Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 19 2023 20:04:11 FOUS30 KWBC 192003 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 302 PM EST Tue Dec 19 2023 Day 1 Valid 16Z Tue Dec 19 2023 - 12Z Wed Dec 20 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CALIFORNIA... ....16Z Update... Little change necessary from the previous forecast package as shifts in the short term guidance were minor and mostly noise within the placement of the heavier QPF maximums within the time period. The biggest change was a slight shift south towards the Sacramento Metro with more of an emphasis on the hills to the northeast of the urban area where an increase in the, "at least 1 inch" storm total probability field off the 12Z HREF EAS was significant enough to warrant an inclusion in the MRGL risk area. The heaviest precip is still forecast within the northern coast between Eureka down to San Francisco as the primary low-level moisture transport becomes fixed within the confines of the coast. Secondary maximums the next 24 hrs can be found within the Northern Sierras at locations below 7000ft MSL where the forecast snow level is depicted, as well as within the confines of the higher terrain in the Shasta-Trinity National Forest where locations like Oak Mountain, below the summit have seen a significant amount of rainfall already, (over 2") where local flooding concerns could arise. The focus will shift south as the upper-low slides southward through the period and the primary IVT signature begins fixating over the terrain north of Santa Cruz, as well as the Transverse Range to the northwest of Los Angeles. QPF forecast a general 1-2" with localized max of up to 3.5" in the more persistent rainfall areas with rates topping at 0.5"/hr based on latest neighborhood probabilities. Kleebauer ....Previous Discussion... 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 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-tv173yrLJli7d-4nP-FceiUvGN8Uv5_QYrAvCY-CT2S= GN_9SaRscmoXNeKoi5B9MLAvUdjUga0NUVBGoK4KUCXEeUU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-tv173yrLJli7d-4nP-FceiUvGN8Uv5_QYrAvCY-CT2S= GN_9SaRscmoXNeKoi5B9MLAvUdjUga0NUVBGoK4KfjqfOek$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-tv173yrLJli7d-4nP-FceiUvGN8Uv5_QYrAvCY-CT2S= GN_9SaRscmoXNeKoi5B9MLAvUdjUga0NUVBGoK4KvfPmtKo$=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) .