Thursday, April 7, 2011

Powerful Windstorm in Alaska

First, let's get one thing out of the way.  Yes--there is pretty good agreement among the global models (the GFS and the ECMWF) that a severe weather event will impact the central US on Sunday and Monday.  I plan to talk about this upcoming event soon...but not this evening.

This evening I want to talk about this amazing storm impacting southern Alaska.  Here's a visible satellite image from a polar-orbiting satellite from 2332Z this afternoon:
Fig 1 -- POES AVHRR Channel 1 visible image from 2332Z, April 7, 2011.
As I've noted before in my blog, it's rather difficult to get the same quality and coverage of data in Alaska as you get in the lower 48 states.  Nevertheless, we can get some good satellite images every once in a while from POES, like what you see above.  There is a huge surface cyclone spinning around in the Bering Sea west of Alaska.  A powerful cold front can be seen moving through the Gulf of Alaska as the sharp, western edge of the clouds in that area.  Note that that frontal zone seems to be well east of the center of the low-pressure in the Bering Sea.  When frontal boundaries seem to separate from their low-pressure centers, it's often a sign that the surface low is filling in, or occluding.  One way to verify that this is an occluding low is to see if it has lost its upper-air support.  Here's the 18Z northern hemisphere analysis:
Fig 2 -- Northern Hemisphere 18Z analysis of surface mean sea-level pressure (contours) and 500mb height (colors) on April 7, 2011.
That is a very, very deep surface low off the western coast of Alaska.  But look at the 500mb heights--the color shadings.  The surface low is directly underneath a deep 500mb trough.  For a surface low to be strengthening, it would be better for it to be on the edge (particularly out ahead) of an upper-level trough, because that's where the jet streaks (and the upper-air divergence) would be.  Since this surface low is directly underneath the 500mb trough, it's probably not under a very divergent region aloft, so this supports the conclusion that the low is occluding.  The HPC surface analysis from this afternoon agrees, showing an occluded front stretching out from the low, connecting it to the true cold front:
Fig 3 -- HPC surface analysis at 2100Z, April 7, 2011.
You can see in their analysis that the meeting of the cold front with a stationary front--usually where we'd expect to find the low pressure center--is somewhere over the western Alaska interior and far from the actual surface low way out in the Bering Sea.  Note the lowest pressure on their analysis in the center of the low--their lowest contour is 956 mb.  That's very deep for a sea-level pressure.

But just because this low is occluding and weakening doesn't mean it's not packing a punch.  High winds behind that still-powerful cold front are impacting all of southern Alaska.  Anchorage is under a high-wind warning, with wind gusts up to 60 mph already reported in the city itself.  At the Glen Alps area up in the Chugach foothills east of town, winds up to 100 mph have been reported--greater than hurricane force winds on the east side of Anchorage.  Combine this with heavy snow falling to the east and further enhanced by the lift provided by the front and it's a rough weather day in south-central Alaska.

We can see evidence of these winds if we zoom in on the visible satellite image:
Fig 5 -- POES AVHRR Channel 1 visible image from 2332Z, April 7, 2011.
The cold front is sharply marked by the western edge of that cloud bank.  Note how there are lots of small cumulus clouds in the otherwise clear air behind the front.  Small clouds scattered like that over the ocean are typically evidence of cold air moving through.  As cold air moves over the relatively warm ocean, small-scale convection can develop which causes lots of small cumulus clouds to form.  But generally, convection is suppressed as the air behind the front is very stable.  However, as air flows over mountains in stable conditions, mountain-induced waves often develop downwind of the mountains.  See the small-scale alternating lines of clouds and clear sky downwind of Kodiak Island (under the light blue arrow)?  Those clouds are marking waves in the air flow downwind of the high terrain on Kodiak Island.  Since they occur on the northeast side of the Island, we know the air must be flowing out of the southwest.  Furthermore, because the waves persist for quite some time downstream of the island, the wind speeds must be rather strong.

You can see how quickly things changed when the front came through.  At 1:42 PM Alaska time, this was the view looking southeast across Anchorage:
Fig 6 -- FAA Southeast Anchorage webcam at 1:42 PM, Alaska time, April 7, 2011.
Lots of low clouds and very overcast.  But an hour later at 2:42 PM, the front had just come through:
Fig 6 -- FAA Southeast Anchorage webcam at 2:42 PM, Alaska time, April 7, 2011.
You can see the rather sharp back edge of all those clouds in front of the cold front receding into the distance.  Since near-surface air is colder behind the cold front, the bases of the clouds rise behind the cold front and that general subsidence causes the clouds to begin breaking up.  It actually looks kind of nice--some sunshine getting through between the clouds behind the front.  But remember--behind the front came the winds as well.  So it's a mixed blessing, I guess.

I will talk about the severe weather potential for the central US late this weekend in the next day or so.  I promise...

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