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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.

Nenana is a small town almost in the center of Alaska (about a hour’s drive south of Fairbanks), and so has a continental climate: very cold in winter, quite warm in the summer and fairly dry all year. The top line on the graph shows the temperature as reported hourly by the automated weather station at Nenana’s airport. Below that are dew point, weather occurrences, cloud cover, and wind speed, all of which bear on the diurnal effect. The critical sun influence is charted here in the vertical colored bars: yellow represents the time of the day the sun is up and grey when the sun is down. By May the grey bars have become quite small indeed. The date and time is along the bottom, in both Alaska Daylight Time (ADT) and Universal Time (UTC), also called “Z” time as in 0600Z=2200 (10 pm) ADT.

Radiation is the driver for diurnal temperature patterns

As a quick review, solar radiation warms the earth when the sun is up, while terrestrial radiation attempts to cool it not only at night, but continuously. Our atmosphere interferes with this radiation exchange. With no atmosphere, the moon undergoes a huge diurnal temperature swing, over 450°F (250°C) between high and low (here on earth we measure the temperature of the air, but since the moon has no air, this is the temperature of the ground). Clouds effectively block radiation from the sun and the earth. Moist air hinders the terrestrial radiation, although to a lesser degree than clouds. Wind does not affect radiation but does mix surface air (where the strongest radiational effects are taking place), with more moderate layers of air above, reducing diurnal range.

Getting back to the graph, you can see all these forces at work. From the left side of the graph, late on May 3rd through May 5th, a weather system is moving into the area. The clouds are increasing and lowering (see the white symbols against the blue band). The diurnal range is getting smaller each day. Snow starts falling in the afternoon of the 5th and there is a corresponding drop in temperature and increase in the dew point (meaning the humidity is greater). The wind varies from brief calms to 10 knots (5 m/s), not too strong but typical for the interior. The snow does not last long and the clouds begin to break and lift. As this happens, the temperature begins another drop (this time due to radiational cooling—the sun is setting—vs the snow-induced cooling a few hours prior) but is stopped abruptly by the formation of fog (note that the temperature and dew point are the same…the relative humidity is 100%). Fog does this not only by blocking further radiational cooling but by releasing the latent heat that was once stored in the motion of the water vapor molecules. The Nenana airport was socked in with fog most of the night and on into the morning. Winds were calm most of the time, but in the middle of the night a 3-4 knots (2 m/s) breeze did briefly improve the visibility from around a half mile to 5 miles (1-3 km).

The sun always wins in the end

Fog after snow is straight out of the forecasting 101 textbook. As you may guess the textbooks don’t always cover Alaska’s situations very well, but Nenana in spring is not so different from many places in the lower 48 states, thanks mainly to the balance of sun and night as mentioned above. So about 10 am on May 6th, after the night of fog, the sun banished the fog and low clouds, beginning 3 days of clear weather. The 6th through the 8th and on into the 9th of May is a great example of an undisturbed diurnal temperature pattern…about as good of one you are likely to find in Alaska, I would guess. Notice how shortly after sunrise the temperature begins a steady climb, going from around freezing (32 °F or 0 °C) to around 60 °F (15 °C) by late afternoon, before falling at least as quickly in the evening and night. The cooling begins quite a while before sunset because the radiational cooling effect is going on all the time, and becomes stronger than the heating of the sun (the sun is at a low angle by then). Notice also how the wind speed pretty much follows the temperature curve. On May 10th another weak weather system moves into the area and again, the diurnal range shrinks as the clouds and moisture and wind increase. After a brief rain, things break up again (no fog this time) and the diurnal pattern strengthens. Spring continues to burst forth.

Haines High School students throw around a football in the school parking lot on 18 Feb.

After a fairly seasonable winter through the end of 2009 in Southeast Alaska, 2010 soon broke into what many old-timers call a “January thaw” that seems to want to be known as a “January-February thaw.” Ketchikan has been warm and wet, double the precipitation but ZERO snowfall in January. Juneau has had about 50 inches so far this winter (1/3 last year’s running total) but it has melted to bare ground yet again. Here in Haines we’ve had above average snow on the ground but people are complaining because it’s wet, rather than the mostly dry stuff of the last three winters. (OK we’re spoiled). What is going on here? There are answers on several levels…I’ll try to hit three:

Why #1 –compared to what?

Above freezing temperatures have never been too rare, even in mid-winter, in SE AK (sometimes referfed to as the banana belt of Alaska). In 1977 Juneau had 29 consecutive days with highs 40F (5C) or higher starting on Jan 30. In January, Juneau averages about 16 days where the high breaks 32F (0C) and 19 in February. Roughly 2/3 of the panhandle is warmer yet. But since the past several winters have been fairly “wintery” with typical or colder temperatures and plenty of snow (record amounts in some cases), the contrast is hitting us. Summary: yes, it’s a long warm spell, and some records are falling, but it’s not an unprecedented or even all that unusual situation.

So why does SE AK get these warm spells and what’s causing this one? Stay tuned…that’s why #2 and we’ll explore that in the next post.

This chart show what’s happening up at the jet stream level, about 30,000 ft (10 km) up. The shaded areas highlight the strongest winds, and you can see the west-to-east flowing jet stream, with large dips and humps in its flow. There is a deep trough (dip) straight south of the middle of Alaska, with the eastern leg of that trough bringing warm air from close to Hawaii right to the northern end of the inside passage at over 100 kts (160 km/hr) in the jet core. You can see for yourself that the term pineapple express is not an exaggeration. Following the patterns downstream shows a large ridge over SE AK followed by a giant trough over the entire lower 48 sates, setting up some active winter weather for the east coast.

The next map is for the same time but shows the low level flow. Not right at the surface but at about 4,500 ft (1,500 m), the 850 mb level. This chart reflects what’s going on at the surface, as far as highs and lows, but is better for looking at low-level temperature, since it is mostly above localized surface influences that complicate the picture. Notice the dashed temperature lines (isotherms) which shows air near and above freezing circling around a double barreled low pressure system off the coast and into SE AK. (Remember, this is the temperature at 4,500 ft, so at the surface it is usually warmer in this kind of situation). Meanwhile a stronger low over Nova Scotia has no doubt dumped a bunch of snow on the northeast US and southeast Canada. A stronger-yet low is scouring the western Aleutian islands. Note how each of these lows is aligned under the downstream leg of a upper level trough on the first map.

When this kind of pattern—deep troughs in the jet stream throwing warm air up from nearly the tropics—sets in for a long visit, Southeast Alaska gets plenty of melt weather and on-again, off-again rain. The warm air hose can also whip itself west and warm up Southcentral Alaska (Anchorage and neighbors) and even the interior and more northerly locations (Fairbanks etc), though the effects are a little different. Now, some of you are thinking… “Why does this warm pattern set up like this,”or more pointedly, “Why is it staying this way, this long?” That is why #3, to be tackled in a third post on this topic.

The maps in this post are from the University of Wyoming’s excellent weather briefing site. To pull up maps like the ones above click on “Upper Air Observations” then “Upper Air Maps.”

Taku Winds blow in Juneau

I’m in Juneau for the Alaska Math and Science Conference and enjoying the sun and the beautiful blue sky. Blue sky does not necessarily equate to “fair” weather here. The Taku winds were blowing yesterday (10/13) and probably still are, though weakening. The photo above looks east across the Channel at Downtown. It is a wide shot to show the layout of the land but you still can see the numerous vigorous whitecaps coming toward the camera (click on image for a larger version). I’ve posted some closer shots below the weather map and explanation.

Surface map from approximate time of photos.

On the map you can see a high pressure area well inland of Southeast Alaska and a healthy low off the coast of Vancouver Island. The strongest pressure gradient (indicated by where the pressure contours, or isobars, are packed the tightest) is right over Southeast Alaska. Taku winds are named after the Taku Inlet (a little south of Juneau) and famously ferocious there and in Juneau.

The temperature of the source air of these winds is a big factor in their strength. In winter, the air over the continent is usually colder than that over the ocean. Other factors aside for the moment, cold air is more dense than warm air, and gravity is going to take advantage of that fact. As the air is pulled off the continent by the low pressure area, the gravity is aiding its trip down the slopes of the coastal mountains. If the air over the ocean were relatively more dense, the down-moving air would not have as easy a time displacing it. This is more often the case with the Anchorage area Chugach Mountain winds. When the pressure gradient sets up to draw air over those mountains, the source air (usually coming off Prince William Sound) is usually warmer than the air at the base of the mountains (in Anchorage) it is trying to replace. When that happens the mountain winds blow over the top of the cold surface air.  In many cases, however, the up and down wave action set up by the mountains can eventually erode the cold air, allowing the winds to reach the surface.

Some more photos (click on photo to see larger version):

strong blast off mountainside racing across Gastineau Channel

This next shot was taken from the east side of the Channel (near the library) and show how the downdrafts hit the water and spread out in most if not all directions. A windsurfer out there would have to watch his back!

radiating downdraft gust on Gastineau Channel