Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 13 2023 20:01:01 FOUS30 KWBC 132000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 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 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8KKVB0Pw61ERJkEp0Y2oRNDrorzTIYYtqlxWTqfl2Bst= 8zKOMupQRS_S_EvSzj3u3WHdbMK7NqgwvnJhC0rrol6ZF0k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8KKVB0Pw61ERJkEp0Y2oRNDrorzTIYYtqlxWTqfl2Bst= 8zKOMupQRS_S_EvSzj3u3WHdbMK7NqgwvnJhC0rrRjwk5j4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8KKVB0Pw61ERJkEp0Y2oRNDrorzTIYYtqlxWTqfl2Bst= 8zKOMupQRS_S_EvSzj3u3WHdbMK7NqgwvnJhC0rrjIIhXVg$=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) .