09/850 AM.
Temperatures will cool region wide through the weekend and into early next week. While Tropical Storm Rachel is expected to track well south of Los Angeles County, periods of light to moderate showers with a low chance of thunderstorms is expected from Saturday evening into early next week. Another warm up is likely Wednesday into the weekend.
(tdy-Sun), 09/853 AM.
***UPDATE***
A more robust marine layer from coastal Ventura County northward along with lowering 500 mb heights aloft and increased onshore flow will lead to region-wide cooling, a much welcomed relief from a prolonged, dangerous heat wave. Further cooling can be expected this weekend into early next week, with periods of light to moderate showers, a low chance of thunderstorms, and locally breezy winds. We will take a closer look at numbers for potential rainfall for this afternoon's forecast package, but generally we will likely see periods of light to moderate rain showers, with Sunday morning through Monday evening having the highest chance.
Tropical Storm Rachel is expected to track well south of Los Angeles County.
***From Previous Discussion***
Overall, 00Z models in good synoptic agreement through the short term period. Main feature of note will be an upper low that drops southward off the CA coast today/Saturday then move inland across far Southern California on Sunday. As this low drops southward, it will interact with moisture from Tropical Storm Rachel late Saturday and Sunday.
Main concern in the short term will be the chances of rainfall across the area as the upper low sucks up moisture from Rachel. At this time, no major changes from previous thinking. The best moisture will remain south of the CWA, but we will still get some decent rain. Rain will begin over LA county late Saturday, spreading across all areas Saturday night/Sunday morning then the rain chances will shift eastward Sunday afternoon/evening. Rain totals through Sunday evening look to generally range from 0.10-0.50 inches across Ventura/LA counties with up to around 1.00 inch across the San Gabriel Mountains. For Santa Barbara and San Luis Obispo counties, rain chances are much less and any measurable rainfall should only amount to a few hundredths of an inch. Looking at the latest ensemble solutions, these QPF numbers look on track. Of course any change in the track of Rachel could result in changes to these QPF numbers.
As for thunderstorm chances, current convective parameters do not look too impressive for the area. However, given the complexity of the interaction between the upper low and Rachel, it seems prudent to keep a low chance (5-10%) of thunderstorms on Sunday. Probabilities are not high enough to mention in the official forecast, but should be kept in the back of everyone's mind.
Other than the rain issues for the area, the forecast is pretty straightforward. Onshore flow will continue across the area through Sunday. With lowering H5 heights, this will allow for the marine layer to become deeper and more widespread through the weekend. Additionally, there will be good cooling trend each day, giving the area a refreshing change from the heat earlier this week. The increasing onshore flow will generate gusty southwesterly winds across interior sections with a low chance of advisory-level gusts on Sunday across the desert foothills.
(Mon-Thu), 09/213 AM.
For the extended period, 00Z models exhibit some noticeable disagreement, leading to relatively low confidence in the extended forecast. The GFS, and its ensembles, drop an upper low along the CA coast and over the area on Monday then move it eastward over the area and into the Great Basin on Tuesday and Wednesday. Conversely, the ECMWF, and its ensembles, drop the low over the Great Basin Monday and Tuesday, before ejecting it eastward on Wednesday and Thursday.
Forecast-wise, either model solution will generate a threat of showers across all areas Monday and Tuesday. Given the uncertainty of the pattern, forecast takes a middle ground. Rainfall totals on Monday and Tuesday are forecast to range between 0.10 and 0.25 inches for most areas with up to around 0.50 inches in the mountains. If the GFS solutions verifies, the rainfall totals would be on the higher side of that range. As for any thunderstorm threat, the GFS track of the upper low could give some mentionable chance of thunderstorms while the ECMWF solution would not. So, will any chances of thunderstorms under 10% at this time and see how things develop.
For Wednesday and Thursday, both models indicate increasing offshore pressure gradients. The ECWMF is stronger with the forecast gradients, indicating the potential for weak (to possibly moderate) Santa Ana winds. The GFS, with its weaker gradients, would indicate a weak Santa Ana at worst. Either way, with rising H5 heights and some amount of offshore flow, temperatures will be on the upswing Wednesday and Thursday.
09/1032z.
At 1000Z at KLAX, the marine layer was 1100 feet deep. The top of the inversion was 3700 feet with a temperature of 25 degrees Celsius.
For 12Z TAF package, high confidence in KWJF and KPMD. For all other sites, moderate confidence in TAFs as timing of flight category changes through the period could be +/- 3 hours of current forecasts.
KLAX, Moderate confidence in 12Z TAF. Timing of flight category changes could be +/- 3 hours of current forecasts. No significant easterly wind component is expected.
KBUR, Moderate confidence in 12Z TAF. Timing of flight category changes could be +/- 3 hours of current forecasts.
09/840 AM.
For the Outer Waters, moderate to high confidence in current forecast. Today through Saturday morning, high confidence in winds and seas remaining below Small Craft Advisory (SCA) levels. From Saturday afternoon through Monday morning, high confidence in a combination of SCA level winds and seas. On Tuesday, there is a 50% chance of SCA level winds around Point Conception with winds remaining below SCA levels elsewhere.
For the Inner Waters north of Point Sal, high confidence in current forecast. Today through Saturday morning, winds and seas are expected to remain below SCA levels. From Saturday afternoon through Tuesday, SCA level winds are expected (mainly in the afternoon and evening hours) with seas approaching SCA levels on Sunday and Monday.
For the Inner Waters south of Point Conception, moderate to high confidence in current forecast. Today through Saturday, high confidence in winds and seas remaining below SCA levels. For Sunday, there is a 60-80% chance of SCA level winds. For Monday through Tuesday, high confidence in winds and seas remaining below SCA levels except for a 50% chance of SCA level winds on Tuesday across the western half of the Santa Barbara Channel.
Periods of light to moderate showers is likely Saturday evening through Tuesday, with highest confidence for Sunday-Monday. There is a 10% chance of thunderstorms during the same time frame. While showers or thunderstorms are possible anywhere, highest chances exist south of Point Conception.
09/843 AM.
A Coastal Flood Advisory is in effect through Tuesday afternoon for Los Angeles and Ventura Counties, while a Beach Hazards Statement (BHS) is in effect for the Santa Barbara South Coast and the Central Coast.
A very complex series of swells from the south, west, and northwest will impact our coastlines into at least early next week. Energy from Tropical Storm Rachel, southern hemisphere swells, northwest swells, and local wind swells will add up with each other to produce elevated surf and dangerous rip currents at times. Extremely high tides will be common each morning, peaking between 6.8 to 7.6 feet MLLW, highest further south. Additionally secondary high tides will peak each evening, likely most impactful through the weekend with tides reaching 5.5 to 6.5 feet MLLW.
While each day under the Coastal Flood Advisories or BHS should see fairly similar impacts, Sunday will likely see elevated impacts. Rapidly increasing winds will likely add to the coastal flooding potential, creating a wind swell as well as elevating tides slightly. Additionally, if lower pressure moves over the region, water levels could rise, adding to the coastal flooding concern in southern LA County. If all these ingredients overlap on Sunday, a more significant coastal flooding event would be possible. This is a very narrow window, but there is a low to moderate chance for a targeted Coastal Flood Warning to be issued in southern Los Angeles County if all factors line up.
As this is a complex and evolving situation, please continue to monitor the forecast.
Ca, Coastal Flood Advisory in effect until 2 PM PDT Tuesday for zones 87-354-362-366. (See LAXCFWLOX). Beach Hazards Statement in effect from Saturday morning through Monday afternoon for zones 340-346-349-350. (See LAXCFWLOX). PZ, NONE.
