Surprise Convergence Zone snow

Last night a convergence zone formed in the wake of the passing front and was sufficiently intense to drive the snow level down to the surface (see radar image at 2 AM and now). There are reports of a trace to 1-2 inches. It is over now. The Seattle profiler shows the event clearly..with the low-level windshift and the cooling of the air due to evaporation and melting. "Surprise" convergence zone snow showers are a traditional here in Puget Sound country....rarely a year passes without one. To get a handle on such events a coastal radar would be very useful...allowing us to secure much better forecasts 3-9 hr ahead.

The next snow threat includes snowshowers today...which will be brief... and then this evening when the convergence zone should reestablish itself according to the latest high resolution model runs. The air is cold enough that heavy showers can bring wet snow down to the surface.

Tuesday looks as before....incoming weather system that will bring snow to the Kitsap and SE Olympics slopes...as well as the Chehalis River Valley. Near the Olympics the snow could be heavy...6-12 inches are possible. Tonight I will analyze this threat in some detail.

Weak Front, Colder Air, and then Hood Canal Snow

Yesterday, there was bright sun in the mountains, but the low clouds and fog held in the southern Sound for most of the day--with Seattle northward breaking out in the early afternoon. The trouble this time of the year is that the sun is not strong enough to effectively mix out the nighttime inversions that often form under high pressure conditions. The low clouds are also self-maintaining--since they reflect the feeble winter rays back to space and the cloud tops are effective radiators in the infrared (and thus promote cooling). In the spring, the sun is much stronger, daylight is longer, and the atmosphere is less stable...allowing more mixing out of the thin veneer of cold air.

Today a week front is approaching the coast and the high clouds associated with it are already over western Washington (see satellite picture). The mountains are relatively cloud free and low clouds fill the basin of eastern WA. Current radar indicates dry conditions over the interior, with a few showers to the west (image). Today should remain dry and with the low clouds dissipating over most of the area..there will be some filtered sun for many. As the afternoon progresses, the clouds should thicken and lower...and showers should occur tonight for most.

Tomorrow, there will be some residual showers...mostly over the mountains and in Puget Sound convergence zone. The air aloft will cool substantially...opening the door for some mixed precipitation on Tuesday.

Tuesday will be interesting...a stronger frontal system will approach in the afternoon. Cool, but not cold, air will be place. Places of higher elevation (above 500 ft) and where heavier precipitation occurs (SE of the Olympics) could see some snow. In fact, the computer models are indicating several inches over Kitsap and SE of the Olympics..with some snow shower extending east to some higher elevation areas (see figure). The models also sugget snow over SW Washington...south of Olympia. As the system moves in the air will warm and precip will turn to rain in the lowlands. So the salt trucks may not be needed! The mountains will pick up several inches of snow...but they really need much more to make skiing fun again.

Heading up to Bellingham soon to give a talk at Village Books at 4 PM--general intro to NW Weather.

Finally, there have been a lot of interactions about the comments section. Let me ask those who are making comments a favor:

1. Stick to weather or its impacts
2. Please no editorial comments about wording or grammar. A few (a suspect a very few) get hot under the collar about my use of words like "nice weather." If you don't like my use of language go to another blog.
3. I can't answer all the questions, all the time...I simply don't have the time. Please understand.