Area Forecast Discussion

SYNOPSIS

30/248 AM.

Rain chances continue through much of the weekend, with the peak of the rain likely occuring Saturday evening into Sunday morning. Warmer and drier conditions expected next week.

SHORT TERM (FRI-MON)

30/744 PM.

***UPDATE***

Generally quiet wx covered the forecast area early this evening. A deep moist layer persisted with breezy onshore flow and a few light showers here and there, especially over interior SLO/SBA Counties. Otherwise partly to mostly cloudy skies prevailed. Little change can be expected overnight as a broad upper level trof will continue over the E Pac and approach the area. Additional light showers will be possible just about anywhere from the mtns to the coast overnight as well. Showers are still expected to increase later Sat and Sat night, but exact rainfall amounts are still in question pending further model runs. Current forecast is on track and no evening updates are expected.

***From Previous Discussion***

Overall, current models indicate no major changes to previous forecast thinking. Synoptically, cyclonic flow aloft will prevail through Saturday morning then an upper low will develop offshore of Baja California Saturday afternoon/evening then move inland across northern Baja on Sunday. With this pattern, it will be a wet Halloween weekend for Southwestern California.

Through Saturday morning, the combination of a deep moist layer and the cyclonic flow aloft will continue to generate scattered light showers just about anywhere. Outside of an isolated moderate shower, rain totals with this activity will only be a few hundredths of an inch. So, no significant issues are expected in this time frame.

By Saturday afternoon/evening, the previously mentioned upper low will develop and move eastward, moving across northern Baja during the day on Sunday. So, late Saturday afternoon through Sunday morning will be peak period of widespread rain for the area with the rain chances tapering off quickly Sunday afternoon. Current models continue the trend of keeping the most significant rainfall south of LA county. However, latest models also continue to exhibit consistent differences in the amount of expected rainfall. Essentially, the ECMWF and its various ensembles indicate heavier rainfall than the GFS and even the RRFS, mainly across LA county. Based on QPF guidance from WPC, current expected rainfall totals between 0.25 and 1.00 inches (with local amounts up to 2.00 inches across the foothills/mountains) look reasonable for a best case forecast. However, given the persistence of the ECMWF, will continue to stress a 20% chance of double the amounts as well as a 20% chance of half the expected amounts.

As for rainfall rates, do not anticipate anything too significant. With the best 'energy' remaining south of the area, rainfall rates are expected to be in the 0.10 to 0.33 inch/hour range. So, other than some nuisance urban flooding, do not anticipate any hydrologic issues.

By Sunday night and Monday, an upper level ridge will build over the area. At the surface, weak offshore pressure gradients will develop. So, will expect mostly clear skies and warmer conditions on Monday. Fortunately, any upper level support will be minimal. So, only weak offshore winds are expected.

LONG TERM (TUE-FRI)

30/1214 PM.

For the extended, models start out in good agreement on Tuesday, but then start to diverge noticeably Wednesday through Friday. On Tuesday, the upper level ridge will still be over the area with weak offshore surface flow. So, will expect clear skies, comfortable temperatures (upper 60s and 70s) and some weak offshore winds.

For the Wednesday through Friday time period, forecast confidence wanes dramatically. This decrease in forecast confidence is due to noticeable uncertainties in the track of a couple of upper lows. At this time, there is high confidence in a gradual cooling trend through this period. However, confidence in rain potential is very low. The GFS family keeps the area dry while the ECMWF family indicates the potential for some shower activity across the area. If any shower activity should develop, it would likely be on the light side in terms of amounts. So, the forecast will have some slight chance POPs for some areas through this period with a hope of more model agreement in future runs.

AVIATION

31/0616Z.

At 06Z at KLAX, there was no inversion or marine layer.

Moderate confidence in 06Z TAF Package. VFR CIGs with occasional periods of MVFR conditions except for KPMD/KWJF.

KLAX...Moderate confidence in TAF. VFR CIGs with occasional periods of MVFR. Isolated -SHRA before 31/10Z, followed by occasional to intermittent -RA thereafter. 60% chance of east wind component reaching 7-8kts & 25% chance of 10 kts.

KBUR...Moderate confidence in TAF. VFR CIGs with periods of MVFR expected. -SHRA chances increase after 31/12Z.

MARINE

30/836 PM.

Moderate confidence in winds and seas staying below Small Craft Advisory (SCA) through Saturday evening, except for a low chance of brief gusty downdraft winds with any heavier showers Saturday. A marine weather statement (MWSLOX) has been issued to highlight the very low, but present risk for isolated waterspouts through Saturday morning across the inner waters south of Pt Conception including nearshore (PZZ650/655).

SCA-level NW winds likely to form Saturday Night and Sunday from the Central Coast to San Nicolas Island, including the western Santa Barbara Channel. NE winds under SCA-levels likely to follow on Monday. SCA-level NW winds will return Tuesday afternoon roughly across the same waters.

LOX WATCHES / WARNINGS / ADVISORIES

CA...NONE. PZ...NONE.