One-Two(-Three-Four...) Punch for LA

Last night's storm was a bust. As the models started to indicate late, a surface low over the SoCal bight (the area of ocean between the shore and Catalina) induced some low-level drying (easterly flow) which kept all the rain confined to the upper layers of the atmosphere. Radar returns showed rain, but surface observations showed nothing which is very common with this setup.

But now that that is behind us, two powerful and dangerous storms are on our doorstep this week!

STORM 1 (Tuesday) – LA: 1.5"

A very powerful frontal system associated with a strong low off the Oregon coast will drive heavy rain and winds through the region beginning around daybreak on Tuesday. Strong southerly flow out ahead of the front combined with very moist lower levels sets up the perfect scenario for a 4-6 hour period of heavy rain and possible flash flooding. Those in foothill communities should especially be aware as 30 knot southerly winds at the lower levels will crash up against the mountains and wring out twice as much moisture on the foothills as we receive in the basin. Here's a map showing wind direction and speed at 10am on Tuesday. The patch of yellow indicates strong 30+ knot winds for a 4-6 hour period out ahead of frontal passage:

What can we expect?

I expect a 6 or so hour period of moderate to heavy rain beginning around 7am in Ventura, spreading to LA around 9am and eventually Orange County around 10am. During this time, rain rates may approach 1" / hour in some foothill communities. Those living on or near hillsides, please prepare. The front should exit the area around rush hour, leaving behind anywhere from 1-2" in the coastal basin communities and 2-5" in the foothills. Particularly vulnerable foothills will be those foothill communities stretching from Altadena to Glendora and  Rancho Cucamonga. The hills behind Malibu will also be particularly vulnerable.

STORM 2 (Wednesday-SATURDAY) – LA: 1.2"

Right on the heels of Tuesday's storm is another, very similar storm slated to arrive late-morning on Wednesday. The parent low that has been spinning around near Oregon will slowly move eastward bringing with it another potent cold front. Again, strong southwesterly winds combined with moist lower layers and a fairly dynamic upper atmosphere (the subtropical jet is screaming fast, right now!) will bring a big slug of moisture into the region beginning in Ventura around 10am Weds, spreading to OC by 1pm. Here's a map of simulated radar reflectivity at 1pm on Wednesday showing moderate rain throughout the region:

The difference between the second storm and the first is that the initial frontal passage of the second storm will not be followed by relative dryness as the first one will. Instead, a zonal flow of moisture off the Pacific is likely to keep on-and-off showers continuing throughout the day and night on Wednesday, followed by a second (and final?) frontal passage sometime Thursday afternoon. After this second frontal passage, the region may be subject to a third wave of rain, with showers and moderate rain continuing through Friday and Saturday. But, if the models are any indication, we may not actually dry out for several weeks.

What can we expect?

This second storm should bring anywhere from 1-2" in the urban areas and 2-4" in the foothill communities. West-facing slopes in Orange County and San Diego will be particularly vulnerable to flash flooding during this second storm.

Here's a map showing modeled rainfall totals through Saturday afternoon:

Mika ToscaComment