Monday, April 8, 2013

Strong Winds Buffet California

It is well known that Californians are weather wimps compared to tough, hearty Northwesterners.  So when I started getting calls today of powerful, damaging winds hitting the Bay Area and southern California, I was ready to dismiss it as the baseless complaints of hot-tub lovers in lotus land.    No strong oceanic cyclone was in sight!  But when I looked into the matter more carefully I was surprised by what I found and the somewhat unusual nature of the winds.

Let's start by looking at the maximum winds over the past 24-h around the Bay Area (see graphic).   Lots of places experienced forty to fifty mph, with some sites far higher.   San Francisco Airport hit 60 mph, tens of thousands of homes lost power, some major roads (Highway 84) were closed, and trees/power poles were downed all over the area.  These are serious winds.
  

What about southern California?  The 24-h max gusts there (see graphic below) is even more impressive.  Loads of winds of 30-50 mph, with huge winds (75-100 mph) in the mountains.  Impressive.


Most of these winds were from the north and northwest, a somewhat unusual direction for powerful winds in that region.  No Santa Annas or Pacific cyclones.

Lets look at a series of upper air maps from the UW WRF model system starting yesterday at 5 PM to figure out what happened. These are for 500 hPa (about 18000 ft), with the lines representing the height of that pressure surface.  You can think of them as pressure.  The winds are parallel to the lines and the closer they are to together, the stronger the winds.

At 5 PM Sunday, there was a ridge over the eastern Pacific and a low/trough over the Northwest (yes...it passed over us first!)
Twelve hours later the trough made it to California and a huge height (pressure) gradient developed over coastal California.  Such gradients are associated with very strong winds at this level and below. 

 By 5 PM today, the trough had moved towards Arizona, bringing strong winds to southern CA and Arizona.

 The upper trough was associated with a low pressure area at the surface and a strong pressure gradient was observed during the day over California--here is the surface chart at 11 AM to illustrate this.  Strong pressure gradients bring strong winds. There were reports of some serious dust storms over Arizona later today.

Well, gorgeous weather is in store for the Golden State after this storm passes, and our weather looks decent for a few days.  But the latest model runs suggest that NW weather will turn cool and wet over the weekend, with the potential for very low snow levels by Sunday and Monday.  Perhaps the last hope for TV news snow hypesters!



Residents of California and the Pacific Northwest have different attitudes for dealing with weather.

3 comments:

Buzz's Marine Life Puget Sound said...

255 mph winds at Huntington Beach and nothing in the news about it being blown to bits?

JewelyaZ said...

Good thing there was no wildfire burning anywhere nearby!

I would assume that the 255 mph reading is from a broken sensor (or even a decimal point human error) as nothing nearby went over 25 mph.

Amazing winds, to be sure. Glad to see that it appears noone was hurt.

Walter Harley said...

Cliff, you say "with the lines representing the height of that pressure surface. You can think of them as pressure. The winds are parallel to the lines..."

This continues to perplex me. Why aren't the winds *perpendicular* to the isobar lines? Shouldn't air move from higher pressure to lower, just as a marble would roll downhill on a topo map? I would think that parallel to the pressure lines, pressure would be equal and there would be no reason for air to move in that direction.

Is this just a typo, or is there something interesting going on?