Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 06 2023 20:15:21 FOUS30 KWBC 062015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Thu Jul 06 2023 Day 1 Valid 16Z Thu Jul 06 2023 - 12Z Fri Jul 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS AS WELL AS THE ARKLATEX REGION INTO MISSISSIPPI... ....16Z Update... On the central/southern Plains, the Slight Risk area was trimmed on the southern end to remove much of OK with this update. The dying MCS over eastern OK failed to produce much flash flooding through the overnight, and as the overall MCS weakens and shifts eastward, any potential for it to produce flash flooding is expected to continue decreasing. Additional storms that will develop over the remaining Slight Risk area from eastern CO through northern OK are likely to result in a fast-moving squall line over OK right before 12Z, as depicted in many of the CAMs, but this itself is also unlikely to cause much flash flooding, especially given the 18+ hours break in between the two systems. On the flip side, the remaining Slight risk area is considered a higher-end Slight for portions of southwest KS, as recent heavy rains in this area have already primed the soils for additional flash flooding potential as the next round of storms move through this afternoon through the overnight. The Slight Risk area over northern LA/southern AR was expanded North and west to cover the southeast corner of OK, northeast corner of TX, and more of southwestern AR with this update. Multiple CAMs are suggesting a corridor of heavier rains associated with storms that may refire this afternoon and evening across the area, at least partially from the remnants of the ongoing MCS over eastern OK. There remains fair disagreement in the CAMs guidance as to how these storms will organize, whether they form as a series of discrete clusters of storms or backbuilding individual cells, but the forcing and the overall meteorology favoring the storms in this area remains largely unchanged. The Marginal Risk area for eastern VA through the coastal Carolinas was dropped. The heaviest rainfall in the area has been largely staying offshore, with only light to moderate rain impacting eastern NC. With much of the area being swampy, resulting in very high FFGs, think the flash flooding risk for the next few hours will be very low. After that the bulk of the rain will move offshore, with no significant rainfall expected through tonight. Finally, the HREF guidance showing a 30-45% chance of 5" of rain for the day across south FL prompted a small expansion of the Marginal westward to cover more of the southern interior of the state, north of the Everglades. Wegman ....Previous Discussion... .......North/Central High Plains into the Central/Southern Plains... Current RADAR, Satellite and observational trends suggest a few clusters or larger complex should be ongoing across central OK with some embedded training thunderstorms capable of producing 2-4" totals and flash flooding. Though late morning diurnal minimum should result in a reduction in rainfall rates and flash flooding potential, especially in crossing areas of recent dryness of SE OK. As such, the inherited Slight Risk area was adjusted slightly eastward to account for some of this timing.=20 By afternoon/evening, strong upslope flow will return with new clusters of thunderstorms developing toward nightfall across E CO with strong southerly to southeasterly LLJ increasing to 35-40kts after 00z. Moisture flux values become very anomalous given the 45 to 60 degrees of directional convergence and increase of 1.25 to 1.75" total Pwats as the shortwave height-falls reach the Rockies. 00z Hi-Res CAMs (and ongoing convection) suggest an expansion of the Slight Risk toward the south and west. There did remain some guidance suggesting of a secondary enhanced band of mid-level convection along the northern edge of a developing MCS toward the end of the Day 1 period. Guidance did also shift slight west and south and so have adjusted the Slight Risk area into the OK/northeast TX panhandle to account for this trend.=20 A second cluster further north is expected to evolve out of N WY/SE MT with strong thunderstorms and potential training profiles crossing into southwest SD and western Nebraska. Greatest QPF signal appears to align with the Sand Hills, and so a broad Marginal Risk encompasses the region with the exclusion of the core of the Sand Hills. ....Southern Arkansas/Northern Louisiana... Evolving convective clusters/complex mentioned in section above will be down-trending by late morning crossing E OK/AR, but the MCV will continue to remain providing some structure for enhanced convergence and perhaps some weak DPVA for ascent as instability once again builds downstream. Banded convection along the leading edge may support short-term training as well as producing some outflow boundaries for additional development through the evening/early overnight hours. This is important, as a surge of western Gulf moisture is forecast to arrive across E TX/N LA by evening increased deep moisture availability as well as potential orthogonal moisture flux convergence to outflow boundaries or ongoing convection in proximity to the MCV. Strong collision/cell mergers may allow for short-term increase in rainfall rates up to 2.5-3"/hr (as 00z HREF probability for 2"/hr rates is about 40-60%) with 60-70% probability of 3" across N LA/SW MS. As such, in coordination with local forecast offices, have placed a small Slight Risk across the area of enhanced probabilities and Hi-Res CAM QPF signal. ....Texas Gulf Coast... GOES-E SWIR shows a fairly compact MCV/low level tropical wave south southeast of Brownsville with the start of a north-south line of convection along/just west of the circulation axis.=20 Strong south-southeasterly low level flow is expected to be generally convergent along the eastern side of the circulation aligned with early morning instability axis within the Western Gulf. There is ample uncertainty to its further evolution which is contingent on the next cycle of this convergence/deep convection is just starting to manifest with the north-south line of convection. There is Hi-Res CAM signals to suggest best convergence and convection will remain along and just offshore the south Texas coast, though a wave of thunderstorms is highly plausible to slide northward toward the Upper Texas coast by mid-morning into early afternoon with spotty rates of 2-2.5"/hr; eventually sliding north-northeast into LA. Additionally, depending on how close early morning convection develops, much of Southern Texas may have increased low-level Theta-E, while remaining cloud free increasing overall instability along the Coastal Plain. Aggressive models such as the HRRR suggest bands of thunderstorms rotating onshore, while other solutions suggest perhaps one band. Still, trends are increasing for enhanced rainfall across S TX, with fairly high consensus for 30-50% of 3" or greater from Corpus Christi and south. However, the placement and timing remain uncertain enough to expand the Marginal Risk of Excessive Rainfall to the Mexico boarder without delineating a Slight Risk area at this time. The potential exists for a future upgrade to a Slight Risk if trends continue to increase in coverage/magnitude of rainfall in the area. ....Central Great Lakes into Ohio and Tennessee Valleys... At the base of broad synoptic scale trough, that currently dominates northern and central Canada, is a shortwave sliding through the central Great Lakes with an attendant cold front that extends into the mid to lower Mississippi Valley. Deep layer moisture plume resides along/ahead of the front with 1.5-1.75" of total PWats; and ample day-time heating should support stronger thunderstorms with the capability of increased rain-rate potential (2"/hr). A weak surface wave should exist in the LP of MI just ahead of the core of the shortwave, this should allow for the best enhanced potential for repeating thunderstorms and highest localized rainfall totals throughout the length of the front with isolated 2-3" totals possible. HREF probability fields denote a slightly higher potential in the thumb and toward Detroit.=20 Currently, the signal appears to fall just below level to justify an upgrade to Slight Risk at this time, but observational and 12z guidance trends will be monitored for any upgrade later today. ....Coastal North Carolina/Southeastern Virginia... At the start of the forecast period, a few thunderstorms may be ongoing in the vicinity of the Outer Banks in proximity to the core and southeastern quadrant of an exiting shortwave trof. Very high moisture exists with Total PWats AoA 2.0-2.25" with slightly lower values but still at or above normal through the Mid-Atlantic. As the shortwave presses eastward, increased insolation from clear skies along the western edge will allow for the return of increased instability. Weak but deeper layer northeasterly flow should provide some enhanced convergence to break out additional round or two of thunderstorms from the Necks of eastern VA southward across E North Carolina (generally east of I-95). Weak steering flow should allow for some increased duration and isolated spots of 2"/hr rates; 00z HREF depicts 2"/3hr of 40-50% across these portions of VA/NC to support a Marginal Risk. To add to the potential, AHPS 2-week precipitation anomalies are 200-300% of normal in spots across the area, suggesting above normal soil saturation for reduced infiltration and increased runoff in those intense rainfall rates. ....Southeastern Peninsular Florida... South Florida resides in the trailing end of an exiting weak mid-level trof centers off the Outer Banks of NC, while only Surface to boundary layer southeasterly trades will resist eastward progression. Overall moisture is at or slightly above normal with 2.25" total PWats within the trof. Activation of the western sea-breeze should be the focus for early thunderstorms, while the eastern/southeastern sea-breeze will be weaker and likely later to develop by mid-day. As such, increased instability and slow eastward progression of the thunderstorms will start to merge/collide across the eastern quarter of the peninsula toward late afternoon. 00z HREF signals driven by ARW members, suggest strong 3"+/3hrs probability signals between 21-00z across the highlighted Marginal Risk with 25-35% potential of exceeding 5" in the vicinity of the I-95 corridor from Cape Canaveral southward. Gallina Day 2 Valid 12Z 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 NORTHEAST... ....2030Z Update... ....Plains... Several changes worth noting with this afternoon's update: Starting over the Plains, model guidance continues to shift the primary axis of heavy rain southward, with northern OK now in the bullseye for getting the most rain from an expected MCS that will develop overnight Friday night. Due to the southward shift, portions of northern Kansas through Kansas City will largely be spared most of the heaviest storms, and have been removed from the Marginal Risk area accordingly. This remains a high-end Slight for Oklahoma though, as repeated rounds of storms from MCS's have been lashing the area repeatedly for a few days now. Friday night's storms may be the strongest of the set as far as heavy rain potential goes. There is some potential a Moderate Risk may be needed in this area with future updates, but would prefer to see how the CAMs are handling this round of storms before any such upgrade would occur. A second and much weaker MCS will impact areas further north into eastern SD, NE and western IA with a developing low. This will actually occur before the main MCS further south on Friday morning. As this area has had somewhat lower rain in previous days, the antecedent conditions aren't quite as favorable, so the Marginal Risk was maintained. ....Northeast... The previous Slight Risk area was expanded quite a bit with this afternoon's update, now including much of the northern Appalachians and the DC and Baltimore metro areas. Continued increases in forecasted total rainfall for the area, previous' days indications of flash flooding from disorganized pulse convection in this area, PWATs over 1.5 inches, and potential for upsloping in the mountains from the Catskills northeast to western ME, are all contributing to the flash flooding potential Friday. For now, it appears the potential for eastward progress will be very limited with the storms, and Philadelphia and the rest of the Northeast Urban Corridor are not expected to be impacted by this event. Across the Southern Tier of NY through the Catskills and northeast Pennsylvania, flash flood guidance is particularly low due to recent rainfall and terrain speeding up potential flash flooding onset, so that area is considered a high-end Slight. ....East Coast of Florida... A repeated round of afternoon storms with access to ample moisture on the sea breeze will once again pose a threat for flash flooding along the urban corridor from the Space Coast south through Miami. Since the storms will be capable of 2+ inch per hour rainfall rates, those rates occurring in urban areas may be enough to overwhelm local drainage systems to cause isolated flash flooding. Wegman ...Previous Discussion... ....Northern/Central High Plains into the Central/Southern Plains... Broad zonal flow across the central US will likely have two ongoing convective complexes by the start of the day 2 period (07.12z) a northern more associated with a weak inflection/shortwave crossing the Sand Hills of Nebraska starting to spread across the mid-Missouri Valley and the second and much stronger complex across SW KS/N OK. The former will have ongoing slowing thunderstorm activity with localized 1-3" totals, though Nam and Nam-Nest continue to depict a much higher qpf signal, this seems less and less likely given other 00z Hi-Res CAMs and global guidance...it still warrants a Marginal particularly as a northern stream cold front drops out of Canada, allowing for any new development to occur along its tail, where moisture/instability may pool.=20=20 Further south, a very strong MCS with 90-95th percentile moisture flux convergence values will be ongoing through S KS/N OK. While, morning convection will wane by midday, floating outflow boundaries and returning upslope flow and some clearing skies should allow for a recharge of the local environment. 00z GFS/ECMWF/UKMET and CMC all continue to highlight a broad area of 3-6"+ QPF mainly across SE KS, though trends continue southward and a tad westward. Additionally, the ECMWF has a bit more WNW to ESE orientation compared to other guidance but still shows fairly good timing agreement. With apex of the ridge just west the apex of the Rockies, upslope flow is likely to redevelop thunderstorms across portions of central WY through central CO toward 00z, stronger severe cells are likely to again grow upscale across SE CO crossing areas affected over the prior few days, enhancing the risk for flash flooding. As such, small southward adjustments were made to the Slight Risk as a whole...narrowing the axis, while also expanding a small sliver to the west to account for the likely repeat event over SE CO.=20 ....Northeast, Mid-Atlantic into Carolinas... A cold front on the leading edge of the base of the large scale closed low in Canada, will continue to slowly advance into N NY and the Appalachians. Enhanced moisture pooled along the front, in combination with increasing instability from morning heating, should allow for a break out of thunderstorms along the length of the front or pre-frontal environment as far south as the Carolinas. Slightly stronger low to mid-level convergence further north across N NY/PA will exist along/ahead of a subtle shortwave crossing the region. Slightly backed flow should enhance moisture convergence and with ample 1.75" total PWats, will allow for slow moving but efficient rainfall production. 00z HREF shows 2"/3hr signals across much of central NY to N VA, with highest 3" probability of 40-60% across N NY to N PA. A Slight Risk of Excessive Rainfall was introduced where this signal best overlapped lowest FFG values across S NY into N PA. Some consideration was made across N NY/NW VT as well as central Western VA where recent heavy rainfall has fallen as well, with out HREF/00z Hi-Res CAMs extending past 08/00z, confidence afterward on better placement was not enough and will be reassessed with 12z HREF/CAM cycle for further upgrades/expansion. Further south in the Carolinas...a Marginal Risk of Excessive Rainfall was considered across the central Piedmont/Fall line across the Carolinas, but again, 00z Hi-Res CAMs and HREF probability signals were not available and global guidance was less reassuring over locally reduced (2-3"/hr) but regionally higher FFG values along this axis. An upgrade to Marginal Risk may be needed with subsequent updates. Gallina Day 3 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... ....2030Z Update... The Slight Risk area continues from far southeastern CO to the middle Ohio and Tennessee Valleys on Saturday. A front sagging southward will have ample Gulf moisture ahead of it for storms to develop and move along. There will be 3 separate forcing mechanisms for the heavy rainfall. First, an ongoing MCS from Day 2/Friday night will track east-southeastward from Oklahoma to the TN Valley by Saturday night. A digging shortwave trough in the upper levels will enhance an 80 kt jet streak moving over the Ohio Valley Saturday afternoon, which in turn will help to fire up additional thunderstorm activity. Finally, yet another MCS is expected to develop over southwestern KS and the OK/TX Panhandles Saturday night which in turn will cause additional heavy rainfall along the nose of the nocturnal LLJ. All of these forcings are subject to additional changes with upcoming updates as they're all mesoscale in nature and model agreement is still somewhat tenuous. Of late, the trend in the guidance is southward with each of these features, which makes sense given the storms like to develop into the source of moisture and instability. Oklahoma and Kansas have already been hard hit for the past several days, so additional southward adjustments may result in a gradually decreasing flash flooding threat. Elsewhere, another round of afternoon showers and storms are expected in the same corridor as Friday again on Saturday. However, the forcing should be quite a bit weaker Saturday afternoon, as much of the Mid-Atlantic will be between upper level shortwaves rotating around the longwave negatively tilted trough. Thus, the higher flash flooding threat Saturday will be up into northern New York and New England. A second shortwave doesn't arrive into the Mid-Atlantic until the predawn hours Sunday morning, which will substantially limit any resulting storms ability to grow strong enough to pose a flash flooding threat due to lack of diurnal heating/instability. That said, due to previous days storms priming the soils in this area and locally lowering FFGs, even the more pulse convection expected Saturday afternoon evening in this area could pose an isolated flash flooding risk, so the Marginal Risk area was expanded southwestward generally following the Day 2/Friday Slight Risk area in the Mid-Atlantic. Wegman ....Previous Discussion... ....Central High Plains through the Ohio River Valley... An expansive mid-level ridge centered over the Desert Southwest/Northern Mexico and an amplifying trough digging out of Canada will create squeezed northwest flow from the High Plains of WY/CO through the Mid-MS Valley on Saturday. Within this pattern, a 300mb jet streak arcing southeast from the Pacific Northwest will intensify over the Central Plains placing its diffluent LFQ across the region, which will combine with at least subtle height falls to drive deep layer ascent Saturday into Sunday. This will combine with subtle embedded shortwave impulses and local vorticity maxima dropping through the mean flow, and while timing and intensity of these features is uncertain at this time range, each one will likely result in enhanced local ascent to drive showers and thunderstorms, especially as they overlap the modest low-level baroclinic zone associated with a wavering frontal boundary. This setup will support waves of convection throughout D3, possibly manifesting as multiple convective clusters rotating over the ridge to the south. Again, timing of these features remains in question, but it is most probable these will occur early Saturday morning and again Saturday night. While these clusters could pose the most significant flash flood risk, additional diurnal convection driven by the ample ascent into robust thermodynamics characterized by PWs approaching +2 standard deviations according to the NAEFS ensemble tables overlapping MUCAPE of 1000-2000 J/kg will support rainfall rates in excess of 1"/hr, likely much higher at times. While storm motions D3 may be progressive noted by 0-6km mean winds around 20 kts, these are progged to be generally aligned to the surface front indicating the potential for training, especially during periods of enhanced 850mb LLJ which will impinge into the boundary from the SW. Despite the uncertainty inherent by D3, and noted by a large spread in the greatest axis of ECENS/GEFS/SREF probabilities for 1" and 3" of rainfall, the inherited SLGT risk remains warranted, although some modest adjustments were made. These adjustments were to account for the most recent ensemble probabilities for 24-hr rainfall, especially where they overlap the highest PW anomalies and regions of greatest 14-day rainfall according to AHPS. Further refinements will likely be needed as the event approaches, but the continued favorable setup for heavy rainfall in some areas that are also forecast to receive heavy rainfall D1-2 suggest a higher threat for instances of flash flooding, and it is possible that a higher threshold outlook may be needed if D3 rainfall can overlap perfectly heavy rain events from D1-2. Additionally, while the SLGT risk was tailored to focus the highest risk areas, the MRGL risk was also adjusted northward a bit into the High Plains where recent rainfall has been excessive producing more sensitive soils, but also trimmed out of the Mid-Atlantic States due to a forecast dearth of instability and recent downward trends in model QPF. ....New England... A MRGL risk remains for much of Northern New England and parts of Massachusetts Saturday and Saturday night as a favorable setup for heavy rainfall develops atop saturated soils. A surge of moisture noted by PWs reaching 1.5 inches (nearly +2 standard deviations according to NAEFS) will combine with 500-750 J/kg to produce an environment that can support rainfall rates in excess of 1"/hr. Forcing for ascent will be provided by an approaching front from the west, driven by subtle height falls as a shortwave embedded within the larger scale trough centered over Manitoba ejects northeast towards New England. This deep layer ascent impinging into these robust thermodynamics should create scattered to widespread showers and thunderstorms during the aftn/eve, with storm motions of 10-15 kts aligned to the approaching front suggesting the potential for training. While coverage of thunderstorms D3 may be somewhat scattered as indicated by relatively modest 1"/24 hr ensemble probabilities and within the available simulated reflectivity, this is likely at least somewhat underdone due to a lack of higher res models. Regardless of coverage however, the possibility of training 1"/hr rain rates atop sensitive soils driven by 14-day rainfall that is 200-300% of normal resulting in USGS streamflow anomalies that are almost without exception above the 90th percentile, suggests even scattered convection could result in rapid runoff and instances of flash flooding. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6OaVYIuXYC8d0xzBOCe4yNH1hXuOi971rR-qMsUc4TCX= 8O1lVYqoDnyNQ86wuTXDe_P1W-fmgpIXWizh_Qio1Lzu7Tk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6OaVYIuXYC8d0xzBOCe4yNH1hXuOi971rR-qMsUc4TCX= 8O1lVYqoDnyNQ86wuTXDe_P1W-fmgpIXWizh_Qioa5_rne4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6OaVYIuXYC8d0xzBOCe4yNH1hXuOi971rR-qMsUc4TCX= 8O1lVYqoDnyNQ86wuTXDe_P1W-fmgpIXWizh_QiovhpsEpI$=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) .