21/238 AM.
Temperatures will remain above normal through Sunday in most areas except the coast. A cooling trend will begin Monday as a strong storm approaches with increasing clouds. Rain will begin Tuesday in most areas with periods of heavy rain Tuesday night and Wednesday as well as very strong winds. Showers are expected to continue on Christmas day and through next weekend.
(tdy-Wed), 21/111 PM.
***MAJOR STORM COMING WITH SIGNIFICANT HYDROLOGIC IMPACTS***
For the short term period, 12Z models in very good synoptic agreement through the period. And with that being said, Southwestern California will be dealing with a major winter storm.
For this afternoon through Monday night, conditions will be rather benign. Through Monday night, there will be an increase in mid to high level clouds across the entire area along with pretty extensive marine layer stratus west of the mountains. With this pattern, there will continue to be a threat of light rain across northern San Luis Obispo county.
On Tuesday and Wednesday, things will dramatically change. Based on deterministic and ensemble IVT guidance, a moderate to strong atmospheric river will take "dead aim" on the area. Through Tuesday afternoon, rain will gradually increase in coverage and intensity across the area. For Tuesday night through Wednesday, the first, and most impactful, surge of the AR will bring heavy rainfall to all areas. At this time (through Wednesday evening), 2-5 inches of rain is expected across the coasts/valleys and 5-10 inches across the foothills and mountains. In terms of rainfall rates, hourly rates of 0.75 to 1.25 inches will be possible (based on latest high resolution data). So, needless to say, there will be significant hydrologic issues, including widespread urban flooding, mud and debris flows and very hazardous driving conditions. Given this potential, a FLOOD WATCH has been issued for all four counties from Tuesday afternoon through Wednesday evening. START TAKING PROTECTIVE ACTIONS NOW.
Given the subtropical nature of this system snow levels through Wednesday night will remain very high (above 7500-800 feet). So, at least through Wednesday evening, no significant winter weather issues are expected.
Finally, winds will also be a significant issue with this system. From Tuesday afternoon through Wednesday, moderate to strong southeasterly winds are expected. Many coastal and valley areas can expect advisory-level winds, gusting 35-55 MPH. Across the mountains and foothills, warning-level winds, gusting up to around 70 MPH, will be possible. Future shifts will likely need to issue various wind products (both advisories and warnings).
(Thu-Sun), 21/111 PM.
For the extended, a wet and unsettled pattern will continue across the area through the weekend. 12Z models agree on the details for Christmas Day, but start to deviate Friday through the weekend.
For Christmas Day, the second surge, associated with the AR, will impact the area. This second surge will bring additional moderate to locally heavy rainfall to the area. At this time, additional rainfall totals for Christmas Day are expected to range between 1 and 3 inches. So, there will continue to be flooding and debris flow issues and the FLOOD WATCH may need to be extended into Christmas Day. Snow levels will begin to drop on Christmas Day, down to around 7000 feet. So, there will likely be a White Christmas at the resort level and Winter Weather products may need to be considered. As for winds, they will shift to more southerly direction and decrease in intensity. However, there will still be a chance of warning-level winds across the mountains and advisory level winds elsewhere.
For Friday, both the GFS and ECWMF continue to indicate wet weather continuing across the area, but the GFS is more significant with rainfall totals than the ECMWF family. Snow levels will drop to the 5500-6000 foot range and winter weather conditions will likely impact more mountain communities.
For Saturday and Sunday, models diverge noticeably. The GFS is much more bullish, keeping some decent shower activity across the area into Sunday. However, the ECMWF is much drier. Forecast currently leans towards the GFS and will keep that going.
IN SUMMARY, TUESDAY THROUGH SATURDAY LOOKS TO BE AN EXTENDED PERIOD OF SIGNIFICANT RAINFALL. BY SATURDAY EVENING, RAINFALL TOTALS WILL RANGE FROM 4-8 INCHES ACROSS COASTAL AND VALLEY AREAS TO 8-12+ INCHES ACROSS THE FOOTHILLS AND MOUNTAINS. PLEASE TAKE THE NECESSARY PROTECTIVE ACTIONS AS FLOODING AND DEBRIS FLOW WILL BE A MAJOR ISSUE THIS UPCOMING WEEK.
21/1752z.
At 1706Z at KLAX, the marine layer was 500 feet deep. The top of the inversion was near 2500 feet with a temperature of 17 C.
High confidence in VFR TAFs for desert airfields (KPMD & KWJF).
Low to moderate confidence for remaining TAFs. Timing of CIG/VSBY restrictions could be off by +/- 3 hours, and off by 2 categories at any point. KPRB, KSBP, and KSMX may see -DZ at times through the period.
KLAX, Low confidence in TAF. Generally expecting LIFR to MVFR CIG/VSBY restrictions (OVC003-012) through forecast period. Timing of flight cat changes may be off +/- 3 hours and may bounce at times. 30% chance that CIGs clear or remain intermittent in nature from 20Z Sun to 05Z Mon. Good confidence that any east wind component remains below 7 kts.
KBUR, Moderate confidence in TAF. Generally expecting VFR conditions. However, BR or HZ MVFR VSBYs are possible thru period and CIGs could return after 12Z Monday (20% chance).
21/1227 PM.
High confidence in the current forecast through Monday night, then moderate confidence thereafter. Confidence lower due to timing and seas. Winds and seas will largely remain below SCA levels through Monday night.
On Tuesday, SE to south winds and seas will are expected to rapidly increase to dangerous levels as a storm system approaches the coastal waters. There is a 90+ percent chance of SCA level southerly winds developing by Tuesday afternoon with a likely (80-90 percent) chance of widespread Gale Force Winds through at least Tuesday night with a 20-40 percent (highest northern waters) chance of at least brief Storm Force Winds. Large short-period seas are likely to develop Tuesday night and should linger Wednesday into Thursday. There is also a 5-15% chance of thunderstorms Tuesday night through Thursday morning.
Boaters are urged to monitor the latest weather forecasts. If marine weather conditions deteriorate as advertised, boaters should remain in safe harbor during this significant storm. Unprotected south facing harbors may be especially vulnerable to the south swell and winds.
Ca, Flood Watch remains in effect from Tuesday afternoon through Wednesday evening for zones 38-87-88-340>358-366>383-548. (See LAXFFALOX). Flood Watch in effect from Tuesday afternoon through Wednesday evening for zone 362. (See LAXFFALOX). PZ, NONE.
