CSU bi-weekly forecast (Sep 1-14): Average activity

[clfenwi]

With average ACE for this period being 28.4 units, the CSU is forecasting average activity (19.9-36.9) during the forecast period. Reasons cited are "most global forecast models" not developing anything new after Fiona and a weak MJO signal. Earl is expected to generate enough ACE in the balance of its lifetime to bring the total up to normal.

They are currently 1 for 2 on their forecasts. Last forecast for above average ACE verified well as there were 36 units of ACE (with 19 units being the threshold for "above average").

http://typhoon.atmos.colostate.edu/Includes/Documents/Two_Week_Forecasts/september_1_2010.pdf

[clfenwi]

By my quick and dirty calculation, this forecast is about to go into the window for verifying. As of last night the total for the period (which includes peak Earl, a bit of Fiona. Gaston, and Hermine) was 19.0275, which is just below the threshold for 'average activity' . Whether this forecast verifies is going to depend on what Igor does. If it steadily develops, it could come fairly close to causing the forecast to bust by bringing ACE to the 'above average' threshold, but it would take Igor making major status in 5-6 days for a bust to actually occur .

[wxman57]

And I think that it's quite likely that Igor will be a Cat 3-4 hurricane in 5-6 days. Not to mention Julia in its wake or Karl in the western Caribbean during this time frame.

[cycloneye]

Is safe to say now that this two week forecast will not be a bust.

[clfenwi]

If ACE doesn't creep into the 'above average' category, it's going to be really close. By my calculation, ACE for the forecast period, as of 12Z this morning, is 29.655. 36.9 is the threshold for above average. Igor persisting at 120 knots or higher combined with Julia persisting would bring the total over the threshold.

I'm working with atcf best track, assuming the forecast period to be 00Z 01 Sep to 18Z 14 Sep. Our numbers, of course, will be slightly different if they're using a different source or start/stop time. Regardless, barring a collapse in Ivan's strength, ACE for the period is going to be really close to or in the 'above average' category

[cycloneye]

What was the final data for this two week period that ended at 2100z? Did CSU nailed or busted?

[supercane]

Bust. Precise verification numbers not given, but verification of this forecast from the new latest 2-week forecast:

VERIFICATION OF SEPTEMBER 1 – SEPTEMBER 14, 2010 FORECAST
The two-week forecast of tropical cyclone activity from September 1 – September 14 looked to verify quite well until Hurricane Igor’s rapid intensification over the past couple of days. Activity at average levels (70-130%) was predicted, while observed activity was just outside the average category (approximately 140%) during the two-week period, due in large part to Igor’s copious ACE generation. As was the case during the last part of the August, most of the activity that occurred was in the tropical Atlantic’s Main Development Region. It appears that, in general, low-level winds were very favorable for storm formation during the two-week period. For example, low-level horizontal divergence was below-average, indicating increased low-level convergence which helps increase the likelihood of storm formation and intensification.
Several storms contributed to ACE during the two-week period, with most of the ACE being generated by Hurricane Earl during the early part of the period and Hurricane Igor during the latter part of the period.).

[clfenwi]

My calculations had the total ACE for the period at 39.055. They needed it to stay under 36.9 for the forecast to verify. The rapid intensification of Igor was the killer.