Thursday, November 4, 2010

Deceptive Alaskan Weather

Since I lived in Anchorage for a summer two years ago while working for the weather service there, I also have a healthy interest in Alaskan weather, something that is not often discussed down here (except in the context of climate change in the arctic). So for today, we'll take a brief look at what's going on in Alaska.  Here is an infrared satellite image from earlier this morning:
Fig 1--GOES-W IR satellite image from 1030Z Nov 4th, 2010
This looks like a powerful extratropical cyclone, with its center of circulation just about centered on Anchorage.  And look at the size of this cyclone!  The colder IR temperatures, indicated by the yellows and the reds in the image and almost always associated with IR emissions from clouds, almost fill the entire state of Alaska and the Yukon.

This looks like a powerful storm, but just how severe is it at the surface?  It's difficult to get good obsevations in Alaska, mostly because there's a lot of open water and even on land the populated areas are very sparse.  One way we can try to get a more cohesive set of measurements is to use satellite-derived winds.  One simple way to do this is to identify certain cloud features on a satellite image and then see how far they move over a given time.  By estimating the elevation off the ground (or ocean) of these cloud features, we can get an estimate of the winds at that level.  However, since clouds have depth, they tend to cover multiple levels of height.  Therefore, the best we can do is guess at the wind speeds within a layer.  An example of one of these products can be seen here:
Fig 2--GOES W Pacific region IR-derived winds, 1445Z, Nov 4th, 2010.  Yellow indicates 700-1000mb, cyan indicates 400-699mb, and red indicates 100-399mb.
We can see that we don't have data everywhere.  This is because sometimes either the cloud structure is too dense to identify any clear cloud features or there just are no clouds in that region.  Another reason this happens, particularly over Alaska, is that Alaska is just too far north for a geostationary satellite like GOES to accurately calculate winds.  If you think about it, the GOES satellites are parked over the equator and you can see that the view of Alaska is rather squashed and distorted.  Trying to calculate any sort of wind vectors accurately over that distorted of an image can be problematic.  We can try to use polar-orbiting satellites to get better pictures, but their orbital passes are not as frequent as image updates from GOES.  I also don't know of a good place to get POES (the polar-orbiting satellite) data, though I'm continuing to look...

Anyhow, we can see that in the Gulf of Alaska that there are several low-level (yellow) wind barbs in the 30-35 knot range, which isn't too extreme but still could pack quite a punch.  It's clear that there's probably a lot of cloud cover and maybe precipitation associated with this cyclone as per the IR image (fig 1).  So how is Alaska reacting to this storm?
Fig 3--Warnings and advisories from the National Weather Service for the Alaska region from 8:45 AKDT on Nov 4th, 2010.  Pale purple indicates small craft advisory, medium purple indicates gale warning, dark purple indicates storm warning and green indicates a winter weather advisory.
Those aren't as intense of advisories as you might expect with a swirling cyclone of doom that this particular storm happens to look like.  The worst is a storm warning in the marine zone between Yakutat and Prince William Sound, where they're expecting winds to be decreasing throughout the day from 50 kts to 30 kts.  Otherwise, based on those satellite-derived winds we saw, the gale warnings throughout the Gulf of Alaska make sense as a gale warning covers winds of 35-40 kts.  Makes for a pretty straightforward forecast, at least out at sea.  But what about precipitation or areas inland?  Why no warnings there?

Here's the current radar (1742Z) from Anchorage (PAHG):
Fig 4--PAHG BREF1 Radar image from 1742Z, Nov 4th, 2010
And here's the NWS Anchorage WFO's forecast for Anchorage today:

So we can see that they are indeed calling for rain and snow and some wind.  Why isn't this such a big deal?  Two big reasons--

1) Most of Alaska's major populated areas are not located in areas immediately facing the open Pacific Ocean.  In fact, The coastal mountain ranges including the Aleutians, the Kenai, the Chugach and the Wrangell-St. Elias Mountains all provide some measures of shelter to even "coastal" cities and towns.  Most are on some sort of gulf or inlet that actually puts the city pretty far inland from the open water.  Such mountain ranges can also force a lot of lift and consequently lots of precipitation on the windward, ocean-ward side and leave much drier conditions on the leeward side.

2) It's Alaska.  2-4 inches of snow is normal.  Nothing to be alarmed about...

So, there you have it.  It should be clear that just because a storm looks impressive on satellite (or radar..) doesn't meant that the storm necessarily must be extraordinarily crazy or record breaking.  Alaska has its share of impressive-looking storms and some do arrive with more impressive precipitation and winds than most.  Therefore forecasters up there must be very vigilant and proactive to get warnings out there.  However, like we saw with the satellite-derived winds and the squashed images, getting good weather data in Alaska is rather difficult.  It just makes forecasting that much harder, even if the weather can get somewhat beautiful (fig 1).


1 comment:

  1. Very excited to see an Alaska post- we have awesome weather that often gets neglected :) Another cool feature from this storm are the channeled winds through the gaps of the AKPEN that you can see on the water vapor imagery. This was due to strong cold air advection on the backside of the storm. But like you said- this was not a very impressive storm and I don't think any really damaging gusts occurred there. Also- if you are bored- you should totally check out the cyclone from a week ago- impressive pv intrusion in the vicinity of a jet and an upper level front. This results in strong downward vertical motion so if you have a good low level jet, that momentum gets advected to the surface and you end up with strong gusts. Thanks for the fun posts!