Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

826 pm PDT Sat may 25 2019


25/733 PM.

A low pressure system will bring increasing clouds and showers to the area late tonight through Sunday. Isolated thunderstorms are expected for interior areas Sunday afternoon and evening, along with a chance of mountain snow. Warming and drying is expected early next week.

Short Term

(Sat-Tue), 25/817 PM.


Busy afternoon in the mtns as a little trof went over the region and interacted with the usual area of instability over the northern VTA county mtns produced two large near severe TSTMs. These two TSTMs moved to the NNE into Kern county where they all merged with a very strong TSTM that formed there. The resultant TSTM then moved to SE following the path of the very strong Kern county TSTM. This storm advected into Nrn LA county mtns near the Grapevine bringing lightning and brief heavy downpours along with it. The trof has moved to the east and the Sun has set both of which combined to end the TSTM threat for the night.

Moderate onshore flow kept the marine layer stratus over the Central Coast all day long. There was good clearing early this afternoon over the coasts and vlys south of Pt Conception but low clouds are already rapidly filling into the area and by early morning Sunday low clouds should cover almost all of the coastal and vly areas. An unseasonably cold upper low will also dive down through the state overnight and the lowering hgts and weak peripheral lift will bring drizzle or light rain to many areas across the coasts and vlys as well as the coastal slopes.

00Z runs still show that the unusual Sunday forecast is on track with the upper low bringing widespread rain to the entire area. Snow down to 5000 feet (locally 4500 feet) and afternoon and evening TSTMs near the Kern County line.

There will be enough snow and rain with the system to create a multitude of winter weather problems and a winter weather advisory is in effect from 600 am Sunday morning through late Sunday evening.

***From Previous Discussion***

A rather potent upper level low pressure system for this time of year (as low as 543 dm at H5) is forecast to move to the far nrn CA coast by late tonight then SE into central CA on Sun. This system will continue tracking SE into SErn CA Sun night then E into nrn AZ on Mon. A broad NW flow aloft will be over srn CA Mon night. A fast- moving and weak upper level disturbance should move into swrn CA from the N on Tue. The upper level low will quickly lower H5 heights and bring in colder air aloft thru Sun afternoon, down to -20 deg C over nrn L.A. County to as low as - 26 deg C for nrn SLO County. The 12Z NAM does not indicate much instability over the fcst area on Sun likely due to cloud cover, but surface-based LIs were still 0 to -2 over portions of the mtns to nrn SLO County in the afternoon. The area will also be under the left exit region of a 120 kt jet Sun afternoon. Altho the NAM MOS was showing a slight chance of thunderstorms over a large portion of swrn CA, for now will keep a slight chance of thunderstorms over the mtns, Antelope Vly, Cuyama Vly and interior SLO County vlys for Sun afternoon and early evening. Otherwise, showers and mountain snow showers will be likely across most of the fcst area thru Sun afternoon, with decreasing showers Sun evening.

Rainfall totals are expected to generally be under 0.10 to 0.25 inch for the coast and vlys, except up to 0.50 inch in some foothill areas, and 0.25 to 0.75 inch in the mtns with local amounts up to 1.00 inch possible. Snow levels will fall to 6000 to 6500 feet on Sunday, then drop further to 4500 to 5500 feet Sunday night, with accumulating snow expected over a wide area above 5000 to 5500 feet. It looks like up to 1 inch of snow will be possible below 5500 feet, with about 2 to 4 inches between 6000 and 6500 feet, and about 3 to 6 inches with local higher amounts above 6500 feet. The highest peaks may see accumulations up to 8 or 9 inches.

Gusty mainly sub-advisory level south to west winds will also affect the mtns, producing reduced visibilities in snow and blowing snow. Expect wintry driving conditions in the mtns above 5000 to 6000 feet Sunday into Sunday night, with snow and ice covered roads. These conditions are more typical in a mid-winter storm. If you plan on going into the mtns Sunday, be prepared for hazardous driving conditions and dress for winter weather.

With the unusual winter-like storm expected, a Snow Advisory has been issued for the Ventura and Los Angeles County mtns from 6 AM Sun thru 3 AM Mon for the accumulating snow and gusty winds. Please see the latest Winter Weather Message (LAXWSWLOX) for further details.

There will be a slight chance of rain and snow showers thru late Sun night in the mtns, otherwise dry weather can be expected later Sun night thru Mon across the region. Decreasing cloudiness can also be expected thru Mon, with a fair amount of sunshine for Mon afternoon.

Marine layer clouds and fog are forecast to return to much of the coast and vlys Mon night into Tue morning, otherwise mostly clear skies can be expected thru Tue with breezy onshore winds Tue afternoon.

Temps will be well below normal in all areas on Sun, as much as 15- 25 deg or more below seasonal norms. It will be so chilly on Sun that many climate stations will likely have near record to record cold high temps for that date. For example, the lowest max temp recorded for May 26th at downtown L.A. is 63 deg set in 1947 while the fcst max temp on Sun there is 61 deg. Temps for Mon will turn warmer but still be at least 5-15 deg below normal for many areas. Highs will turn much warmer on Tue but remain 3-7 deg below normal overall.

Long Term

(Wed-Sat), 25/218 PM.

The EC and GFS are in generally good agreement in the extended period. Another weak upper level trof will move into the forecast area on Wed, then broad upper level troffiness will persist over the region Thu thru Sat.

There will be a slight chance of showers in the mtns in the afternoon and early evening Wed and Thu, with a small chance of a thunderstorm (less than 15%). Otherwise, clear to partly cloudy skies with varying amounts of night and morning low clouds and fog for the coast and vlys can be expected. Temps will have minor day-to- day changes thru the extended period, but overall remain several degrees below seasonal norms.



At 2330Z at KLAX, There was no marine layer nor was there a sfc based inversion.

Low confidence in TAFs. MVFR level cigs will form over the coastal and vlys TAF sites KSBA and south fairly randomly from 05Z to 11Z. DZ could develop at any cstl or vly TAF site from 09Z-15Z. Light rain will develop across the entire area from 14Z to 20Z. There is a 30 percent chc of IFR cigs and vis at all sites after 14Z.

KLAX, Low confidence in TAF. MVFR level cigs could form anytime between 07Z-12Z. DZ could develop anytime from 11Z-15Z. Light rain will develop sometime between 16Z to 20Z. There is a 30 percent chc of IFR cigs and vis after 14Z. There is a 20 percent chc of an 8 kt east wind component 11Z-15Z.

KBUR, Low confidence in TAF. MVFR level cigs could form anytime between 09Z-14Z. DZ could develop anytime from 12Z-16Z. Light rain will develop sometime between 16Z to 20Z. There is a 30 percent chc of IFR cigs and vis after 14Z.


25/824 PM.

For the outer waters, There is a 60% chance of Small Craft Advisory (SCA) winds through late tonight, strongest over the western half of the zones. Winds may diminish for a few hours early Sunday, but increase again late in the morning through the evening. SCA level winds are likely again Monday afternoon through Wednesday. For the inner waters north of Pt Sal, There is a 30% chance of SCA conditions during the afternoon and evening hours tomorrow through Tuesday, otherwise conditions will remain below SCA levels.

For the inner waters south of Pt Conception, conditions should remain under SCA levels through Sunday night. For Monday afternoon and evening, SCA level winds are likely across the western half of the Santa Barbara Channel and there is a 50% chance of SCA level winds across western portion of PZZ655.

Watches, Warnings, Advisories

Ca, Winter Weather Advisory in effect from 6 AM Sunday to 3 AM PDT Monday for zones 53-54. (See LAXWSWLOX). PZ, Small Craft Advisory in effect until 9 PM PDT Sunday for zones 670-673-676. (See LAXMWWLOX).

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