Friday, April 27, 2012

How Can the Past Winter Pattern Influence Hurricane Season?

Earlier in the year, I explained the drivers of this winter's record setting warmth and lack of snow. At the top of the list are the strongly "positive" behavior of the North Atlantic and Arctic Oscillations. (Read my earlier post which explains the specifics of each oscillation) The effects that each oscillation produces can linger for months even after the numbers positive or negative have stabilized. In most seasons when one or more teleconnection is abnormally strong (as was the case the last 3 winters), the change in air pressure, temperature and ocean temperature, the dynamical imprint on the atmosphere can be much greater.  These imprints seem to trickle down into the large scale wave changes and stochastic processes that drive our weather patterns.

A great example of these changes are seen in the Atlantic Ocean sea surface temperatures. Remember the last few winters consistently cold and snowy pattern?  This was largely driven by record low Arctic and North Atlantic Oscillation levels.  Remember that the North Atlantic Oscillation are caused by changes in pressure of the Icelandic Low and the Azores High. If the Azores High becomes weaker, the Icelandic Low stays north/stronger, the NAO is negative, more frequent cold air outbreaks/more clipper snows.  When the Azores high becomes stronger, the NAO goes positive, not as much cold air.  

What does this have to do with hurricane season?  Look at the pressure patterns over the Atlantic in the spring following a strongly NEGATIVE NAO winter. The Azores High (shown in light green) is smaller and weaker.

Dec, Jan and Feb 2009-2010
Dec, Jan and Feb 2010-2011
The steering currents around the high stay weak.  These weak trade winds from the east don't cause upwelling of deeper colder water. Here are the steering winds over the last 2 winters into early spring.

Now this early spring

The turnover of water is much smaller which allows the sea surface temperature to increase.  Here are the Atlantic sea surface temperatures in early spring in 2010 and 2011 after a STRONGLY NEGATIVE NAO winter.  Very warm water is located in the tropical storm development areas.

Now look at this winter's NAO signature.  Remember our mild pattern?  Look at the Icelandic Low and the Azores High.  The High is very strong which kept cold air locked up in Canada.   

The Icelandic Low stays further north which kept the flow across the US and the Great Lakes out of the west NOT THE NORTHWEST.  The Azores High allowed the steering currents to stay strong which is now causing the upwelling of water off of Africa and into the central Atlantic development region.

Currently, the Atlantic Sea Surfaces are much cooler than in springs past covering a larger area.  
So the positive NAO this past winter was a result of a stronger High in the central Atlantic. This strengthened the trade winds which caused upwelling of cooler water which has led to a cooler Atlantic development area.

Will this result in fewer tropical storms developing earlier in the season?  Will this result in less tropical storm or hurricanes affecting the east coast or Gulf coast?  We shall see...

Monday, April 23, 2012

Late Season April Snow On The Way!

Late April snowfalls are indeed rare birds.  The last time Northeastern Ohio had measurable snowfall after April 20th was in 2005 when more than a foot of wet snow fell between the 23rd and the 25th. Normally, it takes me 35-40 minutes to drive home. That evening after the 10PM news, I didn't get home until 1:45AM!  25 mph along the interstate straddling tracks in the snow made by the semis ahead of me. By far the worst drive through snow I've ever had.


The biggest concern will be for downed powerlines across central PA. Leaves on the trees in early spring have the ability to hold more than leaves in late fall. Copious amounts of heavy wet snow will no doubt bring power lines down.  Luckily in northern Ohio, the amounts of snow will be far less.  We have several factors working against widespread snowfall:  1) The track of the storm steers the moisture north, away from northern Ohio.  2)  The ground temperatures are in the lower 40s.  3) The Lake Erie water temp is 50.  Still, evaporative cooling (cooling process within the cloud as precipitation falls--the chill you feel when you step out of the shower) will cause temps to drop sufficiently enough during the night low enough to allow snow to accumulate in spots.  Here are projections through midday Tuesday.

Here are the times in Cleveland when we had snowfall after April 20th since 1948:
2005     April 23-25th  12.4"
1971     April 24            0.1"
1968     April 25            0.2"
1967     Apri 24             0.1"
1963     April 30            0.3"
1953     April 20-21st    1.7"