SLO County weather forecast: Mild temperatures follow rainstorm

An early season storm last Sunday and Monday (Sept. 18-19) produced 2.8 inches of rain at Vandenberg Space Force Base but just a few hundredths of an inch of precious precipitation in Santa Ynez. The San Luis Obispo County Airport saw 0.86 of an inch, while the Paso Robles recorded 0.31 of an inch. Weather stations in the Arroyo Grande area recorded nearly two inches of rain.

For much of last week, overnight Santa Lucia (northeasterly) winds have kept the marine layer out to sea, leaving behind clear skies, but a change in the weather pattern will develop this week. Persistent northwesterly winds will allow the marine layer with fog and mist to develop in the coastal regions.

Sunday’s gentle to moderate (8 to 18 mph) northwesterly winds will increase from fresh to strong (19 to 31 mph) levels on Monday through Tuesday during the afternoon. These winds coming from the sea will allow the marine layer to form along the coastline and move inland during the night and morning with areas of fog and mist.

Sunday’s high temperatures will range between the mid-60s along the beaches. The coastal valleys (San Luis Obispo) will reach the low to mid-80s, while the mid to high-90s are expected in the inland valleys (Paso Robles).

The northwesterly winds will increase from strong to gale-force (25 to 38 mph) levels on Wednesday into Thursday and should be strong enough to produce clearing along the beaches during the late morning and afternoon from the overnight fog and mist. High temperatures away from the coastline will range from the high-70s in the coastal valleys and low-90s in the inland valleys as cooler marine air filters into the inland areas.

The NOAA’s GFS (Global Forecast System) model is advertising another early season storm developing off the California coast on Oct. 4 that could produce increasing southerly winds and rain.

Surf report

Sunday’s 3- to 4-foot northwesterly (290-degree, deep-water) swell (with an 8- to 11-second period) will remain at this level into Monday morning.

Fresh to strong (19 to 31 mph) northwesterly winds will generate a 3- to 5-foot (310-degree, deep-water) sea and swell (with a 5- to 8-second period) on Monday afternoon through Tuesday, increasing to 5- to 7-feet with the same period on Wednesday into Friday.

A 1- to 2-foot Southern Hemisphere (210-degree, deep-water) swell (with a 15- to 17-second period) is expected along our coastline on Sunday through Tuesday, decreasing Wednesday.

Seawater temperatures will range between 58 and 62 degrees through Monday, decreasing on Tuesday through Friday to 54 and 56 degrees.

This Date in Weather History (September 25):

1939 - A west coast hurricane moved onshore south of Los Angeles bringing unprecedented rains along the southern coast of California. Nearly five and a half inches of rain drenched Los Angeles during a 24-hour period. The hurricane caused two million dollars in damage, mostly to structures along the coast and to crops and claimed 45 lives at sea. “El Cordonazo” produced 5.66 inches of rain at Los Angeles and 11.6 inches of rain at Mount Wilson, both records for the month of September. (David Ludlum)

2010 - North to northeasterly (offshore) winds at the surface combined with a strengthening upper atmosphere high pressure ridge produced warm to hot temperatures throughout San Luis Obispo County. Cal Poly reached 104 degrees today which was one degree short from their previous record high of 105 degrees for the day set back in 1978. The Airport hit 105 which was a record. Diablo Canyon hit 89 degrees.

This week’s temperatures

LOWS AND HIGHS, PASO ROBLES

LOWS AND HIGHS, SAN LUIS OBISPO AND COASTAL VALLEYS

John Lindsey is a longtime meteorologist who lives in Los Osos. Email him at JohnLindseyLosOsos@gmail.com.