Medium Range Forecast Exercise for Tallahassee, Florida:
A Prototype for Forecasting along the Gulf Coast in Fall

 

Discussion

Both answers are correct. We will focus on the errors in the GFS 120-hour prediction with regard to the handling of the longwave trough in the eastern U.S. Clearly, there are major errors in the model's prediction of this system. Instead of a trough with a cut-off over Indiana and with one major region of vorticity along the Appalachians, the verification shows a longwave trough with three embedded short waves and associated vorticity centers, one over North Dakota, one over New York, and a third over southern Texas and northern Mexico. With regard to the southern Texas trough, it appears that the UKMET forecast from 23 October 2003 had the right idea in its handling of vorticity centers crossing over the western North American longwave ridge into the central U.S. Recall that the UKMET tended to dig these vorticity centers southward toward Texas, to form a positively tilted trough, rather than propagating them southeastward into a major longwave trough with one consolidated vorticity center over the eastern U.S. It is possible that the vorticity and residual moisture associated with tropical depression Patricia have been incorporated into the southern Texas trough.

Discussion

The verification will necessitate major changes to the forecast. The forecast of cold, dry, clear, and precipitation-free must be replaced with warmer temperatures, greater moisture, more cloudiness, and the likelihood of precipitation. The cold, dry upper-air vortex centered over Indiana with its associated westerly flow over North Florida (Fig. 6) did not materialize. What we now see is a significant trough over Texas giving North Florida southwesterly flow aloft. This would indicate significantly warmer temperatures and greater moisture influx from the Gulf of Mexico. There is enough PVA ahead of this trough (Fig. 5) to expect that it will propagate eastward toward the forecast area, bringing cloudiness and precipitation. If, indeed, the residual moisture from Tropical Depression Patricia is incorporated into this trough, the rainfall for TLH could be heavy.

With a warmer air mass accompanied by greater moisture and cloudiness, the minimum temperature should be raised from MOS's value of 38° and the ZFP value in the lower 40's. It should be higher than the climatological mean value of 52° as well, considering the southwesterly flow aloft. The maximum temperature forecast for MOS (64°) and the ZFP (upper 60's) may not need much adjustment. This is a case of getting it right for the wrong reason. The MOS and ZFP forecasts were considerably cooler than the climatological mean of 78° for the date because of the expected cool, dry air mass. The maximum temperature will likely be well below the climatological mean, but this will be due to the expected cloudiness and precipitation.

A reasonable revised forecast is: cloudy with rain, possibly heavy at times; low in the lower 60's; high in the upper 60's.