Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jun 18 2023 19:50:59 FOUS30 KWBC 181950 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 350 PM EDT Sun Jun 18 2023 Day 1 Valid 16Z Sun Jun 18 2023 - 12Z Mon Jun 19 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST... 1600 UTC Update -- Fairly minor changes made to the Day 1 ERO, largely based on the observational/mesoanalysis trends this morning along with the latest (12Z) models, especially the CAMs and in particular, the HREF suite of probabilistic guidance. Based on these factors, have expanded the back edge of the Marginal Risk area back to include more of the ArkLaTex region. Have also expanded the Marginal area farther north along the MS Valley in northern IA, southeast MN, and western WI. Storm motions will be relatively slow along this compact northern stream mid-upper shortwave, north of the main vort farther south traversing the lower MO and mid MS Valleys. Deep-layer instability across this region will not be as impressive compared to areas farther south, nevertheless sufficient as mixed-layer CAPEs climb to 500-1000 J/Kg during the afternoon. Hourly rainfall rates of 1-1.5" could lead to localized runoff issues, especially in urban areas in and around Minneapolis-St. Paul. Lastly, based on the CAM consensus (higher forecast confidence), we nudged the southern edge of the Slight Risk a little farther north across far northeast LA and southern MS. WNW-ESE storm track, eventually more NW-SE later this afternoon, will keep the more widespread activity along and north of the surface boundary, where there isn't a strong capping inversion centered ~850 mb compared to areas farther south (including the Lake Pontchartrain North Shore and points south toward the LA Gulf Coast). Hurley Previous discussion below.. ....Lower MS Valley into the Southeast... A 500 mb shortwave driving the ongoing convection across OK/KS/AR/MO will become detached from the main flow and slow down considerably by this evening and tonight. However, its presence along with plentiful Gulf moisture (over 1.75 inches PWATs) and impressive instability (MUCAPE values in the GFS are over 5,000 J/kg over far southern AR by this evening) will be very favorable to additional/continued thunderstorm development all throughout the Slight Risk area through tonight. For northern areas, the main driver will be that stalling out shortwave. Meanwhile, the forcing further south across central MS/AL will mainly be prior outflow boundaries and a reinvigorated nighttime low level jet (LLJ) redeveloping on 30-35 kt southwesterly mid-level winds. While the storms will be moving, Corfidi vectors of 5 kts over much of central MS/AL suggest a high likelihood of repeating convection. As usual, exactly where this axis of repeating convection sets up is the primary uncertainty at this point. Latest HRRR guidance suggests it will be over central AL, while much of the global models and other CAMs suggest MS will be the more likely target. For now just left the entire area in a Slight, but depending on how many repeating rounds of storms move over the same area, especially if there is some urbanization, it's possible locally Moderate level impacts will be seen. ....FL Peninsula... Ongoing showers and thunderstorms this morning will continue sagging south across the southern Peninsula through the day today. An isolated flash flooding threat is possible in any urbanized and poor drainage areas that get repeating rounds of convection, but given the ability for much of the region to handle the amounts of rain forecast, these will be few and far between. ....ID/MT/WY area... An upper level low and longwave trough moving into the Pacific Northwest will become more positively tilted as a vigorous shortwave trough dives southward to the west of the upper level low and trough axis, reaching northern CA by early Monday morning. This will align the upper level pattern in a southwest to northeast straight line configuration across the Marginal Risk area. An embedded 130 kt jet will round the base of the trough, putting the Marginal Risk area in a favorable left exit region of that jet streak. Somewhat anomalous moisture with PWATs approaching 0.75 inches, or 1.5 sigma above normal, along with moisture from recent rainfall will all contribute to repeating shower activity, with potential for some embedded convection, as noted on radar over western MT as of the time of this writing. The mountain ranges that are aligned orthogonal to that flow, so those with southwest facing slopes have the greatest potential to see flash flooding impacts, as upslope further enhances rainfall rates in this region. Wegman Day 2 Valid 12Z Mon Jun 19 2023 - 12Z Tue Jun 20 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST... 2000 UTC Update -- only minor modifications made to the Day 2 outlook areas, based largely on the latest guidance trends. Per collaboration with WFO CHS, have pulled the eastern edge of the Slight Risk farther west across southern SC and eastern GA. More widespread convection and thus concentration of heaviest rainfall (better probability of excessive rainfall) should be farther inland vs. along the coast, given at least to some degree the better forcing (upper difluence/divergence) closer to the mid-upper level circulation. This Slight Risk area lines up very well with the 12Z UFVS version of the Day 2 FV3GEFS first-guess field. Hurley ....Southeast... The slow-moving shortwave will continue strengthening as it evolves upscale into a full cutoff low by Tuesday morning. The shortwave/low starts the day over southern IL and slowly drifts southeast to over central TN by Tuesday morning. The behavior of this low will be the primary determinant as to where the strongest and most persistent convection sets up. The guidance continues to drift the low further east with time, which in turn is shifting the axis of heaviest rainfall also to the east. As a result, the Slight Risk area was trimmed quite a bit from the west, with downgrades for much of MS, eastern LA, northern AL out of the Slight, and northwestern GA also out of the Slight with this morning's forecast update. PWATs over this area will remain very high, with portions of southern SC, southern GA and the FL Panhandle occasionally exceeding 2 inches. Elsewhere, much of the Slight Risk area will remain at or above 1.75 inches. This will be because the LLJ wrapping around the low will draw the plentiful Gulf moisture northward ahead of the low, which will act as the fuel for heavy-rain-producing thunderstorms. Immediately south of the low however, the source region for the air in this area will be out of the Midwest, so atmospheric moisture content will be considerably lower, and thus feature considerably less coverage of storms. This is the primary driving factor for the shift in the ERO risk area. Lesser amounts of rain than areas further east can be expected over eastern TN and the eastern 3/4 of KY. However, there will still be enough moisture and instability, along with the close proximity to the center of the upper level low that localized rainfall amounts to 2 inches are possible, which could cause isolated flash flooding, hence the Marginal Risk for that area. On the other side of that coin, the leading edge of the LLJ out of the Gulf and the Gulf Stream modified Atlantic will encroach into the southern Appalachians, particularly the east facing slopes of the mountains of the Carolinas and southern VA. The predominant southeasterly flow will allow the mountains to upslope additional rainfall out of the LLJ, resulting in heavier rainfall totals. While the area has been dry recently, this may allow for some absorption of a day or 2 of heavy rain, but the nearly stationary movement of the upper low will keep this feed of tropical moisture going into these same areas for multiple days in a row, so it will be seen how much rain the area can take before flooding becomes an increasing problem. Along the eastern Gulf Coast of the FL Panhandle, higher moisture concentration with some upper level energy will also favor renewed training/repeating convection over vulnerable portions of the Panhandle, though likely concentrating a bit further east of the recently hard hit Pensacola. The Big Bend, FL; Tallahassee, FL; and Valdosta, GA areas may be in the core of the heaviest rainfall. As usual this will require the guidance consensus to remain similar to now going forward. ....UT through eastern ID, western WY, and eastern MT... The upper level trough will slowly pivot to be less positively tilted with time, meaning the southern end of the trough will gradually track east, while the northern edge remains largely stationary. Embedded shortwave disturbances will pivot northeastward ahead of the low, and act as the forcing for additional shower and occasional thunderstorm activity over much of the Pacific Northwest. The axis of heaviest rain will stretch from the Great Salt Lake in northern UT through the northeast corner of MT. Once again the heaviest rainfall totals will be along southwest facing slopes along that axis, particularly through the Grand Teton and Yellowstone N.P. areas. Into eastern MT, recent rains have kept the soils moistened, which will support lesser amounts of rain causing isolated flash flooding, particularly with any training and repeating heavier showers or storms. MUCAPE values approaching 500 J/kg should allow convective elements to be embedded within any areas of showers, locally enhancing rainfall rates. Wegman 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!9s6HKLCLN6iduAG1cGgGgXd0Dc4KlwEHbbpBl413D4pB= 5wItEisP_BjypK5krqIqN8-QoUn6RzVVlCFgWB3j0La7drA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9s6HKLCLN6iduAG1cGgGgXd0Dc4KlwEHbbpBl413D4pB= 5wItEisP_BjypK5krqIqN8-QoUn6RzVVlCFgWB3jKbk9Eyg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9s6HKLCLN6iduAG1cGgGgXd0Dc4KlwEHbbpBl413D4pB= 5wItEisP_BjypK5krqIqN8-QoUn6RzVVlCFgWB3jedB3mA8$=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. 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