Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 08 2022 08:35:48 FOUS30 KWBC 080835 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 334 AM EST Tue Nov 08 2022 Day 1 Valid 12Z Tue Nov 08 2022 - 12Z Wed Nov 09 2022 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS THE TRANSVERSE RANGES OF SOUTHERN CALIFORNIA... The meteorology and primary statistics with the rainfall across the Southwest remain largely unchanged, so the previous forecast discussion is below. This discussion will focus on changes from the below. In collaboration with the LOX/Los Angeles, CA and MTR/Monterey, CA forecast offices, the previous Slight Risk area was expanded with this morning's forecast to include downtown San Diego and Los Angeles, and all surrounding coastal areas. Further, the Slight Risk was also expanded up the coast to include all of the coastal mountains up to Monterey Bay. Per the latest HiRes guidance and current forecast trends, a period of heavier rain associated with the leading edge of a vigorous upper level low will move down the coast, but directing internal convection straight into the coastal mountains. Thus, much of the expected rainfall today will likely fall in the few hours the heavier rain band/front is over any particular part of the coast. Expect a broad 2-3" of rain over much of the coastal mountains from Malibu northward. While rainfall won't be as heavy in the lower elevations and urbanized areas of So-Cal, the lower FFGs and still steady rainfall should result in better-than-isolated flash flooding. The Moderate Risk area remains unchanged, as the bulk of the rainfall will be directed at the highest peaks of the Transverse Ranges northeast of L.A. The Marginal Risk area of the Sierras remains unchanged as well. As mentioned below the focus is on local burn scars, though with a large fraction of that precipitation expected to fall as snow in the higher elevations, the best flood threat will be around the lower elevation burn scars. There will be minimal additional flow from the higher elevations. In coordination with the VEF/Las Vegas, NV and SLC/Salt Lake City, UT offices, decided to keep the Marginal risk inherited unchanged. This area will have decidedly less instability than further east, so much of the rainfall, while significant, will fall at lower rates due to the unidirectional stratiform nature of the precipitation. The highest rainfall totals will be on southwest facing slopes, especially on any ridges oriented northwest to southeast. Due to the lack of instability, think the upslope component will be dampened a bit. Finally, lower snow levels will result in any QPF falling in the mountains north and east of the Marginal Risk area falling as snow, which negates the flooding threat. In Florida, the inherited Marginal Risk area was also left unchanged. Rain from the westernmost rainbands of Nicole will just start impacting the Atlantic coast of south Florida overnight tonight. As with all tropical cyclones, there will likely be embedded stronger cells, which when moving into the highly urbanized I-95 corridor could result in isolated flash flooding prior to 12Z Wednesday. Wegman ....Previous Discussion... ....Southern California... On Tuesday, the vertically-stacked low starting the day centered off the coast of northwest CA will begin driving southeastward into central CA by early Wednesday morning. A strong upper jet with shortwaves rounding the base of the low will drive into So-Cal on Tuesday. Overall this will be a very dynamic system, with plenty of large scale forcing for ascent. A moisture plume with PWs around 1" will move into So-Cal, with values around or above the 90th percentile for early November. The combination of anomalous moisture and plenty of upper level support will result in heavy rainfall rates in the mountains especially, but also into the L.A. Basin down to San Diego. Upslope low level flow will be maximized in the Transverse Ranges, which should result in enhanced rainfall totals of 3-6", which will be falling on top of an additional 1-2" prior to 12Z Tuesday. Amounts closer to 1-1.5" are expected in the lower elevations of So-Cal. Given the strong dynamics and height falls over the area, not surprising that some weak instability is also showing up in the guidance with MUCAPE values up to 500 J/kg depicted in the 12Z model guidance. Not much instability, but enough to support some embedded convective elements within the rain shield Tuesday and when combined with peak 850 mb winds of 40-50 kt directly orthogonal to the terrain, locally high end rainfall totals appear likely. The 12z HREF members support 50% probabilities of 0.5" in an hour rainfall in these heavier convective showers/storms starting around 15Z but ramping up to over 90% with a frontal band of heavy rain containing near 30% probabilities for localized rainfall rates around 1" in an hour mainly in the 18-00Z time frame...centered from the western San Gabriel into the San Bernardino Mountains..as plausible as well...mainly in the favored terrain, but even a lower risk along coastal areas. After coordination with LOX/SGX, a targeted Moderate Risk of Excessive Rainfall was introduced for the Transverse Ranges from Los Angeles into southwestern San Bernardino counties. The Moderate includes the sensitive El Dorado and Apple burn areas which had significant debris flows in September of this year. The Slight Risk was expanded westward a bit across Orange County to include the Bond burn scar in the Santa Ana Mountains. Isolated flash flooding, rapid runoff, and urban flooding are also possible for areas along the coast as well. ....Foothills of the Sierra Nevada... A Marginal Risk was introduced for this update into portions of central California where the potential will exist for 0.5+ in/hr rainfall rates below snow levels which are forecast to be as high as 6000-7000 ft at times. The main time frame of concern will be from 15Z to 00Z, in advance of the advancing cold front. Areas at most risk will be burn scars from the past couple of years which dot the region highlighted with the Marginal Risk. ....Portions of the High Desert and Great Basin... Moisture hitting the West Coast will have a much easier than normal time getting past the mountains into interior sections of southern NV, northwest AZ, and southwest UT on Tuesday given the strong dynamics and inland advancement of the IVT plume. Looking like rainfall amounts of 1-2" will fall across this area, with localized amounts around 3" in the favored south southwest facing slopes. Not expecting much in the way of instability this far inland through 12z Wed, which should limit peak rainfall rates below 1 in/hr, but the strong IVT and orographic effects should still push hourly rainfall totals above 0.5" at times. Overall these rainfall totals and rates should not result in widespread or significant flooding impacts. However minor flooding and runoff issues are still probable with rainfall of this magnitude. ....South Florida... The center of the tropical/subtropical system should be well offshore Tuesday and Tuesday night...however the mid level low associated with this system is very broad. Embedded shortwave energy on the periphery of this broader mid level low may track across FL this period, and guidance indicates PWs potentially increasing to around 2". There remains at least some risk of convective bands moving into southeast FL during the day or overnight period, which could potentially pose a localized urban flash flood risk with tall/skinny CAPE profiles in forecast soundings and low level onshore flow of 35-45 kt / coastal convergence axis setting up. Farther north, 12Z members of the 12Z HREF showed the potential for a narrow axis of heavy rain to set up near/just north of Cape Canaveral. Potential for 3-5 inches of rain exists where onshore flow of 30-40 kt at 850 mb is forecast by the latest model consensus. However, the area potentially impacted looks fairly localized and CAPE appears as if it will focus in the lower levels due to mid-level dry air in forecast soundings. Current thinking is that a Marginal Risk need not be extended northward to include the northeastern peninsula as 14-30 day rainfall has been mostly below average...although portions of the St. Johns river valley remain in flood stage. Chenard/Otto Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6C975GVzYZ4WUabyTaP3h-V-_pZU_utgfUi6ANjtfgqS= -wWbwWBaq_q86e3eB3Q5q4dIxEhHj1uCX99woUqC9azMA_0$=20=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6C975GVzYZ4WUabyTaP3h-V-_pZU_utgfUi6ANjtfgqS= -wWbwWBaq_q86e3eB3Q5q4dIxEhHj1uCX99woUqCy2KnchU$=20=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6C975GVzYZ4WUabyTaP3h-V-_pZU_utgfUi6ANjtfgqS= -wWbwWBaq_q86e3eB3Q5q4dIxEhHj1uCX99woUqC1uONyxc$=20=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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