Sprinklers or Not?

A weak offshore flow will begin on Monday, lasting through at least Wednesday.  This will slowly dry us out and start a warming trend through Thanksgiving.  Cold overnight lows and warm days are on tap for the week.

A cold offshore flow will begin Sunday morning.  This will be brief and moderate at best.  With no upper-level support, it will be driven by the thermal gradient solely.  The winds could begin between 4am and 10am on Sunday within the mountains, foothills and passes.  If winds do not start until after 4am, overnight lows Saturday night/Sunday morning could reach into the low 40’s.  Temperatures will be warmer on Sunday and Monday.

Synopsis: A broad trough (with a southwest to northeast axis) over the eastern Pacific will continue to send a fetch of upper level moisture over us Wednesday.  Winds will increase slightly as an onshore push strengthens through Thursday.  However, due to the warm air advection and strong inversion, limited momentum will be mixed down to the surface, even Thursday afternoon.  A weak upper-level front could pass over our region Thursday afternoon.  Partly cloudy skies are on-tap for Thursday with cool and breezy conditions.  Significant precipitation is not expected.

Outlook: High clouds will continue through Wednesday evening.  The very dry lower levels should moisten beginning Wednesday night.  Low clouds are expected Wednesday night through Thursday morning.  Drizzle is possible but, not likely Thursday morning.  Winds will increase slightly Thursday afternoon.  A light shower is possible Thursday evening.  A hundredth of an inch?  One can wish.

Continued dry and mild….Drainage flows expected during the night and early morning hours with locally colder temps within valleys and low-lying areas.  For Halloween evening, clear skies with temperatures will coming down from the mid-80’s quickly:

5pm: 78°F

6pm: 74°F

7pm: 70°F

8pm: 66°F

Temperatures fall through 60°F around 11pm with overnight lows near 56°F.

This morning at Scott’s house, the mercury dipped to 42.1F. Closer to UCI a station showed 44.8F (The influence of the ocean keeps us a bit warmer in setups like this). These are our coldest temperatures of the season thus far! Tomorrow morning won’t be quite as cold, but we can still expect temperatures to dip to or just below 50F.

-Mike

A brief, but classic, Santa Ana event is upon us. Earlier today the tail end of a weakening front brought us some clouds, and even some light showers in San Diego County. Behind the front, strong mid-level winds are blowing from the northeast. Earlier this evening, I was just out in Lake Forest, and I could feel the winds start a-blowin’. Even down here in Irvine, my blinds are rattling a little from some light nighttime gusts. The dew point has already dropped significantly — to about 25F or so. The air temperatures remain relatively high – upper 60s – but are falling quickly. This is a very COLD offshore (Santa Ana) event, so expect a very chilly morning, and below normal temps tomorrow (high 60s to near 70). Mountain regions are heading for the teens (!) tonight.

The setup is fairly classic, as the trough digs very far southward and drags some substantial upper-level support with it. Offshore pressure gradients aren’t that impressive, and so winds, while they will approach advisory strength, will probably not reach hurricane strength.

However, as quickly as they came, the Santa Ana winds and the accompanying cold, dry desert air, will leave us Thursday. In their wake, we will get a gradually warming, dry airmass. Humidity levels should return to normal late Thursday.

-Mike

Stay moist!

Quick update: The rain moved into Orange County in earnest around 2am last night in the form of light to moderate steady showers. Rain rates in the foothills and mountains were a little higher (0.20″/hr) than here on the coastal plain (0.10″/hr). As you can see from the radar image, rain continues to stream onshore from Santa Barbara all the way to Oceanside.

The rain will gradually taper off late this morning into this afternoon, though some island-effect/orographic showers may linger into the evening. Full clearing tomorrow.

Rain totals as of 7am:

John Wayne Airport: 0.34″

Fullerton Airport: 1.01″

Santiago Peak (Saddleback): 1.06″

-Mike

UPDATE: Here is the radar showing the front just north of Pt. Conception, with pre-frontal precip. extending down to about LAX. The orographics in the Santa Monica, San Gabriel and San Bernardino Mtns are apparent as well:

Today’s showers were just a precursor to tonight/tomorrow’s main event. Pre-frontal precipitation has already spread as far south as Santa Monica, fueled mostly by orographics, and some mechanical advection provided by the coast. The southward extent of this precipitation will slowly push into Orange County, affecting the mountain areas first before spilling toward the coast, as the front to our north advances.

The big question about the impacts of this storm still remains: How far south will the front make it before it falls apart? Right now, it seems to be maintaining good forward speed, but is forecast to slow down into the overnight. How far south it makes it will determine whether we get a tenth of an inch or a full inch of rain. Splitting the difference, Scott and I are both in consensus that we should probably get somewhere around 1/4″ to 1/3″ of an inch (I’m the optimist at the 1/3).

Cloud bases have begun to lower through the afternoon. As you can see on the satellite image, the approaching front is pulling up a lot of subtropical moisture out ahead of it. If you look directly offshore from us you see the gathering moisture as it is pulled into the system. Despite a potential northward stalling front, the presence of this moisture, and a very wet atmosphere should be enough to wring out some rain overnight.

Look for rain to be on the increase into the overnight, with a possible tapering off late tomorrow morning…

-Mike

Plenty of moisture up through 700mb and a frontal system (and associated jet dynamics) to our north are allowing for some moderate warm-air advection over our area today. This should result in some off and on showers produced over our region. Light showers should be more persistent along the foothill of the Santa Ana, and especially, the San Bernardino Mountains as a result of the combination of warm-air advection and orographics. Down here on the coastal plain, we could pick up, at most, 0.10″ throughout the day today, with, as Scott suggested, a more vigorous frontal passage tomorrow.

NWS is calling for up to 0.50″ of rain here along the coast. Scott anticipates a little less, with about 0.25″. I’ll split the difference and say anywhere from 0.25″ – 0.50″; nevertheless, expect about 1/3″ when all is said and done.

Enjoy the rain!

Here’s the satellite image showing the vigorous storm to the north, and the warm air advection over the Southland:

-Mike

PS: Here is the corresponding radar image showing the showers in the area:

Synopsis:  A plume of moisture is moving across central California.  Uplift is associated with the jet dynamics and orography.  The central California coast will pickup some decent totals on Tuesday.  For us, there are no upper-level dynamics to speak of and none are expected with this system at our location. However, there is plenty of moisture.  Thus, we have seen a brief light shower or two…..but, not enough to soak the ground.  For Tuesday, I expect more clouds with a chance of a brief light shower (mainly against the foothills and mountains) with high temperatures near 70°F.  However, ground-soaking rain has more of a chance on Tuesday night/early Wednesday morning.  A surface front passage is projected for Tuesday night/Wednesday morning.  This front will provide the surface convergence we need to convert that large fetch of moisture (orange arrow on water vapor imagery below) into rain. Moisture availability will compensate (somewhat) the lack of upper-level dynamics so, the rain will still be likely.  Could we get a 1/4″?

With a lack of upper-level dynamics, the mountains will provide the uplift.  So, areas just to the west of  foothills and mountains and in the mountains themselves will see more rain.

Tuesday morning’s sounding (San Diego) shows a saturated level from 760m to 2200m!