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Winter Expected to be Cooler and Wetter than Normal for Northeast Florida and Southeast Georgia

Winter 2009-10 Outlook for St. Simons Island, Ga
Angie Enyedi, Jason Hess and Steve Letro

General Pattern and Official Climate Prediction Center Outlooks

El Nino conditions have continued in the equatorial Pacific Ocean since June 2009, and the development of the El Nino event may partially explain the slightly below average hurricane season of 2009. Weak to moderate El Nino conditions are expected to continue through the 2009-2010 winter season.

The presence of El Nino (or La Nina) often correlates with higher (or lower) probabilities of wetter (or drier) than normal winter conditions across the Southeast United States depending on the strength of the phase (stronger El Nino or La Nina events show better correlations than weaker ones). However, large scale atmospheric patterns such as the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) can also influence storm tracks. These patterns can be highly variable and occur over shorter time scales, which can make them difficult to predict over the course of an entire season. During a moderate to strong El Nino winter, the NAO and AO can have less influence on the frequency, strength and motion of large scale weather systems impacting the southeast United States. This is because El Nino winters are usually accompanied by a more dominant southern branch of the Jet Stream, which often steers more frequent precipitation-producing low pressure centers across the region. The more active jet stream can also lead to more frequent periods of potential severe weather, including strong squall lines and isolated tornadic events.

Winter season (November-March) climate studies conducted by the Jacksonville National Weather Service office 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 (CPC) official forecast for the 2009-10 winter season (December 2009 through February 2010) is for below normal average temperatures and for above normal precipitation across Southeast Georgia and Northeast Florida (figures 1, 2 and 3).


Figure 1: CPC Temperature Forecast for Dec 2009-Feb 2010 Figure 2: CPC Precipitation Forecast for Dec 2009-Feb 2010 Figure 3: Downscaled CPC Temperature Forecast for Dec 2009-Feb 2010 St. Simons Island

Winter Season Temperature Outlook

Data calculated for the last 35 years at St. Simons Island indicated that the average number of freezes each year is around 11, regardless of the season’s ENSO phase. During strong El Nino and strong La Nina years, St. Simons Island experiences the least number of freeze events. The most St. Simons Island freeze events over a winter season occurred during weak La Nina events (average of 15 freezes per season; figure 4). Over a quarter of winter season hard freeze events were observed during weak La Nina events in St. Simons Island (average of 3 per season; figure 5).

With a weak to moderate El Nino event expected this winter season, we climatologically expect to experience a below average number of freeze and/or hard freeze events. If an active southern stream branch of the jet stream develops and steers more frequent low pressure centers across the region, there could be more cloudiness and less time for cold airmasses to become entrenched over the southeast United States. In addition, the general storm track across the country is usually oriented west to east, which can prevent the coldest air masses from diving southward into the Gulf of Mexico and extreme southeast United States.

Ironically, as seen in the CPC seasonal temperature forecast, despite the likelihood of fewer hard freezes, we expect below normal average temperatures this winter season. While the weak to moderate El Nino events typically produce fewer freeze events than normal, the more frequent storm tracks across the southeast United States usually prevent persistent high pressure systems over region. These highs can block the passage of cold fronts through the area and can bring prolonged periods of warm and dry weather. During El Nino winters, the more active jet stream and storminess over our region usually provide more cloud cover and rainfall, which can keep maximum temperatures below normal, and therefore, overall cooler than normal average temperatures.

Figure 4: Average Number of Freezes based on ENSO Phase Figure 5: Average Number of Hard Freezes based on ENSO Phase Figure 6: Average Extreme Low Temperatures Given ENSO Phase

Winter Season Precipitation Outlook

St. Simons Island averages 15.4 inches of rain from November to March, regardless of ENSO phase. Significant precipitation departures occur during strong ENSO phases. On average, precipitation surpluses of 5.92 inches occur during strong El Nino years, while during strong La Nina years, the average winter seasonal deficits near 5.42 inches. There is little departure from normal during weak ENSO and neutral phases (figure 7).

We climatologically expect to see above normal precipitation values this winter season given a weak to moderate El Nino forecast through early 2010. The departure from normal will be determined by the eventual storm track across the southeast United States and how frequently low pressure centers can develop over the Gulf of Mexico and track across the local area.

Figure 7: Precipitation Departure from Normal based on ENSO Phase