through much of Montana... A front that will move across the Intermountain Pacific Northwest and the high Plains of MT will produce locally heavy rainfall of up to 1 inch per hour across this region. The heavy rains will be supported by a small but vigorous shortwave progressing eastward along the Canadian border. In coordination with OTX/Spokane, WA; MSO/Missoula, MT; and TFX/Great Falls, MT forecast offices, a Marginal Risk area was introduced with this update. ....U.P. of MI and northeast WI... Frontal precipitation increased a little bit with the 12Z suite of guidance, so the Marginal Risk area was expanded a bit to cover more of the U.P. and also into WI, but for now no upgrades were considered with this update. ....Gulf Coast... Frontal and sea breeze convection expected along the Gulf Coast from New Orleans to Apalachicola will pose an isolated flash flooding risk Monday afternoon. At the moment most of the rainfall is expected to be offshore, with immediate coastal areas having very high FFGs, the Marginal Risk was maintained with no significant changes with this update. Wegman ....Previous Discussion... ....Northeast... At the start of the day 2 period (Mon 10.12z), the shortwave will will reaching peak maturity with deep layer DPVA across the Northeast as it starts to close off across N PA/E NY with surface low in proximity to E Long Island with a well developed and focused TROWAL crossing southeast New England with 20-30kts of warm conveyor ascending across New England into a sharpening deformation zone from the Catskills across Northern New England.=20 There are multiple signals for a remaining axis of 500-800 J/kg of MUCAPE along and southeast of the convergence zone, which will remain critical for the elevated, slow moving convection along the TROWAL. Given prior day's heavy rainfall (3-6") and already elevated wet soil conditions (per AHPS 200-300% above normal precip anomalies) and potential for slow/stationary cells resulting in localized additional 2-4"+, the risk of flash flooding remains highly likely and may need to be further increased based on how much rain unfolds in the Day 1 period, particularly across the southeast facing Berkshires and Green Mt Ranges where upslope flow should enhance ascent/rainfall rates where FGEN signals overlap the greatest.=20 ....Gulf States into South Carolina... Deep Layer trough as pressed the cold front and pooled moisture/instability along the Central Gulf Coast with a stronger trend toward a further south solution. This places the best upper-level diffluence and moisture flux convergence overall along and just offshore of the MS Delta toward western FL Panhandle.=20 While synoptic guidance continues to hint a stronger moisture flux convergence within the downstream shear axis across S GA; this looks like a typical guidance bias, stronger thunderstorms closer to the AL/GA and FL line are more likely given proximity to better instability, but should be more scattered in nature and within the area of higher FFG. As such, kept the Marginal Risk to align with the best moisture/instability where isolated stripes of enhanced rainfall could result in isolated flooding concerns. ....Central to Southern Plains... Stronger closed ridge over the Southwest and reduced activity overnight late Monday across Central High Plains, should have allowed for weaker moisture return to build up higher values, increased instability for potential activity into Tuesday evening. 00z guidance along with prior cycles continues to hint at an organized cluster or small complex to form at the upwind side of the cold front in the vicinity of NE NM into the Texas Panhandle/W OK. Storm mode will likely determine localized very intense rainfall rates capable of inducing an isolated few instances of flash flooding, in proximity to saturated grounds across central Oklahoma, a Marginal Risk is extended from the area in the Lower MS Valley back across and north of the Red River.=20=20 Additionally, northern stream cold front as leading edge of larger scale height-falls will be pressing southward across the central Plains and intersecting some of the untapped moisture/instability that rolled back through eastern Colorado on Sunday and Monday.=20 Instability of 2000-3000 J/kg and 1.5"+ total PWats pooled along strengthening (20-25kt) LLJ is likely to converge in early Tuesday for thunderstorm development across Nebraska, eventually propagating into KS. As such an extension of the Marginal Risk is brought northward into southern Nebraska extending toward the Iowa boarder.=20 ....Upper Midwest... There is uncertainty to the level of moisture flux return along/ahead of the cold front dropping out of Canada crossing Lake Superior to N Nebraska in the day 2 period. Stronger storms are likely to dot the front, but cell motions should mitigate the highest totals across portions of Iowa into MN, particularly given recent dry soil conditions. However, timing of the front with peak heating and likelihood of Lake Breeze enhanced thunderstorms off Green Bay/NW Lake Michigan may result in thunderstorm collision for a few spots of intense short-term rainfall rates over 2" and spots of 2-3" overall. The area has seen above normal rainfall over the last few weeks per AHPS and lower FFG values (mainly along the northern U.P.) may overlap enough for an isolated potential for localized flooding. So in coordination with MQT local Forecast Office, a small Marginal Risk of Excessive Rainfall was added for the core of the U.P. of Michigan for this cycle. Gallina 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!_TuDaXPx6UrUGVCr-Z-b3gVojIhV6pwip5bY6xvun3Ac= qEY_Fdim-ugpDFD0TxDtjvpFvQKpDNxEhh1RC5v3HpLRO9s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_TuDaXPx6UrUGVCr-Z-b3gVojIhV6pwip5bY6xvun3Ac= qEY_Fdim-ugpDFD0TxDtjvpFvQKpDNxEhh1RC5v3cnSaLY4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_TuDaXPx6UrUGVCr-Z-b3gVojIhV6pwip5bY6xvun3Ac= qEY_Fdim-ugpDFD0TxDtjvpFvQKpDNxEhh1RC5v3MsqnUxk$=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) .