Friday, September 20, 2013

First Day of Fall, First Big Storm

 Update:  Sunday morning.  Front is moving in faster than models predicted....about 3-4 hr faster.  Rain will be in Puget Sound by 7-8 AM.  Mountains an hour later.

Fall officially begins at 1:44 PM PDT on Sunday, September 22, 2013

Our first major storm of the fall will hit almost exactly at the same time.

After a weak front went through tonight (Friday), we will have a break (but with some showers and clouds) on Saturday.  But Sunday will truly feel like fall as a strong front, with lots of rain and some wind, moves through the region.  According to the UW WRF model, a strong front will be right off the coast at 11 AM (18 UTC) Sunday morning, with strong southerlies along the coast and a BIG wind shift to westerlies behind it (see map). A fairly strong low pressure center is moving into the Gulf of Alaska with this front, producing a decent pressure gradient along the coast and  in the Strait of Georgia.
By 2 PM, moderate rain will spread to the Cascade crest (see WRF forecast for the 3 hr ending 2 PM).  So if you want to get in a late season hike, head east of the Cascade crest, go early, and be done by 3-4 PM.    Notice our old friend, the Olympic rain shadow, is back.  Port Angeles to Victoria could be dry at this time.

Impressively, this front has enough oomph to bring rain east of the Cascade crest.  Here is the 3-h rain totals ending 8 PM Sunday.  You see what I mean!  You can see some weakening in the rainfall just to the lee of the Cascade crest, where air is moving downward.

The "storm total" rain fall will be quite respectable.  The 24-h total ending 5 AM Monday, shows up to 2.5 inches in the mountains, and a third of an inch in much of eastern Washington.

You like a nice blow?  Fairly strong winds (sustained to 35 kt) will precede the front on the coast and equally strong flow will strike the NW marine area, particularly around the San Juans and norther Whidbey Island.  These NW interior strong winds result from the large north-south pressure difference with the front and the effects of the Olympics and mountains on Vancouver Island.

The last month has been considerably warmer (but wetter) than normal.  Here are the temperatures at Sea-Tac Airport for the past four weeks.  The red and blue lines are the average maxima and minima.  The thing that really strikes you is the our minimum temperatures have generally been MUCH warmer than normal.   Temperatures have rarely gone below 60F.  I should not that there is probably some minor issues with the Sea Tac temperature (reads a few degrees warm), but similar warmth is found at other stations on both sides of the Cascades.


ceejay said...

Subsidence in the lee of the Olympics associated with these kinds of systems can create impressive temperature jumps and very gusty winds, often in the Bellingham-Lynden-Aldergrove BC area, just in advance of the frontal trough. This makes it difficult to produce representative forecasts as precipitation amounts, wind intensities and temperatures can vary widely over distances of just a few km. Anyway, probably not a great day for a bike ride.

Oldan Slo said...

I got in a nice 3 hr bike ride this morning. A little drippy around Mercer Island, but surprisingly dry the rest of the time.