Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 13 2023 16:02:27 FOUS30 KWBC 131602 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1201 PM EDT Wed Sep 13 2023 Day 1 Valid 16Z Wed Sep 13 2023 - 12Z Thu Sep 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND NEW ENGLAND... ....Southwest and Four Corners Region into the Southern Plains... Mid-level shortwave/vort max over the southern Rockies beneath the RRQ of a 70-90kt upper jet streak has been responsible for organized activity in a punctuated swath from outside the Phoenix metro eastward through southern NM and entering West Texas. Moisture has been building over the past couple days in flow off the western Gulf, per the 24-hr PW change (>0 from roughly 101W eastward). Slight Risk area focuses on areas east of the expected shortwave track with a southeastward slant into the most favorable deep-layer forcing (enhanced upper divergence and low-level FGEN), with 850-700 mb moisture flux anomalies between +2-2.5 standard deviations above normal. Broader Marginal Risk outline in the Four Corners region encompasses ongoing activity across southeastern AZ and expected afternoon development across southwestern CO. ....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 12Z CAMs continue to support a Marginal Risk 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 (e.g., Houston, where 12Z HREF probs of >3"/hr are 10-20%). ....New England... A mid-upper low over the Upper Great Lakes early this morning will amplify somewhat (become more negatively tilted) into tonight as a 70-90 kt upper level jet streak and associated shortwave energy traverse the trough base. 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 the ongoing convection ahead of the advancing cold front, though the eastward progression will be somewhat slower farther north across New England. 1-hr FFG values are around 1.50"/hr (+/- 0.50") over much of New England except southeast Mass/Cape Cod, which is left out of the risk areas. 12Z RAOBs show PWs in the 70th-90th percentile over the region, but the best forcing will lie generally near and south of the E-W boundary separating the upper 60s to low 70s Tds from low to mid-60s along and north of the MA/NH border. Over Southern New England, rain rates have been heavier (1-2"/hr) and FFG values, lowered from the morning rain, will be easier to exceed should rainfall redevelop this afternoon along the front. Fracasso/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 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... 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!-1o6SmGtzESyDHRN3gGFTel_pZKudwIxu6xK1RVmcdUE= Ai4VoFFpU-8sqJJroTN5J5oz4XMf69GZelfHxgvshrVtgAE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-1o6SmGtzESyDHRN3gGFTel_pZKudwIxu6xK1RVmcdUE= Ai4VoFFpU-8sqJJroTN5J5oz4XMf69GZelfHxgvs5mKDSks$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-1o6SmGtzESyDHRN3gGFTel_pZKudwIxu6xK1RVmcdUE= Ai4VoFFpU-8sqJJroTN5J5oz4XMf69GZelfHxgvsvwItcyY$=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) .