Thursday, August 4, 2016

Seafair Weekend Forecast: A Lazy Low Visits

Few forecasts are more important in Seattle than the one for Seafair weekend, and this weekend will be a mixed bag.  If you need to pick one day for outdoor activity, Saturday should be the better day.


The issue is that our nemesis, an upper level trough of low pressure, will be approaching and greatly influence our region Sunday and Monday.   Here are a series of 500 hPa (about 18,000 ft) upper level maps.

Friday morning (at 5 AM)....a low to our northwest.


Saturday morning, it slowly easing southeastward.

 Sunday AM, this lazy low is slowly heading south.

 Finally, on Monday morning, this sleepy low has barely moved past us.


What a contrast to last year, when ridging (high pressure) was the rule.

With a low hanging around, temperatures will be 5-10F below norming (upper 60s).

Precipitation?   Nothing on Saturday and early Sunday before early afternoon will be dry.  Thus, the Seafair festivities will generally be fine.  But later in the afternoon, the combination of upward motion from the trough and surface heating will result in the formation of convective showers (including thunderstorms), as suggested by the forecast precipitation for the 3-hours ending 5 PM on Sunday.   The greatest threats will be over the coastal zone and NE Washington.


As the upper low moves by, the showers will rotate northward into western WA.   The 48-h totals ending 5 AM Tuesday are impressive along the coast and over southern BC and Alberta, with some areas getting more than an inch (pink and black colors).


The big worry?  lightning.   The models are suggesting lots of instability on Sunday and Monday and thunderstorms can be expected.   So there may be a lot of smoldering fires ready to rev up with warmth and wind next week.


4 comments:

windlover said...

This question has nothing to do with Seafair weather, but rather our upcoming call/winter. I ran across an Accuweather article from June 16 that mentioned "The Blob" was still off our coast and would prevent our weather from getting cold like it can in a typical La Nina winter. (they are saying it will transition into a weak La Nina from late fall to winter, causing stormier than usual weather for the nortwestern U.S.) My question is....I thought The Blob was all but gone? I know Accuweather isn't the best to listen to for our area but I haven't heard you or local forecasters saying The Blob was still strong enough to impact us like it did the last couple of years....Thanks for your time Cliff! And for always being there for an accurate forecast!

ryamkajr said...

He answered this already

https://cliffmass.blogspot.com/2016/07/is-blob-really-dead.html

MRT said...

Windlover: Cliff did answer this question, and I believe the poster above me posted the link. As for the winter, frankly, if La Nina develops it is looking to be just on the line between La Nina and ENSO Neutral; around -.5 C. I don't see a repeat of the record Seattle rains this year, I really don't. It will probably be wet, but considering many places shattered records last year, partly attributed to the massive El Nino (it was playing a block that sent the jet stream up towards us all winter), I think a normal winter is in store, with normal precipitation amounts.

Organic Farmer said...

If I recall, the blob article never stated the blob still existed. It simply pointed out that the sub surface sea temperature in the same region is still above normal. The blob was all about surface temperature. Point being, if really persistent ridging sets up the blob could return and feed into that "perma ridge".

I would like to hear about similarities and differences between our weather patterns and those the same latitude on the South American Pacific coast.

I understand this winter is colder than normal for that region.