I've tried to be as objective as possible with these forecasts, because while Seattle freaks out over the "s" word, it needs to be made clear to the public that there are drastically different types of snow events in Washington and not all of them are serious.
Now, it is looking more and more likely that we will experience a major snow event starting later tonight and likely persisting into Wednesday or even beyond.
Don't believe me? Check out what the National Weather Service office here in Seattle had to say this morning.
"VERY HIGH IMPACT WINTER WEATHER IS EXPECTED THROUGH
WEDNESDAY NIGHT OR THURSDAY. SNOWFALL AMOUNTS AT SEATAC COULD EXCEED
ANYTHING SEEN SINCE NOVEMBER 1985...A TOP 5 RECORD 24 HOUR SNOWFALL
I wasn't around in November 1985, but I have heard that it was a very snowy time for Western Washington. I did a little research and actually found a surface chart for November 21, 1985. It outlines a nearly perfect scenario for snow here, and although our scenario will be considerably different, the two share some key similarities.
This map shows the surface analysis at 5 P.M. PST, November 21st, 1985 over the Pacific Northwest. There are TWO key things on this map that are telling of a major snow event over Western Washington. First of all, there is a large dome of high pressure up in Canada with very cold air at the surface and a strong pressure gradient through the Fraser River Valley. Second, there is a low pressure system to the south of Puget Sound, drawing even more cold, northeasterly winds from the Fraser River Valley into the state while providing lift for precipitation.
The event being forecast is different from the November 1985 event in that it the air in the area will not be as cold and the storm coming in will come more from the west off of the Pacific instead of coming in from a more northerly track like the 1985 storm. However, we still do have cold air to our north and a big, juicy storm coming to our south. Whenever this happens, there is the possibility for significant snow in the lowlands.
The forecast for the next couple days is very complex, so I am going to break it up into three parts.
Part 1: Today into tonight - isolated snow showers
Part 2: Tonight into Tuesday afternoon - increasing snow showers associated with an arctic frontal boundary
Part 3: Tuesday night into Wednesday - possible major, widespread overrunning event
Today into tonight is expected to be the most benign event of the four for the Seattle area. A small disturbance is directing unstable, westerly flow into our area, generating snow showers in spots. The Olympics, however, are shadowing the Central Puget Sound region, particularly Seattle. Expect these showers to slightly increase throughout the day, but the Seattle area will likely remain largely snow-free for today outside of a couple isolated showers. Take a look at the image below and notice how the Puget Sound area is high and dry while Bellingham and Olympia are getting pounded.
02:03 pm Monday, January 16, 2012
SEATTLE/BREMERTON AREA-TACOMA AREA-HOOD CANAL AREA- 235 PM PST MON JAN 16 2012.WINTER STORM WATCH REMAINS IN EFFECT FROM MIDNIGHT PST TONIGHT THROUGH TUESDAY AFTERNOON... ...WINTER STORM WATCH REMAINS IN EFFECT FROM LATE TUESDAY NIGHT THROUGH WEDNESDAY EVENING... * SOME AFFECTED LOCATIONS...SEATTLE...TACOMA...BREMERTON... SHELTON. * TIMING...SNOW ACCUMULATIONS ARE POSSIBLE TONIGHT THROUGH TUESDAY AS COOLER UNSTABLE AIR CONTINUES OVER THE AREA. LOCAL ACCUMULATIONS UP TO 3 INCHES ARE POSSIBLE. A MUCH MORE SIGNIFICANT WINTER STORM IS LIKELY LATE TUESDAY NIGHT THROUGH WEDNESDAY OR WEDNESDAY NIGHT. SNOW WITH THIS SYSTEM WILL BE WIDESPREAD AND HEAVY. * SNOW ACCUMULATIONS...AT THIS TIME UP TO 3 INCHES ARE POSSIBLE ON TUESDAY. TEMPERATURES WILL REMAIN STEADY OR GRADUALLY FALL DURING THE AFTERNOON HOURS...SO ROADWAYS MAY FREEZE. A MUCH MORE SIGNIFICANT WINTER STORM IS LIKELY LATE TUESDAY NIGHT THROUGH WEDNESDAY EVENING. WEDNESDAYS STORM MAY BRING 6 TO 14 INCHES OF SNOW. * MAIN IMPACT...EXTREME TRAVEL DIFFICULTIES ARE POSSIBLE.
If you read closely, you'll notice that the National Weather Service actually has two winter storm watches over our area right now! One of them is for an arctic front that will sweep through tomorrow, and the other is for the bigger storm late Tuesday into Wednesday. Part 2 is concerned with the first winter storm watch tonight through tomorrow afternoon.
As we move into Monday night, an arctic front will start to move south through Whatcom and Skagit Counties, reaching the Seattle area at or after the morning commute. Even though tomorrow's communte could be tough, it is hard to know if schools will close at this point because the peak of the snow will strike after 5 A.M., which is when most schools have to make their decisions about whether to stay open or close. Schools aside, I'm forecasting anywhere from 1-5 inches of snow across much of Puget Sound from tonight to tomorrow afternoon. Arctic fronts can be very hit-or-miss, so it is definitely possible that some places may see no snow at all. Right now, it looks like the highest amounts will be in Snohomish Country, but we'll have a much better clue of what we can expect snow-wise from this feature later tonight. I don't want to go to classes as much as the next guy, but it'd be smart to plan for school in the morning just because of the uncertainty associated with this feature.
Based on the model below, the front should get to Seattle around 10:00 A.M. tomorrow, but I wouldn't be surprised if it came in earlier. As I said before, these arctic fronts have a mind of their own.
Valid 10:00 am PST Tue, 17 Jan 2012 - 30hr Fcst - UW 4km 12z WRF-GFS 3-hour snowfall
That said, it sure doesn't look like I'll have to do any work Tuesday night. Which leads me on to Part 3.
Oh boy. This is the part of the forecast I have been losing sleep over. And after this afternoon's models, I don't think sleep will be possible.
First off, let's take a look at the models. Remember, the things you need for a major snowstorm are high pressure and cold air to the north and a juicy Pacific low pressure system making landfall to our south, giving us heavy precipitation while drawing in cold air from the north.
Here is the latest 18Z NAM:
Valid 05:00 pm PST Wed, 17 Jan 2012 - 54hr Fcst - UW 4km 12z WRF-GFS 3-hour snowfall
Look at that! A big low pressure system right at the Columbia River, and higher pressure over Southern B.C.!
But it gets even better! Another storm goes to our south, possibly giving more snow Seattle northward.
Valid 11:00 am PST Thu, 18 Jan 2012 - 69hr Fcst - NAM 6-hour precip, 1000-500mb thickness, SLP
Now, the 18z GFS.
Valid 11:00 am PST Wed, 17 Jan 2012 - 48hr Fcst - GFS 6-hour precip, 1000-500mb thickness, SLP
Again, a juicy storm comes right to our south! And once again, another storm comes to our south, possibly prolonging the snow event for many areas.
Valid 8:00 am PST Thu, 18 Jan 2012 - 66hr Fcst - GFS 6-hour precip, 1000-500mb thickness, SLP
Us forecasters have a lot on our plates right now. It's very exciting to even be talking about an event like this, and it will be great to watch it on the coastal radar. These posts are long, but for an event that doesn't come around too often, it's worth it. :)