Archive for October, 2010

Meteorological Bomb Hits Southeast Alaska

Monday, October 18th, 2010

Yes, bomb is a technical term in meteorology. In a general sense it is a rapidly-strengthening extra-tropical storm (the common low pressure disturbances that regularly spread rain, snow wind etc across the mid latitudes), and as the name implies, with the capacity to do damage, usually via wind. Also implied is the element of surprise. To qualify as a bomb the central pressure must drop by 24 millibars in 24 hours. Our specimen is a low that made landfall in Southeast Alaska last week (10/12/2010). Lets see if it fits the general or technical criteria.

This infrared satellite image ought to at least suggest to the layman, and convince anyone with meteorological training that this was a powerful, well formed, yes, even beautiful storm. The spiraling frontal band wraps 1-1/2 times around a tight-looking low center a little northwest of Sitka. This photo does not tell us much about its life story, though, and we’re wanting to know if this storm is of the fast hitting variety.

infrared satellite image

For that let’s look at the surface chart (click for a larger version): The first one is from 4pm ADT on Monday the 11th. The low in question is labeled 1000 (millibars or mb for short). Its the one in the lower right part of the frame which happens to be straight west of Seattle and straight south of Anchorage, but is not aimed at either metropolis, but at the sparsely populated panhandle of Alaska.

sfcmap00

Now, at 10 pm the low has moved northeast and deepened to 993 mb, a drop of 7 mb in 6 hours, a vigorous strengthening by most standards. But this is Alaska in October. Stay tuned…

sfcmap06

At 4am the low has dropped to 976 mb. That’s a total deepening of the requisite 24 mb, accomplished in not 24 but 12 hours! The rapid deepening along with the fast movement of this storm meant that any mariner or landlubber in its path had rapid and major weather changes to deal with. It is a testament to the science that this storm was well forecast far enough in advance that mariners could seek shelter and landlubbers could make provisions. That might not have been the case 20 years ago.

sfcmap12

For just one example of the rapid weather changes, check out the Sitka observations quoted below from the early morning to mid-day when the storm was hitting this coastal burg the hardest. At 5 am conditions were VFR with only 10 kts of wind. A little light rain was falling but, hey, when is it not? By noon the visibility is down to a mile in heavy rain and the wind is 23 kts with gusts to 57 kts, the next hour 64 kts (that’s 74 mph for your land lubbers or about 32 m/s for you scientists.)

Site M/A Day Time Sky Conditions           VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak
PASI  AA 12 0253  OVC080                    10          48  40 13008     962  74%  44
PASI  AA 12 0353  OVC055                    10 R-       48  40 13013     955  74%  42
PASI  AA 12 0453  OVC055                    10 R-       49  39 11010     949  68%  44
PASI  AA 12 0553  BKN070 OVC090             10 R-       48  37 11015G22  942  65%  42
PASI  AA 12 0653  OVC060                    10 R-       50  36 10013G24  930  58%  45
PASI  AA 12 0753  OVC046                    10 R-       50  38 11025G36  917  63%  42
PASI  AA 12 0853  OVC042                     9 R-       49  40 10018G32  908  71%  42
PASI  AA 12 0953  BKN036 OVC050              6 R-       49  42 12016G26  906  77%  43  30
PASI  AA 12 1053  BKN041 OVC048              4 R        49  43 12013G20  906  80%  43  28
PASI  AP 12 1146  FEW028 OVC038              2 R        50  45 14028G37  910  83%  42  40
PASI  AA 12 1153  SCT024 BKN034 OVC042   1 1/2 R        50  44 15023G45  911  80%  43  45
PASI  AP 12 1201  FEW020 BKN029 OVC035       1 R+       50  45 15029G57  912  83%  42  57
PASI  AA 12 1253  BKN025 OVC030          1 1/4 R+       49  44 16036G64  921  83%  39  64
PASI  MA 12 1453  BKN035 OVC043              5 R        50  42 16036G58  932  74%  41
PASI  AA 12 1853                             3 R-F      49  45 19022G32  958  86%  41

Other land stations fared similarly, with automated marine observations at exposed locations clocking 65-90 kts (75-100+) and the high elevation station on Sheep Mountain, near Juneau hitting over 100 kts (120 mph). Here are some specifics:

Metlakatla: peak wind 55 mph. Ketchikan: peak wind 60 mph, and around 2 inches of rain. Hydaburg:  peak wind 70 mph. Port Alexander:  peak wind 69 mph. Little Port Walter: 2.85 inches rain in 24 hrs. Hoonah: 2.55 inches rain in 24 hrs. Juneau: peak wind 68 mph; power knocked out in some areas due to trees on power lines. Pelican:  peak wind 71 measured by the Pelican Elementary School station, plus 4.46 inches of rain in the 24 hrs up to 8 am on the 13th (a record for that date but by no means for the the month or year). Skagway:  peak wind 54 mph.

Coastal marine observation stations are automated weather stations (for reasons which will become obvious) placed at  locations pertinent to marine interests. Usually they have a much higher wind exposure than inhabited places—in some cases they are in locations with highly accelerated winds due to channeling by terrain. Here are some of the more extreme examples out of the SE AK marine obs. These kind of winds are not all that uncommon at these stations.

Cape Decision:  peak wind 74 kts. Sisters Island:  peak wind 67 kts. Cape Spencer: peak wind 86 kts. Eldred Rock: peak wind 64 kts. Lincoln Rock: Winds hit 87 kts then the wind speed went missing for seven hours, coming back online after the height of the storm.

Ocean (or moored) buoys are faithful and indispensable reporters of the weather off the coast. Most of these are away from the small scale channeling effects of the terrain, and may seem understated compared to the coastal marine obs that rack up the wind speeds like those above. You have to read them with that in mind. You can also look at the size of the waves reported in a storm to get a reality check. Here’s a plot of pressure, sustained wind speed and gust speed at buoy 46084, the Cape Edgecumbe buoy, located about 50 miles SW of Sitka, just south of the low’s track. Note the hurricane-like pressure fall and rise on 10/12, and the attendant winds. Significant wave heights were over 30 ft at this buoy near the time of the highest winds. The wind scale is in meters per second (m/s). Double it if you want knots, add another 15% to go to from knots to mph:

46084Mplot_wind_pres

If you are interested in seeing where these coastal stations are located, or getting the data for them, visit the National Data Buoy Center’s excellent web site at http://www.ndbc.noaa.gov/maps/Alaska.shtml.

I’d love to hear any first hand accounts of this storm (mariners and landlubbers welcome). Use the comment form for that or any questions, or suggestions for topics to cover. I’m still working on an outlook for coming winter and hope to post it next week.

Who Flipped the Switch?

Monday, October 4th, 2010

After a once-in-a-decade-or-more dry spell for 2/3rds of Alaska the weather machine in evening the score. Here in Haines we enjoyed 12 straight days with no precipitation and some record warm afternoons (and, yes we did get the hay cut, dried and into the barn, but it did take 5 days to dry. See the hay-cutting-weather post). This was the general case for all of normally-wet Southeast, Southcentral (Anchorage’s drought was broken a up by several days of “trace” amounts of precipitation—if you ignore those they had a 19-day rainless period), and right on into the interior where Fairbanks had 21 consecutive dry days (no traces even). Of course Anchorage is normally quite a bit drier than Southeast, and the interior drier yet. So compared to climatology, the September 2010 dry spell award should go to Yakutat, where the average September doesn’t see more than 8 dry days, consecutive or not. This year there were 14 in a row (by the way, that was it, 14 dry days total!).

HNStemp-precip

Well, on September 23rd the switch was flipped, the omega block gave up, and the low pressure systems started a rampage that is just slowing a bit now, but not stopping. The first weather system was an especially strong one for so early in the storm season…or for any season. The infrared satellite image below shows a vigorous front blasting through Southeast AK, while the low center (looks like a hook or almost an eye) hangs offshore about 100 miles south of the coast with heavy convective cells rotating around the south side of it:

4afm

Below is the surface analysis for a few hours earlier, showing the tightly wrapped isobars:

sfcmap18

The storm was at its lowest central pressure at this point, a very low 953 mb. It slowly weakened over the next couple days, but it still had enough strength to wreck havoc from British Columbia to the Alaska Range. Marine areas were hit with winds up to 90 mph. Many parts of Southcentral AK also had high winds, due to the strong pressure gradient from the interior to the Gulf of Alaska, and the acceleration of the wind squeezing through mountain passes (the Venturi effect). Here are some specifics from the NWS:

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
545 PM AKDT FRI SEP 24 2010

...WIND REPORTS FROM ACROSS SOUTHCENTRAL ALASKA...

THE HIGHEST WIND GUSTS RECORDED TODAY AS OF 5:30PM ACROSS
SOUTHCENTRAL ALASKA.

-------------------------------------------
    SITE                            GUST
-------------------------------------------
BETWEEN WASILLA AND PALMER         78 MPH
VALDEZ AIRPORT                     62 MPH
PALMER AIRPORT                     61 MPH
ANCHORAGE INTERNATIONAL AIRPORT    59 MPH
WASILLA AIRPORT                    52 MPH
MERRILL FIELD IN ANCHORAGE         48 MPH
KENAI AIRPORT                      46 MPH
TALKEETNA                          44 MPH
SOLDOTNA                           28 MPH

STRONG WINDS WILL CONTINUE INTO THE EVENING HOURS...WITH AREAS OF
REDUCED VISIBILITY IN MANY PLACES DUE TO BLOWING DUST.

Since this first storm broke down the gates, there has been a new assault every few days, and most coastal areas are about to surpass the dry spell with an equal or longer wet spell. In actuality, this is much more typical than the dry spell. The main thing that stands out now is that the strength of many of these lows is quite impressive for early October.

What’s next?

Does this progression mean anything? Will it continue? Beyond the 5-10 days (variable) of useful forecasts from the computer models (which right now are calling for a continuation of the current storm cycle), we enter into the realm of general, broad brush forecasts of whether it will be wetter or direr than normal, warmer or cooler etc. averaged over a time period. There is not much to go on, but making some statistical associations with longer-term climate and oceans cycles (such as El Nino and La Nina among others) may have some validity and possible usefulness in predicting what kind of winter we might have, for instance. Am I bold enough make that kind of forecast? Check back around the middle of this month to find out.