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