Pasadena, CA 91107 Weather

Synopsis

28/1034 PM.

Gradual cooling will continue through the week as will coastal flooding issues due to very high tides. The next storm system is expected to bring rain to the area from as early as Friday through Sunday, likely peaking Saturday evening into Sunday morning.

Short term (tdy-fri)

28/715 PM.

***UPDATE***

The quiet weather continued today with a mix of patchy marine layer clouds and high clouds throughout the day. Otherwise there were minimal changes in the sensible weather compared to yesterday. The upper low in the PACNW will slide a little further south through tomorrow, bringing lower 500mb heights and southwest flow aloft to southwest California. This will help create partly to mostly cloudy skies with a mix of high level clouds and patchy marine layer clouds. Temperatures should cool a couple of degrees across most areas tomorrow, continuing the stretch of benign weather.

***From Previous Discussion***

Overall, 12Z models in good synoptic agreement through the short term period. At upper levels, a low will continue to spin off the Pacific Northwest coast with a sharp trough axis extending south. At the surface, weak onshore flow will prevail through Saturday.

Forecast-wise, tonight through Friday afternoon, the weather should be rather benign. With the upper level trough and weak onshore flow, the marine inversion should remain rather deep and marine layer stratus should become more widespread across the coasts and coastal valleys. Additionally, there will be an increase in mid and high level clouds, leading to a partly to mostly cloudy sky conditions. As for precipitation chances through Friday afternoon, there will likely be some patchy drizzle or light marine layer showers across the area. However through Friday afternoon, totals, if any, will be only be on the order of a few hundredths of an inch. With the cloud cover and lowering thicknesses, temperatures will exhibit a cooling trend through Friday.

For Friday night and Saturday, an upper low will pinch off from the parent trough, about 400 southwest of Point Conception. This will set the stage for a wet Halloween weekend. Models continue to exhibit some difference in the speed of this low moving eastward towards northern Baja (the ECMWF is faster than the GFS). With the afternoon forecast, will split the difference between the GFS and ECMWF. So, will expect the rain chances to increase across all areas during the day on Saturday. Given the current trajectory of the low, the highest rain chances and totals through Saturday evening look to occur across LA county. Preliminary totals through Saturday evening are expected to range between 0.10 and 0.33 inches across the area.

Long term (sat-tue)

28/130 PM.

For the extended, main storm action for the area looks to be Saturday night and Sunday morning with rain chances diminishing Sunday afternoon and evening. Based on model trends the last few days, the highest rain totals are expected to remain south of LA county. For our area, storm totals through Sunday are expected to range between 0.50 and 1.50 inches for most areas with local amounts up to 2.50 inches across south-facing mountain slopes. Rainfall rates are expected to be in the 0.10 to 0.33 inch/hour range. However, based on a continued wide range of solutions in the ensemble members, there is moderate confidence at best in the above numbers. There is a 20% chance of totals/rates double the expected numbers and a 20% chance of totals/rates half of the expected numbers. As for thunderstorm chances, the odds are pretty low (10% or less) as the best instability is forecast to remain south of LA county.

Once this storm moves eastward Sunday afternoon/evening, an upper level ridge will develop over the area Monday and Tuesday. This will bring dry and warmer conditions to the area. Additionally, there will be some offshore flow at the surface, but the offshore winds look to be on the weak side.

On Wednesday, the upper level ridge is nudged eastward as a low tracks northeastward into northern California. This will likely bring some increase in high-level clouds and some slight cooling as offshore flow decreases. As for rain chances, models indicate a low chance (less than 15%) of any showers across the area.

Aviation

29/0533Z.

At 0455Z at KLAX, the marine layer was 3600 ft deep. The top of the inversion was at 5800 ft with a temperature of 12 C.

High confidence in VFR TAFs for KPMD and KWJF.

Moderate confidence in remaining TAFs with VFR mid and high level clouds likely through the period. Periods of SCT-OVC 020-030 are possible through the TAF period, especially from 06Z-18Z for coastal and coastal valley sites.

KLAX: Moderate to high confidence in TAF. BKN to OVC020-030 cigs possible through the period, especially 06Z-18Z. Any east wind component is expected to remain under 8 kt.

KBUR: Moderate to high confidence in TAF. BKN to OVC020-030 cigs possible through the period, especially 06Z-18Z.

Marine

28/855 PM.

Conditions are expected to remain below Small Craft Advisory (SCA) levels through Saturday. Southwest flow aloft will produce chances for drizzle and isolated light rain showers Thursday night and Friday.

A storm system is expected to impact the coastal waters this weekend. There is a wide range of outcomes, but there is at least a low threat of a 12-24 hour period of strong winds and steep seas. Worst conditions look to be focused south of Pt Conception.

Watches, warnings and advisories

CA: Coastal Flood Advisory in effect until 2 PM PDT Friday for zones 87-340-346-349-350-354-362-366. (See LAXCFWLOX). PZ: NONE.