A strong Pacific system is coming onshore this afternoon and evening and during the afternoon precipitation will begin over the region (see satellite picture). You will see that this is NOT a pineapple express type event where the moisture extends into the tropics. We will get wet, but only for 6-12hr this evening and tomorrow am.
The air mass over us is cool and relatively dry, but not quite cool enough for snow as is. But that could and will change in some areas due to the "diabatics." Precipitation nearly always starts as snow aloft around here...even during rain events. Consider what happens when precipitation falls into above-freezing air below. If the air is not saturated then there is evaporation, which is a very powerful and rapid cooling agent. You know how powerful it is when you step out of the shower or get wet outside. But evaporation only lasts a few minutes to a half hour..until the air saturates. The temperature that air cools to when it falls into dry air is called the "wet bulb" temperature...you guessed it...we measure it by putting a wet cloth on the bulb of a thermometer and see what the reading is.
Now if that doesn't cool the air enough to reach the surface then melting can do the job. Snow falling into above-freezing air melts...and that takes energy out of the air...and thus cools it. Melting can be VERY effective when precipitation rates are large..and can drive the snow level to the ground. Melting is often associated with Puget Sound convergence zone snows--with snow where the precip is heavy and rain elsewhere.
Well, now you are diabatic, savvy what about today? Our air is marginal for snow, but is unsaturated. There will be moderate precipitation rates associated with the incoming system, which eventually will cause substantial warming aloft. Later tonight it WILL rain. But there could be 1-3 hr at the initial portion of the event when there could well be snow...and in fact the computer models are suggesting it. (see graphic). Greater threat will be the Kitsap area, SW interior and high elevations...but everyone will probably see at least some wet snow. Right now 4-7 PM is the best bet for the white fun. But no plows probably needed for this...warming and rain will follow (but be ready for a wintry mix for a few hours if you live on a hill)....cliff
