As a part of our local event review of Hurricane Irene, we examined the number of missing ASOS METAR observations across eastern North Carolina, eastern Virginia, and the Eastern Shore of Maryland. It has been widely noted before that surface METAR observations often become unavailable during tropical cyclone conditions as the instruments lose power or communications. The loss of ASOS data during an event greatly limits the number of accurate rainfall, wind, and aviation weather observations which can have a detrimental effect on forecast and warning services. In addition, the lack of quality observations can have a detrimental effect on post event research such as the ongoing CSTAR project examining inland tropical cyclone winds. The image below shows the regional radar reflectivity imagery at 00 UTC and 23 UTC on 27 August as Irene was approaching and moving across the region. In the right panel, numerous METARs are not available across central and eastern North Carolina (click on the image to enlarge the view).
Our review only examined hourly ASOS observations which are directly managed and supported by the NWS/FAA. We did not examine AWOS, ECONET, or other sources. After an initial inspection of the ASOS stations across eastern NC, eastern VA, and eastern MD, we focused on the 12 ASOS stations that were within 150 km of the track of Irene. This arbitrarily defined region is shown in the yellow highlighted area in the map shown below. The ASOS stations examined include: KAKQ, KECG, KEWN, KHSE, KMRH, KNHK, KORF, KOXB, KPHF, KRWI, KSBY, and KWAL. Stations KFAY, KLBT, KRDU, and KRIC were excluded as they were just beyond the 150 km range. After initially examining all of the observations across the study domain on 26 through 28 August 2011, we examined the period in which adverse winds and weather impacted the area.
Hourly ASOS observations from a 42 hour period from 18 UTC 26 August through 12 UTC 28 August were examined for the availability of meteorological data from the ASOS. For the 12 stations examined, during the 42 hour period, there are a total of 504 potential hourly observations. During Irene, 144 hourly ASOS observations were missing, totaling nearly 29 percent (.2857) of the potential hourly observations. The breakdown of observations by location is shown in the chart to the right.
The loss of meteorological data from the ASOS sites likely resulted from the loss of power at the ASOS unit. During other tropical cyclones, a loss of communications capability has resulted in missing observations. This issue has been noted previously in tropical cyclone service assessments in our region including Hurricane Fran and