George R. Kasica
METEO 241 Portfolio #4: My Week as Weather Briefer - Day 4 - September 21, 2006

On the fourth day (click to open) of my week as briefer I decided to stay focused on hurricane Helene even though it was weakening, it was also close to becoming an extratropical system and I felt that it might provide some interesting opportunities as this occurred

The first opportunity was in fact one to help clear up how you can't necessarily apply a smaller scale concept to an entire weather system - in this case the one that a high pressure ridge which will accelerate an air parcel in an anti-cycloinc way will accelerate an entire storm. This was first shown in the post by Shawn O'Leary:

re: Tropical Weather Briefing #4, Week of September 18
SHAWN O'LEARY - 9/21/2006 02:12 PM
If Helene is to follow the predicted path (attachment 1) to the north east of her current position, I would think that the ridge of high pressure (attachment 2) that Helene will encounter will aid in the development of anitcyclonic relative vorticity.  Attachment 3 illustrates the 500mb relative vorticity.  The increase in anticyclonic vorticity should accelerate Helene along her projected path.

1_3Day_Cone_145035W_sm.gif    
2_surface_analysis.gif    
3_500mb_Vorticity.GIF  

Steve Seman provided the key insight that you can't apply theories that deal with individual air parcels to entire weather systems:

re: Tropical Weather Briefing #4, Week of September 18
Steve Seman - 9/21/2006 03:38 PM
Yes, individual parcels do as they travel through a ridge in a relatively quiescent atmosphere.  But, you can't apply parcel-theory to an entire weather system--which spans hundreds of miles.  As Lee said, the mid-tropospheric winds are the key for steering.

Lesson learned here: Be careful when you apply smaller scale concepts in a broad fashion as they may not necessarily be true in the broader context.

The discussion then turned to the impending transition of Helene from a hurricane to an extratropical cyclone and what allows it to maintain it's intensity after this occurs and several good posts were made, most notably one by Philp Lutzak that mentioned the fact that the system would be under the right entrance region of a strong upper level jet stream, which Steve Seman provided a graphic below from the 48-hour GDFL model forecast to illustrate (click graphic for full size image):

re: Tropical Weather Briefing #4, Week of September 18
PHILIP LUTZAK - Edited 9/21/2006 09:15 PM
Hi Steve,
  It'll be right under the right entrance region of a powerful (140-160 knot) jet streak. That'll provide lots of upper level divergence.


(48 Hour GDFL Image from 9/23/06 provided by Steve Seman Courtesy of NOAA)

The day again had a good learning point for me:

  • Be careful when you apply smaller scale concepts in a broad fashion as they may not necessarily be true in the broader context.

Now that we've looked at the fourth day of briefing and responses, let's move on to day number five, Friday September 22, 2006(click to continue), or click the following link to go back to Wednesday September 20, 2006.