Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 22 2024 15:58:20 FOUS30 KWBC 221557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Mon Jul 22 2024 Day 1 Valid 16Z Mon Jul 22 2024 - 12Z Tue Jul 23 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES, SOUTHERN PLAINS, MID ATLANTIC, AND THE CAROLINAS... ....16z Update... Split the Mid Atlantic Slight Risk with this forecast update, mainly to exclude the DMV region (due to the combination of lower QPF signal and relatively high FFGs with drought conditions in place). Also removed northern LA from the Slight (for similar reasons). The northern Mid Atlantic Slight and Marginal were extended a bit northward into NY State (where FFGs are quite a bit lower). In addition, the Marginal was also extended a bit eastward along the south coast of New England, Cape Cod, and adjacent=20 islands (where a Marginal Risk was already in effect for tomorrow,=20 as the event may start for those areas prior to 12z). Overall these changes were not very significant, and adjustments elsewhere were minor (based on observational trends and the new 12z HREF).=20 Churchill ....Previous Discussion... ....Southern Plains... A quasi-stationary front will bisect much of TX during the period with a succession of mid-level disturbances entering the region beginning later this morning, carrying through the rest of the period as they pivot south-southeast around the western flank of a mean trough to the north. A deep moisture presence will be situated across Southwest TX through points east with a stronger low-level convergence pattern and favorable anomalies tied to the lingering front. Scattered convection will form across the Davis Mountains and Stockton Plateau in the early afternoon time frame with cells across the Edwards Plateau initiating not long after. Considering the moist anomalies and relatively slow storm motions off the terrain, some cells will be able to produce locally enhanced rainfall with rates between 1-2"/hr on average with some of the stronger cores capable of breaching 2"/hr anywhere across the aforementioned area(s). Numerous multi-cell clusters will develop by mid-afternoon with outflow generation likely considering the higher DCAPE environment forecast across the western half of TX. Outflow propagation will make progress to the east with more cell initiation forming in the favorable environment away from the cold pools. Storms should develop initially over Hill Country to the I-35 corridor, but more organized convective clusters will enter the picture by early evening leading to more widespread heavy rain chances and flash flood concerns as rates can sufficiently hit 2-3"/hr as reflected in the HREF hourly rate probability fields. There's a very high spatial coverage of higher probabilities for at least 1" of rainfall in the HREF EAS fields (30-50%) with the neighborhood probabilities for at least 3" and 5" depicting a formidable areal extent of 60-80% and 25-40% respectively for each threshold. There's even some low-end >8" probabilities reflected in the HREF output as well, indicative of a locally significant impact potential across portions of Western and Central TX. The best probabilities lie within the Edwards Plateau through Hill Country with a small bullseye showing up in the means to the I-35 corridor. This is well within the bounds of a SLGT risk with higher end SLGT risk wording necessary for the current forecast. There is a non- zero chance for an upgrade somewhere across the above region, but there is less of a true, organized heavy rain risk to pinpoint, but the area that will be impacted will see those locally significant flash flood concerns arise. ....Mid Atlantic and Carolinas... Persistent troughing to the northwest across the Great Lakes will edge eastward with southwesterly flow and increasing diffluence for much of the Mid Atlantic and Carolinas, moving into the Northeast U.S by the back end of the period. At the surface, a lingering quasi-stationary front will settle over the VA Tidewater down through the Carolina's and points west with a stronger low-level convergence footprint tied to the boundary. PWAT anomalies of 1-2 deviations above normal will spread north as we see a bit more backing of the flow within the Mid Atlantic with the strongest ties across the Southern Mid Atlantic thanks to a surface wave riding up the front once again, reaching the VA/NC coastal areas by the late afternoon hours. The area(s) of highest interest are located across the VA Tidewater and adjacent Northeast NC, the eastern side of the Carolina Piedmont, and up into the Susquehanna Valley of PA where the HREF neighborhood probability fields are all lit up for the potential of seeing >3" in spots with even a strong >5" signature located along the I-64 corridor in Southeast VA (50-60%). The increased upper forcing due to better aligned mid-level diffluence and some right-entrance region of a developing upper jet streak just off to the north will allow for a blossoming of scattered convection across portions of the Mid Atlantic through Central PA. The primary initiation points will likely be tied to the terrain at first, but as cells propagate off the terrain, they will experience some modestly favorable shear that would help sustain updrafts that could lead to stronger cell cores and locally heavy rainfall. PWAT anomalies are most favorable for heavy rain along and east of the I-99 and US-15 corridors leading to a lot of CAMs members ratchetingup the potential with isolated pockets of 2-4+" totals within the deterministic output, and reflected in the HREF blended mean data set showing the expanse of higher totals and increasing flash flood concerns. The threat will also reside within the urban corridor from DC to Philadelphia, but there is some disagreement within the CAMs on the potential due to a small cull in the precip field with initiation away from the major metro areas. The UFVS ML First Guess Field is still insistent on a widespread SLGT risk through the urban areas extending all the way up to the PA/NY border. The QPF footprint within ML output is also insistent on potential maxima within the metro areas, so the best course of action was have that area within the new SLGT risk proposal with areas to the north into PA outlined as well given the better signals and agreement in the First Guess Fields. Further south into Southeast VA and the Carolinas, the threat is more defined thanks to the presence of the stationary front leading to ample low-level convergence potential coinciding with fairly stout theta-E indices located along and south of I-64. The highest threat will reside over the Hampton Roads area up to Williamsburg where several days of heavy rainfall have degraded the FFG indices considerably leading into today. This signal of degraded indices is documented down through the NC Piedmont all the way into Columbia, SC where back-to-back days of significant convective impact have led to flash flooding reports all across southern and central SC. The threat for today extends into those areas again thanks to weak mid-level perturbations ejecting northeast within the mean flow correlating with the stationary front to provide another focused area of heavy thunderstorms from basically the GA/SC border and points northeast. As a result, the SLGT risk addition was also extended to include part of the Delmarva down through Southeast VA into portions of the Central and Eastern Carolinas. ....Southwest and Southern Rockies... Continued Monsoonal convective pattern will impact the Desert Southwest this period with focus along the terrain in AZ and NM, including the hotter spots of the Mogollon Rim over into the Sangre de Cristos and the Sacramento Mountain chains. Probabilities continue to be modest for 1-2" maxima with some higher end potential in stronger cores that linger within the terrain. The continued mid and upper ridging in place has led to slower storm motions with some training concerns under the weak mean steering flow. Considering the onslaught of scattered to widespread convection for the past several periods leading to many Flash Flood Warning issuances by the WFOs across the Southwest, another SLGT risk was maintained within the Southern Rockies where the impact potential is highest thanks to remnant burn scars and persistent flooding over the past week. A MRGL extended to much of the Southwestern U.S with the Mogollon Rim as the secondary candidate for higher totals and impacts for localized flash flooding concerns. The threat probabilistically is still on the middle to high end of MRGL for the aforementioned area, but a short term upgrade is plausible if coverage ends up more sufficient than currently forecast. ....Upper Midwest... Shortwave trough across Manitoba will pivot southeast into the Arrowhead of MN with increasing large scale ascent and accompanying positive vorticity advection (PVA) within the confines of the area. Relatively steep mid-level lapse rates within a corridor of modest instability will result in scattered thunderstorm initiation once the disturbance crosses the border into the U.S. Progressive storm motions will limit the threat of flash flooding to more isolated signals, however some heavier cores with rates between 1-2"/hr will be plausible given some of the low-end probabilities within the latest HREF output. Totals are generally within the 0.5-1" in areal coverage, but the matched mean does indicate a few cells capable of dropping 2-3" near the northern shores of Lake Superior, including near DLH where urban flooding is more probable. A MRGL risk was introduced in coordination with the local Duluth WFO for low-end flash flooding concerns, but enough to warrant the targeted risk. Kleebauer Day 2 Valid 12Z Tue Jul 23 2024 - 12Z Wed Jul 24 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL AND SOUTHEAST TEXAS... ....Texas... Quasi-stationary front will push north and eventually wash out across the Southern Plains as surface ridging across the Gulf begins flexing more northwest allowing for deeper moisture presence to advect into Southeast TX. Scattered convection will develop from the Lower Trans Pecos through all of Central TX by the afternoon due to diurnal destabilization with lingering outflow boundaries from previous periods convection becoming a potential focal point for heavy rainfall and training over much of the area. Areal QPF averages are between 0.5-1" across West-Central and Central TX with some of the upper quartile outcomes closer to 3-4" within different deterministic forecasts. This seems to be towards the upper threshold of the potential across the region, but after what is expected today, there's expected to be a lingering flash flood threat after much of the area becoming primed from previous rainfall. This was sufficient for a maintenance of the previous SLGT risk. Further to the southeast, the additional surge of low-level moisture out of the Gulf will be accompanied by a weak mid-level disturbance that will advect north out of the Bay of Campeche, already causing some convective flare ups down that way when assessing the latest IR satellite. Ensemble means are becoming more bullish on the threat of heavier rainfall tied to the Middle and Upper TX coast with some deterministic outputs pushing 3-5" between Corpus to the far Upper TX coast, including coastal Houston. The threat is gaining favorable within the ensemble probability fields as well with the NBM now depicting a 25-40% chance of >2" within the zone encompassing Matagorda up towards Port Arthur. Considering this is working off the mean QPF of the blend, that is impressive at 2-day leads. The extension of the SLGT was made to encompass that area of the coast from CRP up to the Southwestern corner of LA. ....Southwest and Southern Rockies... Scattered convection will occur once again across much of the Southwestern U.S with isolated threats of flash flooding within the flashy complex terrain, remnant burn scars, and slot canyons located within the Great Basin. The ridge axis will shift a bit further to the west aligning more of the convective potential through eastern CA and much of NV leading to more coverage of the MRGL risk across those areas. QPF means are generally light, but some of the higher end outputs within the deterministic suite are upwards of 1-2", especially across the Mogollon Rim. This is a classic Monsoonal setup with the favored terrain being the primary focus for the period. The MRGL risk inherited was maintained with full continuity as ensemble mean QPF did not change much in terms of magnitude and precip placement from previous forecast. ....Midwest... Shortwave trough over the Northern Great Lakes will dip further southeast with trailing mid-level vorticity pivoting around the base of the trough situated over Ontario. Scattered cells in the more will move southeast, but lose fervor as they enter into MI. A secondary pulse of convection is expected later Tuesday with a more organized area of thunderstorms expected to form upstream near Lake Superior, dropping southeast through WI and the western portions of the UP. The storms will remain on the progressive side, but the threat of rates up to 2"/hr could cause some isolated flash flood concerns in more urbanized zones over Central and Eastern WI. This includes places like Green Bay, Northern Milwaukee, Oshkosh, and Sheboygan. The previous MRGL risk was maintained with some minor adjustments on the western flank of the risk area to account for QPF shifts in the ensemble means. ....Southeast through the Northeast U.S... Scattered thunderstorms will continue across the Southeast all the way up into the Northeastern U.S thanks to persistent synoptic scale forcing and a relatively modest thermodynamic environment in place. Smaller mid-level perturbations will be entrenched in the mean flow and will help trigger some smaller, organized cell clusters capable of isolated flash flooding basically extending from the Deep South into the Mid Atlantic. Any cell generation in the deep, moist environment will be capable of flash flooding concerns with the highest threat likely within the Carolina's due to the antecedent wet conditions in place over much of the eastern 2/3's of the region. A broad MRGL risk is in place extending from the Southeast through the portions of the Northeastern U.S given the threat. Kleebauer Day 3 Valid 12Z Wed Jul 24 2024 - 12Z Thu Jul 25 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA... ....Texas and Louisiana... Disturbance off the Gulf will continue to press north around the western flank of the ridge located within the Gulf. There's some discrepancy within the deterministic on the exact placement on the core of the mid-level energy as it enters out of the coastal waters into the U.S. Despite the lack of consensus on the where for the energy, the deep moist advective pattern across Southeast TX into portions of the Central Gulf Coast is not so much a question with only where the general precip maximum will focus. There's growing consensus that significant rainfall on the order of 2-5" with locally higher will impact somewhere within the Middle and Upper TX coast, over into Southwestern LA. ML output from the ECMWF AIFS and GFS Graphcast are closely aligned to the Upper TX coast which makes a bit more sense synoptically given the favored mid-level ridge positioning and western weakness aimed for the Upper coastal areas. The previous SLGT risk was not changed much due to the modest uncertainty in the precip maximum with the setup, however the threat is more on the higher end of the SLGT threshold due to the potential for significant rainfall in any area of expected impact with the highest impacts focused where the mid-level disturbance makes its presence closely known. This is a period to monitor as an upgrade to a higher risk is plausible considering the upper quartile output of ensemble QPF and the tropical connection. ....Southern Rockies and Southwest... Monsoonal convection will encompass much of the Southwest U.S with primary coverage in the Great Basin as the mid-level ridge pattern shifts focus to the west of the Four Corners. Modest moisture anomalies and relatively formidable instability across much of the region will allow for scattered thunderstorms with isolated heavy rain cores that could spell issues if they fall along complex terrain, burn scars, urban footprints, and slot canyons. A MRGL risk is in effect across much of the climatologically favored areas in the Southwestern Monsoon. ....Great Lakes and Ohio Valley... Surface low will shift eastward over northern MI, eventually pushing northeast into neighboring Ontario with a cold front trailing the surface reflection. The combination of marginally better upper forcing with increasing surface convergence in-of the cold front moving eastward will create a period of scattered showers and thunderstorm along and ahead of the front. Some isolated pockets of heavier rainfall within the urban corridors spanning from Chicago up through Southern MI will have the opportunity for flash flooding as the system progresses through the area. The signal is fairly weak overall and could very well fade in later updates. Considering the synoptic pattern expected and some time to delve into greater detail, maintained much of the continuity in the forecast with some minor adjustments on the northern flank of the risk area. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5GjrZO_CW9zY2-7pMFEhCZM0z6KVZj09NHOa5x2Nh0hk= KPF9bokwRL9Wk_z9x20jC-1X9prTzX5iEgbXPdXRyYwLtLU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5GjrZO_CW9zY2-7pMFEhCZM0z6KVZj09NHOa5x2Nh0hk= KPF9bokwRL9Wk_z9x20jC-1X9prTzX5iEgbXPdXRWPZLt58$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5GjrZO_CW9zY2-7pMFEhCZM0z6KVZj09NHOa5x2Nh0hk= KPF9bokwRL9Wk_z9x20jC-1X9prTzX5iEgbXPdXRf4TYiaM$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .