Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 14 2023 00:52:09 FOUS30 KWBC 140052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Wed Sep 13 2023 Day 1 Valid 01Z Thu Sep 14 2023 - 12Z Thu Sep 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... ....Southwest and Four Corners Region into the Southern Plains to the Middle/Texas Upper Coast... Largely maintained the Marginal Risk area as earlier amended over the Southwest region as low-amplitude shortwave makes its way from the southern Rockies eastward towards Texas. The Slight risk area was adjusted to the east and expanded a bit due to concern the broad scale ascent associated with the shortwave could set up elevated convection...and the expansion covered on-going convection that was moving out of the hill country towards even deeper moisture. Debated how far eastward to take the Slight..but felt the very high flash flood guidance resulting from prolonged dryness will keep it mainly and urban and small stream flooding concern that should generally be covered by a Marginal Risk area. Farther south and west...changes to the Marginal Risk area was small and largely based on trends in radar and satellite imagery from late afternoon/early evening. Think much of the instability should allow rainfall rates to taper off quickly by 03Z or 04Z but that it was too early to remove at this point. ....New England... Much of the activity from earlier in the day has weakened and shifted east...with early evening radar showing mainly light to locally moderate rain in the far eastern/southern portion of New England with better coverage and rates farther north. The downward trend allowed for removal of the Slight Risk and trimming of the western portion of the Marginal risk area. However...with south to southwest flow persisting in the eastern part of the area...and the approach of a shortwave from the west...felt it was too early to completely remove the area from Maine southward to eastern Massachusetts and into Rhode Island.=20 Bann Day 2 Valid 12Z Thu Sep 14 2023 - 12Z Fri Sep 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....Central Rockies/High Plains through the Southern Plains to the Mid-Upper Texas Coast... Positively tilted trough over the West shifts east to the Four Corners area by Thursday night. Embedded shortwave energy ahead of this trough axis will traverse along/south of the Red River Valley. Increased low-level southerly flow will increase 850-700 moisture flux with PWs ranging from 1.5" southeast of the OK-TX Panhandles to 2-2.25" along the Middle and Upper TX Coast. These PW values are around +1.5 sigma for mid-September. Given some overlap with the recent rainfall (earlier this week and today), expect the FFG and soil moisture percentiles to lower/increase respectively, which should boost the probabilities of exceeding FFG values even if deep-layer instability is limited somewhat by the extensive cloud cover and widespread precip coverage. 12Z HREF neighborhood probabilities of exceeding 2"/hr are roughly 20-40%, split between morning and afternoon convection. Ultimately, the lead-in rainfall evolution may affect the future shape of this outlook. Farther northwest, extended the Marginal risk into south-central UT per coordination with WFO SLC as lingering convection has potential to exceed the typically lower FFG values (<1"/hr) in this region during the afternoon before drier air intrudes. This was supported by the 12Z HREF probabilities as well. Fracasso/Hurley Day 3 Valid 12Z Fri Sep 15 2023 - 12Z Sat Sep 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER CENTRAL AND SOUTHERN PORTIONS OF THE ROCKIES AND PLAINS... ....Central/Southern Rockies/Plains... The broad Marginal Risk area across parts of the central Rockies-Plains south through much of TX changed very little from yesterday's Day 4 ERO, i.e. with minor adjustments along the eastern and southern peripheries to reflect the latest guidance trends. Mid-upper level shortwave across the Four Corners region to begin the Day 3 period (12Z Fri) will slide eastward into eastern portions of the central-southern Plains by the end of the period (12Z Sat). Ribbon of higher (anomalous) TPW across the outlook area is largely unchanged from Day 2, although as the shortwave/surface push east, lower PWs will begin to advect across the central Rockies-High Plains later in the period. Models show broad scale forcing, including the upslope low-level flow into the High Plains and eastern portions of the Central-Southern Rockies. However as is reflected in the QPFs, model signals continue to be fairly diffuse with respect to exactly where the best focus of highest rainfall totals will be, so confidence remains too low to depict any embedded Slight Risk area (though again TX would be favored). A "known unknown" is the rainfall amounts/footpring in the D1-2 period (now through early Friday) that will affect the FFG values. The latest CSU-based first guess fields also support no higher than a Slight Risk for now (and an unrealistically small one at that); however, with the anticipation of better model consensus/less QPF spread with time, especially once within the high-res CAM windows, would expect an uptick in forecast confidence for perhaps a targeted Slight Risk area at some point with subsequent EROs. ....Cape Cod... In southeastern Massachusetts, Lee will be making its approach toward the Gulf of Maine by the end of the period, with rainfall expected into at least Cape Cod, if not most areas east of I-91 and along I-95. Amounts may be light to modest but for now can leave off a Marginal risk area for the Cape given their higher FFG values. Should the pace of Lee quicken enough and/or its path jog closer to the coast, an area may need to be introduced. Fracasso/Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4UfSXAEieSEd76Zm65teqeYD9v4xjR90ZG0inslCQtxx= T_hFAjGuONalXMsVZ_hMuqdGV5b9AA7aLOnJrIE_iMRRqZE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4UfSXAEieSEd76Zm65teqeYD9v4xjR90ZG0inslCQtxx= T_hFAjGuONalXMsVZ_hMuqdGV5b9AA7aLOnJrIE_iyHDZbg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4UfSXAEieSEd76Zm65teqeYD9v4xjR90ZG0inslCQtxx= T_hFAjGuONalXMsVZ_hMuqdGV5b9AA7aLOnJrIE_dFDXRxM$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .