12/906 AM.
Temperatures will increase each day into mid-week, resulting in widespread Moderate to Major HeatRisk Tuesday through Thursday. Monsoonal moisture moving into the area will add to the discomfort, bringing shower and thunderstorm potential each day, highest in the mountains.
(tdy-Tue), 12/937 AM.
***UPDATE***
A hot and muggy week expected as an upper level high over the northern plains tops out around 600dam, and is also favorably placed to import tropical moisture from Mexico. PW's peak tonight and Monday around 1.5" and slowly fall the remainder of the week but remaining as high as 1.35" Thursday. The best chance for any precip reaching the ground remains today and Monday but can't rule out a few drops through mid week. And there also remains a 10-20% chance of thunderstorms though that time, but the best chance of those would be in the mountains.
The heat and humidity will be the biggest concern this week with temperatures peaking on Wed. Extreme Heat Watches will almost certainly become Warnings on Tuesday for most areas except possibly along the immediate coast and continuing through Thursday, though there is a 20-30% chance that warnings will extend to the coast as well, especially the Santa Barbara South coast where hot and strong Sundowners are expected to peak Tuesday and Wednesday evenings, possibly bringing temperatures at the beach into the 90s. Current projections indicate a 30-40% chance of 110 degrees in the western San Fernando Valley on Wednesday and a 60-80% chance of 95 or higher in Downtown Los Angeles.
***From Previous Discussion***
Satellite loops indicate a well-defined MCV centered near Yuma, Arizona early this morning -- the remnants of diurnally enhanced convection from yesterday that formed across southern Arizona and adjacent northwest Mexico. In the past hour, convection has been re-generating in the western semicircle of the MCV, reflective of the focused ascent intercepting monsoonal moisture. It is unlikely for the ongoing convection to persist into the local area. However, these recent trends suggest that the MCV will remain sufficiently in-tact as it pivots around the broader western-CONUS anticyclone and eventually into portions of the forecast area later today, to produce at least isolated sprinkles across a broader portion of Los Angeles, Ventura, and Santa Barbara Counties this afternoon. The forecast in these areas has been updated to account for higher coverage of light-precipitation mention this afternoon. The greatest chance for a few lightning strikes will remain over interior sections of these counties, though an isolated lightning strike cannot be completely ruled out closer to the coast.
Aside from the potential for a few lightning strikes, significant dry air in the lower troposphere and related sub-cloud evaporation will minimize precipitation amounts, with little in the way of wetting rains expected. However, ample sub-cloud evaporation in the steep lapse-rate environment may also accelerate downdrafts and boost the chance for locally gusty winds with the afternoon round of midlevel convection. Local wind gusts of 30-40 mph from variable directions will be possible.
Otherwise, increased cloud coverage associated with the approaching MCV will tend to restrict the degree of diurnal warming today. However, ample humidity -- characterized by lower 60s dewpoints across the Los Angeles basin -- amid the warm air mass already in place will support the continuation of heat headlines.
Ample mid and high level clouds are streaming into the region from the south. Few (if any) observations sites have reported any rain, thus most moisture is not reaching the surface and will likely continue to fall mostly as virga through this morning. Light showers are possible though, especially for San Luis Obispo and Santa Barbara Counties. There remains around a 5% chance of dry lighting through this morning. Today through Tuesday there is a 10-20 percent chance of thunderstorms and showers over the LA, Ventura, and far interior Santa Barbara County mountains, while a 5-10 percent chance exists everywhere else. Storms are expected to produce more measurable rain starting today, and especially Monday and Tuesday. The influx of moisture will increase the humidity, making conditions muggier and increase any heat discomfort. The upper level clouds have disturbed the marine layer clouds, which are patchy and have been scattering and reforming overnight (the high clouds will also make for good sunset conditions).
Through Tuesday, 500 mb heights will experience minimal changes, but onshore flow will become much weaker. Some model guidance even show weak offshore flow during the mornings staring on Tuesday. This will dampen the cool seabreeze and allow for temperatures to rise more each the day. A warming trend is anticipated for the next several days, reaching widespread Moderate to Major HeatRisk on Tuesday. This will pose a threat of heat related illnesses to much of the population, not just those most vulnerable. All headlines (Advisories and Watches) remain in effect with no recent changes to the timings. Heat Advisories continuing into Tuesday morning, then the Extreme Heat Watches are in place Tuesday morning through Thursday evening.
Tuesday night, expect moderate Sundowner winds across southwest Santa Barbara County, and increasing northwest winds across interior areas.
(Wed-Sat), 12/319 AM.
Heat will be the primary hazard next week, and an Extreme Heat Watch is in effect Tuesday through Thursday for all areas except the beaches along the Central Coast. Widespread moderate to major HeatRisk is anticipated for the region. Over the next couple days, some locations are likely to to be converted to Heat Advisories, while others may become Extreme Heat Warnings. High temperatures in the warmest valleys and deserts have the potential to be between 95 to 110 degrees. Even the beaches and coastal plains may see highs between 85 and 95 degrees. Temperatures are favored to trend down Friday into the weekend, though there is a fairly large range of outcomes as to how much. The most likely scenario would bring temperatures back down to near normal for mid-July by the weekend. However, the humidity is poised to stay through the extended period (and beyond), and this would still present Minor to Moderate HeatRisk across much of the region, especially away from the coasts. Ensembles suggest PWATs hover around 1.5 inches during the period and even into the next week, so muggy days are in the near and long term future here is Southwest California.
The periphery of the high pressure centered over the Plains will nose into the Southern California region starting Wednesday. This will alter the flow pattern to be more southwesterly compared to the southeasterly flow expected earlier in the week. This will likely cut off the moisture source for the region, thus chances of thunderstorms and showers will likely drop to around 10 percent across the region for Wednesday and Thursday. However, starting Friday, other influx of monsoonal moisture from the southeast will be possible, and thunderstorm chances will likely bump back up to around 20 percent. The thunderstorm forecast will be complex with generally low confidence for any particular location, as storm development will be highly dependent on mesoscale features and even potential outflows form storms well outside of our forecast area. The greatest chances will be during the afternoon and evening hours across the mountains of Los Angeles and Ventura Counties.
Moderate Sundowner winds for southwestern Santa Barbara County and northwest winds over the interior are expected Wednesday night and potentially Thursday night (although weaker Thursday). Otherwise winds will trend slightly more onshore Thursday into the weekend.
12/1742z.
At 1721Z at KLAX, the marine layer depth was was 1700 ft. The top of the inversion was at 3200 ft with a max temperature of 24 C.
High confidence in TAFs for KPMD, KWJF, KBUR, KVNY and KPRB.
Moderate confidence in the remaining TAFs. Mid and high clouds will likely continue to disrupt the marine layer cloud formation through tonight, and any cigs that do form are likely to be patchy and scatter and reform frequently later tonight into Mon morning.
KLAX, Moderate confidence in TAF. There is a 40% chance of no MVFR cigs forming late tonight into Mon morning, and if any do form they may bounce between BKN015 and BKN250. High confidence that any east wind component will be below 6kt.
KBUR, Generally high confidence in TAF. There is a 10%-20% chance of brief OVC008-015 cigs 12Z-16Z Mon.
12/826 AM.
For the outer waters, winds and seas are largely expected to be below Small Craft Advisory (SCA) levels through Tuesday, then there is a 50%-60% chance of SCA winds and seas developing Tuesday night through Thursday night.
For the nearshore waters along the Central Coast, mainly light winds are expected through Tuesday, but winds will begin to increase on Tuesday, likely reaching SCA levels Wednesday afternoon through Thursday night. Short period seas may also reach SCA levels.
For the inner waters south of Point Conception, conds are expected to remain below SCA levels through the forecast period, except for a 50% chance of SCA wind gusts over the western portions of the Santa Barbara Channel Wednesday and Wednesday night.
12/830 AM.
A south to southwest swell will continue to affect the coastal waters through this week. With this longer period swell, the potential for larger surf and a greater risk for rip currents will continue.
Evening high tides are expected to increase this week with Monday expected to be the highest tide of 7.1 to 7.7 ft above MLLW. There is the potential for some minor coastal flooding this week with the evening high tides and south swell. One thing that will need to be watched late this week into the weekend will be the potential of a greater south to southwest swell from potential tropical cyclone over the eastern Pacific.
Thunderstorms will be possible along the beaches today into Monday (10-20% chance). For more information, please refer to the Beach Hazard Statement.
Ca, Heat Advisory remains in effect until 10 AM PDT Tuesday for zones 38-88-343>345-353-368-372-373-375>380-548. (See LAXNPWLOX). Extreme Heat Watch in effect from Tuesday morning through Thursday evening for zones 38-88-341>345-347>358-362-366>376-378>383-548. (See LAXNPWLOX). Beach Hazards Statement in effect through Wednesday evening for zones 340-346-349-350-354-362-366. (See LAXCFWLOX). PZ, NONE.
