textproduct: San Diego

SYNOPSIS

Marine layer low clouds over the coastal areas this morning will become patchier with less inland extent tonight into Monday morning. A warming trend begins today with unseasonably hot weather through the week. Temperatures peak Thursday and Friday when highs will be as much as 30 degrees above normal. Both daily and monthly high temperature records are likely to be broken this week.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...

Marine layer low clouds have spread into the western valleys this morning with mostly clear skies elsewhere. Low clouds will clear towards the coast by late morning as offshore flow develops. The HREF has a 60% chance of low clouds and fog returning to the coastal areas tonight into Monday morning, with lesser inland extent as the marine layer becomes more shallow. Surface pressure gradients remain weakly onshore to the local deserts as of 2 AM, but are forecast to transition offshore later this morning as surface high pressure builds over the Great Basin. This will produce locally breezy winds below the passes and on the coastal mountain slopes with gusts to around 25-35 mph. Northeast to east winds continue into Monday, strengthening slightly Monday morning when peak gusts will be closer to 40 mph.

A large upper level ridge of high pressure amplifies off the West Coast and moves slightly eastward today and Monday. This building ridge combined with the offshore flow will begin the warming trend, especially for areas west of the mountains. Today, highs will be as much as 15-20 degrees above normal in the valleys (80-95 degrees), and around 10-15 degrees above normal elsewhere. For Monday, highs will increase to around 15-25 degrees above normal, with the greatest departure west of the mountains where highs will be in the low 80s along the coast, and mid 80s to mid 90s across the inland coastal areas and valleys. High temperatures in the low desert will approach 100 degrees. A Heat Advisory begins for the coastal areas and valleys Monday morning.

The upper level high slowly shifts eastward through the week, becoming centered over CA Tue and Wed as it continues to strengthen. Ensembles are in good agreement through this period, with high confidence of the warming trend continuing and high temperatures upwards of 20-25 degrees above normal. The upper high then shifts further east towards the lower Colorado River Valley Thursday and Friday, peaking in strength. There are still some discrepancies in how strong the high will get. On the extreme end of the forecast (NBM 90th percentile), high temperatures could exceed 105 degrees in the Inland Empire and 110 in the Coachella Valley on Thursday, and close to 110 in the Inland Empire and 115 in the Coachella Valley on Friday. Current forecast is around the NBM 50th percentile, which exceeds the EC MOS 90th percentile by a few degrees. Given the likelihood of temperatures around 30 degrees above normal and moderate to high HeatRisk, an Extreme Heat Watch was issued for the mountains below 5500 ft and the low deserts for Thursday and Friday. The ridge finally begins to break down Saturday and Sunday, allowing for gradual cooling.

AVIATION

151200Z...Coasts/Western Valleys...Largely uniform low clouds (based 800-1100ft MSL) have filled in through 15 miles in across coastal San Diego County, but remain somewhat patchy across Orange County. The cloud bases may sink a few hundred feet after 12-15z as the coastal eddy weakens, which could bring VIS reductions down to 4-6 SM, largely across coastal mesas and inland valleys. CIGs expected to scatter for the coastal TAF sites by 17-18z this morning, with clouds retreating offshore for the afternoon. Patchier low clouds closer to 400-800ft MSL look to spread back ashore after 06z Monday, spreading up to 10-15 miles inland.

Patchier low clouds 400-800 ft MSL develop and move inland after 06Z Mon.

Inland Valleys/Mountains/Deserts... Breezy offshore winds develop this afternoon after 18z on the coastal mountain slopes, through mountain passes, and locally stretching into eastern valleys. North to northeast wind gusts of 20-25 kts expected, locally up to 35 kts in mountain passes. Gusts weaken after 03z Monday.

MARINE

No hazardous marine conditions are expected through Thursday.

BEACHES

A long period (17-19 second) south-southwesterly swell (190-200 degrees) builds to 2-4 feet Monday afternoon into Tuesday. Breaking waves of 4-6 feet are expected, with sets up to 7 feet possible on south-facing beaches. This swell is expected to persist into Wednesday, waning Wednesday night.

SKYWARN

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

SGX WATCHES/WARNINGS/ADVISORIES

CA...Heat Advisory from 10 AM Monday to 8 PM PDT Friday for Orange County Coastal Areas-Orange County Inland Areas-San Bernardino and Riverside County Valleys-The Inland Empire- San Diego County Coastal Areas-San Diego County Valleys- Santa Ana Mountains and Foothills.

Extreme Heat Watch from Thursday morning through Friday evening for Coachella Valley-Riverside County Mountains-San Bernardino County Mountains-San Diego County Deserts-San Diego County Mountains-San Gorgonio Pass near Banning.

PZ...None.