Satellite imagery from multiple sensors captured the development of a subtle low over far northern Alaska from May 8 to May 11. NWS forecast discussions describe the minor impacts from the system:
From NWS Fairbanks, AK on the morning of the 9th: “A round of snow this morning for the western North Slope as a surface low tracks west across the North Slope. This same low was responsible for overnight snowfall along the central and eastern Arctic Coast and is expected to move off shore tonight. North winds 15 to 20 mph are expected west of this low with no blowing or drifting snow concerns.”
From NWS Fairbanks, AK during the afternoon of the 10th: “Cyclonic flow was evident on VIIRS satellite along the Arctic Coast near Utqiagvik and Wainwright and sporadic snow showers were reported. Look for clouds and periodic snow showers to persist tonight and Saturday morning as the area of low pressure moves farther north into the Arctic Ocean. Fog may redevelop along the Arctic Coast late tonight due to radiational cooling with mainly clear skies.”
Imagery to go along with the latter discussion is shown in Fig 1, spanning from 12Z on the 8th through 23Z on the 11th. A cyclonic circulation is indeed analyzed in the VIIRS IR imagery, but further details about potential troughs, ridges, shortwaves, jets, cyclogenesis, etc, that can be readily diagnosed in moisture channel imagery, are absent. Of course, VIIRS lacks a water vapor band.
Figure 1: 08 – 11 May 2024 VIIRS IR Imagery (M15). From CIRA SLIDER.
GOES-18 ABI water vapor imagery can be useful over Alaska, as is shown for the same period in Fig 2. The basic development of the small-scale circulation can be gathered from the imagery. At such a high latitude, GOES pixel size becomes quite large, and moisture channel imagery appears “cooler” overall as the instrument senses through more atmosphere. Further poleward, the imagery becomes unusable.
Figure 2: 08 – 11 May 2024 GOES-18 ABI Water Vapor Imagery (ch09). From STAR GOES Imagery Viewer
Another, new, option is to view imagery computed from the JPSS CrIS Instrument. CrIS is available on NPP, NOAA-20, and NOAA-21 alongside VIIRS, and is a hyperspectral infrared sounding instrument. Infrared Imagery at wavelengths similar to those from ABI, but absent from VIIRS, is now being generated from CrIS at high latitudes. For example, water vapor imagery from CrIS at the high latitudes fills a data gap due to its absence from VIIRS and degradation of GEO. An example from this event is shown in Figure 3. The imagery becomes more useful the closer to the poles you get, where GEO is completely unavailable.
Figure 3: 08 – 11 May 2024 CrIS Water Vapor Imagery (Similar to AI Ch09). From CIRA SLIDER > JPSS Satellite > Northern Hemisphere Sector.
The imagery has coarse spatial resolution relative to VIIRS, but is still able to capture important features such as drying/cooling that are used to diagnose troughs and ridges, shortwaves, cyclogenesis, etc. With this case, at the beginning of the animation on the 8th, a region of cloudiness/better moisture/lift and localized stronger upper-level easterly flow is apparent over northern/northeast AK. During the first half of the 9th, a small area of cyclogenesis is apparent over time, considering not only the beginnings of cyclonic motion, but given that this is occurring along a gradient in the moisture field. By the end of the 9th, a clear circulation has developed with an obvious cold/moist/ascending and warm/dry/descending couplet. During this period, one can conclude more active weather with cloudiness and potential precip over the western north coast under the ascending branch, and dry weather to the east under the descending branch but where overnight fog may develop with radiational cooling. By the 10th, the system has become occluded as dry air has wrapped into the center of circulation. The low accelerates north off the coast on the 11th.
This evolution can also be captured in CrIS Airmass RGB imagery, another product not available from VIIRS. The RGB reveals the region of cyclogenesis as relative red to dull yellow in the context of clouds and other airmass features (Fig 4).
Figure 4: 08 – 11 May 2024 CrIS Airmass RGB Imagery (Similar that from ABI). From CIRA SLIDER.
Operationally, the imagery can be used as a supplement to VIIRS IR imagery, which captures clouds in detail but does not reveal the important moisture features, and GOES Imagery which provides degraded water vapor imagery over northern Alaska and no imagery toward the poles.
Currently, CrIS Imagery can be accessed on CIRA SLIDER here. More info on the CrIS Imagery is available here.
Bill Line, NESDIS/STAR
