Seattle Snow?

Sunday, November 6, 2011

7:13 P.M.

Seattle, November 22, 2010

Now, I don't want people to get too excited, but take a look at what the NWS said at the end of their forecast discussion this afternoon.

LONG TERM...
CONFIDENCE IN THIS PERIOD REMAINED SOMEWHAT LOW DUE TO DIFFERENCES
IN THE MEDIUM RANGE MODELS. IT APPEARED THAT THE MAIN MESSAGE WAS
THAT A COLDER PATTERN WILL BECOME ESTABLISHED OVER THE PAC NW THE
END OF THIS WEEK. THE ECMWF SEEMED TO BE THE COLDEST AND WETTEST OF
THE SOLUTIONS. IF THE ECMWF SFC PATTERN PREDICTION FOR LATE SATURDAY
IS CORRECT...I WOULD BE WORRIED ABOUT THE POTENTIAL FOR SIGNIFICANT
SNOW ACROSS PARTS OF THE LOWLANDS. FOR NOW...I FELT IT WOULD BE
PRUDENT TO COMPROMISE BETWEEN THE EXTREMES AND INDICATE A TREND
TOWARD MUCH BELOW NORMAL DAYTIME TEMPS. 

What does this mean? It means that there is a slight possibility that we could see snow this weekend.

 Valid 04:00 am PST Sun, 13 Nov 2011 - 168hr Fcst - ECMWF (Euro) 12z 500mb height and SLP

First, let me explain what the model above shows. It shows the SLP (sea-level pressure) and the height of the atmosphere at 500mb. Because cool air is denser, the 500mb level is lower where cold air resides. You can see a lot of cold air over Siberia, northern Canada, and the Pacific Northwest. On the other hand, you can see higher heights and  warmer air in the Eastern Pacific due to a huge ridge of high pressure. Higher heights also reside closer to the equator, where the air is warmer and less dense. 

If you look closely, you can see lots of low heights over our area. But it takes more than just low heights to give us a snowstorm. You need precipitation. That is where the SLP part of our chart comes in.

You can see a HUGE low pressure system - 975 millibars to be exact - coming in near the mouth of the Columbia. This location is very, very important. If this system was to our north, we would get wind and rain because it would draw in lots of warmer air from the south. As shown, it is just to our south, giving us a blast of cold air to keep all the precipitation falling over us as snow. If this scenario was expected in 24 hours, I would be confident that we would see a major snowstorm. 

But alas, it is not. The general trend with the models is to move to a colder regime, but the ECMWF is by far the coldest and snowiest models. The other models show plain ole' Seattle rain and low snow levels in the mountains. Here is the GFS model from this morning... it also shows the 500mb height and SLP, the GFS just calls it the 1000-500mb  thickness.

Valid 04:00 am PST Sun, 13 Nov 2011 - 168hr Fcst - UW 36km 12z WRF-GFS 1000-500mb thickness, SLP

Valid 04:00 am PST Sun, 13 Nov 2011 - 168hr Fcst - UW 36km 12z WRF-GFS 3-hour precip, SLP

These models paint a much more humdrum situation than the ECMWF. There are relatively high heights over our area, indicating warm air, and, apart from some precipitation to our south, there is no real storm entering the area. 

See how hard weather forecasting is? 

I made up a weather forecasting system last November called my "LAWES" system (Likelyhood of an Arctic Weather Episode with Snow). Here is the scale, printed below for your convenience

So, lets go through this scale.
LAWES level 0: You are on a planet that has the ingredients necessary for snow
LAWES level 1: Models are showing a chance of snow beyond a week out, 0-20% chance of snow
LAWES level 2: Models are showing a chance of snow within a week, 20-40% chance of snow
LAWES level 3: Models are all showing a scenario that would easily produce snow within a couple days, 40-60% chance of snow
LAWES level 4: Places around the area are already getting snow AND snow is forecasted for Seattle, 60-99 % chance of snow
LAWES level 5: Snow is occurring, 100% chance of snow (derp)

Now, notice how there are no measurements for how strong a Seattle snowstorm could be. But have no fear! That's what suffixes are for.
Ending in:
.1 - 0-2 inches of snow
.2 - 2-4 inches of snow
.3 - 4-6 inches of snow
.4 - 6-8 inches of snow
.5 - over 8 inches of snow
.6 - board the next plane to Panama

At this point, I'd forecast a LAWES level 1.4. A snow event is not too likely, but if it does happen, it could be quite significant. 

Last November, we had a major snowstorm on November 22. My second-most popular post, "Yes, I failed!!!" recounts this major November snowstorm. As you probably guessed, I butchered the forecast. For once, more snow fell than was forecast. November 22 was very early for a snowstorm. I hope we beat it this year.

In the words of Mark Twain, "It is best to read the weather forecast before praying for rain." I'll be updating this blog as new information on this possible snowstorm comes in. Praying couldn't hurt, though. :)

Charlie Phillips