Sunday, August 30, 2009

WE'VE ENTERED THE ZONE

SYNOPSIS
The Atlantic basin is entering the historical annual peak of tropical cyclone activity, represented by the two week period on either side of September 10.
Relatively few hurricanes in 2009 may have permitted areas of the western Atlantic and Gulf of Mexico to experience warmer-than-normal sea surface temperatures.
El Nino is present in the equatorial Pacific and expected for duration of the 2009-10 Northern Hemisphere winter.
These and other indicators may signal a snowier winter for the Eastern United States, concurrent with the possibility of an earlier than usual start to the winter storm season.

10:00 AM Sunday, 8-30-09 The Atlantic Basin is nearing the climatological "high water mark" of tropical cyclone activity, historically peaking on September 10. The next two or three weeks may turn out to be the most active period of the entire season. However, a strengthening El Nino signal in the equatorial Pacific may continue influencing westerly shear enough that any developing systems spend more time looking like a threat than turning into one. On the cautionary side, sea surface temperatures (SST) in the Gulf of Mexico are at or above the seasonal peak, with large areas 86 F or greater. All of us coastal-types are not only entering the school zone, but also the hurricane primetime zone. With the 4-year anniversary of Katrina a painful reminder, anything tracking across the Altantic for the next 2-3 weeks bears close monitoring.


For snow-starved powderhounds, the current SST trend is a strong indicator of what dreams may come this winter. I closely monitor Global SST anomalies in the fall as a key signal of what winter may bring. A prime example of this hypothesis is the winter of 2002-03. Hurricane Isabel was the only notable system to significantly affect the East Coast, during a weakening El Nino with Pacific temperature anomalies averaging 1.0 C. Moderate shear influenced by those Pacific anomalies finally relaxed in mid September 2003, permitting Isabel to make her unstoppable westerly beeline for the East Coast. This year, El Nino is stronger, with "sea surface temperatures across the equatorial Pacific .5 to 1.5 C above average," per the Climate Prediction Center's weekly report. The CPC goes on to say, "Current observations and dynamical model forecasts indicate El NiƱo is expected to strengthen and last through Northern Hemisphere winter 2009-10. For snow-starved Mid-Atlantic powderhounds, this could be a very good sign, and a source of my hypothesis.


This hypothesis is centered on two seemingly unrelated events: (1) A strengthening El Nino increases potential for disruptive westerly wind shear in the Altantic basin; and (2) Wind shear that reduces or deflects the impact of tropical cyclones near the coast permits SST's to reach warmer-than-normal levels in certain areas. Undeniable evidence is plain to see in the current Caribbean SST anomaly map as of 8/27/09, as denoted by the "blue swath" indicating the path Hurricane Bill followed in mid-August.


What's the connection to a snowier winter? Just keep reading, we're almost there. But compare the Atlantic SST data to the Gulf of Mexico on that map. Notice that large areas of the Gulf are at or near 86 F (30 C). Whatever energy not removed from the water by tropical activity during summer may persist into winter. That energy can be tapped by developing winter storms crossing the southeast in the much the same manner of the President's Day Storm in February 2003. Although 60 or more days remain for the tropics to "stir things up," with some water temperatures already well-above normal, it will take several strong systems to put a dent in those levels. There are several more factors that must take their place in the "Circle of Life" as it pertains to a snowier signal for this winter, among them trends in the North Atlantic Oscillation, Northern Hemispheric snow cover, Arctic sea ice, and most importantly tracking the snowline descent from Canada each fall. The National Ice Center has links to useful daily imagery.

The long and short of it is this: I hypothesize a high probability winter 2009-10 in the Mid-Atlantic produces at least the climatological "normal" amount of snow (for example, a 30-year average of 19.2 inches at BWI). If the trends continue, perhaps we will see a whole lot more. The real answer that administrators and district-level employees seek is simply: "That's nice for the teachers, but will it be ENOUGH to close schools AND offices? ;-)

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