Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 13 2023 08:17:34 FOUS30 KWBC 130817 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 416 AM EDT Wed Sep 13 2023 Day 1 Valid 12Z Wed Sep 13 2023 - 12Z Thu Sep 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND NORTHEAST... ....Southwest and Four Corners Region into the Southern Plains... An impulse currently over NV shifts ESE across the southern Rockies Wednesday morning with organized activity in the southern High Plains/Caprock spreading through north-central TX where moisture has been building over the past couple days in flow off the western Gulf. 00Z CAM guidance focuses heavy rain a bit farther northwest of the TX Hill Country, and as such we have trimmed back the eastern portion of the Slight Risk area accordingly. The Slight Risk area continues to be aligned with the most favorable deep-layer forcing (enhanced upper divergence and low level FGEN within the right-entrance region of a 70-90kt upper jet streak), with 850-700 mb moisture flux anomalies between +2-2.5 standard deviations above normal. ....Middle-Upper Texas Coast... Continued onshore flow from the northwest Gulf of Mexico brings PWs above 2" and the approaching impulse should allow evening/overnight heavy convective development along the northern half of the Texas Coast into southwest LA. Model consensus from the 00Z CAMs (including HREF deterministic means and exceedance probabilities) continue to support a Marginal Risk extension to the Middle-Upper TX Coast, despite the area being in exceptional drought, given the higher sensitivity of 2-3" hourly or sub-hourly rainfall rates over urban areas.=20 ....Mid-Atlantic into Eastern New York and through New England... A mid-upper low over the Upper Great Lakes early this morning will amplify somewhat (become more negatively tilted) Wed and Wed night as a 70-90 kt upper level jet streak and associated shortwave energy traverses the trough base. Strengthening deep-layer forcing ahead of the low/trough will get a boost from a southerly 80-100kt upper jet streak on the eastern flank and support more widespread, organized convection ahead of the advancing cold front. This cold front fairly progressive per the latest observational trends (satellite and mosaic radar loops) and is progged to remain as such Wed-Wed night, though the eastward progression will be somewhat slower farther north across New England. As was noted yesterday, the extent of higher PWs (over 1.75") ahead of the trough/front is generally limited to south/central New England, and as such the northern extent of the Slight Risk is across southern VT-NH-ME. The Slight Risk also encompasses the highest HREF exceedance probabilities, while also aligning with the areas of heaviest rain Mon-Mon night and thus lower FFGs/highest soil moisture percentiles currently. Hurley 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 above normal for mid September. Given some overlap with the wet weather Mon-Mon night and and again on Day 1 (Wednesday), 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. Hurley Day 3 Valid 12Z Thu Sep 14 2023 - 12Z Fri Sep 15 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER CENTRAL AND SOUTHERN PORTIONS OF THE ROCKIES AND 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). The latest CSU-based first guess fields also support no higher than a Slight Risk for now; 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. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_f8v3lGJ_wsmDCKKx8QjzDWtN2HkarJYMlejs9MUaJfu= h_Auw6T8X6s1gKVtwqEXTdfQ4I9V3aRndTBO5pnYUlO_NPk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_f8v3lGJ_wsmDCKKx8QjzDWtN2HkarJYMlejs9MUaJfu= h_Auw6T8X6s1gKVtwqEXTdfQ4I9V3aRndTBO5pnY8ddnPWA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_f8v3lGJ_wsmDCKKx8QjzDWtN2HkarJYMlejs9MUaJfu= h_Auw6T8X6s1gKVtwqEXTdfQ4I9V3aRndTBO5pnYHLPluaA$=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) .