A Clearer Picture

Nowcast at 3 PM Saturday.   I am going to try doing some nowcasts for this event....first this afternoon.... Just finishing a paper on this subject if anyone is interested.  Found here.

We are going into a period of very interesting and active weather.

I have just finished looking over the latest forecast model output from the U.S. and foreign centers, as well as the UW high-resolution simulations, and the conclusion one draws is that most of you are going to see some snow during the next few days--ranging from a dusting to several inches.  If anything the threat of snow is greater for the latest series of runs, but they are also emphatic that we are not facing a major, widespread snow event.  Instead we are talking about snow showers for many, with a some lowland areas that might get several inches (such as in the convergence zone). 

Right now the air above us is quite warm, except for a veneer of cool air near the surface (see plot for Seattle below).  A strong inversion caps the cool air, resulting in poor air quality and the current burn ban. No chance of lowland snow with this temperature structure!

But things are going to change rapidly tomorrow as a strong front with very cold air moves through (see figure), with a switch to northwesterly winds behind (see figure).  Relatively

steady rain from the front should be over around lunchtime and then with the cold air behind, we will enter the normal post-frontal showers.  You could see the front and following showers lurking out there during the afternoon (see image).  Interested fact--in the eastern U.S. when a cold front goes by there are few

showers...it dries up.  Why?  We have relatively warm water out there and cold air over warm water is unstable and produces cumulonimbus clouds.  Over most of the continent the ground is cold, and cold air over cold ground does not initiate convection.

The atmosphere will progressively cool later in the afternoon and early evening, with 850 mb (roughly 5000 ft) temperatures dropping below -6C by Saturday evening and -8C on Sunday.  Such temperatures are cool enough for snow even with onshore flow...which we will have...Certainly snow above a few hundred feet ASL  and down to the surface when there is any real precipitation intensity.  By midday Sunday it will be all snow no matter what. North of the border there could be some light snow with the initial frontal passage on Saturday morning, according to the WRF model.

The issue is that most of the showers over lower elevations will be random, hit or miss affairs, except for a few locations where they will be more persistent and stronger.  One such location could be the Puget Sound convergence zone that could well develop under northwesterly flow.  The CZ could set up anywhere from Seattle north to Everett.  Or could be in a band radiating from the Olympics over  to Whidbey Island.  The exact location is difficult to predict.  Here is the latest UW WRF prediction of 24-h snowfall ending 4 PM on Sunday:

And here is for the next 24-h:

This is not snowmagedon, but several inches from Seattle northward, snow over SW Washington, and a foot or more in the mountains.  Appreciable snow over NW Washington. You notice a lack of snow over the Kitsap and south of Seattle...that is the Olympic rainshadow with NW flow.  The higher elevations of eastern Washington pick up some light snow.  Snow comes and goes as a series of disturbances move through during the period through Tuesday.  If one of the them is stronger than predicted and sets up a low center over SW WA, we could get much more snow.

But the best is yet to come.  In the NW we often get snow going into cold AND going out of cold.  The models indicate the mother of all warm fronts to approach early Wednesday (see below--sea level pressure, near surface winds, and temperature in the lower atmosphere--shading).  Wow...that is quite a warm front!  As it comes in, warm air and lifting will come in aloft.  The low level cold air will hold for a short while, giving a burst of snow...perhaps several inches...before it all turns to rain.  Just in time for the commute!

And did I mention that the strong winds and moist air is predicted to cause VERY heavy rainfall later in the week.  Here is the 48h rainfall ending 4 AM on Friday. Some places...like in the Siskiyous near the Oregon/CA border will get more than 10 inches if this is true, and virtually all the coastal mountains and Cascades will get 5-10 inches.  Flooding on some rivers is quite possible if not probable.

Well, after one of the most boring winters I can remember in a while, we are about to start experiencing some interesting times.  And with the new Langley Hill radar we will have a new view of the action and should know whether the forecasts are going wrong in time to provide some warnings and updates.