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.
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.
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…
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.
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:
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.