Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 20 2023 01:01:00 FOUS30 KWBC 200100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 759 PM EST Tue Dec 19 2023 Day 1 Valid 01Z Wed Dec 20 2023 - 12Z Wed Dec 20 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CALIFORNIA... 01Z Update... An anomalously deep closed low currently offshore of the West Coast centered near 41N and 133W continues to dig down to the southeast with a gradual approach on the coastal ranges of California. The height falls associated with this energy should be centered offshore of central California by early Wednesday morning and will allow for an increase in locally heavy shower activity for the northwest California coastal ranges and extending south down to around the Bay Area as increasingly divergent flow aloft works in tandem with an uptick in Pacific moisture transport. This coupled with some increase in orographic forcing out ahead of an offshore frontal occlusion should favor an increase in coverage of shower activity going through the overnight hours. IVT values are forecast to increase to locally as much as 300 to 500 kg/m/s and this will support rainfall rates increasing to as high as a 0.25"/hour with some spotty rates perhaps near a 0.50"/hour with some occasionally heavier convective showers given presence of 100 to 250 J/kg of MUCAPE. Some 1 to 2 inch rainfall amounts will be possible over the coastal ranges north of the Bay Area by early Wednesday morning. The Marginal Risk is maintained for these areas. Farther south, the Marginal Risk area was slightly tweaked to account for the latest HRRR trends down the coast involving the Transverse Range. A slow increase in IVT values coupled with some transient low-amplitude shortwave energy arriving from offshore will favor some increase in shower activity overnight. The HRRR guidance supports some spotty 1 to 1.5 inch rainfall totals, with potential for spotty 0.50"/hour rainfall rates where some locally heavier convective showers advance inland up over the higher terrain. The larger scale flow will be becoming increasingly orthogonal to the terrain with time, and this will favor greater orographic ascent as well. Some very localized runoff problems/flooding may occur with these rains. Over the foothills of the Sierra Nevada and adjacent areas of the Central Valley, the latest radar imagery shows show broken bands of modest convection currently with some locally heavy rains occurring. Some spotty 0.50"+/hour rainfall rates will be possible going through the evening hours as the activity takes advantage of lingering diurnally driven instability and modest low-level convergence. A Marginal Risk area has been depicted to address these short-term rainfall concerns with a very low-end threat for some flooding. Orrison Day 2 Valid 12Z Wed Dec 20 2023 - 12Z Thu Dec 21 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... ....20Z Update... Updates to the forecast were made within the previous forecast below...A Moderate Risk has been introduced for the Transverse Range in south-central California with coordination and agreement from the local WFO. 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 south-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. Latest 12Z HREF depicts a pocket of elevated MUCAPE with indices reaching between 500-1000 J/kg which is very significant for any disturbance in the confines of SoCal. 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. HREF probabilities have jumped significantly for both the 1"/hr rate potential, as well as the forecast total rainfall of at least 3" (90% neighborhood probability), as well as even 5" now approaching 70% neighborhood probability within the Santa Ynez and San Rafael Mtns. This rainfall and expected rain rates will likely 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. Considering the increased signal for significant rainfall over the mountains adjacent to LA proper, and with coordination from the WFO Los Angeles, have upgraded the aforementioned area to a Moderate Risk. Taylor/Kleebauer Day 3 Valid 12Z Thu Dec 21 2023 - 12Z Fri Dec 22 2023 ....THERE IS A MODERATE RISK OF FLASH FLOODING ACROSS PORTIONS OF SOUTHERN CALIFORNIA... ....20Z Update... Little to no changes to the previous forecast as the Moderate Risk remains in effect over portions of Southern California, including the Los Angeles Metro area. Signals remains steadfast for locally heavy rainfall on Thursday afternoon and evening as a secondary moisture surge on the lead side of the closed low off the coast will allow for an enhancement of regional precip with onshore winds perpendicular to the coastal plain, extending inland into the adjacent terrain. Latest NBM mean precip is generally between 2-3" for a large chunk of SoCal from Santa Barbara and surrounds down towards San Clemente and points north. Even the QPF for the 25th and 75th percentiles align well with heavy rainfall with the LA metro area settling in a range of 1.5-2.75" with a mean over 2" as of this time. This is the upper echelon of climo for the date in question with several higher impact events showing up in local analogs. Areas to the northwest of LA will approach up to 8" within the two day span which would be a compounding issue and exacerbate antecedent conditions inherited from the previous day of heavy rain. With consensus holding firm on the risk, wanted to maintain the previous forecast with little deviation. Kleebauer ....Previous Discussion... 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!4x-hWWh8F7wJqIyXTloa5G7kyxXjIJpK77Wkb5NWpXec= -FDUr0Bm1eqsgVC5EK52ZdoPlXigu_v_jpHRLykC2e-6ZQs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4x-hWWh8F7wJqIyXTloa5G7kyxXjIJpK77Wkb5NWpXec= -FDUr0Bm1eqsgVC5EK52ZdoPlXigu_v_jpHRLykClObvDLY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4x-hWWh8F7wJqIyXTloa5G7kyxXjIJpK77Wkb5NWpXec= -FDUr0Bm1eqsgVC5EK52ZdoPlXigu_v_jpHRLykCPcdclOc$=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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