Archive for February, 2011

A Far West Alaskan Blizzard

Friday, February 18th, 2011

I probably blog too much about Southeast Alaska (it’s where I live), so for this post I’m going as far away from SE as one can go and still be in Alaska. Lets look at what Shemya, near the far western end of the Aleutians, and Barrow, on the peak of Alaska’s roof (and points between) have been dealing with weatherwise.

A first class Bering Sea winter storm is just getting through pummeling the far west frontier. This low easily qualified as a meteorological bomb as it deepened rapidly from Japan, across the Aleutians, up the western edge of the Bering Sea and is now weakening northwest of Barrow. Click on the map below to open an animated GIF of the storm’s progress.

bering-low-17-00z

Here’s some of the weather brought by this storm, listed from south to north:

Shemya: “The rock” was hit early and hard and right on the tail of another storm. Late on Tuesday the 15th south winds picked up and were gusting to over 50 mph (80 km/hr) most of the night, all of Wednesday, and into Thursday morning. The storm had a peak gust of 75 mph (121 km/hr) in the wee hours of the 16th.  Intermittent precipitation oscillated between rain and snow.

2011021600.70414.skewt The upper air sounding for Tuesday afternoon at Shemya. Note the strong winds at all levels with 135 kts (150 mph) jet stream winds at around 10 km (33,000 ft). Soundings were missing during the highest winds. Most likely the winds were just too strong for even the Shemya techs to successfully launch a 6 foot (2 m) diameter weather balloon.

The Pribilof Islands: The mid-Bering island group was far enough east to miss the strongest winds, but did have plenty of southerly weather with 30-45 mph (50-70 km/hr) gusts and rain and/or wet snow. This was followed by the cooler westerly flow around the back side of the low, dropping temperatures below freezing and bringing snow and a little drifting and blowing snow.

St. Lawrence Island: Savoonga had around 15 hours Thursday (2/17) with visibilities in snow and blowing snow less than a mile (1/4 mile or 400 meters much of the time) and winds sustained at 30-40 mph (50-65 km/hr) with gusts as high as 69 mph (111 km/hr). Gambell got off a little easier.

Nome: Conditions in the Gold Rush City were much less severe than most in the region. There was some snow and blowing snow and wind gusting to around 40 mph (65 km/hr), but the worst only lasted a few hours on Thursday. Currently however, a secondary low and front behind the first is giving them a 2nd act: Winds have switched back to southeast and increased and snow and blowing snow are back. (see the satellite image below)

s_4f2f Secondary front bullseyeing Nome.

Kotzebue: A slightly subdued version of the Savoonga experience, but with temperatures quite bit colder: from the low single digits (F) (-15 C) on Wednesday rising to the mid 20s F (~-4 C) Thursday compared to 20s and 30s F (~-5 to +5 C) for Savoonga. That kept wind chills down around 15-25 below zero F (-25 to -30 C) through early Thursday.

Point Hope: Fared about the same as Savoonga, but with not quite as high of gusts.

Point Lay: The airport web cam  for Friday mid-day tells the story (compare with stock clear day photo on the right). The low visibility is most certainly caused, as in the other towns, by snow and blowing snow, not fog. Even though many observations report fog during snow storms, it is most often not present — a topic for the future.

PointLay13-19113PointLay-clear10112-clearday

Barrow: By the time this storm got this far north it was weakening, but only slowly. The top of America had winds gusting to around 45 mph (70 km/hr) with visibility around 1/4 mile (400 m) in heavy snow and blowing snow as of the time of this writing Friday afternoon. The temperature, which had been well below zero F (<-18 C) a couple days ago had risen to +16 F (-9 C) around midnight Friday, but has been dropping again as the low tracks into the Arctic Ocean, dragging arctic air around its south side.

Here’s the last 24 hours of observation at Barrow (times are in ADT):

Site M/A Day Time Sky Conditions           VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak
PABR  AA 17 2153  SCT090 BKN150              9          12   8 18018     929  84%  -7
PABR  AA 17 2253  BKN100                     5 BS-      13  11 18020     927  91%  -6
PABR  AA 17 2353  SCT095 OVC150              4 BS-      13  11 19018G26  925  91%  -5  27
PABR  AP 18 0000  CLR                    1 3/4 BS-      12  10 19018G26  925  91%  -7
PABR  AP 18 0021  BKN038 BKN050              5 BS-      16  12 20017     925  84%  -1
PABR  AA 18 0053  BKN032 OVC041              5 BS-      16  14 20018     925  92%  -1  28
PABR  AP 18 0115  FEW041 BKN049 OVC090   2 1/2 BS-      16  12 20019     924  84%  -2
PABR  AP 18 0128  BKN060 BKN085              5 BS-      16  14 20020     923  92%  -2
PABR  AA 18 0153  SCT070                     5 BS-      16  13 19018     923  88%  -1
PABR  AA 18 0253  FEW080 OVC200              5 BS-      15  12 20020     922  88%  -3
PABR  MA 18 0353  BKN075                     5 BS-      14  11 19016G22  922  88%  -3
PABR  AA 18 0453  SCT075                     7          11   8 19018     921  87%  -8
PABR  AP 18 0551  BKN080                   3/4 BS-F     12  10 19018     921  91%  -7
PABR  AA 18 0553  BKN080                   1/2 BSF      13  11 19020G23  921  91%  -6
PABR  AP 18 0555  BKN021 BKN035 OVC045     1/4 BSF      14  12 20020G25  921  92%  -5
PABR  AP 18 0604  BKN026 BKN036 OVC047       3 BS-F     14  12 20020     921  92%  -5
PABR  AP 18 0613  BKN013 OVC036            1/4 S+BS     14  12 20018     922  92%  -4
PABR  AA 18 0653  BKN037 OVC048              1 S-BS-    13  12 21018     922  96%  -5  26
PABR  AP 18 0732  OVC075                     3 BS-      10   9 21020     923  96% -10
PABR  AA 18 0753  OVC075                 1 1/2 BS-      11   9 22022     922  91%  -9
PABR  AA 18 0853  BKN075                     3 BS-       8   6 21021     923  91% -13
PABR  AP 18 0925  BKN075                   1/2 SBS      10   9 21025G30  924  96% -12  30
PABR  AP 18 0938  BKN075                   1/4 S+BS     10   9 21027G32  924  96% -13  32
PABR  AA 18 0953  BKN075                   1/4 S+BS     11  10 22024     924  96% -10  32
PABR  AP 18 1000  BKN007 OVC019            1/4 S+BS     10  10 23026     924 100% -12  30
PABR  AP 18 1039  BKN006 OVC026            1/4 S+BS     10   9 24024G34  926  96% -12  36
PABR  AP 18 1043  BKN010 OVC026            1/4 S+BS     10   9 23027G34  926  96% -13  36
PABR  AP 18 1122  BKN010 BKN019 OVC028     1/4 S+BS      9   7 24027G34  928  91% -14  34
PABR  AA 18 1153  BKN014 OVC019            1/4 S+BS      8   5 24025G33  930  87% -15  34
PABR  AP 18 1201  BKN014 OVC019            1/4 S+BS      7   5 24026G33  931  91% -16  33
PABR  AP 18 1217  BKN014 OVC038            1/4 S+BS      7   5 24024G34  931  91% -16  34
PABR  AP 18 1232  BKN014 OVC038            1/4 S+BS      7   3 24031G38  932  83% -18  38
PABR  AA 18 1253  BKN009 OVC021            1/4 S+BS      6   3 24028G34  933  87% -18  38
PABR  AA 18 1353  BKN015 OVC025              1 BS-       2  -2 25022G30  938  83% -22  35
PABR  AA 18 1453  OVC021                   3/4 S-BS-     0  -4 26022G31  941  83% -24  34
PABR  AP 18 1525  BKN025 BKN032            1/4 S+BS      0  -6 26026G33  941  75% -26  33
PABR  AP 18 1544  BKN012 OVC030            1/4 S+BS      0  -4 25024G31  943  83% -25  33
PABR  AA 18 1553  BKN012 BKN021 OVC035     1/4 S+BS     -1  -5 25024G33  944  83% -26  33
PABR  AP 18 1620  BKN012 BKN021 OVC050     1/2 SBS       0  -6 25023G32  945  75% -25  32
PABR  AP 18 1637  BKN012 BKN021 OVC050     1/4 S+BS      0  -6 25029G34  946  75% -27  38
PABR  AA 18 1653  VV014                    1/2 SBS      -1  -5 25021     947  83% -25  38
PABR  AP 18 1728  BKN025 BKN033            1/2 BS        0  -6 26020     948  75% -23  33
PABR  AA 18 1753  BKN047 BKN130              2 BS-       0  -7 25020G30  949  71% -23  33
PABR  AP 18 1804  SCT055 BKN130              4 BS-       0  -8 24020G26  949  68% -23  26
PABR  AA 18 1953  FEW055 BKN150             10          -2  -9 25018     952  71% -25

What’s the Snow Level?

Saturday, February 12th, 2011

The simple answer: It’s the elevation above which any precipitation is expected to be snow, and below which, rain.

100_7848

It is interesting that the term is not found in the NWS online glossary http://www.weather.gov/glossary/ nor in the AMS’s online glossary http://amsglossary.allenpress.com/glossary/ nor in their printed version, Glossary of Weather and Climate. It is, nonetheless, a term frequently used in official NWS forecasts in Alaska and mountainous areas of the lower 48:

MOTORING FORECAST FOR SOUTH KLONDIKE AND HAINES HIGHWAYS
ISSUED JOINTLY BY NATIONAL WEATHER SERVICE JUNEAU AK
AND ENVIRONMENT CANADA VANCOUVER BRITISH COLUMBIA
1000 AM AKST FRI FEB 11 2011

SOUTH KLONDIKE HIGHWAY - SKAGWAY TO WHITE PASS
.TODAY...RAIN. BREEZY. SNOW LEVEL 600 FEET. HIGHS AROUND 39.
SOUTH WIND 15 TO 25 MPH.
.TONIGHT...RAIN LIKELY THROUGH THE NIGHT. SNOW LIKELY LATE.
LITTLE OR NO SNOW ACCUMULATION. SNOW LEVEL 700 FEET DECREASING TO
300 FEET LATE. LOWS AROUND 32...RANGING TO AROUND 21 NEAR WHITE
PASS. LIGHT WINDS.

But maybe what you really wanted to know is…

How can I determine the snow level?

First let’s talk about a more precise term, the freezing level. It is the elevation where the temperature of the air is zero Celsius (32 Fahrenheit…but Celsius is so much easier when dealing with freezing level and such). It’s not quite as simple as it sounds, though…here are several situations to consider:

  1. Zero freezing level: If the surface temperature is at or below 0 C, and the air above remains 0 C or colder all the way up, the freezing level is zero, or “surface.” This is the usual situation for the colder parts of Alaska during the winter (and spring and fall for the Arctic). The temperature might be way below freezing at the surface and aloft, but we still say the freezing level is zero or surface—even though it really is a non-issue.
  2. Single freezing level: If the surface temperature is above 0 C, and the air cools to below 0 C above the surface and remains below freezing above that, then we have the simplest case of a single above-surface freezing level.
  3. Multiple freezing levels: If there are inversions (layers where the temperatures increase with height) in the atmosphere that straddle the freezing point, then there are multiple freezing levels. This can happen with surface temperatures above or below freezing (the latter being the more common case in Alaska). These situations can be quite complex and can lead to freezing rain or drizzle or sleet. More about this will have to wait for a future post.

In a single above-surface freezing level situation, the snow level will be a certain distance below the freezing level. This distance varies but the rule of thumb is 1,000ft (~300 meters) below the freezing level. This is because as the snow falls into above-freezing air, it does not melt immediately. This is obvious from those times that it is snowing at ground level while the temperature is a bit above freezing, in rare cases by 5 or 6 Celsius degrees (~10 Fahrenheit degrees). Two things come into play. One is that it simply takes some time for the flakes to melt. Another is that melting snow absorbs a lot of heat and cools the air, slowing the melting process. [see my comment update below]

How can I find the current snow level?

Unless you want to carry a thermometer aloft on foot or in an aircraft, you can take advantage of the global 3D weather observing system perfected over the past 70 years or so, and now available to anyone with Internet access. Weather balloons are still the backbone of the system, and you could find the exact freezing level(s) where and when the balloon soundings are taken. This data is ingested into the massive computer models, which not only provide a interpolation of the current freezing level for any location, but then project all weather parameters, including freezing level, into the future. Here’s a map from the Alaska Aviation Weather Unit giving the current day’s freezing levels:

icg click for larger version

You will notice most of the state—almost everything north of the red line—is in situation #1 above, SFC (surface) freezing level. In the Aleutians, the freezing level ranges up to around 5,000 ft (020 means 2,000 ft in aviation speak). In Southeast Alaska it ranges from 2,000 to almost 4,000 ft, but notice how the surface line crosses the green line and runs down eastern part of the panhandle. This is indicating multiple freezing levels…in some places (within the area between the SFC line and the 020 line) the surface is below freezing but there is above-freezing air aloft .

You can find the current version at http://aawu.arh.noaa.gov/index.php?tab=1

How can I know what the snow level will be in the future?

This map from the Alaska Weather TV show gives the next day’s freezing level (but be sure to check the time since the maps are only produced once per day in the afternoon.

Freezing_Level click for larger version

You can find the current version at http://pafc.arh.noaa.gov/tvwx.php. Look under the aviation maps drop down.

If you want to move beyond this simple chart, you can find more detail and longer reach forecasts by looking at the model graphics. Most show lower atmosphere temperature trends, which can be used to surmise changes in the freezing and snow levels, but some include actual freezing level maps. There are also precipitation type maps which show whether rain or snow, or freezing precipitation is forecast. Visit this UAF site and scroll down near the bottom for freezing level maps. Look for “precipitation category” for rain vs snow, and if you know how to read a sounding plot, you can also see forecast soundings for four stations under “Skew-T/Log-P.”

hhttp://knik.iarc.uaf.edu/AtmGroup/ForcastGraphics.htm

Can I estimate the freezing level or snow level from the surface temperature?

Yes, but with plenty of caveats. What is needed is to know how fast the temperature decreases with height (a weather element called the lapse rate). The bad news is that the lapse rate is a) not always constant, and b) not always even lapsing (in the case of an inversion–warming with height). The good news is that we can usually spot those situations where there are probably inversions or otherwise tricky temperature profiles. In most situations where you are dealing with a large scale weather system bringing wind and rain or snow, the atmosphere becomes more homogeneous, rather than layered. It is said to be well mixed. This is more often true on the coast and less often true where mountains block the low level flow from the storms, such as most of inland Alaska. If clear high pressure weather is in place, this method is likely to fail. Caveats done for now, the lapse rate in the mixed situations tends to be about 3.5 degrees F per 1,000 ft, or 6.5 degrees C per 1,000 m. Simply figure how many feet or meters needed to reduce the surface temperature to the freezing point. The snow level will be about 1,000 ft or 300 meters below that. Any error is most likely to be on the low side, since the lapse rate is not usually greater than the figures given, but often less. However if there are lots of cumulus buildups then you have an unstable situation and the lapse rate is probably greater, meaning the freezing level may be lower than calculated. In this case the snow level may be lower yet, since showery type precipitation tends to be more vigorous, with a downdraft to push snow flakes and cool air to a lower level.

Is the snow level ever so high that there is, for practical purposes, no snow level?

Not in Alaska. Virtually all precipitation that falls outside the tropics starts out as snow. Rain is simply snow that melted before reaching the ground. So the freezing level really would be better called the melting level. The one significant exception is drizzle, which can form and fall from shallow, above-freezing stratus clouds.

I hope I answered more questions than I raised, but just in case, the comments feature (link below) is a good place to ask for clarification, or add your own ideas or experiences with snow levels, freezing levels or anything weather.

Snow Talk

Friday, February 4th, 2011

There’s a lot of talk about snow this winter, which makes it just like any winter in Alaska, or say Washington or Massachusetts. If your ski trail looks like the one on the left then the talk’s going to sound pretty down. If it looks like the one on the right, then you’re probably pretty chipper. Click for larger versions.

icy, rocky ski trail snowy ski trail

Well, I took the photo on the left this afternoon, and the one on the right exactly one year ago, from the same spot. So that should tell you how the snow lovers are sounding around here.

Where is the snow (in Alaska)?

Plenty of snow in the Midwest and eastern US, but how about up here where it is supposed to be? For much of Alaska it’s right there under your skis. For us panhandlers it’s up in the mountains. Here’s the Feb 4 map from the River Forecast Center in Anchorage. (there’s a link at the end of the article that will get you the latest, zoomable, interactive map).

snow_pacr

Except for Southeast Alaska, snow depths, and ski conditions are decent. In Southeast, our rain is of course snow in the mountains. Lack of sea-level snow might even suggest more snow up high. This is because the lack of snow is often due not to dry weather but plenty of storms with a southerly trajectory, which means warm and wet. This means plenty of snow above the snow level* and rain below. This seems to be the case recently. Although there is currently little or no snow at low level locations throughout the panhandle, at elevation things are more typical for February. These higher elevations sites are shown on this map and often right next to low elevation stations, so you need to look carefully. For instance, it shows no snow on the ground at the Juneau low level sites, but right nearby there are 3 reports ranging from 30 to 105 inches. These are at Eaglecrest ski area and range in elevation from 1200 to 2600 ft. There are two other reports that stand out on the map, at the north and south ends of the panhandle with 34 and 31 inches. Both can credit their snowiness primarily to being tucked way up long valleys or fjords, closer to the cold interior than the warm coast.

Is there more snow coming?

Don’t count out Southeast Alaska for sea level snow this winter. There are still a couple months left and things can change. Speaking of change, in 2010, after the nice snowy right hand photo above was taken, we had a few days of 40+ weather with copious rain, so change goes both ways around here. The computer models are indicating a continuation of active, fast moving storms, and with a bit of a cooler trajectory, which may be enough to get a least parts of Southeast Alaska into positive snow talk territory again.

more snow info Links:

River Forecast Center snow depth map

downhill ski area reports:
Eaglecrest
Moose Mountain
Alyeska

cross country skiing snow reports:
Statewide
Anchorage
Cooper Landing
Fairbanks
Haines
Homer
Juneau
Seward (can anyone provide a link?)
Skagway (can anyone provide a link?)
Soldotna/Kenai (can anyone provide a link?)
Valdez
Whitehorse

*I’ll go into the snow level in a future blog. Stay tuned, and let me know your thoughts and questions on this post or at least how the skiing is where you live.