Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 31 2022 16:00:06 FOUS30 KWBC 311600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Sat Dec 31 2022 Day 1 Valid 12Z Sat Dec 31 2022 - 12Z Sun Jan 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... Strong atmospheric river (AR) continues to impact northern California this morning with a frontal boundary draped WSW to ENE out of the Pacific. Moisture anomalies (PW and IVT) per 12Z RAOBs/obs are above the 99th percentile from near SFO eastward across the Sierra and will maintain its axis through the early afternoon hours. Weak surface wave along the front will slow the front's southeastward progression over the next several hours, but as that passes through the Sacramento Valley the front will pick up speed as sharper height falls move into the area. Observed hourly rainfall rates have eclipsed 1"/hr along the coast and over the coastal ranges as well as into the Sierra foothills, with lower rates in the Sacramento Valley but still near and just over 0.50"/hr. Strong moisture flux eastward will result in more spillover east of the Sierra than normal, and maintained a small eastward extension of the Marginal Risk into northwestern NV as the weak wave translates ESE-ward early this afternoon. Thereafter (after 00Z), precipitation will be mostly out of NorCal as the AR shifts southward. For central/southern areas of CA, AR will be more progressive this afternoon and evening especially after 00Z (central areas) and 04-05Z (southern areas). 110kt jet will still provide broad lift over the region with plenty of moisture (PWs at least about the 90th percentile) but the speed of the front will limit the QPF. Nevertheless, strong moisture flux and dynamics along with some instability (MUCAPE ~200 J/kg and some negative LIs) will support some rainfall rates > 0.50-0.75"/hr, especially over the Transverse Ranges as the orthogonal flow aides in lift on 30-50kt 850mb wind. By the time the AR reaches SoCal, it will be moving fast enough that only a few hours of heavy rainfall are expected in any one area. Thus, the heavy intensity of the rain will be negated by the fast movement of the AR itself. Nonetheless, urban areas of the L.A. basin, burn scars, and largely saturated soils will all work to lower FFG values such that the inherited Slight Risk was maintained for SoCal, but confined to the terrain east of San Diego. Fracasso Day 2 Valid 12Z Sun Jan 01 2023 - 12Z Mon Jan 02 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN ARIZONA... No changes to the inherited Marginal Risk area were made with this morning's forecast update. The digging trough from New Years night will press eastward into Arizona on New Year's Day Sunday. Expect rainfall to begin early New Year's morning and continue through the day across the Marginal Risk area. Generally expect rainfall totals of 1-2 inches, but with locally higher amounts in the mountains east of Phoenix. Meanwhile further north along the Mogollon Rim and points north, expect snow levels to be low enough that any rain that occurs initially will quickly change over to snow. PWATs of 0.5 to 0.75 inches, or +3 standard deviations above normal, will start out the day in the Marginal Risk area. Thus, moisture amounts will not be a problem on Sunday. Snow levels are expected to begin the day around 8,000 feet...but drop to around 5,000 feet by sunset on Sunday. This should allow many lower elevation areas to see all rain. Wegman Day 3 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. Surface cyclogenesis over eastern CO will intensify as the low moves into the upper Mississippi Valley 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 and evening, 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 suggest 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. There is significant uncertainty as to the northern extent of the thunderstorms. Thus, while the Marginal Risk area extends north to the Chicagoland area, expect that with better agreement in the guidance, this area will likely be trimmed towards the south with future updates. This is largely due to relatively small amounts of instability forecast for this area. Moisture levels will not be an issue with this event, as PWATs to 1.5 inches will be up to +4 standard deviations above normal for the middle MS Valley. Changes from the inherited Slight Risk were minimal with this update, and consisted mostly of small expansions, largely towards the west into eastern TX, and also a bit towards the north for the Marginal Risk to account for the spread in the guidance as to how far north up the MS Valley the thunderstorm activity gets to. Soils in these northern areas are increasingly dry, and have thawed out from the Christmas freeze...which increases FFGs. Thus, there is a decent gradient of flash flood potential into IL/MO/IA. The storms that would cause flash flooding would need to overcome an increasingly dry and more stable environment, unlike their counterparts further south that will have the advantage of already somewhat saturated soils, more moisture closer to the Gulf, and more instability. Really the primary factor limiting southern areas vs. the north is that the main forcing is likely to miss the lower MS Valley as it tracks northeast towards the Great Lakes by Tuesday. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8GJzFWSoN3hLV-m3hiQQllVlH9l_Rc_ajXynB6VlVo06= k-kwiyvhQLkyR99REh1VS0ku2hX9cYwN65yX_RHbTIVI8l0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8GJzFWSoN3hLV-m3hiQQllVlH9l_Rc_ajXynB6VlVo06= k-kwiyvhQLkyR99REh1VS0ku2hX9cYwN65yX_RHbFt38UXA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8GJzFWSoN3hLV-m3hiQQllVlH9l_Rc_ajXynB6VlVo06= k-kwiyvhQLkyR99REh1VS0ku2hX9cYwN65yX_RHbR7sYdlw$=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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