Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 06 2025 15:52:05 FOUS30 KWBC 061551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1151 AM EDT Tue May 6 2025 Day 1 Valid 16Z Tue May 06 2025 - 12Z Wed May 07 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF EASTERN TEXAS INTO THE LOWER MISSISSIPPI VALLEY... ....16Z Update... The Moderate Risk area was trimmed a bit on the northwestern side with some of the latest CAMs guidance indicating that there will be less coverage of convection in between the main MCS to the south and east centered over northern Louisiana, and the upslope focused precip (with help from an upper level shortwave) to the west from the TX/OK Panhandles northwest into southern CO. THe Slight Risk area was expanded to the north and west to include a portion of the I-25 corridor near Colorado Springs, and the surrounding Marginal was expanded well north along I-25 to include Denver and Ft. Collins, with more rainfall the further south you go. This is due in part to rainfall amounts being quite abnormal for this area, and while associated flash flooding should be isolated given dry antecedent soil conditions and heavy snow in the higher elevations of the Front Range, the urban factor should support isolated instances through the night. For CO in general this will be a long-duration stratiform rain event in an area that doesn't have too many of those in a given year, so the abnormality of it could overwhelm flood-prone areas. Around the DFW Metroplex, a line of intense thunderstorms is moving across the metros right now. Once the line clears to the east, that should be the end of the flooding threat for the day. For points south and west of there, the Slight and Marginal Risk areas were trimmed accordingly. The biggest change with this update was to introduce a Slight Risk upgrade for Central New York and portions of the Southern Tier south into northeastern Pennsylvania, in coordination with BGM/Binghamton, NY and BUF/Buffalo, NY forecast offices. Similar to yesterday in the DMV, albeit further north, the same upper level=20 low is expected to produce additional areas of training convection from around the Mason-Dixon line north into NY. For the Slight Risk area, additional upper level energy will allow for greater organization as compared with areas further south, and despite the fast movement of individual cells, the focus of the heaviest rains over areas that have seen multiple afternoons of heavy rainfall prior to this event have left the soils very saturated and therefore unable to absorb much additional rainfall. The surrounding Marginal was expanded back into MD, Northern VA, and the eastern panhandle of WV with this update, as convection is already beginning to break out in these areas. Again, it's unlikely the storms will be nearly as organized today around and south of the Mason-Dixon line as compared with yesterday, so a Marginal should be all that is needed. The Marginal risk out west in southern NV, far eastern CA, and western AZ remains unchanged with this update as widely scattered convection is expected once again today, and at last much of southern NV and northwestern AZ have very saturated soils from yesterday's convection, favoring the redevelopment of additional flash flooding in this area with today's forecast storms. Wegman ....Previous Discussion... ....Lower Mississippi Valley to the Colorado Front Range... A powerful upper low over the Four Corners region is the catalyst for what will be an expansive area of Excessive Rainfall from the lower elevations of the Central Rockies' Front Range to as far east as the central Gulf Coast. The tightening pressure gradient between low pressure in the Southern Plains and a bubble of high pressure over the Tennessee Valley today is fostering a deep southerly fetch of Gulf moisture that is wrapping around the northern flank of the 700mb low over northern New Mexico. The ECMWF SAT shows a 300-400 kg/m/s IVT, topping >99th climatological percentile, oriented at southeast CO and northeast NM. While instability is lacking, the highly saturated profile when combined with strong synopptic-scale forcing aloft and upslope enhancement will support a prolong period of heavy rain below 7,000ft. The Slight Risk remains in place in these areas, as well as farther east across portions of OK/TX Panhandles where the pivoting band of >0.75" PWs will result in efficient rainfall over more saturated soils. Speaking of saturated soils, much of southern OK and northern TX also remain under the Slight Risk given their highly saturated soils and any heavy rainfall rates through the day Tuesday may result in scattered instances of flash flooding. RAP soundings around OKC show highly saturated profiles with warm cloud layers up to 10,000ft deep. Some weak instability (<100 J/kg MUCAPE) may be present as well, which while these values are low, they could result in highly efficient rainfall rates with PWs approaching 1.5". Farther south and east, much of eastern TX and the Lower MS Valley will either be in the warm sector of the storm system or just north of the surface warm front as it tracks just north of the Gulf Coast by Tuesday evening. Along the upper coast of Texas, PWs around 2.0" and warm cloud layers up to 13,000ft deep in some cases will overlap with MUCAPE that ranges between 500-1000 J/kg. A highly sheared environment will also support healthy mesocyclones that can further enhance rainfall rates. The 00Z HREF neighborhood probabilities show low-to-moderate chances (30-50%) for 24-hr rainfall totals >5" from just nor of I-10 in southeast TX on east throughout much of southern LA and southern MS. There are actually some low chance probabilities (10-20%) for isolated totals >8" in southern LA. Overall, this is a setup that is ripe for flash flooding in the Lower MS Valley with some instances of significant flash flooding in areas where training segments of thunderstorms track over sensitive urbanized environments. Residents should ensure they have a reliable way to receive flash flood warnings from their respective NWS WFOs. ....Northeast... The upper low responsible for a multi-day stretch of wet weather throughout the region will begin to make its exit today, but not before it triggers more rounds of heavy rain and thunderstorms. The ECMWF SAT shows the lingering presence of a >500 kg/m/s southerly IVT that is responsible for fostering anomalous >90th percentile PWs above 1.25" in southern New England. Farther west over NY and eastern PA, the 00Z HREF mean shows anywhere from 500-1,000 J/kg of MUCAPE available with PWs ranging between 0.75-1.0". Soils have saturated substantially over the past couple days, resulting in 1-hr FFGs that are <1.00"/hr in the southern tier of NY and northern PA. Given these factors, the Marginal Risk remains in place for the potential for additional isolated instances of flash flooding. ....Desert Southwest... Following a couple days of active weather that has moistened up soils throughout the region, another round of showers and thunderstorms is expected today. As mid-level RH values decrease, the expectation is for more surface-based heating to add to an increasingly unstable environment aloft (250-500 MUCAPE available Tuesday afternoon and evening). What is also aiding in the flash flood potential is added synoptic-scale lift as a 500mb low approaches from the north and the diffluent left- exit region of a 250mb jet streak moves in overhead. 48-hr QPE estimates as much as 1-2" of rain has fallen in portions of the Marginal Risk area. Given the sufficient synoptic-scale support is coupled with increasingly sensitive soils, opted to introduce a Marginal Risk this forecast cycle for the region today. Mullinax Day 2 Valid 12Z Wed May 07 2025 - 12Z Thu May 08 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GULF COAST... By Wednesday, the warm front that lifted through the Upper Texas coast will stall out along the Gulf Coast and be the focus for widespread showers and thunderstorms. PWs remain above the 97.5 climatological percentiles from the Upper Texas coast to as far east as the FL Panhandle. In fact, there are anomalous PWs that stretch up into northern AL Wednesday afternoon. The flash flood threat remains greatest along the central Gulf Coast and Lower MS Valley regions where they are most commonly positioned at the nose of a >500 kg/m/s IVT and PWs that are topping 1.75" in most locations. RAP soundings in southern LA Wednesday morning show low-mid level RH values >90% and warm cloud layers at least 11,000ft deep. These soundings also depict tall "skinny CAPE" profiles with MLCAPE >500 J/kg. Plus, hodographs still suggest the potential for organized convection given modest mid-level shear and low-level helicity (sfc-3km >150 m2/s2). The NBM 75th percentile shows rainfall totals >5" in some cases across the central Gulf Coast, which is on top of the heavy rainfall that occurred on Tuesday. The concern would be the SWrly IVT continues to intersect the stalled frontal boundary in a way that supports training and back-building convection that produces highly efficient rainfall rates >2"/hr over soils that are increasingly saturated. No Moderate Risk was hoisted as of this forecast cycle as there are still some varying scenarios depicted by model guidance, but should guidance come into better agreement on a swath of 3-5" amounts and./or soils grow highly sensitive due to Tuesday's Excessive Rainfall, a Moderate Risk upgrade could be necessary. The Marginal Risk was expanded north into the hills of northern AL where FFGs are lower (<2" for 1-hr FFGs) and guidance showed some modest moisture advection via the aforementioned IVT. Given the saturated soils in the region and the potential for locally heavy rainfall rates, the Marginal Risk was expanded a little farther north closer ot the TN Valley. Mullinax Day 3 Valid 12Z Thu May 08 2025 - 12Z Fri May 09 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GULF COAST... ....Central Gulf Coast... Thursday and into Thursday night is a near carbon-copy setup to Wednesday's where the stationary front draped W-E along the Gulf Coast will continue to act as the initiator in additional thunderstorms across the South. The central Gulf Coast will remain the more primed spot for potential flash flooding given the increasingly sensitive soils in the region after multiple days of heavy rainfall. PWs ranging between 1.6-1.8" will be common with MLCAPE that tops 1,000 J/kg. Thunderstorms could produce >2"/hr rainfall rates across the Marginal Risk area. Should guidance trend wetter in the coming days, it is possible a Slight Risk upgrade may be necessary given the soils more sensitive state. ....Northeast... No Marginal Risk was introduced tonight, but this region bears watching as a cold front approaches from the West. Several guidance members now show 1-2" of rainfall across portions of the region on Thursday, however they vary on where those heavier amounts are most likely to occur. A positively tilted upper trough approaching from Ontario will provide plenty of upper-level ascent while the approaching cold front acts as the surface/low-level trigger for initiating thunderstorms. Plus, some guidance members show the positively-tilted trough cutting off just enough to where an 850mb low forms near the Northeast coast. This is not the case across all guidance and is resulting in rainfall swaths that are farther north over the interior Northeast (GFS/ECMWF/ICON), or as far south and east as the I-95 corridor (EC-AIFS/CMC/UKMET/RRFS). With lingering uncertainty in the forecast, the Marginal Risk upgrade was held off this forecast cycle. But given the highly saturated soils in the region, it is possible a Marginal Risk will be necessary in future forecast updates once confidence increases in where the heaviest swath of rainfall occurs. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8YGEMNH7anaq1fMdyhZrKvBDHOfcToj2iLIDx0U6Tfwp= bJ3JjgfBkFOhj2lrRZNzqED76974CZuH_y1bgMk3EP5Pc84$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8YGEMNH7anaq1fMdyhZrKvBDHOfcToj2iLIDx0U6Tfwp= bJ3JjgfBkFOhj2lrRZNzqED76974CZuH_y1bgMk3IGHMjdE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8YGEMNH7anaq1fMdyhZrKvBDHOfcToj2iLIDx0U6Tfwp= bJ3JjgfBkFOhj2lrRZNzqED76974CZuH_y1bgMk3zClE4Wk$=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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