Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jan 01 2023 15:48:51 FOUS30 KWBC 011548 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1047 AM EST Sun Jan 01 2023 Day 1 Valid 16Z Sun Jan 01 2023 - 12Z Mon Jan 02 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN ARIZONA... 16Z Update... Forecast remains on track per the latest obs and 12Z CAM guidance. Front will push through the region over the next 8 hours with some embedded heavier precip elements, and otherwise the low-end Marginal risk area remains. Fracasso A progressive atmospheric river (AR) and associated jet streak will move eastward across AZ this New Year's Day. The unidirectional flow and highly anomalous Pacific moisture with PWATs up to 1 inch, or +3 sigma will support continued heavy rain through the atmospheric river moisture plume into Arizona. This heavy rain will be offset somewhat by the fast forward/eastward motion of the AR. Thus, only a few hours of heavy rain is forecast for any one location in AZ through the day today. Following behind the main area of rain, falling snow levels and much lighter precipitation will quickly end the flooding threat. Thus, the focus for flooding rains will quickly shift east through the day and will largely end by tonight, so the Marginal Risk is really for the first 12-15 hours or so of the forecast period. Further, once the moisture plume gets to NM, it will largely be cut off from the Pacific as the flow becomes nearly due southerly, resulting in rapid weakening of any associated convection. Rainfall rates between 0.5 and 0.75 inches/hour are possible during this time, but the main concern for isolated flash flooding will be in burn scars, slot canyons, and any urbanized or other typically sensitive areas. Very few changes in the guidance were noted, particularly associated with the falling snow levels behind the cold front/AR. There were no changes made to the inherited Marginal Risk area for this forecast update. Wegman Day 2 Valid 12Z Mon Jan 02 2023 - 12Z Tue Jan 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE MID-MISSISSIPPI VALLEY TO EASTERN TEXAS... On Monday, a deep upper level trough over the 4 Corners region will eject northeastward into the Plains, reaching the eastern Dakotas/MN by Tuesday morning. The right exit region of a 120 kt south-southwesterly jet streak will move out ahead of this trough through the day. Ahead of all of this energetic forcing, a plume of Gulf moisture will be drawn northward up the MS River Valley through the day. This plume will contain PWATs exceeding 1.5 inches in some areas, which is as much as +5 sigma above normal. Surface cyclogenesis over eastern CO will intensify as the low moves into IA by Tuesday morning. The trailing cold front will act as the forcing mechanism for numerous thunderstorms of varying types across the middle MS River Valley Monday afternoon through the overnight, as they develop in the aforementioned plume of Gulf moisture. More on the severe threat can be viewed on the Storm Prediction Center's summary of the active weather. As far as excessive rainfall, much of the guidance suggests a series of squall lines and cell mergers are likely across the Slight Risk area. This will increase the potential for training thunderstorms. Thus, the primary flash flooding threat will be from short bursts of heavy rain from thunderstorms, rather than a long duration of widespread moderate to occasionally heavy rain. The Slight Risk outline basically covered the intersection of GEFS probs for higher QPF and at least some instability, and is also near the CSU First Guess outline.=20 Few changes were made with this update, namely trimming the edges of the Slight and Marginal Risk areas. Much of the guidance consensus is a shade slower than previous runs, so the eastern end of the Marginal Risk was trimmed in MS/LA in coordination with the LIX/New Orleans, LA forecast office. The Slight Risk area was expanded just a bit northeastward into far southern IL. Here, instability of up to 500 J/kg should allow preexisting convection to persist as it propagates across the region. No changes were made to the northern extent of the Marginal Risk in central IL/IN Trimmed a row of counties on the western end of the Marginal Risk area in TX with good agreement on more eastward initiation of convection. Lack of forcing is the primary limiting factor for convection along the immediate Gulf coast in far eastern TX, but with ample moisture in that area and lower FFGs in the Houston metro, left the Slight largely unchanged...but think the threat here is lower than areas further inland. Wegman/Fracasso Day 3 Valid 12Z Tue Jan 03 2023 - 12Z Wed Jan 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE FLORIDA PANHANDLE AND THE SOUTHERN APPALACHIANS... On Tuesday, the eastward progression of the upper level longwave trough will slow as compared with Monday. Thus, the surface low that starts the day in IA only makes it across MI to the Windsor Peninsula between MI and NY by Wednesday morning. Meanwhile a 120 kt upper level jet streak will race eastward from IA into the MS Valley by Wednesday morning. This jet and associated shortwave will reintensify convection across the Southeast overnight Tuesday night after potentially multiple rounds of thunderstorms in this same area during the day Tuesday with the initial cold front. The two Slight Risk areas are highlighting the extra moisture along the Gulf Coast in the FL Panhandle, which will locally enhance precipitation amounts, and for the northern area, the additional help of uplift along the southern Appalachians. Both of these areas will therefore see as locally higher rainfall totals than the surrounding areas in the Marginal Risk. PWATs will remain around 1.5 inches through the Southeast, which is +2.5 sigma above normal. The primary changes with this update are expanding the Marginal Risk westward across LA/MS owing to the initially slower movement from Monday night, and the introduction of a Slight Risk area from northern AL into the western tips of the Carolinas, where despite dry antecedent conditions, expect convection will locally overcome the hourly FFGs due to the highly anomalous moisture content of the atmosphere in this area along with upslope enhancement. Expect with multiple pieces of upper level energy impacting the Southeast on Tuesday that there will be larger adjustments to the Marginal and Slight Risk areas with future updates as the guidance comes into agreement on the dominant forcing for convection in this area. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8DDEHk4Lx532VBNiqPvNeo6HbJBxVv-p5BeBwlEKXAWT= 6W0gUHTlBIBBDV1ZaWT4r-a36U3ckA7u--c2ocGEOq5FUPA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8DDEHk4Lx532VBNiqPvNeo6HbJBxVv-p5BeBwlEKXAWT= 6W0gUHTlBIBBDV1ZaWT4r-a36U3ckA7u--c2ocGE7pL-ycQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8DDEHk4Lx532VBNiqPvNeo6HbJBxVv-p5BeBwlEKXAWT= 6W0gUHTlBIBBDV1ZaWT4r-a36U3ckA7u--c2ocGEIkDmTjI$=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) .