November Hurricanes in the Northwest

Often over looked by major media outlets, fall and winter storms in the Northwest often pack a one-two punch of wind and rain in the lowlands (3 punches if you count snow in the mountains). Well this coming week, we have the potential for 4 storms in a 7 day span, all of which would bring breezy conditions (slightly less than windy conditions) and a large dose of rain for lowland locations. Here are a couple images I have taken off the University of Washington weather models that show the evolution of these low pressure systems

This first frontal system will move in Sunday night bringing gusty conditions to the coast and some breezy conditions inland. The black lines in the image are called isobars, lines of constant pressure. The closer those lines are, the stronger the winds. You can see the lines are packed close together along the coast which is where the strongest winds will be. The next storm....

Appears to be a bit stronger with a central pressure near 970mb by the Northern tip of Vancouver Island. This system moves in Monday evening and would bring winds between 15-30mph for the lowlands and gusts maybe to 45mph. Any remaining leaves on your trees would sure to be down by now. The next system moves in a few days later...

Thursday afternoon and evening. This low center is weaker than the previous one, 985 milibars, but would certainly bring a good amount of rain. Winds would be strongest along the coast during this scenario as well.

This final system of the week may be the most dynamical of the 4. What sets this one apart is the strong high pressure down south towards the central coast of California, just barely visible in this image. It's at about 1027 milibars and the low pressure center, in the Northwest corner of the image is around 970 milibars. Such a large difference in pressure, over a relatively small area can great a large pressure difference field, and since air tends to move from high to low pressure, the winds would accelerate very quickly between the high and low. This is why the isobars in this image span a very large area and are very packed together. Once this system moves in, it could be the windiest of the 4 system for the week.

In comparison, lets look at some images of hurricanes and these storms, known as mid-latitude cyclones.

Here is Hurricane Katrina

Yes, it's quite large and probably has very strong winds (I will get to that in a second).

Here is the Hanukkah Eve Storm that hit the Northwest in December of 2006,

I would beg that this storm is even larger, and has a much larger area of impact.

The best way to compare these systems is the central pressure though. The Saffir-Simpson Hurricane Scale details this

Now, Hurricane Katrina had a minimum central pressure of 902mb...we don't see those pressures with mid-latitude cyclones, it's even rare for tropical cyclones. But what I want you all to see is that NW storms can get into the 970's, even 960's on occasion. That would be on par with a Category 2 or 3 hurricane, but on a much larger scale (the larger scale also prevents MLC's to have stronger winds like Hurricanes have).

As you can see, we are in for a bumpy ride this coming week as these systems approach. I'll leave you with a couple images of the rainfall that will hit us.

This the 24hr accumulated precip ending Tuesday afternoon. The black is between 2.5 and 5" of rain...yikes. The red is between 5 and 10", localized over the central cascades near Oregon. Here is the snowfall at the same time...

Climbing up above 2 feet in some areas!!

This the rainfall ending Saturday afternoon...5-10" possible for the Olympics.

More snowfall for the Northern Cascades.

Flooding will be a major concern this week with lots of rain and rising rivers that won't have a lot of time to recede.

Meteorologically Yours,

Aaron

"S" Word Update

I have already encountered some questions about whether we will see some snow this week. For an update, the models have not favored a scenario for snow around the Pac NW. Over the past 3 model runs, specifically the UW WRF (Weather Research Forecasting) model has 'warmed' temperatures up just enough on Thursday and Friday that any moisture around would probably fall as rain first...and maybe a heavier shower could see some wet snow mixed in.

First off, there is no terrifically organized system moving in with the cold so the moisture maker is limited. Pockets of showers could be expected but once again, those would be limited and sparse. Not to mention, hard to predict exact locations. One brighter side to this is that a Puget Sound Convergence Zone could set up once the upper level trough moves through which could enhance precip and bring down the snow level in localized locations.

Will continue updates through the week as more information can be gathered.

Inkling for Some Fluffy Stuff?

Around the Northwest, the mere mention of the "s" word can bring about a panic only one can experience to understand. Well this time of year, snow becomes a popular topic around communities as people become anxious for cooler weather and just the possibility of not having to go to the mountains to play in the terrific white fluff. In addition, snow becomes a popular topic among meteorologists because weather models begin to turn from the summer time calmness (around the Pacific Northwest) to the stormy fall weather and the hint of colder winter temperatures. Sometimes, the two even come together. ;)

The issue with predicting snow around the Northwest, is that it's hard to get snow. The Pacific Ocean strongly mediates the air temperature here near the coast so it takes VERY cold air temperatures to overcome that heating and get snow.

The next hardest part about forecasting snow, arguably the hardest part, is that forecast models ARE NOT PERFECT! As meteorologists we learn about the atmosphere and it's motions, and these models predict those motions. Well that's a pretty tall task to ask of the models. I learned from my professor at the University of Washington, Cliff Mass, a good picture of how to understand model variability. It looks something like this:

Imagine a ski resort. You have 5 runs that all start at the top of the mountain and go down the mountain in different directions. Now think of each run as a forecast and going down the mountain as going farther out in time for the forecast (this will stretch your thinking a little). Here is a rough picture of my illustration

Now at the bottom of the mountain, the forecasts (runs) are all at different spots. As you go farther up the mountain, closer to the forecast time, the forecasts become closer and closer together until you get to the top, where they become relatively similar and agreeable.

Well in forecasting snow, sometimes models will paint a snow forecast 7 days out, lets say the Green line. You notice how the green line doesn't start and end at the same point along a vertical axis, so it may say snow 7 days out and rain the day before the forecast time. It's a very risky business to trust models 7 days out from a forecast time.

Models have gotten better over the years though. 2 decades these models were just getting started and you might not have been able to trust a forecast 2 or 3 days out. Now, 5 days is reasonable for trusting, although exact details (amount of snow, rain, winds, etc.) shouldn't be taken from a 5 day forecast.

Now to what I really wanted to talk about, but I felt I should give some grounds for my feelings about this. The model run by UW this morning painted a "favorable" picture for snow next Thursday and Friday. It would be a classic Pac NW scenario for snow, which I will get back to in a minute. Here are a couple images that the model outputs

I'll start with the upper level pattern of the atmosphere, at 500hpa pressure. Those pink colors, those are very cold temperatures associated with a trough of low pressure digging south from the Canadian continent. This is an important factor because air coming off the ocean is not very cold, but from the Canadian land, it would be VERY cold.

The next thing that is needed is a low pressure developing at the surface, which is clearly visible here. The low is centered right off Vancouver Island, bringing moisture and "pulling" the cold air at the surface towards it. Air tends to accelerate from High to Low pressure. This forecast is for 147hrs (from this morning at 4am). Here is the forecast for 171 hrs.

The low is centered right over Washington, pulling cold air right into Western Washington. The white color indicates temperatures near 0 degrees celsius.

Here is estimated snowfall for a 3 hour period ending Friday afternoon. This shows some snowfall around South Puget sound and the Eastside. Another 3 hours later...

 Some snow in Northern Western Washington. And here is the 24hr accumulated snowfall totals...

As expected the Eastern foothills would see more accumulation, and only a slight amount around the lowlands.

However, all of this forecast for the end of next week should be taken as a grain of salt, as I noted previously. It's just TOO EARLY to tell.

I felt this would be a good opportunity to explain this aspect of forecasting too far out, while also arousing your interest in some possible snow :) Because I'm sure excited

You can follow me on twitter for even more up to date information regarding Pac NW weather and even weather throughout the nation: @huskyAaron
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I will post updates to this story as I get them :)

Cheers!