Tightly wound low produces interesting winds in SE Alaska

satellite image from 9:30 pm 2/18/21

Oftentimes the most interesting weather situations are from small-scale storms or processes. The low pressure storm recently producing some impressive winds in the Juneau area is not especially small overall, but the area of strong gradient — where the strong winds are found — is fairly small. Here’s what it looked like before heading inland.

surface wx map
surface wx map for 9pm Thursday 2/18/21

Meteorologists would call this a “tight” low, not just for the compact area, but that the tight gradient is wound close around the low center, compared to most lows where there is a relaxation near the center, a bigger weak “eye” area. This has meant some interesting patterns in the area of high winds and the neighboring areas with drastically different winds. Similar to the eye of a hurricane, when an extra-tropical low center moves slowly over a location, the winds can take some seemingly illogical turns, especially when you throw in the topography of Southeast Alaska.

plot of wind from MXAK stations in northern SE AK
winds from MXAK stations at about 10:30 am Friday 2/19/21 (courtesy MXAK)

This low tracked into northern Southeast via the most obvious ocean entrance: Icy Strait and Cross Sound, and so threaded through many weather sensors along the way. Let’s look at Cape Spencer first, at the northern side of the opening in the outer coast. Here’s a graph of the wind and pressure. It covers the last 5 days…the last day and a half is when the low we’re examining impacted the station:

cape spencer wind-pressure plot
Cape Spencer wind-pressure plot

The green line shows how the pressure made a little double-dip bottoming out when the low passed very close to the station. Wind speeds were up and down in response to the double bottom and when the low passed for good the pressure and the wind increased rapidly, from the opposite direction it had been blowing, shifting from E to NW within an hour or so. Similar things happened in Gustavus and Elfin Cove on the south side, though the top gusts were about 35 knots at Gustavus instead of 50 at the more exposed cape; weaker yet at more protected Elfin. But just off Elfin Cove, exposed more to the strait, is George Island, where the Marine Exchange of Alaska has placed an anemometer:

George Is windspeed
George Is wind speed (courtesy of MXAK)

Some interesting dips and spikes. Seems like the double dip low perhaps registered here at around 9 and 10 this morning then when the low moved to the east of the station, the west wind kicked in about 50% stronger than the east wind had, about 30 knots with gusts to around 45 (35/50 mph). Winds in Juneau were gusting to around 50 mph overnight when the low was still approaching, so winds were from the Southeast. When the low moved inland, it started to weaken, so the wind shift effect was not quite as dramatic for Juneau, but after mostly dying mid-morning, Southwest winds did pick up along with some vigorous rainshowers and even sleet.

To the north, Lynn Canal, the northernmost reach of the inside passage connecting Cross Sound to Haines and Skagway, usually outdoes the other areas in terms of strong winds. This low, however, did not bring the winds there until late in the game. In fact, winds at Eldred Rock, and especially Skagway were strangely light given a gale-force low so close…until 2 pm today. Check out the plot for Skagway:

windspeed at Skagway
wind speed at Skagway (courtesy of MXAK)

And check out the cool polar graph (thanks again to the Marine Exchange of Alaska — MXAK) of wind direction at Eldred Rock, showing the flip flopping of the wind between north and south.

Eldred Rock wind direction
Eldred Rock wind direction for the 24 hrs up to ~3pm 2/29/21. The dots closer to the outer edge are the latest observations. (courtesy of MXAK)

Why was it acting this way? The low was positioned so that the isobars were parallel to the channel. When this happens, the wind would tend to blow from the east, but it can’t due to the constriction of the mountains lining both sides of the north-south channel. In other words the north-south gradient was light, and east-west gradients can’t effect much wind due to terrain blocking. But was enough to blow 20 kts part of the time (which is a gentle breeze for Eldred Rock). When the low began to dissipate and be absorbed by a strengthening low near Prince William Sound the southerly gradient set up and Lynn Canal was back to longer lasting south wind at about 30 kts (~35 mph) with higher gusts, impacting Skagway and Haines at about 15-20 kts (~20-25 mph) plus gusts.

2 thoughts on “Tightly wound low produces interesting winds in SE Alaska

  1. Interesting discussion!

  2. Thanks! We have moved to Sitka after 50 years in Ketchikan and are trying to learn about weather on the NE part of the Gulf of Alaska and the outside of Baranof and Chichagof islands.

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