Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jun 04 2025 09:31:35 FOUS30 KWBC 040931=20 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 531 AM EDT Wed Jun 4 2025 Day 1 Valid 12Z Wed Jun 04 2025 - 12Z Thu Jun 05 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT BASIN AND SOUTHWEST, THE MIDDLE MISSISSIPPI VALLEY, AND COASTAL SOUTH CAROLINA... ....Southwest and Four Corners into Central/Southern High Plains... Another active day of showers and thunderstorms expected as the=20 upper low over northern Baja (that helped produce localized 2-3"=20 totals over the past 24 hours across portions of CA/NV/AZ border=20 region) fills and ejects northeast atop the Four Corners. The=20 accompanying ascent will move into continued anomalous PWs which=20 will exceed the 99th percentile (per NAEFS), associated with remnant moisture from former T.S. Alvin. This will fuel widespread convection capable of localized rainfall rates of 0.5"-1.0"/hr (or at times even higher, per 00z HREF 40-km neighborhood 1"/1-hr=20 exceedance probs as high as 20-30%). Storms will likely move slowly beneath the upper trough, leading to a longer duration of localized heavy rainfall. Given prevailing 1-hr Flash Flood Guidance of 0.5"-1.0" from the CA/NV/AZ border region into the Four Corners region, these rainfall rates may have locally significant impacts to sensitive terrain features (such as slot canyons and dry washes)=20 or urban instances of flash flooding could result. While the overall coverage of flash flooding should be somewhat modest (i.e. isolated to widely scattered), decided to introduce a relatively large SLGT given the continued highly anomalous moisture, the overachieving=20 nature of the convection over the past 24 hours, and the relatively high exceedance probabilities from the CAMs. The main limiting=20 factor may be instability, given the prevalence of cloud cover this morning which could significant limit diurnal heating. However, HREF mean suggests at least 500-1000 J/kg of SB CAPE, and the ensemble=20 max is as high as 1000-2000 J/kg (should significant erosion of=20 cloud cover occur, the higher-end may be realized in spots).=20 Farther east into the Central/Southern Rockies and High Plains,=20 Flash Flood Guidance is generally higher and uncertainty remains high with regard to any substantial convective organization. While HREF exceedance probs are relatively high (20-30% for 3" threshold) over the TX Panhandle, the FV3 is an overall outlier to the rest of the ensemble. A targeted SLGT upgrade may be necessary later today, should models come into better agreement on organized convection.=20 ....Southern Plains, Lower-Mid Mississippi Valley, Great Lakes...=20 The cold front over the central CONUS will continue on its trek=20 eastward today, but will start to slow and become aligned more west to east as its tail gets pulled northward by an approaching=20 shortwave over the Four Corners region. Shortwaves embedded within=20 the flow will continue to race northeast, especially from Missouri=20 to Michigan, which is where the greatest concentration of t'storms is expected. Storm motions along the front are likely to be=20 progressive, lifting off to the northeast on 0-6km mean winds of=20 20-30 kts, but weaker Corfidi vectors aligned both to this mean=20 wind and the front suggests a potential for training. A SLGT risk=20 was introduced for southeast MO into southern IL, where HREF=20 exceedance probs for 3" and 5" thresholds are concentrated (30-60%=20 and 10-30%, respectively). Much of these totals may occur over a=20 relatively short period of time, as 1-2"/hr rainfall rates locally=20 train/repeat over the same areas.=20 Farther south into the Southern Plains, thunderstorms from this morning should tend to wane over time, but will present a localized flash flood threat through about 15z (with the HREF indicating some potential for redevelopment after 00z, though FFGs are much higher in this region).=20 ....Southeast... An inverted surface trough crossing the northern FL Peninsula with an accompanying mid-upper low (centered near the Big Bend) will=20 shift northeast along the FL/GA/SC coasts, having already drawn=20 substantial tropical moisture northward (along with accompanying=20 showers and thunderstorms). While the NHC continues to indicate a=20 10% probability of tropical or sub-tropical development in=20 association with this feature, heavy rainfall is increasingly a=20 concern (whether or not a circulation ultimately forms, though a=20 non-tropical low is expected to). While the bulk of the heavy=20 precipitation is still expected to occur mainly offshore of (with=20 the most significant HREF signal offshore of SC, specifically),=20 there are concerns that prolonged heavy rainfall could impact=20 portions of the coastline. A SLGT risk was introduced to coastal=20 portions of SC, where rainfall totals in excessive of 5" are most=20 likely to occur (per HREF exceedance probs of 20-30%, which is also in-line with the corresponding 10-yr ARI and co- located with the=20 best EAS probs, indicating relatively good spatial agreement as=20 well). This includes the relatively sensitive Charleston metro=20 area, and any localized flooding impacts may be exacerbated by=20 high tide. Outside of this specific area, locally heavy rainfall=20 could result in more isolated instances of flash flooding across=20 much of FL and adjacent portions of southeast AL, southern and=20 central GA, and into more of SC/NC (depending with the northern=20 extent of the MRGL largely dependent on timing differences with the northward progression of convection). Local 3" exceedance probs=20 are relatively high throughout this region (20-60%), though absent of significant convective organization these totals should largely be below the relatively high FFGs that prevail (which are as high=20 as 4-5" over 6-hr).=20 Churchill Day 2 Valid 12Z Thu Jun 05 2025 - 12Z Fri Jun 06 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTH-CENTRAL PLAINS... ....Intermountain West to Central/Southern Plains... Thursday/Day 2 should see a similar pattern as Wednesday/Day 1,=20 with a broad positively tilted trough axis across the West and=20 shortwaves moving through. Pacific moisture (continuing to be over=20 the 90th percentile for PWs, and as high as the 99th percentile,=20 per NAEFS) will stream northeastward just ahead of the trough to=20 produce scattered convection across the central Great Basin toward=20 the central Rockies, where the Marginal Risk has been maintained=20 (and adjusted to the latest data).=20 Farther east, an initial shortwave will force convection in the southern High Plains tonight into Thursday morning, then a renewed low-level jet (30-40 kts at 925-850mb) will push moisture and=20 instability a bit farther north into Oklahoma/Kansas Thursday night. This will likely sustain convection (or perhaps reinitiate convection from a remnant MCV) that originates in CO/NM, leading=20 to heavy rain rates (that potentially persist and repeat over the same areas). While there continues to be spread in the guidance=20 regarding whether there are ultimately two separate areas of convection (one near the TX Panhandle and Permian Basin and another near the KS/OK border) or one more consolidated area of convection (likely somewhere inbetween), have maintained and adjusted the associated SLGT risk (which is centered near the highest ECENS 2" and 3" exceedance probs). Further refinements are expected with the addition of the remainder of the CAM suite later today (and could result in higher confidence of upper-end SLGT probabilities, should CAMs come into relatively good agreement).=20 ....Ohio/Tennessee Valleys and Lower Great Lakes...=20 Moisture pooling along a meandering front may lead to locally=20 heavy rain across the Mid-Lower MS Valley and OH/TN Valleys toward=20 the Lower Great Lakes region. A Marginal Risk was adjusted eastward across this region from prior cycles, owing to trends in the placement of the frontal zone. Still considered a Marginal Risk=20 stretching into northern New England near/ahead of the front where=20 above normal moisture and instability are likely, but have judged=20 model QPFs remain not too heavy. There is likely a non-zero=20 probability for localized flash flooding, which will continue to be monitored.=20 ....Southeast/Carolina Coast... A non-tropical low pressure system is forecast to be tracking just onshore in the Carolinas on Thursday, and models continue to indicate significant rainfall totals mainly just offshore in association with tropical moisture. While this system is most=20 likely to remain frontal/baroclinic in nature, there is still a=20 low-end chance (near 10% per the NHC) of a tropical or sub-tropical low consolidating within the persistent tropical convection offshore. Even though the heaviest rainfall totals are forecast to stay=20 offshore, wraparound moisture is expected to stream into coastal=20 areas with PWs near the 95th percentile (per NAEFS). Have maintained a MRGL risk for the new Day 2, owing to uncertainties with regard to how things ultimately develop, but current ensemble probs (available CAMs and GEFS/ECENS) suggest 2-3" areal averages=20 for the NC coast (which has the greatest chance of being locally=20 upgraded to a SLGT with subsequent updates). Farther west and south of the low, lack of instability and focused low-level moisture=20 transport looks to limit rain rates and subsequent totals (though=20 lingering flooding may still be ongoing from D1 rainfall). Churchill/Tate Day 3 Valid 12Z Fri Jun 06 2025 - 12Z Sat Jun 07 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF OKLAHOMA... ....Southern Plains...=20 Convection looks to remain active in the Southern Plains from overnight Thursday into Friday morning, fueled by a 30-40 kt low- level jet and likely ongoing MCS(s). In addition, the trailing frontal zone is unlikely to move much as the upper-level pattern remains blocky. Models indicate that another shortwave could fuel yet more organized convective coverage into Friday night, this time originating from the northern stream and digging southeastward from the central Rockies/High Plains. The LLJ should reinvigorate once again, and this is a recipe for heavy rainfall with training and repeating over many of the same areas a distinct possibility. While there is a fair amount of spread in the global guidance at this range with QPF output, the consensus cluster is pretty well situated over central OK (with some solutions suggesting farther eastward progress of high totals into portions of northwest AR and southwest MO). Maintained the inherited SLGT risk over the best ensemble signal (where 00z GEFS/ECENS 2" and 3" exceedance probs overall quite well). As confidence increases, the SLGT risk region may expand (with potential for a targeted Moderate upgrade in subsequent outlooks, depending largely on how much rainfall=20 transpires on Thursday and how well QPF on Friday overlaps).=20 ....MS/TN/OH Valleys into Great Lakes and Appalachians... Moisture looks to continue to pool along a meandering front in=20 this blocky pattern, which may continue to lead to locally heavy=20 rain across the Lower-Middle MS Valley and OH/TN Valleys toward=20 the Lower Great Lakes region. The MRGL was maintained and adjusted, and subsequent targeted SLGT(s) may be necessary with increasing overall PWs and the potential for localized totals beginning to add up from multiple days. In addition, expanded the inherited MRGL risk eastward into much of the Appalachians, as models come into better agreement that better moisture and subsequent convection will start to move into those more sensitive areas.=20=20 Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6w1O9vzfMZOtYloTjXFkInDyuNSnxbAqVHmlxBXT1z5V= kbIUjxnqAV6ViD2fCna79Fs44ZUnGIslxvFBQykTzqIVPm8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6w1O9vzfMZOtYloTjXFkInDyuNSnxbAqVHmlxBXT1z5V= kbIUjxnqAV6ViD2fCna79Fs44ZUnGIslxvFBQykT8iW7c5U$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6w1O9vzfMZOtYloTjXFkInDyuNSnxbAqVHmlxBXT1z5V= kbIUjxnqAV6ViD2fCna79Fs44ZUnGIslxvFBQykT-u4HT4U$=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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