Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 19 2023 08:21:53 FOUS30 KWBC 190820 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 319 AM EST Tue Dec 19 2023 Day 1 Valid 12Z Tue Dec 19 2023 - 12Z Wed Dec 20 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS COASTAL CALIFORNIA... A strong, anomalously deep closed upper low currently well offshore the Pacific Northwest will drop southeastward toward the central California coast by early Wednesday morning. This system will bring an unsettled and active weather pattern to much of the coastal areas of California through the rest of this week, beginning today/tonight across mainly northern/central California coast where the greatest forcing for ascent exists. A plume of higher moisture, characterized by PWs near/exceeding 1" and IVT values 400-500 kg/m/s (per CW3E), will brush the coastal areas and lead to a threat of hourly totals in excess of 0.5". The 00Z HREF neighborhood probabilities for 0.5"+ in 1-hr peak near 70 percent along the coast north of the Bay Area, which may lead to some isolated flooding, particularly for sensitive and urban areas. 24-hr totals approaching 2-2.5" are expected with isolated higher amounts near 3" possible along the coast. Across the northern Sierra, snow levels will be relatively high, so the expected 2-3" totals may lead to some flooding concerns. Taylor Day 2 Valid 12Z Tue Dec 19 2023 - 12Z Wed Dec 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL AND SOUTHERN CALIFORNIA... The strong closed mid/upper level low will slide southeastward just offshore the California coast through Wednesday and Wednesday night. The position of the upper low will put the coastal areas in the favored area of upper diffluence with a steady plume of moisture nosing into the region from the southwest. Per the CW3E page, the latest IVT forecasts reach near 500 kg/m/s, directed somewhat orthogonal to the terrain areas of the central California coast. Moisture anomalies reach near the 90th percentile. In addition to the higher moisture, some instability is expected to develop given the steepening lapse rates shown in the forecast soundings. The latest model guidance shows a broad area of 2-4" 24-hr totals across the central/southern California coastal areas, particularly for the Transverse Ranges where the orientation of the IVT plume is most perpendicular and hourly totals may reach 0.75-1" at times. This rainfall and expected rain rates may lead to scattered instances of flash flooding, some of which may be locally significant for the Transverse Range areas given some of the higher sensitivities and terrain effects. Taylor Day 3 Valid 12Z Wed Dec 20 2023 - 12Z Thu Dec 21 2023 ....THERE IS A MODERATE RISK OF FLASH FLOODING ACROSS PORTIONS OF SOUTHERN CALIFORNIA... A strong/deep closed upper low will slowly meander off the Southern California coast during the period, moving just slowly east/southeast toward the coast by early Friday morning. This setup will provide a prolonged and continuous area of broad forcing for ascent across central to southern California with the associated moisture plume directed into the coast nearly perpendicular at times. The moisture anomalies with this system are near 3 standard deviations above normal, with PW values between 1-1.25" expected. IVT values per the CW3E reach 500 kg/m/s during the peak of the event Thursday and the favorable southerly flow will work with the orographic ascent to enhance rainfall totals and rates along the Transverse Ranges where localized 3-5" are likely and hourly rainfall rates up toward 1" will be possible. Even some instability developing at times will help to enhance rain rates in the stronger embedded cores. The combined 48-hr rainfall totals ending 12Z Friday morning may exceed 5-7" in places (locally higher) and this has the potential to lead to some significant flooding concerns including flash and urban flooding with some possibility for debris/mud flows in the terrain areas or over burn scars like the South Fire burn scar in Ventura County. Given this increased signal in QPF and intense rain rate potential over the sensitive terrain and urban areas, in coordination with WFO Los Angeles, a Moderate Risk was introduced for the Day 3 Excessive Rainfall Outlook for portions of coastal Southern California.=20 Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!77sTEfO6s2K67FYhi2Y0s12gOk0lyHbN8DOhqzDCZTas= cZq0B7nkP3I0lqJrbl9c0c8p37nHDVC0R_imoEKGXu_Ooc0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!77sTEfO6s2K67FYhi2Y0s12gOk0lyHbN8DOhqzDCZTas= cZq0B7nkP3I0lqJrbl9c0c8p37nHDVC0R_imoEKGXPstCS0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!77sTEfO6s2K67FYhi2Y0s12gOk0lyHbN8DOhqzDCZTas= cZq0B7nkP3I0lqJrbl9c0c8p37nHDVC0R_imoEKGhak7xbU$=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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