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-Tue), 21/846 AM.
***UPDATE***
Latest satellite and surface observations indicate widespread stratus and fog across the coasts/valleys with high clouds drifting overhead. Current sounding data indicates marine inversion in the 1000-1500 foot range for depth.
Forecast-wise for the immediate short term, no significant issues are expected. After stratus dissipates this morning, partly to mostly cloudy skies will prevail through the afternoon, thanks to the high clouds. Across Northwest San Luis Obispo county, light rain will continue through the day, but dry conditions will prevail elsewhere. Compared to Saturday, high temperatures today will be very similar although a couple degrees cooling is likely across interior sections.
Overall, current forecast has great handle on the immediate short term. So, no significant updates are expected. For the afternoon forecast suite, attention will remain on the major winter storm set to impact the area, beginning on Tuesday and continuing through the week.
***From Previous Discussion***
***MAJOR STORM COMING WITH SIGNIFICANT HYDROLOGIC IMPACTS***
Enjoy the above normal temperatures and dry weather because southern California is in for at least 5 consecutive days with at least some rain each day. In the short term, very little impactful weather. As mentioned, temperatures are above normal, and in some cases 15-20 degrees above normal. Minimal changes in the weather are expected through Monday other than a very slight cooling trend for the interior. The Antelope Valley has been particularly above normal with highs in the mid to upper 70s when normal is in the mid to upper 50s. There may be some light showers along the northwest portion of the Central Coast but any amounts would be a quarter inch or less.
On Tuesday this will all change. The Atmospheric River that has been pointed towards northern California the last few days will shift south and take aim at SoCal. Rain will move south around Pt Conception by afternoon bringing at least light rain to most areas by evening if not before. Heavier rain will develop Tuesday evening along the south facing slopes of the Santa Barbara mountains, boosted by strong south to southeast winds as high as 60 mph that will double or even triple the rain rates in those areas. Rain amounts will already be close to an inch in those areas before midnight. Elsewhere rain amounts through Tuesday evening will be under a half inch. Will likely need a High Wind Watch for some or all of the Central Coast and southern Santa Barbara County Tuesday in addition to Flood watches.
(Wed-Sat), 21/352 AM.
The heaviest and most impactful part of this AR storm will occur after midnight Tuesday night into Wednesday when IVT values reach close to 900 based on the EC ensembles as the storm taps into deep tropical moisture from the south. If that pans out that could be highest IVT value from a storm in this area in quite some time, if not ever. PW values are expected to reach or exceed 1.2" which would very close to the highest PW measured for this time of year. Hourly rain rates, especially in the upslope areas, should easily top one inch per hour during the peak of the storm on Wednesday. The heaviest rain will be in the south facing mountains, but very heavy rain, possibly close to an inch per hour, is possible at lower elevations as well and anyone in particularly vulnerable areas such as the Santa Ynez Range, Ventura Mountains, Santa Monica Mountains or any recent burn scars should start taking protective actions ASAP. There will almost certainly be numerous mudslides, severe flooding either from heavy rain or clogged storm drains, and possibly debris flows near recent burn scars. Could even see an isolated thunderstorm or two, though there hasn't been much of that so far with this storm over northern California. However, the moisture advection from the south could provide a boost in instability Wednesday.
In addition to the rain, very strong winds up to 80mph are expected over the higher peaks in the mountains, with gusts to 40-50 mph across lower elevations.
While the worst of the storm will likely be Wednesday, periods of rain, possibly heavy at times, are expected on Christmas Day. And with the trough remaining offshore, moist south to southwest flow will continue to usher in off and on showers to the area Friday and Saturday, though rain amounts by that time should be under a quarter inch per hour.
Snow will not be a major factor with this event. During the peak of the storm snow levels will be at or above 9000 feet. They will come down Thursday and Friday to around 7000 feet but that will have minimal impacts.
21/1217z.
At 10Z at KLAX, the marine layer was 1000 feet deep. The top of the inversion was 3000 feet with a temperature near 17 deg C.
High confidence in desert TAFs. Low for remainder of TAFs. Timing of flight cat changes could be off by +/- 3 hours, and off by 1 flight cat at any point. KPRB, KSBP, and KSMX may see -DZ at times through the period.
LIFR to VLIFR conds possible at all sites, and may bounce flight cats at times. Little to no clearing is possible for KPRB, KSMX, KSBP, KSMO, and KLAX.
KLAX, Low confidence in TAF. The timing of flight cat changes may be off +/- 3 hours, and off by 1 cat at any point. Little to no clearing is possible. Good confidence that any east wind component remains below 6 kt.
KBUR, Low confidence in TAF. The timing of flight cat changes may be off +/- 2 hours and 1 category.
21/818 AM.
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, NONE. PZ, NONE.
