Cooling will spread inland into the weekend with high temperatures lowering to around average on Saturday. High temperatures for the coast and valleys will be mostly in the 70s for Sunday through Tuesday. One low pressure system from the southwest on Sunday and a second from the north on Monday and Tuesday combined with tropical moisture will bring showers for late Saturday night through Sunday night with continuing chances for showers through Tuesday night. There will also be stronger southwest to west winds on Sunday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
, UPDATED AVIATION DISCUSSION,
As of 12:30 AM local time, a few low clouds were covering parts of coastal San Diego County. Perhaps some very localized fog exists where clouds and terrain are intersecting, reducing visibility's below 1 mile. Nearly 800 nautical miles to the southwest, Tropical Storm Rachel has begun its turn northeastward moving at less than 10 miles per hour. Its path will have a significant influence on the upcoming weather as its remnants and surface low pressure approach the west coast of North America.
Temperatures, Today marks the last day of significant heat (especially in the low deserts) before a significant pattern change to the region, regardless of the tropical activity. What is most certain in this forecast is that temperatures are bound to cool off significantly into early next week. Highs today fall a few degrees from yesterday - 80s coastal / mid-90s inland / 100s low desert - as a closed upper low and broad west coast trough dip into California. Broad cloud coverage from Rachel's remnants during the day Saturday drop highs around 10 degrees from Friday. The trough continues to deepen Sunday, dropping highs another 5-10 degrees and then finally the trough axis moves over SoCal on Monday, bringing the coolest temperatures some areas have seen since last April. The current forecast has high temperatures Monday in the mid/upper 60s across the coastal basin and high deserts with highs in the 40s in the mountains and 70s in the low deserts. If the closed low positions itself over Socal instead of the Great Basin as the GFS is showing, we could see 700mb temperatures around -5 C and highs could be even a few degrees cooler than forecast. Minimum temperatures are also expected to drop a few degrees each night inland, from around 70 this morning to around 50 for early Tuesday (and to the upper 50s at the coast).
Rainfall, Tropical Storm Rachel approaches the coast this weekend, bringing a chance for an initial soaking rain Sunday. The broader trough very slowly moves through Monday with its own moisture and mid-level vorticity from the trough axis could bring another round of showers Monday into Tuesday. The first wave comes from TS Rachel, which is timing periodic rainfall between 1 AM-5 PM Sunday. While there isn't much background instability (almost no CAPE), PWAT's will generally range from 1.3-1.8 inches (>2 inches for high end scenario) and the surface low could generate periods of persistent moderate rainfall with rain rates over 0.5 in/hr in the mountains. Rainfall amounts from TS Rachel will largely depend on its track, especially for some of the mountain areas. If the surface low from TS Rachel landfalls farther south (>100 miles south of SD) and it continues to track east, the amount of rainfall especially in the coastal basin and desert slopes would be very limited as there is initially not much instability to generate rain. Based on NBM probabilities, this scenario represents around 25% of solutions, yielding to rainfall less than 0.25" for the entire coastal basin and less than 0.10" for areas farther west like Orange County. The most likely scenario, accounting for around 50% of solutions, brings the coastal basin/deserts a soaking rain between 0.25-1" during this first wave with 1-2 inches in the mountains. Assuming the low tracks close enough to the mountains, easterly flow over the desert slopes could yield surprisingly high rates (>0.5/hr) and totals (1-2 inches). The higher end case (~25% chance) brings 1- 2" over the coastal basin/deserts and 2-4" over the mountains, which could result in more numerous impacts across the region. The remnants of TS Rachel should move out of the region quickly Sunday evening, yielding breaks in rainfall. The following waves of showers Monday and Tuesday will be driven by the upper level dynamics of the trough and will feature a much drier airmass. Totals Monday/Tuesday should look more similar to a light wet season system: 0.10-0.25" for the coastal basin, higher amounts in the mountains and less than 0.10" for desert communities. Since the trough will drive in much colder air, light snow accumulations are possible with this second round above 8,000 feet.
Winds, There is similar uncertainty with the winds. A track farther southeast would yield to much lighter winds gusting below 20 mph through the coastal basin while the most likely scenario (~50% chance) that the surface low makes a close approach yielding to wind gusts 20-40 mph through the coastal basin, with gusts 50-70 mph in the mountains and passes during the day Sunday. Winds remain onshore Monday but much less strong than Sunday.
Once the trough passes through the region, temperatures rebound through the end of next week as the active weather clears.
091820z, Coast, Patchy low clouds returning to the coast after 03Z Saturday with bases closer to 1000-1500 ft MSL, best chance for cigs in San Diego County. Lower confidence on clearing times for low clouds, most likely sometime between 16-18Z Saturday.
Otherwise, Mostly clear with unrestricted vis through this evening. BKN high clouds AOA 20k ft moving into the region tonight into Saturday.
No hazardous marine conditions are expected through Saturday. Gusty northwest to west winds 20-30 kts developing Sunday then weakening on Monday. Choppy seas with swells up to 8 feet and 8 second period will produce conditions hazardous to small craft on Sunday.
Surf of 3-5 feet with sets to 6 feet will continue on south and southwest facing beaches through this evening. Elevated surf and strong rip and longshore currents will create hazardous swimming conditions. This elevated surf will combine with high tides during the mornings and evenings to produce local coastal flooding and beach erosion.
Another south and stronger west swell arriving Sunday along with wind waves and continued morning high tides near 7 feet will maintain a threat of coastal flooding and beach erosion into Tuesday. Surf on Sunday is currently expected to be 6 to 9 feet, decreasing to 3-6 feet for Monday and Tuesday. There is still a lot of uncertainty in exact surf heights. The Beach Hazards Statement and Coastal Flood Advisory contain more details.
Ca, Coastal Flood Advisory until 2 PM PDT Tuesday for Orange County Coastal Areas-San Diego County Coastal Areas.
Tropical Storm Watch for San Diego County Coastal Areas-San Diego County Deserts-San Diego County Mountains-San Diego County Valleys.
Extreme Heat Warning until 8 PM PDT this evening for Coachella Valley-San Diego County Deserts-San Gorgonio Pass near Banning.
PZ, Tropical Storm Watch for Coastal Waters from San Mateo Point to the Mexican Border and out to 10 nm-Waters from San Mateo Point to the Mexican Border Extending 10 to 60 nm out including San Clemente Island.
