Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

347 am PDT Tue jul 17 2018

Synopsis

16/258 PM.

Temperatures will remain hot across the region for at least another week though onshore flow will provide some overnight and morning relief along the coast. There is a slight chance of thunderstorms Wednesday and Thursday afternoon across the Antelope Valley and the mountains of Los Angeles, Ventura, and eastern Santa Barbara Counties.

Short Term

(tdy-Thu), 17/337 AM.

Latest GOES-16 Fog Product imagery indicated low clouds already filling in across the Central Coast and moving into the Santa Ynez Valley. For areas S of Point Conception where the LAX-DAG surface gradient was trending a bit stronger probably the reason why low clouds have already spread across the L.A. Basin and will likely move into both the San Gabriel and San Fernando Valleys this morning. The latest AMDAR sounding near LAX was around 1600 ft deep.

Synoptically, an elongated W to E upper level ridge from over the eastern Pac over California to Texas will begin to pivot to a more NW to SE position later this evening into Wednesday. The upper level winds will begin to move east to west and draw in some high clouds today from collapsing thunderstorms over the Colorado River. With high pressure continuing to strengthen some, high temps will increase a few degrees for most inland areas while coastal areas remain mild under the marine layer influence today. There will be a slight chance for elevated thunderstorms over the eastern portion of the San Gabriel Mountains and Antelope Valley as latest Soundings look like there will be enough mid level moisture and marginal instability.

The overall weather pattern will begin to change tonight as high res models as well as the NAM WRF were continuing to indicate some embedded upper disturbances within the easterly wave approaching the southern portion of the coastal waters late tonight or early Wednesday morning. There will be a slight chance for showers or thundershowers over the L.A. and Ventura County Coast and valleys late tonight into Wednesday morning. 850 mb temps (Monsoonal front) will rise up to 15 degrees Celsius Wednesday morning as the juicier air moves into Los Angeles and Ventura Counties.

By Wed afternoon the main focus of convection will move over the L.A. and Ventura County Mtns and Antelope Valley. Convective Parameters look impressive with CAPE over 1000 j/kg, K index over 40 and and Lifted Index also showing the unstable bulls-eyes over the LA and VTU Mtns. There are some concerns with high precipitable waters (PWAT) values between 1.75" to 2.00" that when storms initiate, heavy rain is likely to occur over the mountains and Antelope Valley initially. The steering winds are fairly strong which normally would keep storms moving rapidly and keeping chances for flash flooding at a minimum. However, if storms begin to train and back-build over the same areas, then there will be the potential for flash flooding across L.A. and Ventura County recent burn areas. With this easterly flow aloft, storms that initiate over the mountains will likely move into the adjacent valleys and coastal areas late Wed afternoon and evening. Have added slight chance POPs for all of L.A. County through Wed evening and valleys of Ventura County.

Confidence is low that areas in the Montecito Debris flow areas will be affected on Wednesday. However if the mid level moisture advects further west into SBA County, then the threat would increase. Later shifts will have to monitor this situation. The upper steering flow will begin to move more out of the SE by Wednesday night into Thursday. There will continue to be some residual mid level moisture over the mountains, including the Santa Barbara County Mtns. With the more southerly component to the steering flow and a lack of short waves, it's likely afternoon convection will remain confined to the mountain and desert areas and not be a threat to the Montecito Debris Flow area.

The added monsoon moisture will certainly make things feel more muggy, especially for southern areas and overnight lows will be warming up as a result. mid 70s to lower 80s for the Antelope Valley and 70s in the valleys and some coastal areas. There will likely be some hit on the marine layer south of Pt Conception though that's always a tough call with these more marginal monsoon cases. Temperatures expected to warm up slightly at least for coast/valleys as onshore flow weakens slightly along with possibly less overall stratus coverage with the monsoon moisture moving in. At the very least it will feel warmer as humidities increase.

Long Term

(Fri-Mon), 17/345 AM.

Monsoon moisture starts to dry up Friday though the flow aloft is still favorable should additional moisture sources develop before then. So the current forecast maintains dry conditions though given the continued southeast flow aloft this certainly could change. A slight increase in onshore flow Friday along with dip in the temps aloft should lead to slightly cooler temps but still above normal and probably less humid as well.

Not much change Saturday. Still a somewhat favorable upper level pattern for monsoon moisture but so far models are keeping things pretty dry. However, some instability is possible over the Ventura County Mtns and have added slight chance thunderstorms for the afternoon hours. Otherwise will probably see some afternoon cumulus over the mountains but not much more unless additional moisture moves in.

Sunday and Monday look to be the beginning of a warming trend across the area that will last well into next week, though for now not expected to even approach the numbers we saw on July 6 as models not showing any significant northerly flow developing. Triple digit temps expected in the warmer valleys and 105-108 in the AV by Monday and possibly low 90s in the inland coastal zones. Despite being 10+ degrees cooler than the previous major heat wave these numbers still put a pretty large portion of the area under at least heat advisory criteria with some localized warning level temperatures possible.

Aviation

17/1046z.

At 10Z, the marine layer was around 1600 ft deep at KLAX. The top of the marine inversion was around 3800 feet with a temp of 28C.

Widespread low clouds in all coastal areas and the Santa Ynez Valley. Conds were mostly IFR, except low MVFR across L.A. County. Expect IFR cigs to push into portions of the VTU County valleys, the San Gabriel Valley and the San Fernando Valley by daybreak. Expect clouds to scatter out by mid morning in the valleys and by late morning on the coastal plain.

There is a slight chance of thunderstorms in the mtns of L.A. County and eastern portions of the Antelope Valley this afternoon and evening, this afternoon and evening and for much of Los Angeles and southern Ventura County Wed morning, even to the coast.

Expect low clouds tonight to be confined to the Central Coast with LIFR conds expected.

KLAX, Moderate confidence in the 12Z TAF. There is a 20% chance that cigs will drop into the IFR category between 12Z and 16Z. There is a 20-30% chance that cigs will linger through 19Z or 20Z. There is a 20% chance of a thunderstorm near KLAX between 12Z and 18Z Wed.

KBUR, Low confidence in the 12Z TAF. There is a 40% chance that IFR cigs will not arrive and conds will remain VFR this morning.

Marine

17/315 AM.

Small Craft Advisory winds are not forecast across the waters through Sat. However, there is a 20-30% chance of SCA level winds across the outer waters this afternoon and evening.

Choppy short period seas will linger across the waters thru Wed.

There is a slight chance of thunderstorms across the southern inner waters late tonight, across the southern inner and outer waters, the SBA Channel out to San Miguel Island Wed morning, and across the southern inner waters late Wed afternoon/evening.

Watches, Warnings, Advisories

Ca, NONE. PZ, NONE.

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