Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jan 01 2023 19:59:25 FOUS30 KWBC 011959 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 258 PM 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 Four Corners region will eject northeastward onto the Plains, reaching the eastern Dakotas/MN by Tuesday morning. Upper jet will mostly lift to the northeast, placing the Mid/Lower-MS Valley into the Mid-South in the right entrance region of the jet, with another jet streak nosing in off the Gulf Coast. This will provide broad-scale lift to the region in the warm sector in advance of a cold front from the west. In addition, a plume of Gulf moisture will be drawn northward up the MS River Valley through the day. This plume will contain PWs exceeding 1.5 inches in some areas, which is as much as +5 sigma. The 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. Where recent heavy rain occurred in northwestern LA, there may be a higher risk of flash flooding, but the guidance shows a split in higher QPF probs to the south of northeast of this area. Can re-evaluate with the next CAM cycle. CSU First Guess fields based on a machine-learning algorithm were attuned to this sensitivity. Wegman/Fracasso 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!9UVOZauDzBmArB_8BDN1Hxgs666wIL1D3SIN7iYpjqHG= 3A28IxuiObDudWlgNH0zlN56QH6F-Cno2s6E6qQ7I83n1eg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9UVOZauDzBmArB_8BDN1Hxgs666wIL1D3SIN7iYpjqHG= 3A28IxuiObDudWlgNH0zlN56QH6F-Cno2s6E6qQ7cFMjAGk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9UVOZauDzBmArB_8BDN1Hxgs666wIL1D3SIN7iYpjqHG= 3A28IxuiObDudWlgNH0zlN56QH6F-Cno2s6E6qQ7ZUEeC0Q$=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) .