Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jul 07 2023 19:49:36 FOUS30 KWBC 071949 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 348 PM EDT Fri Jul 07 2023 Day 1 Valid 16Z Fri Jul 07 2023 - 12Z Sat Jul 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL/SOUTHERN PLAINS AND MID-ATLANTIC INTO NORTHERN NEW ENGLAND... ....16Z Update... ....South Texas... The convection across the area has diminished in intensity and coverage as cloud tops warm this morning. Since the heaviest rainfall has ended in the Slight Risk area, it has been downgraded to a Marginal with this update. Very high PWATs remain, so the surrounding Marginal Risk area for the entire TX Gulf Coast and nearby inland areas was maintained for this update. Much of the CAMs guidance continues to decrease storm coverage through the day with nine depicted in any of them for the overnight period, but with the abundance of deep tropical atmospheric moisture, any storms that form from here will potentially produce rainfall rates exceeding 2 inches per hour. ....Plains... No major changes were needed to the inherited Slight Risk area. The squall line ongoing across eastern OK will continue to weaken as it races eastward, but scattered convection is already developing from the TX Panhandle north to central KS. Stronger storms will develop later this afternoon that will congeal into yet another MCS by this evening. The southward trend in the guidance as regards the movement of the MCS noted 24 hours ago has stopped, and the guidance has settled on a nearly stationary east-west oriented line of storms developing overnight along the KS/OK border, with periodic lines of storms quickly tracking across central OK. This scenario would suggest the KS/OK state line area being the focus for the most prolonged heavy rains, and therefore the highest flash flooding threat. However, antecedent conditions would still favor the Slight Risk area being maintained over central OK as well, since the repeating storms that have been moving over these same areas for multiple days have primed the soils. ....Northeast... No significant changes were made to the Slight Risk Area in the Northeast as the main event gets started this morning over western NY and PA. Lower FFGs are noted in the DC area and central MD from recent days=C2 heavy rains, along with a much larger area of low FFGs straddling the PA/NY border over to western NJ. These areas are under a higher end Slight Risk given the favorable antecedent conditions, but New England has better potential for training as the storms move northward, with limited eastward progress expected. ....Florida... No changes made to the Marginal Risk area along the east coast, as sea breeze convection taking advantage of abundant moisture may locally cause hourly rates to approach 2 inches/hour along the I-95 corridor. Wegman Previous Discussion... ....Mid-Atlantic through Northern New England... Upper-level pattern denotes a sheared shortwave along the southeast portion of the global-scale Canadian closed low, continuing to lift into a stubborn shortwave ridge along the New England Coastline. Deep layer moisture transport continues to be favorable from the SSW with above average moisture values of 1.75" as far north as the Rooftop of Maine, with 2"+ from Baltimore south. Early morning heating will allow for 1000-1500 J/kg of CAPE and within modest but sufficient convergence should spark numerous thunderstorms across the Mid-Atlantic through northern New England. Slow cell motions and storm outflow collisions will support localized very intense rainfall rates with 2"/hr probabilities well above normal (20-40% per HREF) and 2"/3hr probabilities nearing 50-60% by 21z across much of N PA to NW ME.=20 Cells should be a bit more scattered to isolated across the Mid-Atlantic into the Northern Carolinas, respectively; however, the risk remains for scattered incidents of flash flooding, which may be of greatest concern over the lower FFG of urban centers from DC to Philly and again through Upstate New York and Burlington. As such, small adjustments were made to include all of N ME and broadening the Slight Risk across central MD. The Marginal Risk was also brought further south across central and E VA including Richmond, Newport News/Norfolk. While clustering/most intense rainfall appears to be over higher FFG east of I-95 in N NC, lower FFG along the I-85 corridor, allowed for some broadening of the Marginal across this region for today as well. ....Central Plains and northern Southern Plains... With the southern Ridge retrograding toward the west and the strong Canadian global-scale closed low anchored across the north-central portion of North America; the persistent parade of convective complexes developing in the lee of the Rockies with upslope and strengthening of the LLJ at nightfall will continue through much of the short-term period. Again tonight, an MCS is developing and is expected to be ongoing across SW KS into NW OK toward the start of the forecast period (08.12z). Pooled moisture up to 2" with stronger LLJ (given stronger shortwave emerging from WY/SD) will enhanced moisture flux convergence along the leading edge supporting a warm advection wing along/downstream of the MCV, weakening should be taking place by 18z resulting in an enhanced boundary across S KS, while the main squall line races out toward AR. By evening, instability will have built up again with maturing thunderstorms breaking out across SE CO/NE NM once again congealing/growing upscale into a MCS. However, as the northern stream shortwave is sliding east, the LLJ will be broader and likely to converge orthogonally across the lingering boundary in S KS/N OK. As a result additional thunderstorms are expected with slow motions. Global models have been supporting at a broad area of 3-6" across this area, with a continued trend southward into NE OK. Hi-Res CAMs have been much less aggressive with the WAA wing with exception of the more synoptically forced guidance such as the FV3, CMC_regional, and Nam-Nest...which even hint toward some higher totals. The ARWs and HRRR solutions are more focused on more intense rainfall totals/rates near the surface-based intersection with the squall line/bow echo expected over central to southern OK. All in all, there is a very solid signal for maintaining the high-end Slight Risk with very small adjustments, totals upstream into SE CO will be less than further east, but the overlap with multiple days of heavy rainfall support maintaining the extension of the Slight Risk further west, but the coverage/intensity is more on the low-end to average Slight Risk compared to further east. ....Mid-Missouri/Mid-Mississippi Valley toward Chicago... A shortwave trough is exiting from Wyoming into the northern Central Plains at the base of the global scale trough anchored in north-central Canada. Favorable cyclonic curvature and divergence aloft has been feeding ongoing convection across W SD that will further feed back in maturing the cyclone. Ongoing thunderstorms and showers are likely to be ongoing through th Missouri Valley but should be generally down-trending and fairly progressive.=20 Some training near along/downstream of the shortwave center may help increase rainfall totals, but rates may be a limiting factor. The NAM/NAM-Nest are quite aggressive with the totals in the deformation zone so a low-end, longer duration flooding condition or two may occur. Residual boundaries along/ahead of the wave will build up insolation to destabilize the environment along the southeast quadrant across MO/IL by late evening. Modest moisture (1.50-1.75") and instability to 1500 J/kg along with favorable height-falls/low-level convergence should spark some stronger thunderstorms, that may maintain through evening, perhaps encroaching on the Chicagoland area toward the end of the forecast period. Spots of enhanced rainfall totals supports a broad Marginal across the area for isolated flash flooding incidents.=20 In the wake of the wave as it shifts east, a stronger northern stream cold front will be dropping southward across the Red River Valley into the Mid-Missouri Valley helping to the residual moisture (and with some clearing) instability across the area.=20 Slow moving scattered thunderstorms across E SDAK into MN may have strong enough rates for an isolated flash flood condition as well. ....East Coast of Florida... Deep layer moisture plume remains draped across much of the FL peninsula within the tail end of the exiting shortwave trough.=20 Confluent mid-level westerly flow should allow for slower moving/anchored convection along the eastern Florida sea-breeze with 00z Hi-Res CAMs depicting an above average coverage of intense rainfall totals, likely due to the deeper available moisture and warm cloud layer with highest probabilities of 2"+ between 20-01z around 50% per 00z HREF. Given multiple days of scattered heavy rainfall and proximity to urban corridor from I-95 ear the Space Coast and south, a Marginal Risk remains in place again today. Gallina Day 2 Valid 12Z Sat Jul 08 2023 - 12Z Sun Jul 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL/SOUTHERN PLAINS INTO THE MIDDLE MISSISSIPPI VALLEY... ....20Z Update... ....Plains to Mid-MS Valley... A few small changes were made for this afternoon's update. The Slight Risk area from the Plains to the Mid-MS River Valley was trimmed on the northeastern side in IN, which has somewhat drier antecedent conditions, such that the heavy rain associated with the thunderstorms should result in less widespread flash flooding. Meanwhile, in hard hit OK, there are very favorable antecedent conditions but less total rain expected. The heaviest rains are expected around the OK Panhandle and the southwest corner of KS, and also over northern MS and western TN. Guidance trends have been slowly drifting southward, so the northern end of the Slight was trimmed, where very little if any rainfall is forecast, whereas expanded a row of counties southward to include the northernmost LA counties. ....Mid-Atlantic to Northeast... The Marginal Risk area across the Northeast was expanded southward to cover the Piedmont of central NC and northeastern SC with this update. Locally heavier convection is expected across the Carolinas on Saturday as a subtle shortwave riding the southeastern periphery of a negatively-tilted longwave trough over the Midwest tracks across the Carolinas during peak heating. PWATs will be over 2 inches, or about 2 sigma above normal, which will provide plenty of fuel for any developing storms to be capable of heavy rains with local rainfall rates exceeding 2 inches/hour. High FFGs should prevent some flash flooding from occurring, but if storms move over cities such as Raleigh with locally lower FFGs, then the flash flooding threat from those storms will be considerable. ....Southeast FL... A small Marginal Risk area was introduced across the urban corridor of Southeast FL with another round of afternoon sea breeze convection with abundant moisture available to produce locally heavy rainfall resulting in an isolated flash flooding risk. Wegman ....Previous Discussion... ....Central High Plains to Mid-Mississippi Valley into Ohio Valley... Retrograding ridge in the southwest starts to amplify into the Four Corners concurrent with global scale trof dropping south with sharpening base across the Upper Midwest into the Great Lakes.=20 The shortwave at the base of the trof has been tapping some broad southwesterly flow to advect above average moisture back through the Ohio Valley increasing to 1.75". Otherwise, the latest in a parade of overnight MCS should be weakening at the start of the period across MO/AR bringing along with it, the highest concentration of Theta-E and deep moisture up to 2" into the Mid-Mississippi Valley area. Both waves will be generally weaker and more progressive, but will support redevelopment through Saturday evening into the overnight hours but with weaker/less focus allowing for numerous clusters of strong thunderstorms to progress across the Ohio, W TN and Lower MS Valleys supporting intense short-term rates over 2" but likely spotty clusters of 2-4". Upstream, another upslope day is likely to support return moisture through SE CO and with backing LLJ, new thunderstorms will develop in the lee of the CO Rockies. Moisture is going to be less robust and flux will be reduced compared to prior days, but scattered 1-3" streaks and short-term training in northwesterly flow is likely to once again induce localized flash flooding potential across SE CO, SW KS into N OK. As such, in combination across the OH, TN and MS Valleys, the inherited Slight Risk of Excessive Rainfall has been left unchanged though a small southward shift was made to the West while expanding the surrounding Marginal for the potential for a stronger cluster of thunderstorms across NE NM into the TX panhandle, as increasing members of guidance suggest more robust signal. Through the Tri-River Valleys, the Slight was expanding ever so slightly eastward to overlap slightly faster convective solutions into more rough/lower FFG terrain of SW OH, central KY.=20 .....Mid-Atlantic into New England... More active thunderstorms in the D1 period and slightly faster transition of shortwave energy northward into eastern Canada, has narrowed the moisture plume across New England toward the Mid-Atlantic as the shortwave ridging over the northwest Atlantic remains stubborn/amplified. The moisture axis has narrowed, but a narrow ribbon of 1.75-2" of moisture remains across N ME to the Megalopolis from NYC to DC. Southerly 850mb flow will be intersecting westerly down slope flow to support a narrow corridor of stronger ascent and scattered thunderstorms, but given the narrowing of the plume, the overall coverage should be reduced.=20 Very intense rainfall is likely to occur under these slow moving cells and spots of 2-3" are probable. However, there is limited consensus in placement of increased activity or higher magnitudes to support placing a Slight Risk at this time, though one may be warranted given the potential, magnitude and placement particularly relative to urban areas, lower FFG and areas affected in the D1 period. Gallina Day 3 Valid 12Z Sun Jul 09 2023 - 12Z Mon Jul 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN OKLAHOMA TO THE WESTERN FLORIDA PANHANDLE AS WELL AS THE MID-ATLANTIC INTO NEW ENGLAND... ....20Z Update... Overall changes to the ERO for this afternoon's update were also relatively minor on D3/Sunday. From the Plains to the central Gulf Coast, expect an MCS ongoing Sunday morning in OK to track southeastward, clearing a line of storms that develops out ahead of it across the lower MS Valley on down to the central Gulf Coast. The storms out ahead of the MCS will themselves be slow-moving and capable of heavy rain before the heaviest rains associated with the MCS move through. A perhaps much more impactful round of rain is expected across the Northeast as a large negatively tilted shortwave trough rotates eastward into the Northeast from the Midwest. A corridor of heavy rain featuring training convection and above normal PWATs is likely to develop near the I-95 corridor from DC to NYC, then up the I-87 corridor through western New England. This area has already been hard hit from rains ongoing today and on previous days, but Sunday is shaping up to be considerably wetter on average and over a larger area than all the previous days, which mainly featured scattered thunderstorms. Sunday's rain looks to be frontal in nature, but the storms will be riding largely parallel to the front moving northward, which drastically increases the chances for training storms and a prolonged period of heavy rain. The area most likely to see those heavy rains will be from Philadelphia north through fear eastern NY and into western VT. This portion of the Slight risk is considered on the higher end of the risk category. While the heaviest rainfall for now appears to remain west of NYC, with an areal average of over 2 inches of rain expected in the city, likely to fall within a 6-12 hour period, the city is also included as part of the higher end of the Slight Risk category. Any additional eastward shift in the axis of heaviest rainfall would likely require an upgrade to a Moderate Risk from the city west through northern NJ, which remains a distinct possibility at this point. Meanwhile further south across the DelMarVa, antecedent conditions are also favorable for flash flooding across the DC and Baltimore metros, which have been hard hit in recent days, but less rain is expected here as compared with areas further north. Wegman ....Previous Discussion... ....Southern Plains to Deep South/Central Gulf Coast... As D3 period starts, the Southwest U.S. Ridge continues to strengthen and broaden centered along the New and Old Mexican boarder presenting broad strong northwesterly flow with 70-90kt 250mb upper-level jet streak from the Northern Rockies across becoming increasingly diffluent across the Southern Plains into the Lower MS River Valley before rounding the base of the larger scale troughing dominating the Great Lakes. The associated cold front will continue to drop south, while remaining deep layer moisture is pooled in 2"+ concentrations from the Red River across AR/LA into MS. Low level flow will increasingly confluent/convergent across the Lower MS Valley to develop into multiple clusters with some training elements along it, particularly across AR into S MS, given higher potential for upstream backbuilding. Further east, stronger thunderstorms will develop in areas of enhanced instability/convergence in proximity to the front; though are likely to be limited in duration without upstream recharging through advective processes from ongoing upstream complex(es) likely. Guidance continues a stronger amplification trend in the southwest and deeper trough eastward, this has resulted in the best QPF signal to continue a south and west trend and so have adjusted the broad Slight Risk south and west. Given multiple rounds over the prior 2-4 day period, a Slight Risk extends back into central OK, though adjustments are likely to be made depending on how the soil conditions worsen in the coming days.=20=20 ....Central Appalachians to Northeast/New England... The narrowed plume of moisture in the Day 2 period will see a recharge and broadening with the arrival of the next MCV/shortwave lifting northward through the Cumberland Plateau late in Day 2 into early D3 period. A tight gradient of moisture will exist across the Allegheny Plateau ranging from 1.25 to 2.0"+ from central PA to Long Island, this is due to strengthening low level flow, while weakly convergent, there is 30-45 degrees of fetch from the Carolinas narrowing toward the Delaware and Lower Hudson River Valleys. Increased moisture flux convergence will allow for numerous intense (up to 2-2.25"/hr rates) for thunderstorms across the region narrowing across northern New England with strong FGEN forcing given aforementioned flow. While the orientation of the QPF signature is dependent on timing of the approach and strength of the shortwave, there remains increasing signal for streaks of 2-4" totals across E PA to W MA, even with ensemble mean solutions reaching 2.5"+ values, driven by the GFS/UKMET and NAM, with the 00z ECMWF a tad slower but still near 3" in spots. At the fringes of the highest QPF streak, there remains solid clustering for risk across the Southern Mid-Atlantic into VA as well as northward across northern New England. At this point, a high-end Slight Risk is solidly in place, and given trends may even require a Moderate Risk in subsequent outlook particularly accounting for where grounds get further saturated in the D1 and D2 period. Further south across the Carolinas into the central Appalachians, forcing is a bit more scattered/isolated in the broader overall flow. Still ample moisture and unstable environment will exist, especially nearer E NC where highest moisture will be persistent.=20 As such, a broader Marginal Risk will remain in place even connecting over to the South across E TN/Upstate NC.=20 Gallina Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5CdTvacChYrbw-N4Oo9hf3h7SemktjLrkd7LKrpeIOon= LSMk7IrJmgIvTDrYTfdSgTsK6ZLUNffbmbFD6ElhuZNCRFE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5CdTvacChYrbw-N4Oo9hf3h7SemktjLrkd7LKrpeIOon= LSMk7IrJmgIvTDrYTfdSgTsK6ZLUNffbmbFD6ElhQV0FXrU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5CdTvacChYrbw-N4Oo9hf3h7SemktjLrkd7LKrpeIOon= LSMk7IrJmgIvTDrYTfdSgTsK6ZLUNffbmbFD6Elh4mgG44Y$=20 $$ --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .