Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jun 18 2023 15:59:24 FOUS30 KWBC 181559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1158 AM 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... ....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 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 Valid 12Z Tue Jun 20 2023 - 12Z Wed Jun 21 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE WESTERN CAROLINAS... ....Western Carolinas and the Southeast... The upper level low continuing to drive the overall soggy weather pattern across the Southeast will continue drifting southward through the day Tuesday, starting in the TN Valley early Tuesday morning and drifting to east central AL by Wednesday morning. This slow movement will allow for a now eastward-oriented LLJ of Gulf Stream moisture off the Atlantic to encroach well inland, to the mid-Mississippi River. However, the greatest moisture and strongest easterly flow associated with the LLJ will be over the eastern Carolinas, where PWATs are likely to exceed 2 inches. By Wednesday morning, the easterly flow will be across all of NC and far southern VA. This flow will upslope up the Appalachians of the western Carolinas, which in addition to instability to around 1,000 J/kg will upslope along the mountains, which will locally enhance rainfall rates. The Slight Risk remains in effect for this area, perhaps shifted a little to the north given the latest guidance of the track of the aforementioned upper level low. Assuming this area is settled upon in future model runs, the Slight Risk area may need to be expanded northeastward into central NC. South of this axis of strongest easterly flow from MS through SC, much weaker flow will work to minimize the heavier rainfall threat a bit, as storms will not have as fast a resupply of moisture. Dry air advection will move into MS and western AL, and as such the rainfall forecast has diminished in these areas, while much of GA will have light and variable flow, despite plentiful atmospheric moisture and instability. This will favor slow-moving pulse convection that will struggle to organize into any MCS's capable of a more widespread flash flooding threat. Thus, the inherited Slight Risk for this region was downgraded to a Marginal. Further south into FL, while the LLJ will be strong at 30-35 kt, this area has very high FFGs, requiring prolonged periods of heavy rain. Confidence this will materialize, despite somewhat favorable moisture and instability is low. Expect a similar storm pattern over this portion of northern FL as occurred overnight last night. Thus, think the flash flooding threat is more isolated, despite a reasonable possibility of local rainfall totals to 4 inches. Should this shift westward into the FL Panhandle, which is more vulnerable to flash flooding, the Slight may need to be reintroduced. ....Portions of the Pacific Northwest... The upper level trough will begin to retreat to the northeast back towards Canada Tuesday and Tuesday night. Before it does so however, a couple more disturbances and jet streaks will move across the Pac NW, resulting in additional rainfall. A Marginal Risk was introduced with this morning's update for portions of northeastern WA through south-central MT. While the rainfall expected will be lesser than previous days, making this a low-end Marginal Risk, MUCAPE values in this region will be between 100 and 500 J/kg, which will allow for convective showers to develop. If those showers move over areas hit in previous days with heavy rain, isolated flash flooding may develop. ....Northern Plains... A strong southerly LLJ will transport Gulf moisture north well into the Canadian Prairies. For the portion over the Dakotas, PWATs may approach 1.75 inches. This will allow MUCAPE values to exceed 2,000 J/kg. A lack of forcing will be the primary inhibiting factor, as Corfidi Vectors will be near 0. Any storms that develop will have the capacity for producing heavy rain, but they will be isolated enough and moving over an area of high FFGs that for now a Marginal Risk was not introduced, but the area will be monitored for future upgrades. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!42ultmYLWrZtbYxPk6Tx8p8BbfyfXCDIHiXAGejniiV8= FZO3QAaTqe5o6GKiHZCFFgiNhgfPcTtpmj2WKHfE_nXWhJM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!42ultmYLWrZtbYxPk6Tx8p8BbfyfXCDIHiXAGejniiV8= FZO3QAaTqe5o6GKiHZCFFgiNhgfPcTtpmj2WKHfEK8HTX4o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!42ultmYLWrZtbYxPk6Tx8p8BbfyfXCDIHiXAGejniiV8= FZO3QAaTqe5o6GKiHZCFFgiNhgfPcTtpmj2WKHfE5hfnBWo$=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) .