Klotzbach and Gray: "the next two weeks should be active"

Guest post by Ric Werme

The CSU Klotzbach/Gray Aug 18-31 hurricane forecast is out. Based on the NOAA historical tropical storm and hurricane frequency below, they are betting on an uptick in activity.

They’re still looking for active weather:

We expect that the next two weeks will be characterized by heightened amounts (130 percent or more) of activity relative to climatology.

More:

The above-average forecast is due to a combination of factors. The primary factor is that most of the global models are very enthusiastic about TC development over the next few days. Most of the models indicate that the wave currently moving off of the west coast of Africa will develop into a classic Cape Verde-type hurricane in the next few days. These systems typically propagate across the tropical Atlantic and generate large amounts of ACE in the process. Most of the models indicate that the system should be entering an environment favorable for storm intensification, and therefore, the likelihood of this system having a long lifespan across the basin looks good. Models also indicate that TC development in the western Caribbean as well as additional development in the tropical Atlantic also is possible over the next week or so. We do not expect the MJO to play much of a role in modulating storm activity, and since we overall have very favorable climate conditions for an active season, we believe that the next two weeks should be active.

So, things are primed and ready to fire, it’s just that the trigger isn’t here yet, but it’s coming. (Barring unforseen events. There’s always that out….)

They also look at the past two-week forecast:

The two-week forecast of tropical cyclone activity from August 4 – August 17 did not verify well. Activity at above-average levels was predicted, while observed activity was at below-average levels. The primary reasons why we believe activity was reduced during the two-week period were due to dry mid levels in the atmosphere and increased vertical stability This dryness was not expected. Also, several upper-level cold lows intruded into the tropics during the period, imparting upper-level westerly shear and hindering storm formation. However, it is not unusual for the first half of August to be almost tropical cyclone-free.

The only ACE generated during the period was by the remnants of Tropical Storm Colin that regenerated on August 5 but was then torn apart by shearing from an upper-level low on August 8. Our forecast was for an above-average ACE value of 10 or more units (>130% of climatology), while a below-average ACE of 2 units (22%) occurred. The Madden-Julian Oscillation was of a fairly weak magnitude throughout the period and likely did not play much of a role in modulating TC activity.

So, a good collection of suppressors – dry mid-level air and upper level shear

blowing the cloud tops off. While K&G have said the short time frame makes it

difficult come up with meaningful comparisons, predicting > 130% and getting

only 22% must be tough to see.