A (Promising!) Long-Range Look at Seattle's Weather This Weekend (for all you AMS visitors heading in...)

As many of the meteorologist readers of my blog probably well know, the American Meteorological Soceiety's annual meeting for this year is being held next week in Seattle.  Many people I know will be flying into town this weekend to attend the conference.  Some (including me) also have plans to do some sightseeing and enjoy all that Seattle has to offer this weekend before the conference starts.  However, as any Seattleite will tell you, the best time to enjoy Seattle is when it's sunny and warm out.  Which, in the middle of winter, is a hard type of weather to find.  So what are we expecting for this weekend?

There's actually rather good agreement in terms of the overall synoptic pattern in our global models, which gives me greater confidence in the forecast overall. Let's start by looking at the setup on Thursday.  Seattle is currently (and will be for the next few days) under a trough of sorts that will linger through Wednesday and Thursday:

Fig 1 -- GFS 18 hour forecast of 500mb heights (contoured) and winds (shaded) for 06Z, Wednesday, Jan. 19, 2011. From the HOOT website.

This trough is contributing to several rounds of showers moving through the area, which will continue to exacerbate the areas already hit by high water and flooding this weekend.  The amplified ridge-trough pattern seen in the above image begins to flatten out by Friday.

Fig 2 -- GFS 72 hour forecast of 500mb heights (contoured) and winds (shaded) for 12Z, Friday, Jan. 21, 2011. From the HOOT website.

Notice that even in the more zonal (east-west) flow off the Pacific northwest coast, there is still a prominent shortwave indicated approaching the coast (annotated on the figure above with a red dotted line).  The ECMWF also picks up on this feature:

Fig 3 -- ECMWF 72 hour forecast of 500mb heights (contoured) and winds (shaded) for 12Z, Friday, Jan. 21, 2011. From the HOOT website.

The ECMWF solution brings the trough a little further south and slightly deeper.  Regardless of the exact depth and orientation of the trough, this translates to some cloudiness and rain on Friday.  Our local UW extended 12km WRF model picks up on these conditions Friday nicely.  First, the outgoing longwave radiation forecast (think a forecast infrared satellite image) shows the increased cloud cover:

Fig 4 -- UW 12km extended WRF 78 hour forecast of OLR for 18Z, Friday, January 21, 2011. From the UW Mesoscale Modeling Group.

And the 24-hour precipitation totals from Saturday morning show quite a bit of precipitation the day before (don't take these numbers too seriously--they're usually way off this far out).

Fig 5 -- 96 hour forecast of the previous 24 hours of precipitation accumulation from the UW extended 12km WRF at 12Z, Saturday, Jan 22, 2011. From the UW Mesoscale Modeling Group.

The highest precipitation amounts are on the coast and along the Cascades (that swath of greens and pinks going up the middle to right side of the image).  Note that there is a relative minimum of precipitation on the eastern side of the Olympic peninsula just left of center on the image.  This is because the prevailing low-level winds during this event will be out of the west and, as such, the eastern sides of the Olympic peninsula is getting rain shadowed by the Olympic Mountains.  If the deeper trough with stronger winds indicated by the ECMWF is closer to what will happen, then that rain shadowing may be enhanced and decrease the precipitation totals all the way out over Seattle on the eastern side of the sound.  It's a possibility...and it happens here every so often.

By Saturday, the trough will be moving out and a ridge looks to begin building in:

Fig 6 -- GFS 96 hour forecast of 500mb heights (contoured) and winds (shaded) for 12Z, Saturday, Jan. 22, 2011. From the HOOT website.

While this ridge will shift winds to the northwest in the low levels, the wind speed rapidly decreases.  The upwind pattern also does not extend too far north, so we're not expecting strong cold air advection behind the shortwave on Friday.  Clouds look to still be lingering on Saturday evening, but beginning to break up...

Fig 7 -- UW 12km extended WRF 108 hour OLR forecast for 00Z, Sunday, Jan. 23, 2011. From the UW Mesoscale Modeling Group.

...and precipitation on Saturday will definitely be less than it was on Friday.

Fig 8 -- UW extended 12km WRF 120 hour forecast of the previous 24 hours of precipitation accumulation for 12Z, Sunday, Jan 11, 2011. From the UW Mesoscale Modeling Group.

Much of the rain will have stopped by Saturday morning and, in fact, the Seattle area shows no precipitation on Saturday in the above image.  That swath of greens to the north of Seattle could represent a Puget Sound convergence zone event (more on that in another post...) which is a fun local weather phenomenon that people may be interested in...

Anyhow, by Sunday that ridge has really built in.

Fig 9 -- GFS 120 hour forecast of 500mb heights (contoured) and winds (shaded) for 12Z, Sunday, Jan. 22, 2011. From the HOOT website.

Under such strong ridging aloft, subsidence would set in across all of western Washington and southern British Columbia.  As a result, few deep clouds, if any clouds at all:

Fig 10 -- UW extended 12km WRF 129 hour forecast of OLR at 21Z, Sunday, Jan. 22, 2011. From the UW Mesoscale Modeling Group.

And no real precipitation in the area, except out over Vancouver island and maybe a brief shower or two in Vancouver:

Fig 10 -- UW extended 12km WRF 144 hour forecast of the previous 24 hours of precipitation accumulation for 12Z, Monday, Jan 24, 2011. From the UW Mesoscale Modeling Group.

A look at a forecast surface map for Sunday shows light winds and temperatures in the upper 40s to low 50s (degrees Fahrenheit):

Fig 11 -- UW extended 12km WRF 129 hour forecast for 925mb temperatures (shaded, in degrees Celsius), surface sea-level pressure (contoured) and winds (barbs) for 21Z, Sunday, Jan. 23, 2011. From the UW Mesoscale Modeling Group.

 So, I'd definitely bring a good jacket, but it looks like if anyone has plans to explore Seattle on Sunday the current forecasts are calling for it to be a pleasant day.  Probably the best you could hope for in the middle of winter, actually.  I'm suspicious with such light winds and under ridging aloft that fog might roll in in the mornings of Saturday and Sunday depending on the humidity levels.  A forecast for relative humidity Sunday morning does show greater than 75% relative humidity in much of the Puget Sound lowlands:

Fig 12 -- UW extended 12km WRF 120 hour forecast for Surface relative humidity (shaded) at 12Z, Sunday, Jan. 23, 2011. From the UW Mesoscale Modeling Group.

But even if there is some fog, it will hopefully clear out by the afternoon.

Remember these are still long range forecasts--the potential for them to be inaccurate is rather high.  We're still out of range of the ensembles to get good probability estimates too.  The general pattern of the global models seem to agree, though, so my confidence in this forecast is higher than usual for this far out. So, for all those of you headed into Seattle this weekend, the current forecast looks very promising for a good beginning to the AMS conference!