Tuesday, January 17, 2012

Models Pessimistic, Charlie Optimistic

Tuesday, January 17, 2012
10:05 P.M.

So, the latest model runs have come in, and they are moving the storm even further south. For snow lovers who live in Seattle, this is bad news. However, I've been watching the radar, and I believe that we will see more snow than is currently forecast by the models. I'm still expecting less than 4 inches north of Seattle, 4-7 inches from Tacoma to Seattle, and over 7 inches south of Tacoma, despite the fact that the newest WRF-GFS model gives Everett to Tacoma a paltry 1-3 inches of snow. The southern Cascades in southern Washington and northern Oregon will receive feet of snow with this storm, so if you are bummed out that we didn't end up getting one of the biggest snowfall events in years around here, just head to the mountains. You can be standing in feet of snow, maybe bring a picture of the Space Needle along with you, and you can at least pretend that Seattle is experiencing a true "Snowmageddon."

Many of you are probably frustrated with how the forecasts have changed so much, particularly how they have gone back down after first proclaiming that Seattle would be in for the snow event of a lifetime. I'm not sure if these next few images will calm your nerves or just get you angry at our weather models, but I think it is important for the populace to understand how much a situation can change in such a short amount of time.

Let's take a look at the progression of this event on the UW WRF-GFS model. Here, I am showing the different 24-hour snowfall forecast maps starting from when the event was 84 hours out and ending with the current snowfall forecast over our area over the next 24 hours.

Things were starting to look interesting on Sunday morning's run. At first, the models showed the low coming north, quickly scouring out any cold air but also dumping several inches in northern spots like Bellingham.

Valid 04:00 pm PST Wed, 18 Jan 2012 - 84hr Fcst - UW 12km 24 hour snowfall - initialized 12z Sun, 15 Jan 2012

Sunday night? uh oh, major snow. Still a lot of snow to our north.

Valid 04:00 pm PST Wed, 18 Jan 2012 - 72hr Fcst - UW 12km 24 hour snowfall - initialized 00z Mon, 16 Jan 2012

Then comes Monday morning's run. Close to a foot over Seattle. Simply amazing.

Valid 04:00 pm PST Wed, 18 Jan 2012 - 60hr Fcst - UW 12km 24 hour snowfall - initialized 12z Mon, 16 Jan 2012

Monday night's run was very different, but it still suggested a lot of snow for Seattle. This was probably our worst-case scenario. The low was shown to go to our south, bringing down cold air, yet still be close enough to give lots of snow to Seattle. Honestly, this was when I started freaking out. A major snowstorm, possibly the biggest since 1996 in Seattle, and maybe before that for Sea-Tac.

Valid 04:00 pm PST Wed, 18 Jan 2012 - 48hr Fcst - UW 12km 24 hour snowfall - initialized 00z Tue, 17 Jan 2012

Then came this morning's run. A major letdown. All of the heavy snow has moved to our south. Still, we had pretty significant amounts nonetheless... I'd take 2-6 inches any day!

Valid 04:00 pm PST Wed, 18 Jan 2012 - 36hr Fcst - UW 12km 24 hour snowfall - initialized 12z Tue, 17 Jan 2012

Tonight's run was a crusher though... the snow has moved even further south!

Valid 04:00 pm PST Wed, 18 Jan 2012 - 24hr Fcst - UW 12km 24 hour snowfall - initialized 00z Wed, 17 Jan 2012

And that's the evolution of our snowstorm prediction. Looking at last night's model run, it's easy to get pretty excited, but the models had a poor handling on this storm and are now giving us paltry amounts as compared to what they were showing before.

Forecast models are our best tool of predicting weather in the future, and model predictions will get better in the future around here, especially as ensemble forecasting continues to be developed. Still, models are models. Let's take a look at what is actually happening out there!!!

Valid 10:47pm PST Wed, 18 Jan 2012

We've got a FAT blob of moisture headed up our way from the southwest, and the good thing is that it is looking more impressive here than it did on our models. Ever since the two WRF-GFS runs today, I've been thinking that the models have been underestimating the strength of the jet stream propelling this thing into our area. You can see some heavier precip around the edge of the front, and if I had to guess why these features are there, I'd say it is because the warm air is rising over the cold wedge of air that I was showing to you in the cross-section diagrams of my previous post. It's a great thing to see, and I'm very happy that the precipitation is this far north this early. The mark of a really good forecaster is the ability to pin down amounts in the short term when models are of little use just by looking at radar imagery. By looking at this radar image, I'd expect snowfall amounts exceeding those shown in the WRF-GFS above, which is why I have forecast 4-7 inches of snow from Everett to Tacoma. Taken at face value, the WRF-GFS shows anywhere from 1-3 inches from Everett to Tacoma.

Now, let's take a look at the bigger picture.

I'm going to show three different images here from different times throughout the day. These images show the water vapor in the atmosphere, which is often more useful than standard infrared satellite maps for picking out centers of low pressure, as low pressure centers tend to develop a "dry slot" right by the low as they develop.

01:00 pm PST Tue 17 Jan 2012

This image was taken at 1 P.M. this afternoon. That huge blob of moisture is what is headed towards us. 

06:00 pm PST Tue 17 Jan 2012

5 hours later, the front is moving in. Notice how defined the leading edge has become as it has moved into our area. Again, I am hypothesizing that the reason for this increase in definition is because of the warm, moist air riding above the cold wedge of arctic air extending back into Canada. I've only taken atmos 101 though, so don't quote me on that.

10:30 pm PST Tue 17 Jan 2012

Now, the high level clouds are completely over our area, and we are just waiting for the surface precipitation to catch up! In case you didn't catch on, this system is moving pretty dang fast, being driven by a very strong jet stream aloft.

Let's go over some quick gradients and surface observations now, and then I gotta get out of here!!! 

These gradients were given to me by my friend and fellow weather nerd Nicholas Efthimiadis.

The BLI (Bellingham) to YWL (Kelowna) gradient is currently -10.7, meaning the pressure in Bellingham is 10.7 degrees lower than the pressure in Kelowna. Remember, wind always flows from high pressure to low pressure, and over terrain, it follows a fairly direct, "as-the-crow-flies" path. Hence, tons of frigid air is making it's way from the interior of BC to Bellingham. Indeed, the last time I checked, the temperature at the Bellingham airport was 11 degrees Fahrenheit. Cold stuff!

The OLM (Olympia) to BLI (still Bellingham!) gradient is currently -1, meaning that Western Washington is now experiencing light northerly winds. Expect those to become pretty strong tomorrow, especially up by Bellingham.

Now, the surface observations in our state as of 11 P.M.

11 degrees, clear, and light (~5mph) winds out of the NNW at BLI, with temperatures slightly above freezing at Sea-Tac. Right now, the Willamette Valley is getting snow and they have temperatures in the low 30s, but they will change over to rain in the next couple hours.

My weather station is reporting a balmy 36 degrees with 255% humidity. I wouldn't put too much stock in it.

Do some snow dances, I have a feeling they might be working. :)


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