Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 15 2023 20:15:51 FOUS30 KWBC 152015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 415 PM EDT Wed Mar 15 2023 Day 1 Valid 16Z Wed Mar 15 2023 - 12Z Thu Mar 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS ARIZONA... ....16Z Update... A few changes were made with this morning's update. The atmospheric river (A.R.) event is finally winding down in southern CA, so the Marginal Risk area there was removed with this update. Certainly some lingering showers are possible into the afternoon, especially if any instability develops from the newly saturated soils in that area, but none of them are expected to be strong enough or long enough in duration to result in any flash flooding. For the first time since March 6, no part of California is under any flash flood risk! Unfortunately this is unlikely to last more than 2-3 days as another A.R. is slated to impact CA early next week. The break will certainly be welcome! Further east across AZ, the inherited Slight Risk is largely the same, save a few tweaks to adjust to the latest trends in the guidance. The biggest change was to extend the Slight Risk area to cover recent flash flooding occurring near the UT/NV/AZ junction point. Otherwise, rain is ongoing across much of the western half of the Slight risk area largely in northwestern AZ. As the main plume of moisture pushes east over the next few hours, afternoon showers and thunderstorms are likely to develop in instability up to 500 J/kg over the Slight Risk area, which individually may produce heavy rains capable of causing flash flooding, particularly in any burn scars, dry washes, and slot canyons. Wegman ....Previous Discussion... ....Southern California and the Interior Southwest... A shortwave trough and its associated surface low will continue to move across the Intermountain West and Rockies, with a trailing cold front extending back across Southern California this morning. As drier air settles in across Northern to Central California, moderate to potentially heavy rains may continue across portions of Southern California as the front, accompanied by a plume of deep moisture, settles south through the region this morning. PWs of 1-1.25in on the leading edge of 30kt low level inflow, along with favorable mid-to-upper level forcing, and orographics may contribute to additional heavy rain this morning along the coast, before decreasing in coverage and intensity by the afternoon as the axis deeper moisture advection settles farther south. Farther to the east, southwesterly flow ahead of the advancing front will focus deeper moisture across Arizona, southern Nevada, and southern Utah later this morning. PWs of 0.5in or more (3-4 standard deviations above normal) will spread across the region.=20 This moisture, combined with favorable mid-to-upper level forcing, is expected to support widespread precipitation, with locally heavy rain rates possible. Some of the heaviest amounts are expected to focus in the upslope regions along and south of the Mogollon Rim and across portions of northwestern Arizona, where a Slight Risk was introduced with this update. In addition to the heavy rain, melting snow may also contribute to runoff concerns.=20=20 Pereira 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 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!93OkNdf_uCifMUrBxQSQe4d8N3JKpO4_pXbHmJQOnFgL= 2kGegAsCL3DbWrfCiBhBtyVifTWv2rpv0j8JF1Y6sfNKI9M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!93OkNdf_uCifMUrBxQSQe4d8N3JKpO4_pXbHmJQOnFgL= 2kGegAsCL3DbWrfCiBhBtyVifTWv2rpv0j8JF1Y638Qkkw0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!93OkNdf_uCifMUrBxQSQe4d8N3JKpO4_pXbHmJQOnFgL= 2kGegAsCL3DbWrfCiBhBtyVifTWv2rpv0j8JF1Y6o4u40c8$=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) .