Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 13 2023 00:49:58 FOUS30 KWBC 130049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Tue Sep 12 2023 Day 1 Valid 01Z Wed Sep 13 2023 - 12Z Wed Sep 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST U.S. AND MID-ATLANTIC... ....Southwest to the southern High Plains... The Slight Risk was expanded westward across portions of Arizona with this update given ongoing convection and expected late evening/overnight activity ahead of a shortwave trough seen in the latest water vapor imagery now entering into southern California. Ahead of that feature, favorable instability has developed along with a fairly impressive axis of anomalous moisture, characterized by PWs between 1.4 and 1.7 inches across southern to central Arizona. Given the favorable forcing for ascent and instability/moisture, fairly broad area of thunderstorms has developed late this afternoon. Current trends and the most recent hi-res guidance continues this activity for the next several hours across portions of southern Nevada, southern Utah through Arizona and New Mexico with isolated 1-2"/hr rain rates. Then as the main shortwave energy arrives, a more concentrated/narrow axis of heavier precipitation is likely tonight across central Arizona through central New Mexico. This is where the 18Z HREF EAS probabilities are highest for 1" with isolated pockets of near 2" totals possible. This may lead to additional isolated/scattered instances of flash flooding through early Wednesday morning. ....Carolinas, Appalachians, into the Northeast... A Slight Risk was introduced for portions of the Mid-Atlantic for late evening through early morning convection that is expected to develop on the nose of the surging moisture transport. Convection is already beginning to develop across portions of northern Virginia this evening, roughly on the gradient of the leading 1.5" PW gradient. Increasing south/southwest flow this evening will pull northeastward those anomalous PWs, which are likely to rise toward 1.75-2.0" by tonight. Meanwhile, favorable forcing for ascent will increase aided by the height falls and approaching frontal boundary. Latest guidance shows upwards of 500-1000 J/kg MUCAPE developing tonight as well, highest from eastern MD through southern/central NJ. The environmental setup will favor some organized convection capable of producing hourly totals up to 2" at times and there's also a slight signal (10-25%) in the 18Z HREF neighborhood probabilities for spotty 3"+ totals through 12Z. Given some of the wetter antecedent conditions (lower FFGs) and urbanized corridor sensitivity, a Slight Risk was introduced for isolated/scattered instances of flash flooding. Elsewhere, less intense or less organized convection is expected across portions of Upstate NY as well as southern Virginia into the Carolinas where a few pockets of more intense convection may be lingering this evening for a few more hours. Across either area, isolated/localized flash flooding will remain possible. Taylor Day 2 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. 12Z CAM guidance focuses heavy rain a bit farther south/west which is where the Slight Risk in northeast NM into central Texas was shifted. This Slight Risk area is coincident with the +1.5 sigma PW anomaly. ....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. Decent model agreement from the 12Z HREF CAMs warrants an introduction of a Marginal Risk here despite the area being in exceptional drought due to sensitive urban areas and particularly heavy rates expected. ....Mid-Atlantic into Eastern New York and through New England... An upper low over the Upper Great Lakes this afternoon opens into a trough that takes on a negative tilt over eastern Ontario/Quebec Wednesday night. 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 developing late tonight ahead of the advancing cold front. This cold front is progged to be a bit more progressive and the extent of high PW (over 1.75") is generally limited to south/central New England, so the Slight Risk was trimmed a bit from the west, limited to southern VT/central NH, but also expanded a bit east in Mass/RI which are more sensitive after heavy rains yesterday. Jackson Day 3 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 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 around 2.00" (+1.5 sigma from normal). Given some overlap with the wet weather today/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. The 12Z model QPF consensus is farther south, leading to the Slight Risk adjustment to be nearly exclusively over TX. Jackson Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_ExHp8Ky-SeggFmfU5f81bv3af7gxpqjrqZhtv7WN9ja= YO-trLFnDjylHKwz-lEZqqCFVHkckYXWXJhyEJoRljUwvZY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_ExHp8Ky-SeggFmfU5f81bv3af7gxpqjrqZhtv7WN9ja= YO-trLFnDjylHKwz-lEZqqCFVHkckYXWXJhyEJoR4i6ZtbY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_ExHp8Ky-SeggFmfU5f81bv3af7gxpqjrqZhtv7WN9ja= YO-trLFnDjylHKwz-lEZqqCFVHkckYXWXJhyEJoRJUFXIgs$=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) .