Sunday, October 25, 2009
The Disappearing Windstorm
Yesterday, a few of you were noting the potential for a major windstorm later in the week, with the National Weather Service GFS (Global Forecasting System) model indicating an amazingly deep low pressure system right off our coast. The graphic (sea level pressure-solid lines) is shown above for you amusement (132 h forecast starting 5 AM Saturday). SCARY! Hurricane-force winds offshore! Gales in the Strait of Juan de Fuca! The TV stations would have field day if this was true.
But one thing forecasters learn quickly is to restrain themselves that far out and to look carefully at run to run continuity. Is this forecast stable? Did several model runs (forecasts) show the same feature. Did the forecasts of several modeling systems produce the same solution? For this storm, each run has been very different. Want to see?
Here is the next run starting 12-h later...much less scary--and farther north.
And the run starting 12-h later (this morning at 5 am, 108 hr forecast)... its nearly gone.
Or the run starting a few days (Thursday, 5 AM) before the scary one....the low is weaker and far inland.
The bottom line is that forecasters have little confidence in such erratic predictions that far out. Generally forecasts settle down when one is less than about 108 h out...but not always. The current situation is a particularly tricky one, since some of the energy of a tropical storm in the western Pacific is being injected into the midlatitudes...and that inevitably decreases the reliability of forecasts. The technical terms is extratropical transition...or ET.
Posted by Cliff Mass at 2:56 PM