Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jun 17 2023 19:31:08 FOUS30 KWBC 171931 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 PM EDT Sat Jun 17 2023 Day 1 Valid 1737Z Sat Jun 17 2023 - 12Z Sun Jun 18 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF NEW ENGLAND ALONG WITH NORTHERN AND CENTRAL FLORIDA... ....Portions of the Southeast... A digging shortwave in the upper levels associated with the left exit region of an upper level jet streak will help with forcing a renewed round of showers and thunderstorms across the Gulf Coast and especially the northern Florida Peninsula on Day 1/Saturday. Plentiful moisture is in place with PWATs over the entire FL Peninsula over 1.75 inches, locally exceeding 2 inches. For this part of the country, this isn't unusual with climatological deviations about 1 to 1.5 sigma above normal. With the approaching upper level support, the MCS over the Panhandle may help support additional convection later this morning over the northern FL Peninsula, with daytime heating and the shortwave's arrival resulting in much more plentiful convection capable of heavy rainfall rates and training this afternoon through this evening. This general idea is supported in many of the CAMs, with the best chances of heavy rainfall this evening through much of the overnight. While FFGs across the Peninsula are very high, the high potential for strong storms and likely training resulting in multiple rounds of strong storms moving over the same areas, particularly along the Gulf Coast, should be able to overcome these unfavorable antecedent conditions for flash flooding, particularly over urban and other poor drainage areas. Both the Tampa and Orlando metro areas are included in the Slight Risk region. ....Much of New England... 1600 UTC Update -- Based on the observational and model trends with the 12Z guidance, including the HRRR and FV3-based CSU first-guess fields, have upgraded a portion of New England to a Slight Risk. This includes parts of northeastern MA into much of NH and parts of eastern VT and western ME. Instability isn't as much of a limiting factor across these areas per the latest SPC mesoanalysis trends (current MUCAPEs 500-1000 J/Kg). Latest (12Z) HREF neighborhood probabilities of 6-hourly QPF exceeding the 2 year ARI peak between 60-75% within the Slight Risk area. Previous discussion... A strengthening low moving up the coast of New England today will pump Atlantic moisture into New England, as PWATs exceed 1.25 inches along the NH and ME coasts. This will support locally heavy rain, particularly in the mountains of New England where upsloping will support locally heavier rainfall rates on east-facing slopes. This is considered a high-end Marginal risk, as antecedent conditions support at least much of the expected rainfall to be soaked into the soils and limiting runoff. However, a widespread 1-2 inch rainfall could very well result in isolated flash flooding in poor drainage areas, canyons, and other sensitive areas. ....The Plains... 1730Z UTC Update -- Pulled the southern boundary of the Marginal Risk area farther south across central-eastern TX and central LA, and also nudged it a little farther east into portions of western MS. While the airmass remains capped between 800-700 mb, a considerable amount of mixed-layer CAPE (4000-5000+ J/Kg) will make for intense rainfall rates (likely over 2" in less than an hour) with any isolated or scattered cells that can propagate farther south of the best upper level forcing (right entrance region of an upper level jet streak and eventual MCV track). Scattered convection is expected to redevelop once again over many portions of the Plains again today. The MCS currently moving over west KS is expected to slowly weaken as it tracks northeast into eastern NE. Additional flash flooding resulting from this MCS is possible with a southerly LLJ becoming established from western MO through much of western MN. The heaviest rains are expected across northern OK as a potent shortwave ejects out of the mountains and once again takes advantage of the plentiful moisture in place across the southern Plains. For Texas, it looks like a repeat day where widely scattered convection redevelops over portions of northern and central TX, including possibly the Dallas-Fort Worth Metroplex area. While additional rainfall is likely over the hard hit areas of CO and western KS, at the moment the strongest forcing is expected to miss these areas. Any northward adjustments however may require a Slight Risk upgrade. ....Portions of ID/MT... A potent upper level low and associated shortwave will move into the ID/MT areas this evening through tonight. Ahead of this feature, expect a renewed round of showers and thunderstorms from central ID through western MT, which will continue into the overnight. Anomalous moisture will move into the area as PWATs approach 1 inch tonight. Corfidi Vectors gradually aligning parallel to the wave and the associated surface cold front will support training showers and storms. The mountainous terrain of the region should support locally heavier rates due to upsloping. Hurley/Wegman Day 2 Valid 12Z 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... 1930 UTC Update -- Subtle changes were made to the outlook areas based on the latest guidance trends. Most notably was a slight tweak to the Slight Risk area over the Lower-Mid MS Valley, TN Valley, and Gulf Coast region. 12Z HREF exceedance probabilities are highest within the Slight Risk area, along and east of the amplifying shortwave trough and thus in a region where synoptic support for deep-layer ascent will become increasingly favorable. Combination of robust deep-layer instability (mixed layer CAPEs 1500-2500+, especially south towards the Gulf Coast) and PWs of 1.75-2.00" will support hourly rainfall rates of 2+ inches underneath the more intense, isolated cores. Hurley Previous discussion... ....Lower MS Valley into the Southeast... A strong LLJ pumping plentiful Gulf moisture with PWATs locally exceeding 2 inches will provide plenty energy for widespread showers and thunderstorms to develop over the MS Valley on Sunday. 2 shortwaves will phase into a much stronger one by Sunday night. Finally, MUCAPE values will be extreme in some areas as values approach 5,000 J/kg over southern AR and LA. At least some of that instability will advect northward with the LLJ. An MCS is likely to be ongoing in eastern OK/western AR, which will persist as it tracks southeastward across southern AR and into MS. Additional showers and storms will merge with the MCS during the afternoon, supported by the diurnal heating, then the eastward shifting LLJ will support the training showers and storms into the hard hit areas of far southern MS and AL Sunday night. Despite the rather progressive nature of the convection, even with some potential for training, the favorable antecedent conditions due to multiple prior days of storms will support additional flash flooding potential. Thus, the Slight Risk remains in effect, with only a few minor expansions. Once again, given the favorable both atmospherics and hydrology, it's possible a further upgrade to a Moderate may be needed with improved agreement in the CAMs guidance, most likely along the Gulf Coast from MS through the western FL Panhandle. ....FL Peninsula... Convection ongoing at the start of the period over northern portions of the state will sag south through the morning and into the afternoon on Sunday. This convection is likely to be locally heavy, with the heaviest rains favoring the Gulf Coast/western side of the peninsula. Northern portions of the Marginal Risk will see some overlap from the Day 1/Saturday period, especially in the Tampa area, which will compound any lingering flash flooding in that area from today. ....ID/MT/WY area... The longwave trough/upper low that caused heavy rain over a relatively narrow corridor over ID/MT on Saturday will gradually shift southeastward to include more of southwestern MT and into the Yellowstone/Grand Teton N.P. areas of northwestern WY on Sunday. The longwave trough will have plenty of shortwave energy rotating around it, but the overall longwave will stretch to a southwest-to-northeast orientation across this region through Sunday night. This will favor the narrow corridor of potential heavier rainfall rates to persist again through Sunday. As portions of this area have been hard hit in previous days and weeks, the soils in this area are closer to saturation, and therefore are more supportive of flash flooding. On the large scale, a bit less rain is expected broadly on Sunday as compared with Saturday. 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!_n0ZigFwG6Rv3T0AbLOhMUFjqJsIvJUxRD9iXJpQJ13e= 1JH-ieW9s6QVGTWZgy7Isl1dKoezWwVcUg-ihxKBHgmrKe4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_n0ZigFwG6Rv3T0AbLOhMUFjqJsIvJUxRD9iXJpQJ13e= 1JH-ieW9s6QVGTWZgy7Isl1dKoezWwVcUg-ihxKBmyuyP3Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_n0ZigFwG6Rv3T0AbLOhMUFjqJsIvJUxRD9iXJpQJ13e= 1JH-ieW9s6QVGTWZgy7Isl1dKoezWwVcUg-ihxKBcKsqJ_w$=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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