Almost a bomb cyclone — powerful storm hits Southcentral
For more information on the wind, read this article by Chief Meteorologist Melissa Frey:
High wind warning for Anchorage Hillside and Turnagain Arm Friday
The storm responsible for the strong wind Friday afternoon in Anchorage moved in quick, dropping atmospheric pressure rapidly across Southcentral.
Storm Strength and Atmospheric Pressure
A way to measure the strength of a storm is through atmospheric pressure. The lower the pressure, the stronger the storm. For instance, a relatively beautiful day might have a sea level pressure of about 1034 millibars. A strong storm might be somewhere in the 990 mb range. Hurricanes range between 980 mb in a Category 1 storm to as low as 920 mb in a Category 5 storm.
As a storm moves into an area, the pressure will drop. The pressure drops as air in the area rises. Rising air not only relieves pressure, but also forms clouds and eventually storms. The rate at which the air rises determines the strength of the storm. The faster the air rises, the lower the pressure underneath the rising air.
If a storm strengthens fast enough, it can be classified as a bomb cyclone. In order for that to happen, it must strengthen quickly. The central pressure must drop by 24 millibars in a 24-hour time period.
This rapid intensification occurs most often in nor’easters, but every now and then it happens in other locations. Most notably, a spring storm recently strengthened rapidly over eastern Colorado. While that one wasn’t a full 24 millibars in 24 hours, it did meet a corrected criterion for mid-latitude storms.
As the storm responsible for Friday’s wind moved into Southcentral, atmospheric pressure rapidly dropped in Anchorage. Between Thursday morning and Friday morning the atmospheric pressure in town dropped by about 25 millibars.
Ted Stevens dropped by 25.2 millibars between 8:53 a.m. Thursday and 8:53 a.m. Friday. Merrill Field dropped by 24.9 millibars in that time period.
That doesn’t mean the storm itself is a bomb cyclone — just that a powerful storm moved in quick.
The storm itself was close to meeting bomb cyclone criteria. It officially strengthened by about 23 millibars in a 24-hour time period. An incredibly impressive strengthening for a storm in any location. That was enough to prompt warnings in advance of its landfall in Southcentral.
The storm prompted high wind warnings around Anchorage as it moved inland. Wind gusts will near 80 mph on Upper Hillside, 50 here in town.
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