textproduct: San Diego

SYNOPSIS

Tropical Storm Rachel will approach the region Saturday evening into Sunday. Cooler weather will continue to spread inland this weekend into early next week as a stronger trough moves in from the north. These systems will bring showers and locally heavy rainfall, gusty winds, and coastal impacts to the region. Quieter and warmer weather will begin to move into the region for the latter half of next week.

..UPDATED AVIATION DISCUSSION FOR 12Z TAF ISSUANCE

Thin mid/upper level clouds from TS Rachel are passing overhead which will yield to bright overcast skies today, with a slight (10- 15%) chance for isolated thunderstorms Saturday afternoon and evening east of the mountain peaks into the deserts, ahead of the more widespread unsettled weather. High temperatures in the valleys and high desert will only reach into the 80s while temperatures at the coast remain in the upper 70s. In the low deserts, temperatures will likely only reach the mid-90s. The weather theme over the next few days will be of cloudy skies, rain and wind at times from TS Rachel and an upper-level trough from the north. Let's take a look on how it will affect our weather...

Rainfall...As mentioned above, there is a slight chance for thunderstorms over the desert slopes and deserts later today ahead of TS Rachel. With high resolution models now in play, confidence on timing has increased quite a bit, while confidence on amounts remains low. The three rainfall scenarios that seem to be common across models are highlighted below this section. From a timing standpoint, generally models agree on bringing the main swath of rainfall from the TS around 10 PM - midnight this evening into San Diego County, expanding quickly northward through the rest of the region overnight (1-4 AM). The entire region should expect periods of rain through the morning, scattering to showers near the coast in the late morning (8-11 AM). In the afternoon, Rachel's surface low will likely be far north or east of the region; shower activity will then become isolated through most of the region, with steady rainfall continuing in the foothills and coastal facing mountain slopes through Sunday evening. Peak rain rates in the mountains are broadly expected to range between 0.5-1"/hr. Here are the three rainfall scenarios for Sunday, broken down by forecasted track:

Scenario #1 (most likely ~ 60% chance): The system tracks around 50- 100 miles south of the US border. We see periods of moderate and locally heavy rainfall across the region during the day Sunday, yielding to increasingly scattered showers in the lowlands during the afternoon. Totals in this scenario range from 0.75-1.5" in San Diego County west of the mountains (around 2" in the foothills). North of SD County, there will be a rainfall gradient from west to east, ranging from ~0.3-0.5" in northwestern OC to ~0.75-2" in the IE. The deserts and desert slopes will be quite a wild-card depending on the flow but generally 0.5-1.5". For the mountains, around 2-3" with consistent rain expected all day, interspersed with locally higher rain rates. Areas of urban flooding and flash flooding in the mountains could occur in this scenario.

Scenario #2 (more rain ~ 20% chance): In this case, we see very high PWAT's over 2" with the storm core making a close approach to San Diego, which would give higher rain totals and potential for more impacts. This would entail 1.25-2.5" through the coastal basin (more east, less west), 3-4" in the mountains, 1-3" along desert slopes and 1-2" over the deserts. Urban flooding impacts could be more numerous in this instance, especially in the lower deserts. Flash flooding concerns would be particularly elevated for the desert slopes.

Scenario #3 (less rain ~ 20% chance): This is the case where the low level circulation makes landfall well south of the US/Mexico border and tracks northeastward. Rainfall amounts would be much lower: around 0.25-1" across inland areas and desert with amounts less than 0.25 for much of Orange County and northwestern SD County. Amounts in the foothills and mountains would be limited to 0.75-2" and impacts would be much more isolated.

During the day Monday, the trough axis moves through, bringing a much more traditional round of light rainfall of around 0.05-0.25" through the coastal basin, with higher amounts in the mountains (light snow above ~8,000 feet) and rain shadowing the deserts. Shower activity looks to persist until Tuesday morning before clearing out Tuesday afternoon.

Wind...As of 08z today, the ECMWF/EPS and GFS/GEFS are narrowing in on a landfall of TS Rachel right around Ensenada, Mexico while the Google FNV3 and other AI models show the storm making landfall much farther south. There are also three scenarios for wind on Sunday:

Scenario #1 (most likely ~ 50% chance): The system reaches land around 20-100 miles south of the US border, around Ensenada, Mexico. The low pressure would be close enough to induce gusty winds near the circulation, especially on the east side of the circulation. In this case, peak gusts move through the region in the early hours of Sunday around 25-45 mph through the coastal basin and 50-70 mph near the mountain ridges. Strongest winds would begin easterly on the SD mountain slopes very early on Sunday, then turn westerly by sunrise, affecting the ridges and desert slopes. These gusty winds would continue throughout the afternoon, weakening by the evening.

Scenario #2 (30% chance): The system makes landfall more than ~100 miles south of the US border. Winds would be much less significant (gusts < 30 mph) for most of the coastal basin. Winds would remain westerly everywhere and only pick up by Sunday afternoon with still very strong (50-70 mph) gusts in the mountains and desert slopes.

Scenario #3 (20% chance): Some ensemble members of the GEFS and EPS bring the tracks approach very close to the US/Mexico border, which would bring very strong winds to San Diego County, including urban areas, with gusts on the order of 40-60 mph (locally higher). The wind forecast for OC and the IE wouldn't change too much - still ranging around 30-45 mph.

Temperatures...Sunday and Monday will be dominated by cloudy skies, rain and an approaching trough from the north. As a result, temperatures will once again cool dramatically, down 10-15 degrees regionwide (widespread low 70s through the coastal basin, 50s to low 60s in the mountains). Monday, the upper trough axis moves through with 500-mb heights around 565 decameters, bringing high temperatures down to the mid-40s in the mountains and into the 60s across the rest of the region (except low deserts - low/mid 70s). As the trough moves east and as clouds clear out, Tuesday's temperatures rebound to around Sundays temperatures. The warming trend continues through the end of the week, bringing seasonable highs back by the end of the week (perhaps a return of widespread 80s/90s next weekend, including the coast). Note that low temperatures are expected to drop into the 20s for high elevation valleys early this week due to the cold air aloft. Elsewhere, lows will fall to the low-50s inland and low 60s at the coast with coolest morning temps Tue/Wed as clouds clear out.

AVIATION

101200Z...Coast...Patchy low clouds along the immediate San Diego County coast will remain patchy and intermittent through this morning. Highest chances for low cloud cigs are for southwest SD Co VCNTY KSAN. Bases will be 1000-1500 ft MSL. Low clouds likely to clear between 13-15Z. Low clouds with CIGs below 3000 ft MSL are unlikely to return in the next 24-36 hours.

Otherwise...BKN clouds AOA 20k ft with unrestricted VIS through tonight.

MARINE

No hazardous marine conditions are expected today. Gusty northerly winds develop early Sunday morning, turning westerly late morning into the afternoon. Winds 20 to 30 knots expected throughout the coastal waters Sunday. Large fresh swell from the northwest will combine with south swell associated with Tropical Storm Rachel and wind waves to create combined seas 6 to 10 feet Sunday. Choppy, steep, mixed seas and gusty winds will produce conditions hazardous to small craft on Sunday. Winds and seas subside for Monday, though gusts will locally approach 20 knots Monday afternoon.

BEACHES

Surf of 3-5 feet with local sets to 6 feet will continue on south and southwest facing beaches through today. Elevated surf and strong rip and longshore currents will create hazardous swimming conditions. This elevated surf will combine with high tides near 7 feet during the mornings and evenings to produce significant coastal flooding and beach erosion.

Fresh south swell associated with Tropical Storm Rachel will arrive late today into Sunday. This will combine with a larger northwest swell and strong winds to produce elevated to high surf Sunday for all area beaches, peaking at 5 to 8 feet with sets to 9 feet. This will pose a hazard to swimmers and lead to very strong rip currents. In addition, morning high tides will continue to be 7 to locally 7.5 feet, continuing the threat for significant coastal flooding through Sunday afternoon. A Coastal Flood Warning is in effect for these hazards through Sunday at 3 PM. Take the necessary actions to protect flood-prone property.

Both winds and swells subside for Monday-Tuesday, and surf will decrease to 3 to 6 feet, though strong rip currents will continue. Morning high tides will continue to be near 7 feet, meaning that minor coastal flooding will continue to be an issue through this period. The Coastal Flood Warning will be replaced with a Coastal Flood Advisory at 3 PM Sunday and continue through 2 PM Tuesday.

SKYWARN

Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions.

SGX WATCHES/WARNINGS/ADVISORIES

CA...Coastal Flood Warning until 3 PM PDT Sunday 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.

Flood Watch from this evening through Sunday evening for Apple and Lucerne Valleys-Coachella Valley-Riverside County Mountains-San Bernardino County Mountains-San Diego County Coastal Areas-San Diego County Deserts-San Diego County Mountains-San Diego County Valleys-San Gorgonio Pass near Banning.

Beach Hazards Statement from Sunday morning through Sunday evening for Orange County Coastal Areas-San Diego County Coastal Areas.

Coastal Flood Advisory from 3 PM Sunday to 2 PM PDT Tuesday for Orange County Coastal Areas-San Diego County Coastal Areas.

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.