Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 27 2024 00:07:27 FOUS30 KWBC 270007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 807 PM EDT Fri Jul 26 2024 Day 1 Valid 01Z Sat Jul 27 2024 - 12Z Sat Jul 27 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDDLE/UPPER TEXAS COAST, THE COASTAL CAROLINAS, AS WELL AS COASTAL SOUTHEAST LOUISIANA AND MISSISSIPPI... ....01z Update... Diurnal convection is waning across much of the country, but a Slight Risk has been maintained where convection remains active across coastal portions of LA/MS where convection (see MPD #740).=20 While this activity is poised to weaken and diminish with the loss of daytime heating, there should be reinitiation of convection=20 late tonight over portion of the Middle and Upper TX Coast (where a Slight Risk has been maintained and expanded). The 18z HREF=20 indicates 40-50% odds for localized 3" exceedance (per a 40-km=20 neighborhood method), mostly after 06z. Broader Marginal Risk areas were maintained across the Southeast, Mid Atlantic, Southwest, and Intermountain West (where localized flash flooding will continue for at least a few more hours).=20=20 Churchill ....16Z Update... Several changes to note with today's midday update: ....Southeast Texas & The Mississippi Delta... The Slight across southeast Texas has been expanded to include much of Louisiana and the Mississippi Gulf Coast. Ongoing heavy rainfall has resulted in numerous Flash Flood Warnings across east Texas and northwestern Louisiana. A plume of deep tropical moisture is surging northward across the Slight Risk area. CAMs guidance shows with peak heating this afternoon, numerous showers and thunderstorms will form over the Gulf and move inland into southern Louisiana and the Mississippi Gulf Coast. With unidirectional flow, it's probable there will be training storms embedded within the broader field of storms. Meanwhile, the plume of moisture over northern Louisiana now should decrease in intensity as peak heating results in universal increases in instability, resulting in dissipating storms reliant on a coherent plume of moisture. PWATs are near their climatological maximum across Louisiana as they approach 2.25 inches this afternoon. This will allow the expected afternoon convection plenty of moisture to translate into efficient rainfall rates. Urban areas including New Orleans, Gulfport, and Baton Rouge are at a higher threat for flash flooding. ....Carolinas... In coordination with RAH/Raleigh, NC; CAE/Columbia, SC; and ILM/Wilmington, NC forecast offices, the Slight for the coast was expanded westward to cover the ongoing convection along the NC/SC border and aligned with MPD 734. While the convection has been shallow, very efficient warm rain processes and training has resulted in multiple inches of rain in the impacted areas. The focus going into this afternoon will be much closer to the coast, but any disorganized afternoon convection that impacts this same area...with sufficient clearing of the cloud cover between now and then, could result in widely scattered instances of flash flooding. ....New Mexico... In coordination with ABQ/Albuquerque, NM forecast office, a Slight Risk upgrade was introduced with this update for the afternoon convection in the Sacramento Mountains. Increasing instability this morning and continued above normal atmospheric moisture should result in a renewed round of afternoon storms. The Sacramento Mountains have been both hard hit in recent weeks with heavy rain, and have very sensitive and vulnerable burn scars around Ruidoso that would enhance the impacts of any potential flash flooding in that area. Wegman ....Previous Discussion... ....Portions of Southeast Texas... Radar was showing increasing coverage of showers in a region of confluent flown off the Gulf of Mexico in the early morning hours...and the expectation is that convection will be persisting beyond 12Z. Given some overlap with a region of lower Flash Flood Guidance values and potential for 1 to 2 inch per hour rates redeveloping later in the day as shown by the HREF 40 km neighborhood probability guidance, maintained a Slight Risk from coastal Texas northeastward along the axis of highest precipitable water/instability. There is some upper support in the form of a trof axis between 200 mb and 300 mb with a 50 kt speed max rotating around the east side of the trof axis that looks help draw the moisture towards the Southern Mississippi valley late in the period, ....Portions of the Southeast US Coast... Surface low pressure will linger along the Carolina coastline for much of the period with an associated frontal boundary helping provide some focus for convection as mid-level height falls approach from the north and west. Much of the operational guidance showed the heaviest rainfall along or immediately off the Carolina coastline...although the NAM maintained some threat of 2 to 5 inch amounts falling inland. Between that and the fact that the 26/00Z HREF showed 40 to 60 pct neighborhood probabilities of 1- and 3-hour QPF exceeding flash flood guidance along the immediate coastline at time of maximum daytime heating...maintained the Slight risk but continued to shrink the area somewhat from the placement of the inherited Slight Risk area. Surrounding the two Slight Risk areas was an expansive Marginals Risk area. Models depict abundant moisture and instability in this area but weak forcing mechanism and meager steering flow. Thus cells that form will be slow moving and efficient rainfall producers. ....Southwest... Enough moisture and instability will still be in place over portions of the Southwest and the Intermountain basin to support the development of convection in the afternoon and which persists into the evening...although the best focus will have shifted eastward since Thursday. There is still concern that any storm which forms will have the potential for 1+ inch per hour rates and isolated storm total amounts in the 1 to 2 inch range...enough to result in flooding and run off problems with the greatest risk being over recent burn scars and in normally dry washes. Some minor adjustments were made to the previous MRGL risk area but overall there was a fair degree of continuity. Bann Day 2 Valid 12Z Sat Jul 27 2024 - 12Z Sun Jul 28 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE MIDDLE TEXAS COAST... ....20Z Update... In coordination with HGX/Houston, TX and CRP/Corpus Christi, TX forecast offices, a Slight Risk upgrade was introduced with this update. Continued onshore southerly flow of deep tropical moisture off the Gulf will bring yet another day of occasionally heavy rains to much of the Middle Texas Coast on Saturday. This area has been hard hit with heavy rain over the past few days, resulting in well below normal FFGs for much of the coast from Corpus Christi to Galveston. Fortunately, the rainfall expected on Saturday should be much less organized as compared with this morning and previous days. However, given the saturated soils along the coast and deep tropical moisture allowing any convection that forms to be capable of very heavy rain rates, there still is a slight risk of flash flooding, whereas this pattern would likely only favor a Marginal if the area were much drier. Elsewhere, only modest adjustments were made to the expansive Marginal Risks across the middle of the country, Southeast, and West. As is typical for both July and previous days' anywhere in the Marginal risks for the Mississippi Valley and Southeast will have storms capable of organizing or producing local outflow boundaries that may initiate additional convection, so isolated instances of multi-inch rainfall totals are quite possible given the plentiful moisture that remains across this region. Wegman ....Previous Discussion... ....Southern and Southeastern United States... The 26/00Z suite of numerical guidance showed deeper moisture getting drawn north and eastward from the Texas coast towards the western part of the Tennessee Valley given persistent south to southeasterly flow on the east side of an upper level trough/closed low. With mid-level height rising from the western Great Lakes to eastern Tennessee Valley, opted tom limit the eastern extent of the Marginal. Elsewhere...enough instability and sufficiently deep moisture will be in place for almost any storms that develop to produce isolated flash flooding due to slow moving downpours. ....Southwest United States... Confidence remains below average in the potential and placement of flash flooding across the West...with the areal coverage shrunk yet again from the previous issuance. Given the terrain and lingering deep moisture in much of the West, it's going to be really tough to completely eliminate the flash flooding potential. Bann Day 3 Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF NORTHERN MINNESOTA AS WELL AS MUCH OF THE TENNESSEE VALLEY... ....20Z Update... ....Northern Minnesota... Rain associated with a slow moving front will be ongoing across northern Minnesota Sunday morning. It is likely to diminish in intensity from mid-morning through early afternoon. Then much stronger showers and storms will move into the area following along the front Sunday afternoon, with multiple rounds of storms likely to follow through much of Sunday night. The storms will be capable of heavy rainfall as PWATs rise as high as 1.75 inches. Since the front will be slow-moving, the storms are likely to train along the same areas, likely in a southwest to northeast swath across much of northern Minnesota, but possibly starting in the eastern Dakotas. Portions of Minnesota remain with wetter than normal soils, so multiple rounds of heavy rainfall are likely to cause widely scattered instances of flash flooding. ....Tennessee Valley and Surrounding Areas... A stationary front turned warm front will track northeastward up the Tennessee Valley on Sunday through Sunday night. Increasing atmospheric moisture associated with the advance of an airmass originating off the Gulf will allow for potential training and backbuilding of strong storms capable of increasingly heavy rainfall. The surface warm front will likely slow as it approaches the southern Appalachians Sunday night. This will support slower moving storms that with interactions with the terrain will be capable of flash flooding. Further, some of these areas, especially the mountains along the NC/TN border have seen heavy rain in recent days. The advection of increasing moisture will support storms capable of very efficient warm rain processes which could result in multiple inch per hour rainfall rates. Urban areas are at a higher risk for localized flash flooding with any repeating storms. Wegman ....Previous Discussion... ....Gulf Coast to parts of the Tennessee Valley... A closed mid- and upper-level low that started to take shape late Saturday night/early Sunday morning should begin to lift northward during the day on Sunday and then become an open wave again by Sunday evening. Lowering mid-level heights should help increase the coverage of convection within an an atmosphere sufficiently moist to support isolated downpours in a corridor from the middle Gulf coast to the Tennessee valley during the afternoon and evening, ....Southwest US... The risk of excessive rainfall continues to get less conducive with time and gets shunted eastward as mid-level westerly flow strengthens over much of the West outside of Arizona and New Mexico. Maintained a Marginal Risk here given the lingering moisture. ....Upper Midwest... Scattered convection should develop along a cold front extending into the northern tier of the US from a system in Canada. With increasing precipitable water and instability values increasing as a result of southerly flow ahead of the front...isolated downpours could result in isolated flooding or run off problems in regions of poor drainage. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_O5ysnD-f6hChg5p3TnQRvzn6sJVMtYgoWr-OAZ3pK0X= UnHVchaO34JWAKDcc67xr9XwiiTvoLYNwRxWTlQGZBPc5pY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_O5ysnD-f6hChg5p3TnQRvzn6sJVMtYgoWr-OAZ3pK0X= UnHVchaO34JWAKDcc67xr9XwiiTvoLYNwRxWTlQGLFx8rgs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_O5ysnD-f6hChg5p3TnQRvzn6sJVMtYgoWr-OAZ3pK0X= UnHVchaO34JWAKDcc67xr9XwiiTvoLYNwRxWTlQGXqECuwI$=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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