22/159 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), 22/334 AM.
***MAJOR STORM COMING WITH SIGNIFICANT HYDROLOGIC IMPACTS***
The latest forecast has no significant deviations from earlier forecasts as most of the ensemble models continue to advertise a very strong system tapping into subtropical moisture to create copious amounts of rain across much of the area along with very strong winds at times.
In the meantime, another relatively quiet weather day Monday with temperatures 5-15 degrees above normal across valleys and interior areas and near to slightly below normal at the coast.
On Tuesday most areas will start out dry through at least 10am, but winds will be quickly increasing as the trough offshore deepens and causes surface pressures to fall rapidly across northern California. Models are indicating a 15-18mb gradient between LAX and SFO by Tuesday evening. The coastal waters off the Central Coast will notice this first with winds gusting up to 50kt across a wide area. Winds will move inland through the day across SLO and Santa Barbara Counties as well as all the mountain areas through Ventura and LA Counties with widespread winds between 40 and 60 mph in those areas and local gusts up to 80 mph in some of the more favored mountain areas. To that end, a high wind watch has been posted across these areas.
Since this system is approaching from the southwest light showers will begin to develop in many areas around the same time, roughly mid afternoon to early evening. Rain rates at least through around midnight Wednesday will under a tenth of an inch per hour.
However, much heavier rain is expected Wednesday as a 60-70kt low level jet at a right angle to the Transverse range generates a huge upslope rain rate enhancement. The storm is just starting to get into the range of the higher res models but between midnight and 4 am Wednesday there is a 70-80% chance of 1 inch per hour rain rates in the mountains between Santa Barbara and LA County and even around a 20% chance of rates as high as 1.5" per hour. Outside of the mountains there is some uncertainty with regard to intensities, particularly Ventura and LA Counties where rain rates at least through Wednesday morning are mostly under a third of an inch per hour. However, given the high IVT values and a long duration of moist south flow it seems likely that even lower elevation areas will see extremely high rain rates at times on Wednesday. With this in mind, residents near burn areas as well as areas that are prone to mudslides need to take protective actions now. While this system has not had a history of widespread convective activity, and models are not indicating much locally, an isolated thunderstorm is possible as well.
Winds are expected to decrease from west to east across the area, starting along the Central Coast in the morning and by late afternoon and evening across LA County.
(Thu-Sun), 22/353 AM.
After a little respite in the rain rates and winds after the first impulse moves through on Wednesday, a second burst of rain and wind is expected on Christmas day. Initially it was thought that the Christmas day rain would be fairly benign but more recent models are indicating a similarly strong low level jet of 60-70kt returning that morning. IVT and PW's do drop off Thursday and the lower res models are showing a marked decrease in rain rates (mostly under a half inch per hour), but given the uptick in the southerly flow there is a possibility for at least short burst of much heavier rain. A second burst of wind is expected as well so the previously mentioned High Wind Watch will remain in effect through Thursday in the same areas.
The trough remains west of the state through at least Friday afternoon, then moves onshore Saturday, maintaining a chance of showers through the period, though generally much lighter with increasing periods of no rain. Dry weather finally expected by Sunday, ending what could be 5 consecutive days of rain across southern California.
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.
22/1052z.
At 09Z at KLAX, there was a surface based inversion. The top of the inversion was near 1900 feet with a temperature of 19 C.
High confidence in VFR TAFs for desert airfields (KPMD & KWJF).
Low confidence for remaining TAFs. Timing of CIG/VSBY restrictions could be off by +/- 3 hours, and off by 1-2 categories at any point. KPRB, KSBP, and KSMX may see -DZ at times through the period with other coastal TAFs joining in this potential after 06Z.
Rapidly deteriorating conds likely Tuesday afternoon to evening as significant winter storm enters the region.
KLAX, Low confidence in TAF. Good confidence that any east wind component remains below 7 kts through 08Z. There is a 30-50 percent chance of east component wind of 6-8 kt much of the day Tuesday. Rapidly deteriorating conditions are likely late Tuesday afternoon or evening with south cross winds potentially gusting 15-25 kt, MVFR/IFR cigs/vbys with -RA/dz at times.
KBUR, Moderate confidence in TAF. Generally expecting VFR conditions through at 00Z.
22/255 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 tonight. Patchy dense fog will be possible through Tuesday morning.
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. SCA level southerly winds will develop by Tuesday afternoon with a likely (80-90 percent) chance of widespread Gale Force Winds through at least Tuesday night with a 50-70 percent chance of STORM FORCE winds 45-55 kt for the waters north of Point Sal. 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.
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, High Wind Watch in effect from Tuesday afternoon through Thursday afternoon for zones 38-340>353-376>383. (See LAXNPWLOX). 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, Small Craft Advisory in effect from 4 AM to 1 PM PST Tuesday for zones 645-670-673. (See LAXMWWLOX). Small Craft Advisory in effect from 8 AM to 1 PM PST Tuesday for zones 650-655-676. (See LAXMWWLOX). Gale Watch in effect from Tuesday afternoon through late Tuesday night for zones 650-655-676. (See LAXMWWLOX).
