The period of 09/27/21 to 10/02/21 was very stormy stretching from the western Bering Sea to the British Columbia and Alaska panhandle regions. Three storm-force lows moved generally west to east with a final storm achieving hurricane force before making landfall just north of Sitka, Alaska. The AirMass RGB imagery from Himawari and GOES-17 along with scatterometer (ASCAT) data available from the Metop-A/B/C satellites provides forecasters at the NWS Ocean Prediction Center with some powerful tools to monitor and predict this series of high impact storms. This blog post will show some examples of imagery, surface maps, and ASCAT data used during this time.
A picture or in this case, an animation is worth a million words. There is so much happening in this animation above that it’s hard to explain any one feature without your eyes being drawn to another feature. There are two storm force lows, a couple gales, and a recurving typhoon. Can you tell which is which? I will break down what is happening here in the following images.
Now with the NWS/OPC surface analyses overlaid, you can tell where the surface low pressure systems, high pressure systems, and recurving typhoon (lower left) are located throughout the animation. For our first storm-force low of interest, I’ll show some AirMass RGB and ASCAT imagery along with surface maps to show the portion of the storm that is the strongest. For the AirMass RGB imagery, the red colors indicate descending dry air that is rich in ozone and potential vorticity. This means the brighter the red coloring, the stronger the warming (drying) in the 350 mb to 700 mb level and the more ozone and potential vorticity available to the storm system. The green coloring is higher moisture and typically, a warmer mid-troposphere, while the bluish, purple coloring is higher moisture, but cooler mid-troposphere.
As you see in the above images, the OPC surface map at 0000 UTC on 09/30/21 was very active with two storm-force lows, a developing storm force in the eastern Pacific, a couple gales, and a recurving typhoon near Japan. The strongest storm force is the one in the northern Bering Sea, west of Alaska with winds of 55-60 kt (65-70 mph) near the Siberian coast (white circle in ASCAT image) as a barrier jet forms as the air quickly converges into a meso- to micro-scale jet due to the strong storm circulation and terrain near the coast. This storm would fill and drop southeast rather fast (>30 kt/35 mph) and begin to merge with the developing gale (near the forecast 998 mb label) south of the Aleutians.
The two animations above show the next phase of this multi-day evolution ending with the strong hurricane-force low that moves towards the Alaska panhandle. Note how fast features are moving south of the Aleutian Islands into the Gulf of Alaska thanks to a strong upper-level jet. I’ll break down this series of storms below.
In this next set of images, another storm-force low is developing as it approaches Queen Charlotte Island, British Columbia, Canada at 0000 UTC on 09/30/21. At this time the storm is producing winds to 45 kt (50 mph), but strengthening rapidly. By 0600 UTC on 09/30/21, the storm achieves storm-force verified by excellent ASCAT coverage with winds 50-55 kt (60-65 mph) nearing the southern tip of Queen Charlotte Island and the Hecate Strait. The storm makes landfall the following hour and later dissipates over the mountains of British Columbia.
Although I won’t cover this next storm-force low beyond the ASCAT image above, winds did approach 45-50 kt (50-55 mph) near southwest Kodiak Island (white circle) which just meets the storm-force warning criteria. This low would eventually provide additional vorticity advection for the hurricane-force low farther south.
The above sequence of GOES-17 AirMass RGB imagery shows the evolution of the hurricane-force low that develops along an active baroclinic zone. These types of storm evolutions are called Shapiro-Keyser cyclogenesis and can lead to some of the strongest (wind perspective) extratropical cyclones.
Thanks to some very timely ASCAT-C overpasses, the hurricane-force low was verified with winds peaking around 75 kt (85 mph), which is very intense for a non-tropical cyclone.
The wind/wave analysis above shows significant wave heights associated with the hurricane-force approaching 41 ft! It is possible individual waves were significantly higher.
Although not available in real-time for forecasters, the above SAR image shows some incredible detail in the winds field associated with the hurricane-force low as it approached Sitka, Alaska during the early morning hours of 10/02/21. Note the strong wind gradient to the northwest of Queen Charlotte Island which depicts the powerful cold front. This was associated with a squall line that produced some rare lightning strikes and wind gusts of around 95 kt (109 mph) at the Ketchikan Airport in the southern Alaska panhandle. Please see the NWS Juneau twitter feed for more information. Meanwhile, multiple wind gusts of 70-90 mph were reported along the coast, while the hurricane-force sustained winds slowly diminished as the storm filled and approached land.
This opening act to the fall/winter baroclinic season in the North Pacific was very impressive and with a Sea Surface Temperature Anomaly map that looks this warm and a strong temperature anomaly gradient across the North Pacific, I expect we will be seeing a plethora of storm and hurricane-force lows in the coming weeks. In the meantime, it has quieted down, though I assume it’s very temporary.
Thank you for reading!