Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
933 AM PST Sat Feb 3 2024
SYNOPSIS...03/532 AM.
A powerful atmospheric river will arrive this afternoon or evening for northern areas spreading through the southland by Sunday morning. This storm will bring very heavy rain and potentially life threatening flash flooding, and high elevation mountain snow to much of the region through Tuesday morning. This storm will be followed by showers and lowering snow levels through the week.
SHORT TERM (TDY-MON)...03/830 AM.
UPDATE
Latest satellite and surface observations indicate partly to mostly cloudy skies with a mish-mash of low and high level clouds. As for winds, nothing of significance currently observed.
For the immediate short term, it`s just the "calm" before the storm. Through the day, will anticipate increasing clouds for all areas. Models still on board with developing a warm front later today which will bring the first bit of rain to the area. Through this evening, best chances for light to moderate warm frontal rain will be across San Luis Obispo/Santa Barbara counties with lesser chances across Ventura/LA counties (but still the possibilty of measurable rainfall for all areas). Also, will see a gradual increase in southeasterly winds later this afternoon and evening, especially across the Central Coast.
Overall, current forecast has great handle on the immediate short term with respect to developing precipitation chances and wind. So, no significant updates expected this morning. For the afternoon forecast suite, attention will be laser-focused on the incoming storm. Initial looks at latest guidance indicates everything looks on track with current forecast, although some details will likely need to be tweaked.
From Previous Discussion
Overall, skies were partly cloudy across the forecast area this morning, with some patches of low clouds, and some high clouds streaming into the area from the west. Locally gusty northwest winds in the mountains have diminished. Residents of southwestern California will generally awaken to the calm before the storm this morning.
A strong and cold upper low was centered about 350 nm west southwest of Portland, Oregon at 3 AM PST this morning. This upper low will slowly southward today, reaching a position about 300 nm west of Crescent City, CA late this afternoon. Strong westerly flow on the southern periphery of the upper low will take aim at the forecast area later today, and will transport increasingly moist Pacific air into the region. All models strongly suggest warm frontal development along the Central Coast later today. Skies will become mostly cloudy this afternoon, expect possibly remain partly cloudy across southern and eastern portions. As the warm front forms and moisture and lift increase, rain will likely develop this afternoon across San Luis Obispo County and most of Santa Barbara County, extending into the mountains of Ventura County and possibly the mountains of Los Angeles County. In coastal and valley areas, there is a chance of rain as far south as Ventura County late in the day, with a slight chance in far western Los Angeles County.
The upper low will become elongated in a north to south fashion as it continues to drop slowly southward tonight, and its associated cold front will begin to approach the west coast. The mid and upper level flow will become more southwesterly tonight, and as it becomes closer to parallel with the front, the front will slow. However, increasing moisture and lift in advance of the front, along with increasing diffluence aloft will cause rain to overspread the forecast area tonight.
As the flow aloft becomes more southwesterly, an atmospheric river will begin to approach the West Coast, tapping into some subtropical moisture with its origins in the vicinity of the Hawaiian Islands. At the same time, flow from 925 mb through 850 mb will become increasingly strong from the south. This will cause rain to become heavy at times across San Luis Obispo County and much of Santa Barbara County late tonight, especially on and below south and southwest facing terrain where orographic lift will provide extra enhancement of the rain, increasing rainfall rates.
Southeast to south winds will increase tonight, reaching damaging levels (gusts to 60 mph or higher) across most of SLO and SBA Counties by midnight. The strong winds will translate eastward, spreading into the mountains of Ventura County and Los Angeles County including the Santa Monica Range by late tonight. Winds are also expected to reach High Wind Warning levels through the I-5 Corridor, in the Santa Clarita Valley, in the Santa Susanna mountains, and in the Antelope Valley and foothills, and in the northern Channel Islands. In the northern mountains of Ventura County and in the eastern San Gabriel Mountains, the High Winds are currently covered by a Winter Storm Watch there, and that Watch will likely be converted into a Winter Storm Warning later this morning.
Wind Advisories have been issued for the remainder of L.A. and Ventura Counties beginning around daybreak on Sunday.
On Sunday, rain is expected across the entire region, and will be heavy at times, likely continuing to increase in intensity in most areas through the day. It may take much of the day for the heavy rain to reach all of Los Angeles County.
Increasing instability with the approaching front and the increasingly diffluent height pattern will bring a chance of thunderstorms to all of SLO and SBA Counties Sunday, with the chance of thunderstorms extending southward into Ventura County in the afternoon. Low level wind profiles, with significant directional and speed shear, will become increasingly favorable for low level rotation of any convective cells Sunday. This will yield a slight chance of waterspouts Sunday across the northern outer coastal waters, the inner waters north of Point Sal, and across the Santa Barbara Channel. It is not out of the question that some of these could come ashore briefly as small tornadoes. Also, with very strong winds from 950 mb and above, it will not take much to transport strong winds to the surface in any convective activity Sunday, especially from Ventura County northward.
The heavy rain and thunderstorm activity will continue to translate southeastward through the coastal waters and into Los Angeles County Sunday night.
There is a significant threat of widespread and life-threatening flash flooding across the region late Sunday into Monday. At this point, it appears that rain will peak in intensity across San Luis Obispo County Sunday morning into early Sunday afternoon, across Santa Barbara County from mid morning Sunday through Sunday night, across Ventura County Sunday afternoon through early Monday, and across Los Angeles County Sunday night through early Monday afternoon.
The elongated upper low will remain off the West Coast Mon night and Tue, maintaining moist cyclonic southwesterly flow aloft over the region. Therefore, behind the period of heaviest rain associated with the front, periods of rain will continue Monday night into Tuesday night. There is a slight chance of thunderstorms just about anywhere across the forecast area Mon through Tue.
The forecast models continue to waver with respect to precipitation totals, with some runs increasing rainfall totals, and others showing less rainfall. In general, the fundamentals of this storm system look very impressive, and suggest widespread significant rainfall across the region. By Tuesday night, rainfall totals should average 3 to 6 inches in most coastal and valley areas with 6 to 12 inches in the foothills and mountains. The highest totals are likely to extend from southern Santa Barbara County to western Ventura County, and on south facing slopes of the San Gabriel mountains, where 60-80 kt of south wind will generate a huge orographic enhancement of the rainfall. Rain rates during the peak of the storm are expected to average one half inch to one inch per hour, with isolated higher rates in upslope areas and near any thunderstorms. Rainfall at these rates may persist for several hours in a row, generating some very impressive 6 hourly rates, especially in the foothills and mountains.
Flooding issues will not be confined to just the foothills, mountains, and burn ares. ALL AREAS, INCLUDING HIGHLY POPULATED URBAN AREAS, WILL BE AT RISK FOR LIFE-THREATENING FLOODING. Streams and small rivers, as well as the Los Angeles River through the San Fernando Valley and metro LA will rise quickly and turn into very dangerous raging rivers. Many roads will be impassable due to flooding. Numerous mud and rock slides are expected in the foothills and mountains. Evacuations may be required in some areas, especially near recent burn scars.
As mentioned previously, in addition to the rain, very strong winds will accompany the storm, especially across SLO and SBA Counties and in the mountains and Antelope Valley. Wind gusts up to 80 mph are possible in the mountains, including near the Grapevine, and as high as 60-70 mph across parts of SLO and Santa Barbara Counties and the Antelope Valley. Elsewhere winds gusts of 35 to 55 mph are expected.
Heavy snow accumulations are possible at higher elevations, and Winter Storm Watches will likely be converted to Warning for the higher portions of the Ventura and LA County mountains. Up to 2-4 feet of snow may fall 7000 ft with 8 to 16 inches possible between 6000 and 7000 feet, and lesser amounts down to around 5000 feet. Snow levels will start out on the lower side, generally 5000 to 6000 feet, then rise above 7000 feet late Sunday into early Monday. On the back side of the storm later Monday into Tuesday snow levels will fall back down to 5000 feet.