17/256 AM.
A weak to moderate Santa Ana event gusty winds and critical fire weather conditions to portions of Southwest California today. In addition, the very warm temperatures bring a high risk of heat- related illness to all coastal areas. A significant cool down is expected Sunday with a good chance of rain Tuesday or Wednesday.
(tdy-Tue), 17/151 PM.
For the short term period, 12Z models are in good synoptic agreement through the period. At upper levels, weak trough will develop over the area tonight/Sunday then a low will work its way south, off the CA coast, ending up about 400 miles west of Point Conception on Tuesday. At the surface, offshore flow will continue to weaken tonight/Sunday with onshore flow increasing through Monday and Tuesday.
Forecast-wise, no major changes to previous thinking. With the pressure gradients relaxing and upper support waning, the Santa Ana winds will continue to weaken through Sunday morning. So, have let all WIND ADVISORIES expire at 200 PM this afternoon with sub-advisory northeast winds expected tonight/Sunday morning. By Sunday afternoon, the winds will begin to shift to an onshore direction with the onshore flow increasing Monday and Tuesday. Do not anticipate any issues with the increasing onshore winds as speeds should remain below advisory levels.
As for temperatures, the combination of increasing onshore flow and lowering H5 heights/thicknesses will bring a cooling trend to all areas through Tuesday. The current HEAT ADVISORIES for the coastal plain should be good to expire at 800 PM this evening. For Sunday through Tuesday, no heat issues are expected.
As for clouds, there will be some partly cloudy skies across the area through tonight as high clouds drift up from the south. For Sunday and Monday, mostly clear skies can be expected although some marine layer stratus/fog could return to the Central Coast. On Tuesday, mid and high level clouds should be on the increase ahead of the upper low, expected to impact the area on Wednesday and Thursday. Additionally, there should be more widespread marine layer stratus/fog Monday night/Tuesday morning across the entire coastal plain and coastal valleys.
(Wed-Sat), 17/151 PM.
For the extended period, 12Z models are in good synoptic agreement. Main feature of note is a decent, early-season, upper low. Models move this system east across the area on Wednesday and Thursday, bringing the high likelihood of wet and unsettled weather for the area. Admittedly, models will likely be challenged with such a system in mid-October, so the forecast details could change noticeably over the coming days.
That being said, there is rather high confidence in wetting rainfall across the area in the Tuesday-Thursday time frame, especially on Wednesday and Thursday. Given the current track of the upper low, it should be able to tap into some moisture from the remnants of Trudy. Based on current WPC guidance and ensemble solutions, a widespread 0.25 to 1.00 inch event is very likely for the area. However, ensembles indicate, depending on model, about a 20-40% chance for totals greater than 1.00 inch. Southerly flow, associated with this low, will have the potential to generate some decent orographic enhancement which could help totals exceed 1.00 inch per those ensemble members. Additionally, there looks to be some decent instability with the upper low. 500 MB temperatures drop to around -20 degrees Celsius and CAPE/MUCAPE values are forecast to be in the 400-800 J/kg range. So, there will be a chance of thunderstorms on Wednesday and Thursday.
In terms of rainfall rates, at this time, rates are expected to be under 0.50 inches/hour. However, if any thunderstorms do develop, locally higher rainfall rates (0.50-1.00 inch/hour) would be possible. So, there may be some hydrologic concerns as we approach the event (mud/debris flows near burn scars, urban flooding, etc.). Stay tuned over the coming days for the latest details.
By Friday and Saturday, the upper low will have move eastward. In its wake, weak offshore flow is likely. So, expect dry and locally breezy conditions along with a warming trend.
17/1800z.
At 18Z at KLAX, there was no marine layer nor an inversion.
High confidence in VFR conditions and moderate confidence in winds.
KLAX, High confidence in VFR conditions. E wind near 7 kts should switch W between 17/19-21Z. Then, light E wind 18/10-17Z should remain below 8 kts.
KBUR, High confidence in VFR TAF. Weak to moderate LLWS and Turbulence is possible through 17/21Z.
17/156 PM.
Nearshore observations have indicated that Small Craft Advisory (SCA) offshore winds have persisted longer than expected. Onshore wind reversal is expected to occur within a few hours along the Ventura coastline. These winds will remain near SCA levels during this transition. Because of this have extended the SCA for the Santa Barbara Channel through 4 PM Today.
Thereafter, conditions will remain relatively benign through Sunday morning across all coastal waters. Small Craft Advisory NW winds will increase Sunday Night and Monday, especially off the Central Coast. Have issued SCA products to reflect this. A moderate- strength storm system is becoming more likely in the Tuesday through Thursday timeframe.
Ca, Heat Advisory in effect until 8 PM PDT this evening for zones 340-341-346-347-349-350-354-355-362-366>368. (See LAXNPWLOX). Beach Hazards Statement in effect from Sunday morning through Tuesday morning for zones 354-362-366. (See LAXCFWLOX). Red Flag Warning in effect until 6 PM PDT this evening for zones 288-375-379. (See LAXRFWLOX). PZ, Small Craft Advisory in effect until 4 PM PDT this afternoon for zone 650. (See LAXMWWLOX). Small Craft Advisory in effect from 3 PM Sunday to 9 PM PDT Monday for zone 670. (See LAXMWWLOX). Small Craft Advisory in effect from 9 PM Sunday to 3 AM PDT Tuesday for zone 673. (See LAXMWWLOX). Small Craft Advisory in effect from midnight Sunday night to 3 AM PDT Tuesday for zone 676. (See LAXMWWLOX).
