Sprinklers or Not?

UPDATE (4pm): Finally. The heavy stuff has arrived. A period of heavy rain is expected for the next hour or two.

UPDATE (noon): Lightning being reported in LA (see: ocwx). Waterspouts being discussed as a possbility over the bight, so we cannot rule out some tornado watches/warnings this afternoon.

UPDATE (1130am): The main band of showers is moving into Los Angeles County around 11am. Embedded within this band are some strong thunderstorms. All indications are that this front will strengthen and amplify as it dips southward – and, already looking impressive- this means we could be in for some heavy showers this afternoon (as expected)…

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Models aren’t handling this one well. Radar and satellite imagery indicate two things that the models missed:

1) The storm is moving a little faster than anticipated. Showers could spread into OC well before 5pm (by several hours).

2) The low is staying offshore, picking up moisture. Thus, we can expect some pretty decent rainfall, periods of moderate to heavy extending into the overnight.

Here’s a satellite image (click to make larger) showing the low swirling off the coast, with the main front now affecting the central coast. Places like Santa Maria have already received 1/4″ of rain…

-Mike

Brief addition to Scott’s detailed forecast below: Intermittent showers will dot Orange County throughout the day out ahead of the main frontal passage this evening. Bring your raincoat!

Update: Some pre-frontal precipitation was occurring around Orange County Tuesday morning (5-5:30am). The atmosphere has already cooled: From Monday night to Tuesday morning at 4am, -20°C level dropped from 17,400 to 16,100 feet, -30°C level fell from 21,200 to 20,800 feet. Freezing level fell 1,000 feet from 6,900 to 5,900 feet.  Currently, 500mb at -26°C and is expected to fall to -28°C or lower Tuesday night (-30°C or lower just to our west).  See earlier post regarding the low snow levels associated with this system.

For the rest of the morning, partly cloudy skies are on tap. The bulk of the rain moves in during the afternoon and evening. The bulk of the continuous rain should occur between 6-8pm. As of now, it appears as though the cold pool will pass just to our west. However, thickness values could still be below 538dm with -28°C 500mb temps between 10pm Tuesday and 7am Wednesday. So, expect convective, intermittent showers after 8pm. An offshore or drainage flow quickly begins after 8am Wednesday, warming and drying out the lower layers.

Forecast: Temperatures will rise into the 50’s Tuesday with breezy conditions and partly cloudy skies. Increasing clouds during the afternoon with rain likely. Showers, heavy at times could occur Tuesday night/Wednesday morning with some thunder. Small hail is possible early Wednesday morning.
Wednesday: Rain ending during the early morning hours. Rapid drying during the late morning with partly cloudy skies (clouds drifting in from the east).

This could be our last storm for a while so, enjoy it while you can.

As Scott mentions below,the storm that’s headed our way for tomorrow is going to be very cold. 850mb heights will be around 1400m ( Tuesday night, with minimum temperates near -4C. This is indicative of a very cold air mass, with the potential for some very low elevation snow. Scott has just run a Bufkit analysis on WRF-NMM model output, which indicates freezing levels around 3000 feet early Wednesday morning. Combining all of this information, we anticipate snow falling as low as 2500 feet. Total precipitation amounts will be determined by the eventual track of the upper low. However, all indications are this could be a significant snow producer. Recent runs suggest as much as 1-2″ of precip falling in the mountains…using a back of the envelope calculation (10:1 snow-precip ratio) this could mean as much as 6-10″ of snow as low as 3000 feet! This means that the two major freeway passes will be a complete mess: Grapevine (Tejon) and Cajon Pass. Cities east of Riverside above 2500′ include: Loma Linda, Redlands, Yucaipa, etc. and some of the inland valleys in SD county… In Orange County, snow may fall just to the east of Foothill Ranch, Lake Forest!

Here’s a rough map I put together showing areas where snow may fall:

Synopsis: A small, upper level low sandwiched in between two stronger systems will move down the California coast Monday. Its small size and the likelihood that its track will straddle ocean and land introduces uncertainties into the forecast for Tuesday/Wednesday. At this time, models place the very cold low (500mb temps of -32C) over southern California Wednesday morning. Rain could enter our region on Tuesday afternoon. Rainfall totals could range from 0.25 to 0.60″. Thundershowers are a possibility Wednesday morning as well.

Those are two words you’re going to start hearing more often. Official rainfall since July 1, 2009 at Downtown Los Angeles is running 160% above normal (12.76″ to date), and here in Irvine, John Wayne is running 155% above normal (11.02″). Mountain areas are doing even better, some recording over 30″ of rain so far this water year.

This past weekend’s storm brought 2.85″ to Los Angeles, and 1.96″ to Irvine. The heavy onslaught was enough to shake loose the saturated barren slopes near La Canada Flintridge. A mudslide triggered yesterday has damaged 41 homes – 9 of them uninhabitable! Latest model simulations indicate even MORE rain headed our way for Tuesday and Wednesday.

But are we really emerging from the drought? Well, for all intents and purposes, yes….for now. In the long term, the past decade was one of the driest ever for Southern California. To fill some of the massive reservoirs on the Colorado River it will take much more water than one above-normal season. Many of the western forests have been ravaged by bark beetle infestation (due to drought), and this damage can’t be undone with one above-normal rainy season. In the short-term, however, the Climate Prediction Center is indicating that, in many parts of Southern California, the drought is “over.” Soil moisture has rebounded, reservoirs are filling up, and streams are running once again. And in places where the drought is still ongoing, the CPC is forecasting some major drought relief – especially in the Sierras when, during the spring, the massive snowpack begins to melt.

This first image is the drought monitor/outlook from January 21:

And this one is from February 4:

The big storms during January, and this weekend’s storm have contributed to a drastic reduction in drought coverage and severity. Good news!

All of this drought improvement is dependent on the rain continuing through the spring. If this week is any indication – that’s looking likely. A new storm is forecast to impact the area beginning Wednesday, bringing as much as 1/2″ or so.

-Mike

We’ve received almost 2″ here in Irvine since yesterday morning, and ~3″ in Los Angeles. Rain rates were as high as 1/2″ per hour at some points during the night, catching a lot of forecasters by surprise. I did mention that a scenario like this was possible, if not probable. Stalled fronts with so much moisture rarely disappoint, and the past 18 hours are no exception. Here’s an image from last night showering the very heavy rain completely filling the region:

The stalled front has finally decided to make its curtain call and its scurrying off to the south. Scott and I both mentioned a brief break in the action this morning. A second, very energetic front is heading our way from the north. Lightning has been detected off the Central CA coast. Rain rates in Paso Robles are approaching 1/2″ per hour. All that said, expect another period of heavy rain and wind this afternoon, before the system finally clears. Intermittent showers will occur overnight.

-Mike

As expected, a break will happen Saturday morning. Rain rates will decrease and we may see skies clear partially before another energetic but, fast moving system moves through this afternoon (sometime after 10am). Clearing from this system is anticipated after 3pm. However, cold upper level temperatures will move in behind this system late afternoon and allow for the opportunity for a thundershower or two into the evening.

Scott

Turns out my initial inclinations from yesterday were borne out. The front has remained stalled over us all night, and the back edge was re-energized by the upper level jet. The wave that I saw on satellite finally caught up with us, and the result was an extended period of moderate to heavy rain through the overnight. Already rain totals in most areas have topped 1″ and are heading toward 2″… We anticipated a break in the action between the first front (the one that has been stalled for 15+ hours) and the second front- slated to arrive later today. The break is still expected, though will likely be much shorter than anticipated. Thunderstorms are possible today as the second front aligns itself with moderately impressive upper-level dynamics.

Rainfall totals today will vary due to the more convective nature of the precip. As the first front FINALLY pulls away, we will get a brief break before another round of moderate to heavy rain this afternoon.

-Mike

UPDATE (930pm): A period of moderate to heavy showers will occur for the next hour or so:

This front isn’t going anywhere fast. It’s been virtually stalled over us all day, bringing mostly light rain showers (with embedded moderate to heavy bursts). So far, rain totals have been “disappointing”, but we may be getting ahead of ourselves. There’s still a lot of moisture out there, and the front isn’t going anywhere. In fact, it will remain stalled over us virtually all night, until a second front approaches us tomorrow afternoon. Downstream, satellite imagery indicates another wave of moisture headed our way. I’m still banking on storm totals exceeding 1″…

Here’s a satellite image showing the train of moisture streaming in…

-Mike