The Big Cooldown
LAST UPDATE (Oct 5, 2:00pm): The main low has moved off to the East, but a vort-lobe max is rotating around and aiming straight for LA from the North. Unusual setup, and usually the convection that is generated on the north slopes of the mountains doesn't make it over into the basin, but the HRRR is very bullish on at least a few showers spilling over the slopes and into the coastal areas. We can probably expect a chance of rain anytime between now and 8pm. Here's a satellite gif showing the current situation:
UPDATE (Oct 4, 3:30pm): Rain moved through early this morning bringing anywhere from 0.3-1.0" to the foothills/mountains and much less (0.01-0.25") to the coastal areas. Based on the satellite and the high-resolution modeling, I expect another band of rain to work through the area between 5 and 10pm this evening. Because of the colder air mass overhead, this band could produce some moderate showers. Here's a high-resolution model forecast of 15-hour precipitation totals ending at 4am Monday. Expect another ~0.1-0.25" area-wide?
UPDATE (Oct 3, 12:30pm): Things are still on track for a big cooldown and some rain for the Sunday-Monday time frame. To the right is an image of mid-level water vapor + vorticity showing the upper level low sliding down the coast, now centered right near the Oregon/California border.
Expect this storm to continue to slide south, and for light rain to begin in the Los Angeles area around midnight/1am tonight (Saturday .. well, actually Sunday morning).
ORIGINAL POST (Oct 2): In case you hadn't noticed, it's been hot. Really hot. September, in fact, was +5.6 degrees Fahrenheit above normal in Downtown Los Angeles. Burbank was even hotter - +7.0 degrees Fahrenheit above normal. In fact, in Burbank the average temperature (including day and night) for September was 80.1 degrees! That's incredible!
The Bad news first.
First, the bad news. We have one last burst of heat coming tomorrow (Saturday). Out ahead of a swirling upper-level low pressure, offshore flow will push the thermometer up past 90 degrees again. :(
But now, the good news.
The good news is that the cooldown that is coming for Sunday, Monday and Tuesday is going to feel epic. An upper level low is, at this moment swirling around near Seattle. The models have been in relatively good agreement since Wednesday that this low will continue to drop due south, dragging with it our first burst of Canadian air for the season. Here is a satellite image showing the low currently over British Columbia:
The "storm" is what we call here in SoCal an "inside slider". This means that the storm really gathers strength as it drops southward mostly over land. The "over land" part is important because here on the West Coast most of our storms derive their moisture from the vast Pacific. Inside sliders are mostly dry....
Sometimes they wiggle just a bit more westward than normal and that trajectory partially over water allows them to scoop up some moisture off the Pacific and bring some rain. Here's an image of 500mb vorticity for Sunday morning showing the swirling upper low just straddling the coast. The reds and purples indicate a lot of upper-level dynamics. The key with this system will be the available surface-level moisture. However, a very warm Pacific should definitely help wring out some rain.
So, what can we expect?
Right now it is looking like the low should enter the picture sometime very early Sunday morning. We should see some light showers off and on during the day Sunday with a potentially stronger round of showers (and maybe embedded thunderstorms?) on Monday as the main upper-level low passes directly overhead.
I think we can expect LA-proper (and OC and Ventura County) to pick up between 0.1 and 0.3" of rain. The foothills could see more. Perhaps most interestingly, however, is the potential for SNOWFALL above 7,000 feet! 700mb temperatures will be below zero, and this could bring some mountaintop snow as the very cold core of the low passes overhead. :) Here is a map of model forecast precipitation totals:
Temperatures will also be very cool.
The high temperature for Sunday and Monday is not forecast to get out of the mid 70s. IF there are clouds and showers around, you can expect that number to be in the low 70s, perhaps hanging out in the 60s all day. Overnight lows will dip down to near-60, but might not go lower because of the saturated atmosphere.