Winter Likely To Be Drier and Warmer Overall Across Northeast Florida And Southeast Georgia
by Jason Hess & Angie Enyedi
General Pattern and Official Climate Prediction Center Outlooks
ENSO conditions have turned negative through much of 2010 with moderate to strong La Nina conditions developing during the summer and fall. This has resulted in a persistent ridge of high pressure over the southeastern U.S. which contributed to hottest ever summer across much of the area. Moderate to strong La Nina conditions are expected to continue through the winter season.
The presence of La Nina often correlates with higher probabilities of warmer and drier than normal winter conditions across the Southeast United States depending on its strength. Stronger La Nina events show better correlations than weaker ones. In fact, since this moderate to strong La Nina has developed, much of the area has entered into amoderate to severe drought situation. However, other factors also play a role. Large scale atmospheric patterns such as the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) can also influence storm tracks. Patterns such as these can be highly variable and occur over shorter time scales, making them difficult to predict over the course of an entire season. In a winter season that features a moderate to strong La Nina event…as is expected this winter… these other factors exert somewhat less control over the frequency, strength and motion of large scale weather systems impacting the southeast United States. This is because La Nina winters are usually accompanied by a more dominant ridge of high pressure over the southeastern United States, that should keep many of the deeper cold airmasses north of our local area.
The presence of La Nina often correlates with higher probabilities of warmer and drier than normal winter conditions across the Southeast United States depending on its strength. Stronger La Nina events show better correlations than weaker ones. In fact, since this moderate to strong La Nina has developed, much of the area has entered into amoderate to severe drought situation. However, other factors also play a role. Large scale atmospheric patterns such as the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) can also influence storm tracks. Patterns such as these can be highly variable and occur over shorter time scales, making them difficult to predict over the course of an entire season. In a winter season that features a moderate to strong La Nina event…as is expected this winter… these other factors exert somewhat less control over the frequency, strength and motion of large scale weather systems impacting the southeast United States. This is because La Nina winters are usually accompanied by a more dominant ridge of high pressure over the southeastern United States, that should keep many of the deeper cold airmasses north of our local area.
Climate studies conducted by the National Weather Service office in Jacksonville revealed that Southeast Georgia and Northeast Florida experience the most freeze and hard freeze events when there is not a significant El Nino or La Nina signal. Conversely, most of our extreme precipitation patterns do occur during strong El Nino (wet) or strong La Nina (dry) seasons.
The Climate Prediction Center's official forecast for the 2010-11 winter season (December 2010 through February 2011) is for slightly above normal average temperatures and for below normal precipitation across Southeast Georgia and Northeast Florida.
The Climate Prediction Center's official forecast for the 2010-11 winter season (December 2010 through February 2011) is for slightly above normal average temperatures and for below normal precipitation across Southeast Georgia and Northeast Florida.
Winter Season Temperature Outlook
Data calculated for the last 35 years at Jacksonville indicated that the average number of freezes each year is around 18, regardless of the season’s ENSO phase. During strong El Nino and strong La Nina years, Jacksonville experiences the least number of freeze events. The most Jacksonville freeze events over a winter season occurred during weak La Nina events (average of 21 freezes per season). One-third of winter season hard freeze events were observed during weak La Nina events in Jacksonville (average of 5 per season).
With a moderate to strong La Nina event expected this winter season, we would climatologically expect to see a below average number of freeze and/or hard freeze events. This is due in part to a persistent high pressure ridge over the southeastern United States. This should keep the coldest and deepest sub-freezing airmasses north of the local forecast area and they should impact the area on a shorter time scale. In addition, the general storm track across the country is usually oriented more west to east, keeping the coldest air masses from diving southward into the Gulf of Mexico and extreme southeast United States. Therefore, in general, fewer freeze and/or hard freeze events occur in our area during moderate to strong La Nina winters. This does not mean that this winter will be without any significant hard freeze events in Jacksonville. For example, the last moderate to strong La Nina Event, during the winter of 2007/08 still had a below normal number of freeze and hard freeze events with 12, but, one significant cold airmass did drop the temperature to around 25 degrees for two consecutive nights in January of 2008.
Winter Season Precipitation Outlook
Jacksonville averages 15.10 inches of rain from November to March, regardless of ENSO phase. Significant precipitation departures occur during strong ENSO phases. On average, precipitation surpluses of 6.37 inches occur during strong El Nino years, while during strong La Nina years, the average winter seasonal deficits near 4.71 inches. There is little departure from normal during weak ENSO and neutral phases.
We would thus climatologically expect to see below normal precipitation values continue this winter season given the moderate to strong La Nina event that is expected to continue through early 2011. The departure from normal will be determined by the eventual storm track across the southeast United States. A persistence forecast of the strong ridge of high pressure over the southeastern U.S. would tend to push the storm track to the northwest of the local forecast area. This would continue the moderate to severe drought conditions in place through the winter months and set the stage for a possible dangerous fire weather season in spring of 2011.
We would thus climatologically expect to see below normal precipitation values continue this winter season given the moderate to strong La Nina event that is expected to continue through early 2011. The departure from normal will be determined by the eventual storm track across the southeast United States. A persistence forecast of the strong ridge of high pressure over the southeastern U.S. would tend to push the storm track to the northwest of the local forecast area. This would continue the moderate to severe drought conditions in place through the winter months and set the stage for a possible dangerous fire weather season in spring of 2011.
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