Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 16 2023 00:49:29 FOUS30 KWBC 160049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 847 PM EDT Wed Mar 15 2023 Day 1 Valid 01Z Thu Mar 16 2023 - 12Z Thu Mar 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF WESTERN ARIZONA, FAR SOUTHEAST CALIFORNIA, AND FAR SOUTHERN NEVADA... Some expansion of the Slight and Marginal Risk areas were necessary with this update, as convection within the warm sector of a slowly progressing low pressure system has significantly overperformed the HREF guidance. This has especially been the case across portions of Southern California, with a couple thunderstorms having managed to form within the Inland Empire. Farther northeast across the Transverse Ranges into the High Desert, even more convection was able to initiate with more impressive short-term localized rainfall totals of as high as 2-3" (per MRMS estimates). Convection has since come to an end across the Inland Empire, with storms across the High Desert organizing into more multi-cell clusters (with a few cells also extending into portions of extreme southern NV). The expectation is that this convection will continue for at least a few more hours (mainly across western AZ), as the downstream environment is characterized by MU CAPE of 500-1000 J/kg, precipitable water of 0.9-1.1 inches (at record territory, being well above the max moving average of VEF and YUM sounding climatology), and ample deep layer shear of 40-60 kts. As instability fades this evening, convection will shift into east-central AZ with the overall coverage of flash flooding expected to decrease as rates generally fall well below 1"/hr. Flooding concerns are greatest burn scars, dry washes, and slot canyons. Please see MPD #110 (and any subsequent MPDs) for more detailed mesoscale information. Churchill Day 2 Valid 12Z Thu Mar 16 2023 - 12Z Fri Mar 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN TEXAS INTO THE LOWER MISSISSIPPI VALLEY... ....2030Z Update... Very few changes were needed with this afternoon's update. The Slight Risk area was trimmed a row of counties north near Houston to account for much higher FFGs in this area. Otherwise, the guidance is in lockstep on the area and intensity of the greatest threat of flash flooding, largely coincident with the highest forecasted rainfall. Of course, there's always some uncertainty pertaining to exactly where within the area the heaviest rain will fall and how quickly, but the broad pattern supporting the heavy rain is largely unchanged, and as such, so too is the ERO forecast. Wegman ....Previous Discussion... ....Eastern Texas to the lower Mississippi Valley... A shortwave trough moving across Southern California and the Southwest today is forecast to lift northeast and interact with a northern stream trough amplifying over the northern Plains on Thursday. This will support the development of a strong surface low that is forecast to track from the central Plains on Thursday to the Great Lakes by Friday morning. A 40-60kt low level jet will tap a plume of Gulf moisture, drawing it ahead of the low's trailing cold front as it settles southeast across eastern Texas and Oklahoma on Thursday. Guidance shows PWs of 1.5+ inches centered across eastern Texas into western Louisiana and southern Arkansas Thursday afternoon-evening. This moisture interacting with mid level energy moving through the base of the broader scale trough is expected to help initiate convection. Showers and storms are expected to continue into the overnight as southerly to southwesterly winds continue to draw ample moisture along the boundary and mid level energy and right-entrance region upper jet forcing support large scale ascent over the area. As the mean flow become parallel to the boundary, training storms may further raise the threat for heavy accumulations. While model spread continues to limit confidence in the details, there remains a good signal for heavy rainfall and the threat for isolated to scattered flash flooding across the highlighted area. Pereira =20 Day 3 Valid 12Z Fri Mar 17 2023 - 12Z Sat Mar 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE LOWER MISSISSIPPI VALLEY TO THE SOUTHEAST... ....2030Z Update... As on Day 2/Thursday, very little has changed in the guidance regarding the progression of the line of storms associated with a strong cold front moving across the Southeast on Friday. The cold front will continue progressing southeastward through the day, though unlike on Thursday, there is nowhere near as much instability expected ahead of the line. In addition to having less instability to work with, resulting in weaker storms, those storms will be progressing into areas that will have increasing FFGs as they approach the coast. Thus, expect just isolated flash flooding in areas where the storms get the strongest and move over more sensitive areas. It's likely that the storms that are moving across central MS, AL, and northern GA early in the day will dissipate as the LLJ feeding moisture into them dissipates, then the storms will reform along the line further southeast along the FL Panhandle and far southern GA...with a relative minimum in precipitation in between. Despite some small expansions in the size of the Marginal risk area as compared with inherited, these were done to cover the variety of different solutions on where the strongest storms will develop and move. The overall threat for flash flooding in this area is low, and potential exists that the Marginal Risk area may be eliminated with future updates, as forecasted rainfall in most areas is well below the FFGs. However, as is typical with most thunderstorm scenarios, there are almost always individual storms that "overperform", so the Marginal Risk covers that possibility. Wegman ....Lower Mississippi Valley into the Southeast... Strong southwesterly flow will continue to support deep moisture out ahead of sharp cold front as it advances east from from the lower Mississippi and Tennessee valleys Thursday morning. Deep moisture and strong forcing will support heavy rainfall rates, however models indicate the front will be fairly progressive -- limiting the threat for heavy accumulations. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9QBq3mcQAkpzSzfAMqK_iBsLhq2dgQ5X0y8xwBsZSFYH= rJP4Aoo8P5jR6NJgssUUK6j3pIX7HHRVKan4kN_NjB6R6as$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9QBq3mcQAkpzSzfAMqK_iBsLhq2dgQ5X0y8xwBsZSFYH= rJP4Aoo8P5jR6NJgssUUK6j3pIX7HHRVKan4kN_N7qF3_xE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9QBq3mcQAkpzSzfAMqK_iBsLhq2dgQ5X0y8xwBsZSFYH= rJP4Aoo8P5jR6NJgssUUK6j3pIX7HHRVKan4kN_NXmNUpNU$=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. 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