Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 12 2023 16:10:40 FOUS30 KWBC 121610 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1209 PM EDT Wed Jul 12 2023 Day 1 Valid 16Z Wed Jul 12 2023 - 12Z Thu Jul 13 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE LOWER MISSISSIPPI VALLEY AND CENTRAL MISSISSIPPI... ....16z Update... A Moderate Risk was introduced over central Mississippi during this late-morning update after assessing recent model trends and current atmospheric conditions near the Lower Mississippi Valley. Heavy rainfall potential is expected to occur overnight (after 06z Thursday) and possibly continue through the morning hours into Day 2. An ongoing and weakening MCS over southern AR/northern LA has produced a weak upper inflection up to 500 mb, with a closed 700 mb low over AR per SPC's mesoanalysis. This upper feature is expected to drift east-southeastward through the day within relatively weak westerly flow and situate over MS by tonight. PWATs in the region this morning were analyzed to be around 2-2.3", which was near the daily max at JAN. By tonight, steady southwesterly 850 mb flow will keep PWs above 2" across MS and allow for efficient rainfall rates where convection occurs. Subtle convergence on the western edge of the circulation will provide the lifting mechanism for developing convection, as well as 25kt southwesterly 850mb flow. MUCAPE is also projected to remain above 2000 J/kg up until convective initiation, with the upstream instability within the northwesterly mean flow also above 2000 J/kg. This will allow for convection to likely maintain for several hours over similar locations. There is low potential for northern energy across the Mid-Mississippi Valley to suppress this mid-level feature and limit the potential, but recent CAM guidance breeds confidence in a locally extreme rainfall event near the central MS region. Lingering uncertainty exists regarding the exact location of heaviest rainfall, but confidence is moderately high for localized 8"+ QPF amounts. 12z HREF guidance has notable neighborhood probabilities (50-60%) for at least 8" of rain within 24 hours, as well as 5" within 6 hours tonight across the introduced Moderate Risk. The Day 1 ERO elsewhere was only modified minimally based off of recent radar trends. Snell ....Lower Mississippi Valley and Gulf Coast States... An impressive vorticity maxima associated with a potent shortwave will be drifting SE from the Arklatex this morning. This feature is progged to drift slowly southeast while weakening through the day, but maintain a modest trough along the central Gulf Coast with weak accompanying height falls. This shortwave will be the primary mechanism for ascent today, but a dissipating surface trough and potentially a lingering outflow boundary from other convection could also serve as a focus for low-level convergence and ascent. The environment will be extremely favorable for heavy rainfall, however, with PWs 2-2.25 inches, +1 to +2 standard deviations above the climo mean according to the NAEFS ensemble tables, with overlapped MUCAPE of 2000 J/kg. These thermodynamics acted upon by the mesoscale features will likely yield scattered to widespread heavy rain producing convection today. The guidance has become more aggressive with thunderstorm potential today, and that led to the (then D2) slight risk upgrade for LA/AR. The area of greatest risk for heavy rainfall will be beneath the aforementioned vorticity max as it will likely manifest as an impressive MCS with slow moving thunderstorms possibly backbuilding along outflow boundaries to prolong rainfall. There is still uncertainty as to how this will evolve through the morning, but it is likely that as the LLJ of 20-25 kts veers it will less efficiently upglide atop outflows and lead to a reduction in coverage and intensity of convection. The timing of this is uncertain, but it is likely there will be several hours of moderate to heavy rainfall with rates 1"/hr or more before waning late morning into early aftn. The remnant MCV that is likely to develop will then serve as an additional focus for convection as it shifts southeast through the day, providing additional rounds of heavy rain as far as the Florida Panhandle. The inherited SLGT was expanded to the Gulf Coast as Corfidi vectors of just 5-10 kts aligned against the mean flow and into the higher instability suggests a good potential for backbuilding storms, and with rain rates of 2"/hr more, this could produce 3-5" of rain in some areas as reflected by HREF neighborhood probabilities. This could result in scattered flash flood instances despite relatively dry antecedent soils, with locally significant flash flooding possible across far southern AR/far northwest LA where event total rainfall (from pre-12Z through the morning) could reach 10 inches. ....Mid Mississippi Valley through the Great Lakes and into the Northeast... A broad but expansive trough centered near the Hudson Bay will maintain lowered heights and cyclonic flow into the eastern 2/3 of the CONUS today and tonight. Within this cyclonic flow, multiple shortwave impulses and accompanying vorticity maxima will rotate west to east in conjunction with a 90-100 kt zonally oriented upper jet streak to produce increasing deep layer ascent. This synoptic lift will impinge into a stationary front that will be draped west to east across the area and waver through the day. This will not only result in locally enhanced convergence for ascent, but may manifest as waves of low pressure in response to the vorticity maxima rotating overhead. While the exact timing of these shortwaves and any low pressure waves are a challenge to resolve even on this time range, it is likely scattered to widespread thunderstorms will be the result. Thermodynamics will be quite impressive across a large portion of this area, suggesting the widespread thunderstorms could all pose a heavy rain threat. PWs south of the stationary front will reach 1.75-2 inches as low-level SW flow reaches 30 kts and impinges into the front. This will supply the high PWs and collocated MUCAPE of 1000-3000 J/kg northward to enhance the environment, while also lifting isentropically atop the front and any residual outflow boundaries. Within the widespread convection, there is a signal for two areas of heaviest rainfall noted by the best convergence of moisture transport vectors along the greatest instability/PW gradients. The first is draped west to east along the stationary front from eastern IA through the thumb of lower Michigan. Here, convergence of the LLJ into the front will help turn the 0-6km mean winds to be parallel to the front, leading to an increased likelihood for training of cells which the HREF indicates will have rainfall rates of 1-2"/hr. This will likely result in waves of convection repeating across the area. The high-res guidance is actually in pretty good agreement and alignment for today, noted by HREF EAS probabilities for 1" and 2" that are quite high from Chicago, IL eastward to Detroit, MI and just north. Other than Chicago, this area has generally been dry the last 2 weeks, which is reflected by high FFG. However, training of echoes to result in locally more than 3" of rain and some exceedance of the FFG, which could result in scattered instances of flash flooding. The other area of concern is around Missouri where the most intense CAPE gradient and best moisture confluence will likely help develop convection in the evening which will then grow upscale into an MCS tonight. Some of the lead convection will likely develop along a residual outflow from prior thunderstorms, so uncertainty in the exact placement remains high. However, once convection begins it should expand rapidly and organize through 30-40kts of effective bulk shear. The guidance again is insistent that an impressive MCS will develop, and as Corfidi vectors collapse to around 5 kts this suggests some training from NW to SE with backbuilding also likely into the higher instability. Much of Missouri has been dry recently which has maintained high FFG, and the HREF exceedance probabilities are modest. However, training of 1-2"/hr rain rates could still produce 3-4" of rain in some areas as noted by the HREF probabilities, and the SLGT risk was adjusted just cosmetically for updated guidance. ....Southwest... The inherited MRGL risk for southeast AZ and far southwest NM was maintained, although the signal for excessive rainfall is modest. The difference today from the prior few days is that the area has received rain recently, so there are pockets of 7-day AHPS rainfall that are 100-150% of normal. FFG is still quite high, but does feature some pockets of 0.75-1"/1hr, and with PWs nearing +1 standard deviation according to NAEFS, the HREF suggests at least a 15-20% for 1"/hr rates. With storm motions progged to be just around 5 kts, and redevelopment probable on subsequent outflow boundaries in the pulse environment, isolated flash flooding is possible, especially in any urban areas. Weiss Day 2 Valid 12Z Thu Jul 13 2023 - 12Z Fri Jul 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY/CENTRAL APPALACHIANS AND THE INTERIOR NORTHEAST... ....Mid-Mississippi Valley through the Central Appalachians and Northeast... Broad mid-level NW flow will continue across Plains and Mississippi Valley, lifting into more SW flow across the Appalachians and Northeast as the broad and expansive mid-level trough centered near the Hudson Bay persists. This will provide an environment waves of convection moving through the flow. Most of this development on Thursday will occur along a slow moving cold front, and leading pre-frontal trough, that will become elongated as it approaches the base of the flattening but still broad and expansive trough, with locally enhanced ascent occurring downstream of weak impulses through PVA and height falls. Overall deep layer ascent appears modest on Thursday, so convection should be generally confined to the front and any mid-level shortwaves, suggesting a generally focused enhanced rainfall risk. Despite modest overall ascent, the thermodynamic environment will be quite favorable, so convection that does develop should be efficient rain makers. PWs will rise to 1.75 to 2 inches on W/SW 850mb inflow of 20 kts, with MUCAPE surging during peak heating south of the front to as high as 3000 J/kg, with the highest ribbon positioned from Arkansas through Ohio. While there is some uncertainty into exactly where the strongest convection will develop due to relying on residual outflows from D1 and timing of shortwaves aloft, the trend has been for activity to push a little south from previous across the MS VLY, and is now weaker/more progressive, prompting a removal of the SLGT from Missouri. However, convective trends among the global models has become a bit more organized and intense from the Ohio Valley into New England. Although much of the activity Thursday, which should be widespread, will be generally progressive, effective bulk shear of 30-40 kts will allow for storms to organize into multicells and then train to the northeast, with additional enhanced ascent occurring where flow can favorably upslope into terrain. The favorable thermodynamics across this region will support rain rates of 1-1.5"/hr, which through training could produce 2-3" of rain or locally higher amounts in some areas. This is reflected by at least modest 3"/24hr probabilities in the global ensembles, and some higher focused 3"/12hr probs from the HREF. While confidence in the exact evolution and development is modest, this region is still quite saturated from recent rainfall that is around 150% of normal in the eastern OH VLY, and 300-600% of normal in New England/Upstate NY, leading to well above normal streamflow and low FFG. The SLGT risk areas were drawn to best reflect the highest QPF probabilities overlapping some of the most sensitive soils. ....Central Plains... Broad NW flow on the upwind side of an expansive mid-level trough will drive multiple impulses through the flow and down across the region on Thursday. These impulses will combine with modest upper diffluence within the tail of a departing jet streak to the east, and convergence along a weakening front/surface trough moving across the area. A departing MCS in the morning could also produce some priming rainfall, but the greater risk for excessive rain will occur in the evening/at night. During that time, ESE 850mb inflow will surge to 20-30 kts as a wave of low pressure develops across the High Plains along a stationary front over Colorado. This will draw PWs above 1.5 inches northward, combined with resupply of MUCAPE approaching 2000 J/kg. This moist ascent will drive a theta-e ridge northward, supporting expanding convection along a surface trough as another impulse moves overhead. Timing of the impulse and its affect on the impinging surface boundary is in question, and if they do not overlap the coverage of storms could be less. However, the ensembles suggest a high probability of at least 1" of rain from near the Raton Mesa northward through northern Iowa, with training of intense rates possible. This rain occurring atop compromised FFG from recent heavy rainfall could produce a few instances of flash flooding. Weiss Day 3 Valid 12Z Fri Jul 14 2023 - 12Z Sat Jul 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR NEW ENGLAND AND PARTS OF THE MID-MISSISSIPPI VALLEY... ....Northeast and Mid-Atlantic... Continued troughing across the eastern CONUS will keep lowered heights across the Mid-Atlantic and Northeast, as shortwaves embedded within the cyclonic flow pivot northward on Friday. One of these shortwaves is progged to lift across New England early in the period, helping to drive a weakening cold front eastward across the Mid-Atlantic and Northeast. Ahead of this cold front, a warm front is progged to surge north into Canada, leaving impressive thermodynamics within the warm sector across the Northeast. SW flow within this warm sector is progged to reach 20-25 kts, advecting anomalous PWs of 1.75 inches and MUCAPE of 1000-2000 J/kg northeastward. As a pre-frontal trough sharpens during the aftn, coinciding with peak instability, widespread showers and thunderstorms should develop and then lift northeast along this trough with rain rates above 1"/hr. Aligned mean winds indicate training is likely, lengthening the duration of rain rates which will otherwise be limited by 20-25 kts of storm motions. This training could result in an axis of 2-3" of rainfall in some areas. The antecedent soils are fully saturated across New England noted by extremely low FFG of just 0.75-1.5"/3hrs, and USGS streamflow anomalies that are uniformly above the 90th percentile, and in some places above all-time records. These vulnerable soils indicate that any heavy rainfall will quickly become runoff and may lead to renewed flash flooding. Although uncertainty remains with how convective evolution will occur Friday, the SLGT risk was adjusted to match the highest ECENS/SREF probabilities for 3" over the most sensitive soils. Some adjustments to this SLGT risk are likely during the next few days. ....Central Plains through the Upper Midwest... A broad SLGT risk exists from eastern KS through western KY for increasing model consensus for organized convection moving across the area. Thunderstorms will be driven by shortwaves and associated vorticity maxima rotating eastward in NW to W mid-level flow, impinging upon an elongated cold front draped west to east through the Mid-Mississippi Valley. The shortwave interacting with this baroclinic zone will drive impressive ascent into favorable thermodynamics characterized by a ribbon of PWs nearing +2 standard deviations according to the NAEFS ensemble tables collocated with a corridor of 2000-3000 J/kg MUCAPE. 850mb inflow out of the SSW at 20-30 kts will robustly resupply these thermodynamics into the area, and as effective bulk shear climbs to 50 kts Friday night, organized convection, possibly a potent MCS, will move across the area. Being D3, and understanding the difficulty in forecasting the placement of these MCS features even on D1, results in lowered confidence than is typical so the the SLGT risk is quite broad. However, it was drawn to best overlap the current WPC QPF and highest ECENS/GEFS probabilities for 3". Even so, additional adjustments are likely during the next few days, but scattered instances of flash flooding are becoming more probable across this region on Friday. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6nMODuqj8ugOaihUQMJCWJmbxcYqE9XG8CybpYF_SM_C= xTg5FtpgwqVTAY1kpawhuJ2A-8u9tIQZtDOiRVgPV0_bnEA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6nMODuqj8ugOaihUQMJCWJmbxcYqE9XG8CybpYF_SM_C= xTg5FtpgwqVTAY1kpawhuJ2A-8u9tIQZtDOiRVgPP27SZZg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6nMODuqj8ugOaihUQMJCWJmbxcYqE9XG8CybpYF_SM_C= xTg5FtpgwqVTAY1kpawhuJ2A-8u9tIQZtDOiRVgPtE_9r1c$=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) .