Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 19 2023 00:48:07 FOUS30 KWBC 190048 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 747 PM EST Mon Dec 18 2023 Day 1 Valid 01Z Tue Dec 19 2023 - 12Z Tue Dec 19 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF SOUTHWEST OREGON AND NORTHERN CALIFORNIA TO THE CENTRAL CALIFORNIA COAST... ....Central California Coast, parts of Northern California, far southwest Oregon... A closed upper low currently analyzed well offshore of California will approach the western U.S. today, opening up as it nears the California coast by this evening. A modest plume of moisture associated with it will be directed to and brush the coastal areas through the period, with 850 mb flow out of the south-southwest to southwest between 30-40 kts and PW values generally between 1-1.5". Enough forcing for ascent and low level convergence along advancing frontal boundary will support unsettled and numerous showers through the period. HREF neighborhood probabilities for 0.5" in 1-hr peak this morning at 30-40 percent along the coastal areas then generally fall to 10 percent or less through the rest of the period as the system weakens/opens up as it moves onshore. Expect areal average totals in the period of 1-2" along the coastal areas and northern Sierra while the lack of any strong signal for intense rain rates should keep flooding concerns mainly toward sensitive/vulnerable areas and poor drainage urban locations. Taylor Day 2 Valid 12Z Tue Dec 19 2023 - 12Z Wed Dec 20 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL MAINLY FOCUSED OVER NORTHERN CALIFORNIA... A second and stronger closed mid/upper level low will approach northern California during the period, bringing an unsettled/active period of widespread precipitation this this area. As the low approaches, fairly modest values of IVT will mainly brush the coastal areas where precip rates on the favored western facing terrain may approach 0.3-0.5" hourly totals. In the northern Sierra, snow levels will be relatively high, which may result in some excessive runoff/flooding concerns there as well due to snowmelt. Otherwise, general poor drainage and nuisance flooding is expected and overall and a Marginal Risk remains on today's update across parts of northern California as well as southward down the central California coast. Santorelli/Taylor Day 3 Valid 12Z Wed Dec 20 2023 - 12Z Thu Dec 21 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... The closed mid/upper level low is expected to slide southward just offshore the California coast during the period. Despite some uncertainty still in the details of timing and placement of this feature, there is a growing signal that its proximity to the coast and an increasing plume of higher moisture transport will nose toward the central/southern coastal areas of California. The latest IVT values show modest values (~500 kg/m/s) per CW3E, with moisture anomalies nearing the 90th percentile and some instability. The 12Z guidance QPF continues to suggest broad 2-3" 24-hr totals across parts of the central and southern California coast, with locally higher amounts near the Transverse Ranges where the orientation of the IV plume are most perpendicular and would lead to the greatest orographic ascent. Given the synoptic setup and expected ingredients, hourly totals may approach or exceed 0.5", particularly the latter half of the period into Thursday morning, resulting in some areas of flooding, particularly for any sensitive/vulnerable locations and urban areas. Per collaboration with WFO LOX and consistent with an increase in this cycle of the WPC QPF, opted to include a Slight Risk on this afternoon's ERO update for the Transverse Ranges, just north of Los Angeles. A marginal risk continues northward up the coast to San Francisco and southward toward San Diego. Santorelli/Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4uOegy-EGNEsaWCQIE4j0xMGYVFQpnvNRZioqZ0B86yB= 9pVhtvhAh5L9UZfALkC_KEWxFowM2MPFpQIZeoAnG3qeTnk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4uOegy-EGNEsaWCQIE4j0xMGYVFQpnvNRZioqZ0B86yB= 9pVhtvhAh5L9UZfALkC_KEWxFowM2MPFpQIZeoAnPhX7rVo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4uOegy-EGNEsaWCQIE4j0xMGYVFQpnvNRZioqZ0B86yB= 9pVhtvhAh5L9UZfALkC_KEWxFowM2MPFpQIZeoAnZHvIFNg$=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) .