Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jan 13 2023 15:44:03 FOUS30 KWBC 131543 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1043 AM EST Fri Jan 13 2023 Day 1 Valid 12Z Fri Jan 13 2023 - 12Z Sat Jan 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN CALIFORNIA COAST AND ADJACENT MOUNTAINS... The upper trough just off the California coast will make landfall later today while a cold front associated with as a potent Pacific cyclone also makes its way ashore. NAEFS by 18Z today showed a 400 kg/m/s IVT oriented SSW-NNE from just off the central Coast of California on north into the Sacramento Valley. PWs are set to range between 0.75-1.0" between 18Z Fri and 06Z Saturday, which are close to the 90th climatological percentile. The heaviest precipitation rates (>0.5"/hr) are expected along the North Coast of California through late morning, then reaching the Shasta just before midday and finally the northern Sierra Nevada this afternoon. These same rainfall rates will extend as far south as Monterey Bay with the 06Z HREF showing 0.5"/hr probabilities as high as 50-70%. Northern and central California (frankly, the vast majority of California) remains exceptionally saturated with most of the Coastal Range, the Great Valley, and along the Sierra Nevada and Shasta sporting 0-40cm soil moisture percentiles >90% according to NASA SPoRT-LIS. With some of these ranges picking up over 20" of precipitation spanning the last 2 weeks, soils remain sensitive enough that rapid runoff and quick rising streams and creeks can occur from even hourly rainfall amounts of ~0.5"/hr. In addition, localized flooding on roads is possible along with the ever present threat for mudslides. Maintained the Slight Risk this forecast period along the North Coast of California, but with rainfall rates gradually diminishing by this evening in wake of the cold frontal passage, it is possible the Slight Risk could be dropped for the 01Z update this evening. Northern and central California should get a brief reprieve in the ongoing barrage of rainfall overnight before the next round of heavy rain arrives early Saturday morning. Mullinax Day 2 Valid 12Z Sat Jan 14 2023 - 12Z Sun Jan 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR AREAS SPANNING FROM NORTHERN CALIFORNIA TO THE TRANSVERSE RANGE AND THE SAN BERNADINO MOUNTAINS... The next atmospheric river (AR) is already on the way, delivering yet another round of heavy rainfall to California on Saturday and Saturday night. A vigorous shortwave trough will be responsible for ushering in the next slug of anomalous Pacific moisture that contains 0.75-1.0" PWs and IVTs are likely to reach 400-500 kg/m/s given the deep moisture being ushered by west to southwest low on the order of 40 kts which the GEFS shows as being more than 3 standard anomalies above climatology. The heaviest rainfall arrives late Saturday morning and into the afternoon hours as the warm front pushes across the state. A cold front passage later Saturday evening will act to cutoff the best moisture inflow. This AR is more progressive than some of the other ARs that have occurred in recent weeks, which should help to limit the extent of the flooding potential. All that being said, just about all of California; from the coast and both the Shasta and Sierra Nevada on south to the Transverse Range feature soil moisture percentiles >95%. Sufficient moisture and lift associated with this storm still supports a Slight Risk along the North Coast of California, as well as windward, upslope areas below 6,000' along the Sierra Nevada. With an uptick in QPF farther south from WPC and NBM plus support from the finer scale NAM...introduced several targeted Slight Risk areas near the QPF maxima in/immediately near the higher terrain. It remains possible that a targeted Moderate Risk may be needed should rainfall amounts continue to increase or initial environmental conditions worsen, Bann Day 3 Valid 12Z Sun Jan 15 2023 - 12Z Mon Jan 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE ALONG AND NEAR MUCH OF THE CALIFORNIA COASTLINE... Yet another area of low pressure over the eastern North Pacific ocean heads towards the northern California coast...with one area of moisture arriving along the central coast Sunday afternoon followed by increasing areal coverage by Sunday evening over north in response to the approaching surface low. The best IVT with this wave looks to get shunted more south and east before reaching the coast of North America resulting in lower QPF than with the systems on Days 1 and 2. There is some model agreement in the models about some places north of the Bay area receiving 0.5 to 1.0 inch amounts but less agreement on amounts farther south.=20 Maintained a Marginal Risk area due to the expectation that there will be rain on or near hydrologically sensitive areas. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5ip_cHn55lZEWYT26XZSVmU7f8kgRECDjV8raYUegVRr= M_dU7HSsO_dV3vCUSMJNbV1VfbM3KW0W7VXvO8wNCP3zhMg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5ip_cHn55lZEWYT26XZSVmU7f8kgRECDjV8raYUegVRr= M_dU7HSsO_dV3vCUSMJNbV1VfbM3KW0W7VXvO8wNAjuKey0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5ip_cHn55lZEWYT26XZSVmU7f8kgRECDjV8raYUegVRr= M_dU7HSsO_dV3vCUSMJNbV1VfbM3KW0W7VXvO8wN38lNrOo$=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) .