Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 03 2022 08:23:00 FOUS30 KWBC 030822 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 322 AM EST Sat Dec 03 2022 Day 1 Valid 12Z Sat Dec 03 2022 - 12Z Sun Dec 04 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL CALIFORNIA COAST, THE WEST-FACING SLOPES OF THE SIERRA NEVADA RANGE IN CENTRAL CALIFORNIA, AND SOUTHEASTERN ARIZONA... ....California... An upper level cutoff low centered off the coast of OR will drift southeastward towards the northern CA coast today. This forcing will team up with a subtropical disturbance and associated Pacific moisture to drive it into central CA today. The front moving into the Point Conception area will continue drifting northward through the morning, with some intensification in the rainfall rates as the upper levels increasingly contribute to the heavier rainfall rates. That said, instability is very low in this area, so the mountains will be the primary driver of heavy rain. This area of rain will stall between Monterey Bay and Point Conception today, which should allow for several (2-4) inches of rain to fall into the coastal mountains. The only real change from inherited was to nudge the southern end of the Marginal a bit north towards Point Conception, since there is a well defined back edge to the rain moving north and is not expected to move back south once that back edge moves through. Hourly rainfall rates have been one quarter inch or less so far, which is well below anything that would cause flooding. Further inland, the unidirectional southwest flow will run up nearly perpendicular to the Sierras of eastern California today. These much higher mountains will then wring out even more precipitation than the coastal mountains. While the highest peaks can expect snow, the high snow levels should allow much of the precipitation to fall as rain. The inherited Marginal Risk area remains unchanged. Expect 3 to locally 5 inches of rain near or over the mountains that will runoff quickly, causing isolated flash flooding in the foothills.=20=20 ....Southeastern Arizona... The inherited Marginal Risk area was trimmed a bit to align with the forecast axis of heaviest precipitation, in coordination with the EPZ/El Paso, TX, TWC/Tucson, AZ, and ABQ/Albuquerque, NM forecast offices. The heaviest rainfall, which could exceed 2 inches in 24 hours in isolated spots, extends from the Mexican border northeast through metro Tucson and then northeast to the=20 New Mexico border. The Marginal Risk was removed from New Mexico as the guidance becomes better aligned further north and west. This separate plume of moisture from the one impacting the California Coast will have some upper level support in the form of a right entrance region of an upper level jet along with highly anomalous moisture featuring PWATs of 1.25 inches, which is over 5 standard deviations above normal for this time of year. Thus, the moisture has very tropical origins. With that said, the lack of instability will be the primary factor reducing rainfall rates over this area. Meanwhile soil moisture in this area is around average for this time of year. Wegman Day 2 Valid 12Z Sat Dec 03 2022 - 12Z Sun Dec 04 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 Valid 12Z Sun Dec 04 2022 - 12Z Mon Dec 05 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR NORTHERN MISSISSIPPI & ALABAMA AND FAR SOUTHERN TENNESSEE... A strong cold front drifting southward across the Plains will draw plentiful Gulf moisture northeastward from the Gulf into the Slight Risk area on Monday. Unidirectional westerly flow through all the levels of the atmosphere will favor training cells that move along a narrow corridor across northern MS & AL. There will otherwise be little in the way of upper level support. The heavier rain will begin around midday Monday and continue through Monday night. There is considerable uncertainty as to where exactly the heaviest rains will set up, and because it's a narrow area, any small change in the the area where the front and the Gulf moisture interact will have a large impact on where the heaviest rains will set up. However, the guidance has been gradually trending southward in where the heaviest rain is expected, which will reduce the threat across northern TN in favor of a greater chance of rains capable of producing flash flooding across northern MS & AL. There's also considerable uncertainty as to how far east into GA the heavier rains will extend, which could require additional changes with future updates. For now the Slight Risk appears to be the best risk assessment for 2 to 4 inches of rain over areas that have gotten some rain, and are therefore not dry, but haven't been overly wet of late either. Once again, given the high uncertainty, placement changes for the Slight Risk are likely with upcoming forecast updates. Compared with the inherited area, the Slight Risk was expanded a bit towards the south and west with this forecast update. Generally these meetings of ample Gulf moisture with the front make for a fairly narrow area of heavy rain, with areas not far to the north of the front getting significantly less rain. Meanwhile areas south of the front see repeating rounds of convection as part of the inflow being drawn into the front. Thus, the Slight and Marginal Risk areas are slightly displaced south of the core of the heaviest forecasted rainfall to account for this. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8nq9gA80XXWGeBmD0gCSxEpD18h5UJ_DiGIyE9E9UtDt= Q2VcYe1REqBYqyIosA8PsC1RHXR4w8sG7_BAtr6UepMHK-A$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8nq9gA80XXWGeBmD0gCSxEpD18h5UJ_DiGIyE9E9UtDt= Q2VcYe1REqBYqyIosA8PsC1RHXR4w8sG7_BAtr6UrSx173E$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8nq9gA80XXWGeBmD0gCSxEpD18h5UJ_DiGIyE9E9UtDt= Q2VcYe1REqBYqyIosA8PsC1RHXR4w8sG7_BAtr6UaIpVW_g$=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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