Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 26 2025 00:53:41 FOUS30 KWBC 260053 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 853 PM EDT Tue Mar 25 2025 Day 1 Valid 01Z Wed Mar 26 2025 - 12Z Wed Mar 26 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHEAST TEXAS INTO SOUTHERN OKLAHOMA AND OVER PORTIONS OF SOUTHEAST FLORIDA... ....Florida... Maintained the Marginal Risk area across the southeast Florida=20 peninsula after realigning it based on even trends in satellite and radar imagery. Even though loss of daytime heating should result in diminishing risk of excessive rainfall due to loss of daytime heating by late evening...it was too early to entirely remove the=20 Marginal risk area as additional showers and thunderstorms capable of producing downpours in a highly urbanized area...a few places which had locally heavy rainfall earlier in the day. The expectation is that the risk of excessive rainfall will diminish by 26/04Z. ....Texas/Oklahoma... Convection has initiated across portions of southern OK near and=20 just north of a stationary front. Southerly flow should continue to advect moisture northward and result in upwards of 1000-2000 j/kg=20 of CAPE with a base just above the boundary layer. This elevated=20 convection will track off to the southeast, generally parallel to=20 the low level convergence axis...which supports some training=20 potential. The 3km NAM remained the wettest model, but we have=20 seen elevated convective setups like this verify towards the higher end of the model spread, and the 3km NAM has performed decently at these events in the past. So while it depicts a lower probability=20 outcome based on all available guidance, and a swath of 1-3" is=20 most likely, can not rule out a localized swath of 3-6" of rain=20 across portions of northeast TX into southern OK late tonight into=20 Wednesday morning. Overall the flash flood risk at any location is low given the dry antecedent conditions and narrow nature of any heavy rainfall swath. However intense rainfall rates and some training could allow for a localized flash flood threat to evolve, particularly if the heavy rain overlaps any more sensitive urban or low lying area. We opted to maintain a focused Marginal risk area to account for this isolated potential. Chenard Day 2 Valid 12Z Wed Mar 26 2025 - 12Z Thu Mar 27 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTH-CENTRAL AND SOUTH TEXAS... Model guidance continues to speed up the timing of the onset of the heavy rainfall event over southern TX. It now appears likely that organized convection will begin growing upscale by later Wednesday afternoon or evening. This area has been very dry and could use the rain, unfortunately this setup looks good enough to be a case where it's beneficial rain initially, but then quickly overwhelms even the dry soil conditions. Model soundings by Wed afternoon/evening show a deeply saturated profile with good directional shear and moderate instability. This should support some convective clusters with some weak mesocyclones and right mover motions are very slow. Combine that with the front nearby and anticipate upscale growth of slow moving convective clusters through Wed night within an environment capable of 2-3" per hour rainfall. Activity may eventually grow upscale enough that it begins propagating east and possibly offshore, however even if that eventually transpires, there will still be a prolonged period where training convection is a possibility. Overall there is above average model agreement on this overall evolution, with only the 3km and 12km NAM really standing out as outliers, and those are two of the less reliable models for situations such as this. While confidence on the event happening is above average, there is still uncertainty on the exact axis of heaviest rainfall, which will probably be somewhat narrow in nature. For now the MDT risk is centered around the highest HREF probabilities, and generally on the southern gradient of the global model QPF solutions. Flash flood coverage over this area is expected to increase Wednesday night into Thursday morning, and locally significant impacts are possible. ....Pacific Northwest... A Marginal risk was added across portions of western OR and WA. Convection is expected to develop over portions of southwest OR by 21z Wed and then track off to the north through the afternoon and evening hours. An anomalous amounts of CAPE is forecast, potentially upwards of 2000 j/kg, and PWs are forecast over the climatological 90th percentile. This combination of CAPE and PWs supports intense rainfall rates. These cells should be quick moving and the bigger hazard is probably severe weather (as highlighted by the SPC Slight risk), however the intense rainfall rates producing over 1" in an hour may be enough for localized flash flood concerns. Areas of terrain, urban areas, and low lying areas will be most prone to localized flash flooding driven by these short duration intense rainfall rates. Chenard Day 3 Valid 12Z Thu Mar 27 2025 - 12Z Fri Mar 28 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTH-CENTRAL AND SOUTH TEXAS AND MUCH OF THE TEXAS GULF COAST... ....South Texas... The event described in the day 2 discussion over south TX will continue into Thursday. There is a bit more uncertainty on the details by this time as convective evolution will likely be impacted by mesoscale influences of the day 2 convection. On the synoptic scale, ingredients are likely even more favorable for heavy rainfall on day 3 then they were on day 2. Large scale forcing should reload and be even a bit stronger as the mid level trough axis and associated shortwave energy moves across the area. The CSU machine learning ERO is showing higher end MDT risk probabilities (albeit probably a bit too far north), likely indicating the favorable ingredients that will be in place for excessive rainfall. However, the extent of the excessive rainfall risk will likely be at least partially dependent on mesoscale factors...such as the outflow location form the day 2 convection and its impact on instability. The expectation is that the outflow boundary will be near the coast by this time, and so that is where the greatest focus for training convection is expected on Thursday, and thus where the MDT risk is in place. There is some chance this is offshore by 12z Thursday, but the current consensus suggests it may struggle to clear the coast, leaving coastal areas susceptible to additional heavy rainfall and flash flooding. Even if instability is eroded by Thursday, the increase in forcing and southerly flow should be enough to destabilize portions of the area again with time, and so do think at least some flash flood risk exists even in a scenario where convection initially propagates offshore. Overall confidence is high in a flash flood event, but confidence in the details by Thursday remains low. In a worst case scenario outflow stalls near the coast allowing for repeat convection and a significant flash flood event could evolve. In a best case scenario convection propagates offshore, but redevelopment later in the day still results in some flash flood threat, albeit probably less significant. When combining day 2 and 3 it seems probable that some areas receive 5-10" of rain over south TX, and localized amounts of 10-15" are certainly possible. Confidence is increasing in the coverage of flash flooding and the possibility of at least localized significant impacts, warranting the Moderate risks on both days 2 and 3. ....MS and OH Valley... A Marginal risk was added across portions of northern MO, southern IA, IL and central/southern IN. A stationary front draped across this region will gradually lift north as a warm front through the period. Southerly flow will result in an uptick in instability and moisture, and at least scattered convection is expected Thursday into Thursday night. There is at least some chance for organized convective development near the front, which could briefly train from west to east. Global model QPF is locally as high as 1-3" along this corridor, and given the expected convective nature of activity, this would suggest isolated amounts over 3" are probable. A localized, and generally minor, flash flood risk could result. ....Pacific Northwest... Another modest Atmospheric River (narrow ribbon of 500+ kg/m/s IVTs) will result in pockets of 3-5+ inch 24 hour rainfall totals along the West Coast from Northern CA northward through OR-WA. Among the 00Z 3/25 models, the Regional GEM is by far the wettest, with isolated totals of 6-8+ inches. Despite the lack of instability, the wet antecedent soils along with the potential of 0.3-0.5 in/hr rainfall rates could lead to localized short-term runoff issues, particularly along small streams and over burn scars. Chenard/Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-yuoHLbc56qURo5AhLBqRGSFda8VJSSVTahtEZZ8WXD3= BsAgyJ42oNZgbfrTdawuPrJkqUcXNgMtDQscyyVJXsTmipg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-yuoHLbc56qURo5AhLBqRGSFda8VJSSVTahtEZZ8WXD3= BsAgyJ42oNZgbfrTdawuPrJkqUcXNgMtDQscyyVJTP67O2s$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-yuoHLbc56qURo5AhLBqRGSFda8VJSSVTahtEZZ8WXD3= BsAgyJ42oNZgbfrTdawuPrJkqUcXNgMtDQscyyVJBlwrrXk$=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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