12/125 PM.
A significant and long lasting heat wave is expected to affect the region today through next week with MODERATE to HIGH heat impacts likely.
(tdy-Sun), 12/123 PM.
Overall, 12Z models in good synoptic agreement through the short term period. At upper levels, the high, just offshore, peaks in strength today then weakens gradually Friday/Saturday before restrengthening on Sunday. Near the surface, weak offshore flow prevails through Friday morning, switches weakly onshore Friday afternoon through Saturday before shifting back weakly offshore on Sunday.
Forecast-wise for the short term, main focus will remain on temperatures. As expected, today has been a very warm day, topping out in the mid 80s to mid 90s. For Friday, the return of the weak onshore flow in the afternoon will allow for some slight coastal cooling, but keep temperatures near persistence across inland areas. So, high temperatures on Friday will still be warm enough (15-25 degrees above normal) to warrant heat products. So, will keep the HEAT ADVISORIES in effect for the coasts and coastal valleys through Friday afternoon. On Saturday, all areas will experience a cooling trend with coastal/valley areas cooling as much as 5-10 degrees. However on Sunday, as the upper level high begins to strengthen again and weak offshore surface gradients develop, temperatures will increase yet again, generally into the 80s to lower 90s. HEAT ADVISORIES could be needed on Sunday for some coastal valley areas.
Other than temperatures, no significant issues are expected through the period. The weak offshore pressure gradients today through Friday morning and again on Sunday will generate some gusty northeasterly winds. However without any noticeable upper level support, will expect winds to remain below advisory levels. As for clouds, mostly clear will be likely for most areas through the period. However, on Saturday and Sunday, there may be some night and morning stratus/fog along the coastal plain (mainly LA county coast and the Central Coast).
(Mon-Thu), 12/123 PM.
For the extended, 12Z models continue to remain in good synoptic agreement through the period. At upper levels, an extraordinarily strong high pressure will build right over the area. At the surface, there is the potential for weak diurnal flow (best case scenario based on the GFS) or weak offshore flow (worst case scenario based on the ECMWF). Either way, nothing in the latest guidance indicates anything but an extremely impressive heat wave next week.
Details that make this heat event very impressive:
1. 500 MB heights are forecast to range between 590 and 595 DM which would be the highest March H5 heights since 1948.
2. 1000-500 MB thickness are forecast to peak in the 580 to 584 DM range. Based on the 25+ year history of TEMP STUDY, the highest thicknesses recorded in March are 571 DM.
3. The Extreme Forecast Index (EFI) indicates values between 0.8 and 1.0 for both Maximum and Minimum temperatures, signifying a very usual or extreme event.
4. If the weak offshore flow, forecasted by the ECMWF, pans out then temperatures west of the mountains could be higher than currently forecast.
5. Based on current forecasts, both daily and monthly March temperatures records could fall for most climate sites.
At this time, there is high confidence in the need for HEAT ADVISORIES for most areas next Tuesday through Thursday, and possibly as early as Monday for some areas. Additionally, there is a 40-50% chance that EXCESSIVE HEAT WARNINGs may be needed for some areas Tuesday through Thursday.
Given the unprecedented length and magnitude of this extreme heat wave, heat stress will be a real threat each day, especially in areas that aren't used to the heat, like the coastal areas, where people may not have methods to cool off their homes. Try to complete outdoor activities early in the day or in the evening, and don't leave people or pets in cars.
As for any rain chances, deterministic and ensembles do not indicate any chances for significant rain through the 28th.
12/1737z.
At 1707Z at KLAX, there was no marine layer. There was a sfc based inversion with a top of the marine inversion was around 1700 feet with a temp of 24 deg C.
High confidence in VFR TAFs. 20% percent chance northeast winds 15-20 kt surface at KSBP during the period. Light LLWS possible through the period at KSBP, highest through this afternoon.
KLAX, High confidence VFR TAF. Moderate confidence in any east wind component remaining below 8 knots.
KBUR, High confidence VFR TAF.
12/116 PM.
Moderate to high confidence in northwest to north winds between 15 and 25 knots through tonight, increasing to 20-30 knots tomorrow and impacting the Outer Waters through at least Sunday night. There is a 50% chance of low end Gale Force winds (roughly 35 knots) across the waters beyond 30 NM from shore late Friday through Saturday night, thus a Gale Watch has been issued. Seas will increase during this period, moderate confidence in seas reaching at least 10 feet across aforementioned areas. Conditions are likely to improve early next week, with relatively small seas expected.
Small Craft Advisory (SCA) level winds will approach the western edge of the Inner Waters along the Central Coast, but chances for these winds to reach nearshore areas are low.
Inside the Southern California Bight, conditions are expected to remain below SCA levels for the foreseeable future, except for hyper-localized SCA level winds across the far northwestern portion of the Santa Barbara Channel Saturday.
Ca, Heat Advisory remains in effect until 8 PM PDT Friday for zones 88-340>342-346>352-354>358-362-366>375-548. (See LAXNPWLOX). PZ, Small Craft Advisory in effect from 3 PM Friday to 3 AM PDT Saturday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 9 PM PDT Friday for zones 670-673. (See LAXMWWLOX). Gale Watch in effect from Friday evening through late Saturday night for zones 670-673. (See LAXMWWLOX). Small Craft Advisory in effect from midnight tonight to 3 AM PDT Saturday for zone 676. (See LAXMWWLOX). Gale Watch in effect from late Friday night through late Saturday night for zone 676. (See LAXMWWLOX).
