Tuesday, March 17, 2009

Cold Wave



Over the weekend and Monday I was busy giving public talks...but I am back again. Nice to meet several of you.

Many have complained about the cold temps...and in general we have seen substantially below normal temps during the past two weeks in western Washington. Above you see the temps for Seattle, Yakima, and Spokane...with the normal highs and lows shown. Except for one day, the Seattle max temps have all been below normal--most well below normal. Eastern Washington was cold early in the period, but the last few days have been near normal. On the 12th Spokane's temp fell to near zero, and Yakima had several days in the teens. It does look that a moderating period is ahead though.

On the road, several people asked me about the interpretation of the weather radar images...so let me try that a bit (see image). What is shown is reflectivity...a measure of how much the target (precipitation) scatters the radar signal (which is in the microwave part of the spectrum). Grey is drizzle (5-10), reds are light rain (15-25), green is moderate rain (30-35), and above that is either absolutely pouring...or wet hail. The concentric range circles are 100 and 200 km from the radar. If you look at the upper left corner you see some additional information. This radar picture is for an elevation angle of .5 degrees, which means the radar is canted slightly up from the horizontal. Thus, the beam is sweeping out a conical surface as it rotates...or in other words...the beam gets higher and higher the farther out it gets from the radar. How how? The second row of numbers tell you. The radar is at 196 meters, and the beam gets to 1068 and 1941 meters by the first and second range circles. That beam is way overhead by the time it gets to the south Sound for example. So shallow drizzle can be missed for such locations.
Now why don't we see the mountains on the radar? Special software remembers where the mountains out and removes that part. In the right corner you see max=44 ..that means the maximum reflectivity anywhere in the domain was 44.

Rain shows up better than snow on weather radar. And big raindrops show up better than small ones. Anyway, enough radar 101.

On Thursday night at 10 PM KIRO TV is repeating their hour-long special on the Dec 3-4, 2007 windstorm/rainstorm, if you are interested.

14 comments:

32.5 East said...

Cliff:
A piece on NPR yesterday on the earmark funds for the radar quoted you as saying:

"The Pacific Northwest has the stormiest coastline of the continental U.S."


I was wondering what that is based on and does it refer to frequency of storms. If the latter how is "storm" defined.
Thanks

mainstreeter said...

The Seattle P-I has archived some great photos from years past on their website,

Here is a photo of a funnel cloud near Seward Park

Cliff Mass Weather Blog said...

32.5 east,
This is based on the work of one of my student's who looked at the temporal variance of sea level pressure....cliff mass

32.5 East said...

Cliff:
Thanks for the clarification of the "stormiest coastline" statement. Can you provide a citation for that? And does the qualifier of "continental U.S." imply that there is a stormier coastline in Hawaii?
Thanks

Cliff Mass Weather Blog said...

32.5 east...not hawaii...but I bet Alaska coastal zone would be way stormier!...cm

32.5 East said...

Cliff,
Then I think you mean the "contiguous" United States (rather than "continental") if you are excluding Alaska.

Cliff Mass Weather Blog said...

32.5
yep...contiguous is better. You seem mighty interested in this issue. Whats up? ..cm

Kevin said...

Cliff,

A ways back, you noted our relatively dry Jan/Feb against averages with a graph...can we see that graph again, updated with the recent precip? I'd like to know how close we are to catching up with normal precip after falling way behind.

Kevin said...

Cliff,

A ways back, you noted our relatively dry Jan/Feb against averages with a graph...can we see that graph again, updated with the recent precip? I'd like to know how close we are to catching up with normal precip after falling way behind.

mainstreeter said...

That graph from last month is
HERE

32.5 East said...

Cliff:
I knew that the part about our coastline being the stormiest in the continental U.S. was wrong as soon as I heard it and made a bet with someone that NPR was wrong on that. I just wanted confirmation.
But it did start me thinking about how one would define "stormiest". It would seem that one could use storm intensity, frequency or duration or a combination of the three. But as someone interested in coastal weather I would like to know the citation for the paper that analyzes the lower 48 coastlines.
Thanks.

CC said...

I again have some questions about the UW probabilistic forecast. It consistently way under predicts precip. For example, for the four days starting on friday (13th) night to tuesday night the expected precip was .41" and the 90% confidence upper limit was 3.55". We actually received 4.79" (we are in Chelan Co, just east of Stevens Pass). During the last stormy period in early March it similarly way under predicted precip. If precip were a normal distribution (I realize it's not) we are talking several standard deviations off here. That is, of course highly improbable.
I'll admit NWAC also underpredicted precip over these 4 days, but they were at least in the ballpark (3"). Is the model not geared for mountain forecasts?
BTW, what are time periods for "day" and "night" in the model?

Cliff Mass Weather Blog said...

32.5...yes, contiguous is a better term. The information is not published...so calculated by a student for me. ...cm

Kevin said...

Cliff or Mainstreeter...that's the one, but do we have an updated version that includes the last 6-7 weeks?