TAMDARat the AAWU

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TAMDAR-at the AAWU

• Wednesday March 5th 2008 TAMDAR Case
• Forecaster Dave Hadley

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• On Wednesday March 5th around 1700 UTC flight
  421397 TAMDAR sounding had a very positive
  impact on the Alaska Aviation Weather Unit's area
  forecast.
• The 1200 UTC sounding at Anchorage International
  Airport showed through the lowest 3,000 feet
  steady southeast winds. These winds were
  producing a very typical downslope event for the
  leeside of the Chugach mountain range.
• Light mixed precip began during the morning hours.

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However, by 1700 UTC the winds had shifted
through the lowest 3,200 feet to a northeast
direction. This switch resulted in cooling of
the lowest layers. The down-sloping was negated
across the lee of the Chugach range causing an
unexpected snow event to occur.
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• The affect of the TAMDAR sounding allowed the
  forecaster to diagnose the rapidly changing
  situation by determining that the cold wedge of
  air had stopped the turbulent mixing process
  below 3,000 feet. This brought an abrupt end to
  the Low Level Wind Shear (LLWS) in the lee of the
  Chugach. The forecaster then was able to adjust
  the SIGMET Juliet 2 forecast of severe turbulence
  from below 10,000 feet to a layer between 3,000
  to 10,000 feet.

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Summary

• The TAMDAR data allowed a SIGMET to be adjusted
  to better represent the meteorological situation
  that was occurring. This adjustment had a much
  lower probability of being detected if the
  forecaster only had the 1200 UTC sounding and
  model data, and then had to wait to the 0000 UTC
  sounding to discover the change in the
  atmosphere.
• The TAMDAR data also helped to explain the
  dominant precipitation type that was occurring
  (more snow occurring than expected).
• The data also gave the forecaster enough
  confidence to drop the LLWS from the area
  forecast and influenced the forecast that was
  issued in the very next forecast package.