You probably know that where the isobars are drawn with the closest spacing is where the gradient is the strongest and therefore where the wind is supposed to be the strongest. So although most of the state should be somewhat windy, SE AK and on north to Cook Inlet should be especially so. Indeed, here is how the Inside Passage looked like at Haines this afternoon:

The interesting thing is that although this extremely large weather system looks fairly simple on the map—nice smooth isobars defining areas of greater and lesser pressure gradient and, presumably, wind strength—the terrain creates a detailed, small-scale patchwork of wind speeds and directions that even some meteorologists have had trouble believing at first. Let’s look closer at the above photo and situation. On the right end of the dock is a wind sock…stretched out horizontally (click on the photo for a larger view). No surprise, the wind was pretty strong there, perhaps 20-25 kts (knots…1 kt = 1.15 mph or .5 m/s) but less, maybe 10-15 kts where I was standing. But look out in the middle of Lynn Canal, or to be more precise, the far 1/3 of the Canal. See what looks like a thin layer of fog? It’s blowing spray…the winds out there are probably a steady 45-50 kts with gusts to 60 or 65 kts. This is not just an exaggerated guess. At the Skagway airport, about 13 miles north (to the left in the photo) the instruments were (and still are as I write this) reporting sustained 35 kts with gusts to 45-50. About the same distance to the south is Eldred Rock where the lighthouse winds are averaging 55 kts sustained, gusts to 70-81 as you can see from the graph below. The state ferry trying to make its rounds to Skagway and Haines today turned around half way and headed back to Juneau.

–graph from the National Data Buoy Center–

Link directly to the Eldred Pock page at NDBC

What’s at work here is channeling. When mountainous terrain has gaps, valleys or channels, which of course it usually does, the wind can flow through those channels to get to the other side, and that flow is often accelerated compared to what it would otherwise be. The wind “wants” to flow through these gaps when there is a difference in pressure between the two sides, which of course there usually is. Here are some rules about channeled winds:

1. The more stable the atmosphere is, the stronger the channeling effect
2. The narrower the channel, the stronger the channeling effect
3. The wind tends to blow along the channel, not across it
4. The wind blows from the end of the channel with higher pressure toward the end with the lower pressure
5. The greater the pressure difference from one end to the other, the stronger the wind
6. The wind is accelerated through, and for a distance downwind of, the channel, but is often light upwind of the channel

Now look a the map of Lynn Canal:

View Larger Map

You can see the extreme channeling leading to these extreme winds. 80 kts is not unusual for Eldred Rock (A on the map).

What about the direction? Look again at the surface pressure pattern, shown cropped and blown up below, Eldred Rock is at A, approximately. From which direction should the wind blow?

Most weather books for the layman, and even many more advanced textbooks would tell you something like this: “The wind blows counterclockwise around the low (N hemisphere) parallel or angled 20-30 degrees across the isobars toward the low.” With this instruction, the wind at “A” ought to be blowing from the east. In fact it is blowing from the north, about 340 degrees true. Many protest and say “the wind can’t circulate clockwise around the low!” Re-read points 3 and 4 in the channeled rules above and look again at the Google map. The channel is oriented right along 340-160 true…just a tad counterclockwise from north-south. Look at the pressure pattern. The pressure is definitely higher to the north, so the wind blows from north to south, right along the channel.

If it makes you feel better, the wind at “B”, the Fairweather buoy, is blowing from the east southeast, at 40 kts gusting to 50. Over the open ocean the coriolis effect is at play and the textbook answer works. Over highly blocking terrain, the coriolis effect is not at play because the air is initially not (hardly) moving, having been blocked by the mountains (it does not want to go over since the air is so stable…rule 1 above). When the air finds a gap or channel, it races through, more directly from high to low pressure. The coriolis effect does not have the time to “turn the wind to the right,” nor can the wind turn to the right since it is constrained in narrow channel of solid mountain! Don’t throw out the textbook, just add some notes in the margin. Few of them acknowledge this exception to the wind direction rules…maybe because most authors and publisher hail from the flatter east coast.

Skagway is in the same channel as Eldred Rock, but the speeds are a little less since the wind is coming off the land and the land has more friction than the water. In the summer, with predominately south winds, there is less difference in speed between Eldred Rock and Skagway. What about Haines? The wind at the Haines airport has been averaging about 5 knots during this event. Why? Look a the map. The Haines airport is a couple miles west of the dot labeled Haines. It’s in the Chilkat Valley, oriented more Northwest-Southeast than north-south Lynn Canal. In this case the isobars hardly cross it–they are pretty much parallel to the channel, so there is little difference in pressure from one end to the other. See rules 4 and 5 above. [If the low were farther east, say over Ketchikan or BC, The wind would blow faster down the Chilkat Valley, and when this happens it often looks on the large scale map like the wind is blowing exactly clockwise around the low, 180 degrees from the textbook solution!] Certainly other parts of Haines, downtown in particular, have more wind than the airport in the current situation, because of their exposure to the edge of the Lynn Canal wind or a wind coming down a smaller side channel. Also, occasional bursts of wind spill over the ridge along the north side of the valley, violating rule 3 above. No absolutes in the weather business.

Now, if the previous material makes any sense at all, get ready for an even more tricky and interesting case. Look up the coast at location “C” on the surface pressure map. The flatlander textbooks would say the wind should be from the northeast, right? Well, by now you know it can’t be right, but you will probably be surprised by this map:

Do the wind speeds and directions look almost random to you? [don’t know how to read the wind symbols? Think of them as arrows, with the point placed on the weather station, the shaft aligned with the wind direction (with the arrow pointing with the wind), and the fletching (called barbs) showing the wind speed: 10 knots for each full barb and 5 knots for each 1/2 barb. Look at Talkeetna for example, the station furthest NW on this map. The wind there is from the north-northeast at 25 knots.] These wind reports are real and they are from the same time as the pressure map above. If you look closely, you will see that almost all the stations with the strongest winds are in, or just at the exit of, a pass, gap or other sort of terrain channel. Most of those have north or northeast wind since those channels will interact best with the large scale pressure pattern as shown on the pressure map above.

The standout exception is Whittier, which has 35 kts from the southwest. Whittier is at location “C” on the pressure map, and it looks like it is not only violating the textbook wind rules but also my channeled wind rules too! What you need to know is that this particular pressure map is too general and smooth to handle this case (other models are more detailed and do show some of these small scale effects). The pressure patterns are affected by the terrain, which in turn gives rise to more possibilities for crazy looking wind directions. In this case the wind is blocked by the substantial barrier of the Kenai and Chugach Mountains and this blockage builds up the air on the west and north sides, the upwind side. This is the higher pressure side to begin with, but the blocking increases the pressure difference, setting up a situation for accelerated gap winds. This damming effect also allows the high pressure to “bend” around the obstacle. For instance, from the pressure map you might conclude that the pressure at Kenai should be about the same as at Whittier, when, in fact, it is a fair amount higher. This bending allows almost any break or gap to be an outlet to the pressurized air, no matter the direction. So the wind at Whittier blows from the southwest whenever there is a low pressure area to the east, southeast or even a fair ways into the south quadrant. Notice the small circle just to the west of Whittier? That is Portage Glacier Visitor Center, a weather station only 9 miles from Whittier as the crow flies, yet so different weatherwise. The wind there is calm. Remember rule 6. There are several other examples of this and the other rules on this map. Can you spot them?

I’d love to hear your questions, comments, etc on this, or other Alaska Weather topics. Click the comments link.

Snow rollers form when the snow possesses a certain layered tackiness that allows the top layer to peel off the underlying layer (or sometimes the ground) and stay together as it gets rolled into a ball or tube. The rolling can be bone by either gravity or the wind. No elves or gremlins are needed. If by gravity, a pretty steep slope is usually needed. If wind, then you can guess that a pretty strong wind is needed.

The rollers in this photo were traveling UP a slight incline, so it had to have been the wind, and some strong gusts to boot. Notice the tracks left by the rollers show that the wind was coming from slightly different directions at different times, consistent with periodic gusts strong enough to move the rollers interspersed with weaker wind. At the very top of the photo you can see the fence around our Haines Little League field. The whole area between the foreground of this photo and the ball field, the entire ball field, and a gravel pit to the right was littered with thousands of these roller ranging up to the size of a large rolled sleeping bag. Here’s a couple photo at the ball field. By then using a flash was the only hope of a photo and the small bright spots are snowflakes caught in the flash.

What weather conditions lead to snow rollers?

There are several ways the layered tackiness needed for snow rollers can come about. One way is a rapid warming of a dry snow pack such as can happen when a strong storm moves into an area. Downslope (Chinook type) winds in mountainous areas usually bring both rapid warming and strong winds. This was the case with once incident I published a photo of in a past Alaska Weather Calendar in the eastern Chugach Mountains in which the photographer actually saw the rollers being formed. In the recent Haines case, wet snow fell on top of a hard frozen crust as a front brought a surge of warmth and moisture. Here are the weather observations for the Haines Airport (about 1.5 miles or 2 km west of where I found these snow rollers). The photos were taken between 1600 and 1630 AST on the 15th. (The times shown on these observations have been converted to ADT – Alaska Standard Time). The wind had been gusting to around 30 kts (35 mph or 15 m/s) since morning, and the had been snow coming down pretty heavy and at a temperature right at, or slightly above, the melt/freeze point…the perfect temperature for a nice cohesive kind of snow. (The temperatures shown on these observations have been converted to Fahrenheit). Remember, the snowpack up to that time had been a hard rain crust, hard enough to jump up and down on (and according to one report, hard enough to drive a small truck on top of!) The new snow was not going to bond to that very easily.

Site M/A Day Time Sky Conditions   VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak

PAHN  AA 15 0954  OVC008             1 S-F      32  31 11012     987  96%  22
PAHN  AP 15 1019  OVC008           1/2 SF       32  30 09011G30  986  92%  22  30 
PAHN  AA 15 1054  OVC008           3/4 S-F      32  31 10013G24  984  96%  22  30
PAHN  AP 15 1111  OVC008           1/4 S+F      32  30 11013G22  983  92%  22
PAHN  AA 15 1154  OVC006           1/4 S+F      32  32 11014G27  981 100%  21  28
PAHN  AP 15 1211  OVC006           1/2 SF       32  30 09015G29  981  92%  21  29
PAHN  AP 15 1219  OVC006           1/4 S+F      32  32 09015G28  980 100%  21  29
PAHN  AP 15 1230  OVC008           1/2 SF       34  32 09020G30  980  92%  22  30
PAHN  AA 15 1254  BKN008 OVC013    1/2 SF       33  32 09015G29  979  96%  22  30
PAHN  AP 15 1342  OVC008             1 S-F      34  32 11012G26  977  92%  25  32
PAHN  AA 15 1354  BKN008 OVC012    1 1/2 S-F    33  32 09013G24  975  96%  23  32
PAHN  AA 15 1454  BKN008 OVC014    1 3/4 S-F    33  32 09013G26  974  96%  23  26
PAHN  AP 15 1510  BKN010 OVC014        2 S-F    34  32 10012G28  975  92%  25  28
PAHN  AP 15 1520  BKN008 OVC014    2 1/2 S-F    34  32 10014G29  975  92%  24  29
PAHN  AP 15 1525  OVC010               3 S-F    34  32 10014G29  974  92%  24  29
PAHN  AP 15 1534  OVC009               4 S-F    34  32 11014G27  973  92%  24  29
PAHN  AA 15 1554  BKN009 OVC013        4 S-F    34  32 11016G27  973  92%  23  29
PAHN  AP 15 1621  OVC011               4 S-F    34  34 11016G31  972 100%  23  31
PAHN  AA 15 1654  BKN011 OVC015        5 R-F    35  33 10020G33  973  92%  23  33
PAHN  AA 15 1754  OVC013               7 R-     35  33 11012G25  976  92%  26  31
PAHN  AA 15 1854  OVC015              10 R-     36  34 11011G25  980  92%  28  29
PAHN  AP 15 1911  OVC013              10 R-     36  34 11009G20  981  92%  29
PAHN  AA 15 1954  BKN015 OVC065       10 R-     36  34 10009G22  984  92%  29
PAHN  AA 15 2054  OVC014              10 R-     36  34 11010G21  986  92%  28
PAHN  AP 15 2114  BKN016 OVC022       10        37  34 11011G19  987  89%  29

It is quite likely there were many more snow rollers in our area that day. There are miles of river flats nearby that get this sort of wind or stronger. I would love to hear of anyone who saw snow rollers on December 15th in the Chilkat Valley. Or any time or anywhere for that matter. Snow rollers are pretty uncommon, but not “once-in-a-lifetime” rare as some websites like to spout. Again, it is a matter of being aware, being outside and keeping your eyes open. Of course it does not hurt to live in a snowy area.

Fair weather rollers

Another way the snow can reach the right consistency for snow rollers is the warming of a cold snowpack by the sun. In this case gravity is likely going to be the motive force to form snow rollers since strong winds and warming sun don’t often come together. Sometime snow falling off trees get the ball rolling but sometimes they start on their own. Here’s a beautiful example of some gravity snow rollers taken in April along the Alcan Highway in the Yukon Territory, courtesy of Alan Sorum of Valdez.

There are many more photos of snow roller on the Internet: Here are some good ones with good information. You may notice most of them are from the plains states…wide open fields, snow, quick changing weather and plenty of wind.

http://www.wrh.noaa.gov/otx/photo_gallery/snow_rollers.php

http://www.crh.noaa.gov/ilx/events/roller/roller.php

http://webecoist.momtastic.com/2011/01/25/snow-rollers-tumblin-tumbleweeds-of-icy-white-delight/

So be outdoors, and look up… and down. Let me know what you see.

1.56 MB animated gif

This gusty south winds impacted more than Skagway. Somewhat gusty wind shifts could be tracked up the panhandle. At Eldred Rock, about 35 miles south of Skagway in the highly channeled northern Lynn Canal, winds switched to south at 37 kts with gusts of 51 kts (43 & 59 mph or 18 & 25 m/s) about an hour before Skagway. Haines got hit with less ferocity, but still enough to get everyone’s attention and kick up a bunch of dust and pollen. Here’s a shot of the pollen being whipped up and blown right up and over the 2000+ ft (600+ m) shoulder of Mt Ripinski in Haines:

Skagway, the real windy city

Skagway, if you’ve never been there, sits in a narrow valley extending off Taiya Inlet—the northern end of Lynn Canal (a fjord, not a man made canal). The wind sweeps through the town proper with pretty much the same strength as it does at the closely adjacent airport (where the weather station is). So we can say the airport ASOS (automated wx station) is quite representative of the town, something that cannot be said for many cities. This day, Skagwegians and thousands of visitors off the 4 large cruise ships in port were no doubt enjoying the 77F (25C) warmth and light winds at 1pm. Yes, the north winds had been blowing–up till a couple hours earlier–around 15 mph (7 m/s), which, incidentally,  is why the temperature had risen so quickly.

Bait and switch

The switch to a cooler south wind in the afternoon is actually fairly common here, but the surprise was the magnitude of the contrast. Check out the progression from the hourly reports:

Site M/A Day Time Sky Conditions           VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak
PAGY  AA 02 0853  CLR                       10          70  39 03014G24  920  32%  70
PAGY  AA 02 0953  CLR                       10          73  38 04011     917  28%  74
PAGY  AA 02 1053  CLR                       10          76  37 05011     915  24%  78
PAGY  AA 02 1153  CLR                       10          76  37 03007     913  24%  78
PAGY  AA 02 1253  CLR                       10          77  39 00003     910  25%  77
PAGY  AA 02 1353  FEW100                    10          62  46 22013     909  56%  60
PAGY  AA 02 1453  FEW110                    10          63  44 22014     908  50%  61
PAGY  AA 02 1553  BKN110                     5 H        64  44 20032G44  912  48%  61  45
PAGY  AA 02 1653  BKN100                    10          63  45 19029G35  916  52%  59
PAGY  AA 02 1753  OVC090                    10          59  47 20017G26  924  64%  55
PAGY  AA 02 1853  FEW055 OVC070              6 R-       56  49 21020G25  928  77%  51
PAGY  AA 02 1953  SCT055 OVC065             10 R-       55  49 19013G19  932  80%  51
PAGY  AA 02 2053  BKN055 OVC070             10 R-       55  50 16007     935  83%  53

The report times are in local time (ADT), so this covers 9am to 9pm inclusive. The switch to a south wind and the drop in temperature happened between 1 and 2pm. But, again, locals would recognize that as the sea breeze kicking in. But a couple hours later the wind doubled, with a peak gust of triple the former speed…enough to wreck havoc with loose items or un-braced lightweight people. The “H” under the weather column at 1553 stand for haze but was really local dust, silt etc being lofted by the wind. The peak wind notes on the 1553 ob occurred at 1540 (2340 UTC) a detail I got from the undecoded ob:

METAR PAGY 022353Z AUTO 20032G44KT 5SM HZ BKN110 18/07 A2912 RMK AO2
    PK WND 19045/2340 WSHFT 2336 SLP860 T01780067 10250 20167 53006
    TSNO=

What caused this blast?

As you can see on the surface chart below (valid at 4 pm, close to the time of the Skagway blast), there is a front cutting across SE AK (click on it for larger version). The front is moving north, and on the previous map (6 hrs earlier), it was drawn at Dixon Entrance (not yet in AK). Sure, fronts cause wind shifts and temperature changes, but looking at this chart and thinking June it is not obvious what was going on in upper Lynn Canal. Three things seem to not jive with the ground truth at Skagway. (That’s the fun of weather.) 1: The front is drawn as a occluded front, meaning the warmest air has been pushed aloft, and such fronts often have a subdued effect on the surface. 2: The front is drawn with a broken line, which indicates the analyst decided it was beginning to dissipate. 3. The front is drawn as barely past Sitka, about 100 miles (160 km) south of Skagway.

One solution

Here’s some factors I think bridge the seeming discrepancy. I think the front intensified and accelerated as is moved up SE AK, due to low air densities in northern SE. The analyst who drew in the fronts can hardly catch every detail when he or she has much more than just Alaska to worry about (the chart you and I see on the internet is drawn at the NWS HQ back east—our local offices draw their own or at least add their own detail.) It could be argued that this was a mesoscale effect and did not need to be drawn on the synoptic scale chart. That may be partly true, but looking at the buoys off the coast, one could track this front both with more speed and power than the map suggests. Here’s a graph of wind and pressure for the Cape Edgecumbe Buoy off Sitka:

Remember, fronts are boundaries between air masses of differing densities (to keep it simple). Higher density air is going to tend to displace lower density air, push it back or up or both. This is pretty intuitive. Lower density air is air which is warmer, more humid, or under lower pressure, or some mix of the three. What happens when an air mass is moving and displacing less dense air ahead of it, and the air ahead of it is getting less and less dense? Just like an army who has broken through the heavily guarded battle front and finds little resistance behind it, it will accelerate. The warming of the land in the northern Panhandle and onshore into Canada created a region of low density air (for you pilot types the density altitude prior to the front was around 2200 ft (>600 m)).

I’d love to hear your thoughts (or direct experiences) on this situation, or at least what you thought of the write-up.