Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 31 2022 20:17:38 FOUS30 KWBC 312017 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 316 PM EST Sat Dec 31 2022 Day 1 Valid 16Z 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... Progressive AR will continue through the Southwest on Day 2 (Sunday into very early Monday) beneath a 110kt jet streak and a sharpening mid-level trough that will be closing off into an upper low. Moisture plume off the East Pacific will quickly move through the region, supporting a relatively brief window for some convectively-enhanced rainfall rates over 0.50"/hr per some CAM guidance (max near 0.75"/hr) outside the terrain. This should be below FFG for the area, but cannot rule out an isolated FF threat with a surge of PW values 0.50-0.75" and some CAPE of a couple hundred J/kg in advance of the frontal passage. Fracasso 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. 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 Trimmed the northern edge of the risk areas to coincide with the best probability of non-marginal CAPE, despite the abundant moisture surge (PWs near 1.50" which are +4 sigma). 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 which lowers from ~1.50-2.00"/hr to 1.00-1.50"/hr from south to north. 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. Fracasso/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4glnF5hu4dfDRS94nfSFgB0v07_GlElLn93CPzKnj6Vk= RMqh2-sA56EDg5u2z5QHoicH2Uv0BMFbzbyvRMO22hWRggg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4glnF5hu4dfDRS94nfSFgB0v07_GlElLn93CPzKnj6Vk= RMqh2-sA56EDg5u2z5QHoicH2Uv0BMFbzbyvRMO2K-qtd6o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4glnF5hu4dfDRS94nfSFgB0v07_GlElLn93CPzKnj6Vk= RMqh2-sA56EDg5u2z5QHoicH2Uv0BMFbzbyvRMO2iBJ_3uo$=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) .