Thursday, September 4, 2014

Tropical–Extratropical Interactions Causing Forecast Challenges

The most recent satellite loop shows that trouble is brewing in the tropics in the form of Hurricane Norbert and a potent slugs of high precipitable water (a measure of the total water vapor content of the atmosphere) over both the eastern Pacific and the Gulf of Mexico.

1200 UTC 2 Sep – 1200 UTC 4 Sep 2014 IR satellite imagery with GFS analyzed precipitable water contours every 5 mm
Model forecasts for the next several days, however, differ depending on how each model handles an approaching upper-level trough in the midlatitude (i.e., extratropical) westerlies.  The upper-level trough digs in the GFS, eventually leading to large scale southwesterly flow over the eastern Pacific and southwest U.S.  As a result, Norbert tracks quickly northward and eventually curves toward SoCal coast, potentially bringing heavy rains to the area on Monday.  If the GFS forecast were to verify, gusty winds would likely accompany the Norbert remnants as they make landfall, but winds would likely be below tropical storm force.  

0600 UTC 4 Sep GFS forecast valid 2100 UTC 8 Sep 2014.  Source: Wundermap.
In contrast, the upper-level trough in the EC remains a bit further north and the flow over the eastern Pacific is weaker.  As a result, Norbert moves more slowly and remains well to the south on Monday. 

0000 UTC 4 Sep ECMWF forecast valid 2100 UTC 8 Sep 2014.  Source: Wundermap.
There are also differences in the forecast for Utah.  With the deeper trough, the GFS taps into a slug of moisture and brings a potent monsoon surge into the state (note the widespread precipitation in the GFS forecast above).  In contrast, the surge is weaker in the ECMWF.

These are the sorts of divergent forecasts that one can get during weak flow situations with interactions between tropical and extratropical systems.  Time will tell if this will be another victory for the ECMWF, an upset by the GFS, or some sort of draw (i.e., Mother Nature splits the difference).  For predictions like this, forecaster's often need a friend.


  1. It looks like the models are having a hard time with the interaction between the very weak trough near our latitude in the eastern Pacific, and a much stronger northern branch system moving into Canada early next week. With the hurricane/monsoon surge it is a pretty complex scenario, although my (somewhat wishful) guess is that a decent chunk of the moisture will reach us. Another interesting factor is that ocean temps along the Baja Peninsula are several degrees F above average, with the 80 degree isotherm right now near the hook-shaped feature about 2/3 of the way up the Peninsula. Might need to start a new batch of Forecaster's Friend.

  2. High CAPE Beer - C? Available Potential Energy? What would the C be?

  3. C = Convective? That would make sense from the sounding on the label. :)