Saturday, May 22, 2010

Cool and Convergence

I hate to say this...but this has been a classic May in some ways: cooler than normal in the beginning, a warm spell mid-month, and a cool down at the end. Take a look at Seattle's temperatures versus the normal highs and lows (below) to see what I mean.The last few days have been roughly 10F below the normal maximum (which is around 65F).

My tomatoes are fading.

The windstorm was of course not the usual May fare. Why so cool and showery? The reason is that a broad upper level trough or low, with associated cold air aloft, that has been parked over the western U.S. (image below). This same pattern is producing a heat wave over the eastern U.S.
And I don't have particularly good news for you..there is no real warmth in sight and another cold trough is headed our way!

Today's situation is made clear by the latest visible satellite photo (below). Sunny east of the Cascades (an excellent place to go during cool spring periods). The influx of cool, moist air has produced clouds and light showers on the western side of the Cascades and the crest, and westerly flow is producing a weak Puget Sound convergence zone, something evident in the latest radar image (below). Cool, unstable air is approaching from off the Pacific. So outside of the convergence zone Saturday will be partially sunny, cool day with highs in the upper 50s.
The convergence zone and showers should rev up later today and tonight (see forecast precipitation below), and more clouds and showers are expected Sunday.
Monday should be the best day as the upper trough moves out.

But THEN another cold upper level trough moves in mid-week (see upper level map for Wednesday)...and you know what that means....

8 comments:

Keith Moore said...

I feel as if my lawn will never get dry enough to mow. I'm starting to like the "wild prairie" look.

Donna & Chris said...

Tomatoes before June 1st--always a risk!

Tom said...

Have things changed over the last decade or so?
I remember back in the 80s when May came around, I'd be sitting in school suffering while the sun was out and it got to the upper 70s. Finally, the day was done and I could go outside and play around. (This was back when kids enjoyed outdoors).
It seems that we just don't have those kind of Mays anymore, and the crud just keeps going with a few breaks in June, then of course after July 5th, summer starts!

Kevin Purcell said...

Cliff: On KUOW and here you comment that the cool early May, warmer mid-May and then a cooler end of May is a common pattern in the PNW. Is that a real pattern ie. is there data that supports it in a statistically significant manner). Or does it just seem that way (with a bit of observer bias remembering those times when this pattern occurred)? I checked your book but don't see anything on this fine a pattern.

Another point is that these perceived qualities of warmth and coolness seem to correlated (in my experience, at least) more with the amount of solar radiation than the air temperature (measured in the shade a 2m above vegetation). The actually air temp may not be that much warmer but when the sun is "out" then one's "surface temp" goes up and it "feels warm or even hot" when sitting in the sun.

The same with Tom's comment: we remember all those hot days when we were in school in May. May is exam taking time in the UK and I can't remember a non-sunny one when I took exams in school or at university but they must have been there.

And on another note: the current NWS AFD has this:

SHORT TERM...RADAR AND OBS THIS MORNING SHOW AREAS OF LIGHT RAIN EAST OF PUGET SOUND FROM SKAGIT COUNTY SOUTHWARD (image). HOQUIAM OB HAS RAIN ON THE CENTRAL COAST AS WELL...BUT WE'LL HAVE TO WAIT UNTIL THE COASTAL RADAR GOES IN IN A COUPLE OR SO YEARS BEFORE WE SEE THAT. TICK TOCK...EVER SEEN AN IMPATIENT FORECASTER??

I love the AFDs :-)

Bham_Guy said...

Mid-May to mid-June was really nice around here last year.

bakuninhong said...

Beautiful afternoon here today in Bellingham! I guess we lucked out. This morning was miserable and cold, though.

JewelyaZ said...

I haven't planted my tomatoes yet... your posts are why! Thanks!

I also noticed the coastal radar comment in the official NWS chatter, and felt pleased about your work all over again. ;-)

scrubjay93 said...

I am ferrying my tomato starts in and out of the house. It's hard enough growing tomatoes up here without the weather messing things up! lol. Next year I will plan on June 1st.