Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 03 2022 20:10:38 FOUS30 KWBC 032010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 309 PM EST Sat Dec 03 2022 Day 1 Valid 16Z Sat Dec 03 2022 - 12Z Sun Dec 04 2022 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL CALIFORNIA COAST... --16Z Update-- The main change to the Day 1 ERO is an upgrade to a Slight Risk in northern San Luis and southern Monterey counties in California. While the overall lack of instability will keep hourly rainfall rates generally below 0.50"/hr most of the day, after 21Z, southeasterly 850-700mb layer winds will increase and aid in a surge in 850-700mb moisture flux over the terrain. The strengthening wind field oriented orthogonally to the terrain will also support stronger ascent via enhanced upslope flow. Recent runs of the HRRR/3kmNAM have shown the potential for elevated MUCAPE by 00Z that could be ~100-200 J/kg just off the coast, which would be more supportive of hourly rainfall rates >0.5"/hr later this afternoon and evening. Coming into Saturday morning, soil moisture percentiles in the 0-40cm layer were already up to 80% in some portions of these counties according to NASA SPoRT-LIS, and will only continue to moisten up as rain continues to fall over the area this morning. The latest 12Z HREF depicted 3-hr FFG exceedance probabilities of 15-20% between 21-00Z this afternoon and 00-03Z this evening, which aligns closely with when the heaviest rainfall rates are likely to transpire. Localized rainfall totals between 16Z today and 12Z Sunday morning could surpass 5", with the latest 12Z HREF's 24hr QPF >5" probabilities as high as 50% within the newly added Slight Risk area. In collaboration with WFOs San Francisco/Bay Area and Los Angeles, a Slight Risk was added along the higher terrain of northern San Luis and southern Monterey counties where the combination of moistening soils and sufficiently heavy rainfall rates could lead to rapid runoff along complex terrain and localized water rises in poor drainage areas. For the other two Marginal Risk areas, made minor tweaks to account for minor adjustments in QPF trends. The forecast reasoning from overnight still appears on track as of this issuance. Mullinax --- ....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 edg .