Downbursts in Alaska?

Yes- downbursts do occur in Alaska- especially in the Central Interior region during the summer. During the afternoon of 23 May 2009, strong convective storms developed over the Central Interior region of Alaska when temperatures were much above normal for the season. Two strong downbursts were recorded in the Fairbanks area during the late afternoon and early evening. The first downburst wind gust of 40 knots was recorded at the Alaska Climate Research Center (ACRC) weather station at 0015 UTC 24 May (1615 AKDT). A weaker downburst (35 knots) was recorded at Eielson Air Force Base (AFB) near Fairbanks about three and a half hours later at 0350 UTC. The pre-convective environment during the afternoon was more typical for the interior northwestern United States. Strong solar heating resulted in the development of a deep convective mixed layer (ML) that was favorable for downbursts. This favorable downburst environment was effectively indicated by the Geostationary Operational Environmental Satellite (GOES)-11 imager microburst product, in which output brightness temperature difference is strongly correlated with microburst risk. The 2300 UTC GOES imager product did indicate elevated microburst risk with wind gust potential of 40 to 45 knots 75 minutes prior to the downburst at ACRC. Further explanation of the GOES-11 imager microburst product is available in Pryor (2009).


Figure 1. Geostationary Operational Environmental Satellite (GOES) imager microburst product at 2300 UTC 23 May 2009 with overlying radar reflectivity from Pedro Dome, Alaska NEXRAD (PAPD) at 0015 UTC 24 May.


Figure 2. Geostationary Operational Environmental Satellite (GOES) imager microburst product at 2300 UTC 23 May 2009 with overlying radar reflectivity from Pedro Dome, Alaska NEXRAD (PAPD) at 0350 UTC 24 May.

Figures 1 and 2 displayed scattered convective storms developing over the central interior region of Alaska. Apparent in both images are downburst-producing convective storms occurring in a region of elevated microburst risk as indicated by the orange shading near ACRC ("X" in Figure 1) and the yellow shading near Eielson AFB ("X" in Figure 2). In general, output brightness temperature difference (BTD) of 35 to 45K corresponded to wind gust potential of 35 to 45 knots. A downburst wind gust of 40 knots was recorded at the Alaska Climate Research Center (ACRC) weather station at 0015 UTC 24 May (1615 AKDT) followed by a weaker downburst (35 knots) recorded at Eielson Air Force Base (AFB) near Fairbanks at 0350 UTC.


Figure 3. Radiosonde observation (RAOB) at Fairbanks, Alaska at 0000 UTC 24 May 2009.

The pre-downburst environment in the Fairbanks area is most effectively illustrated by the sounding profile shown in Figure 3. Similar to favorable downburst environments over the interior northwestern U.S., the boundary layer was well-mixed with low relative humidity near the surface and a high cloud base near the 700-mb level. This environment fosters strong convective downdrafts through evaporational cooling and negative buoyancy generation as precipitation descends in the sub-cloud layer. Thus, the downbursts that occurred in the Fairbanks area were "dry" type associated with relatively light precipitation (radar reflectivity near 30 dBZ) and an "inverted-v" sounding profile with a deep mixed layer (ML) as shown in Figure 3. Noting the elevated output BTD associated with each downburst, it is evident that the GOES-11 imager microburst product effectively captured favorable conditions for strong convective winds in the Fairbanks area during the afternoon and evening of 23 May.

References

Pryor, K.L., 2009: Microburst windspeed potential assessment: progress and developments. Preprints, 16th Conf. on Satellite Meteorology and Oceanography, Phoenix, AZ, Amer. Meteor. Soc.