Sunday, July 22, 2012

Summer 2012's familiar pattern soon coming to an end

This summer is not going entirely as I expected.  June was supposed to be the back and forth month with the wildest weather and the only blast of heat.  July and August was projected to be more reasonable as the last vestiges of the 2 year La Nina had faded away in a transition toward an eventual weak El Nino.  As we know the summer has not gone entirely as planned.

Some good news is that the dog days of Summer will end earlier than last year's Aug 16 date.  Until then a familiar pattern centered further west than 2011 has dominated.  An active and fast Pacific jet stream that is common in a La Nina/Low AAM situation has not yet broken down.  Furthermore, global AAM tanked just in time for the historic heat wave here in the East.  Take a look at the below Water Vapor image from the University of Wisconsin SSEC.
The fast Pacific Jet buckles in the NW US bringing a trough and cool conditions there.
Heat gets concentrated on the SE side of the Jet Stream in a process similar to a heat pump.
In 2011 the jet buckled further inland leading to the epicenter of the heat being over NC.  This year it is further west, locking in the heat over the Central US.  Our region suffered a relatively brief intrusion of outrageous heat at the end of June - early July and has stayed in the "Ring of Fire" periphery since that time.  Therefore since the historic highs most days have been in the mid-upper 90's with plenty of opportunities for rain from the disturbances being transported in by the torrid Central US High Pressure.

Changes are afoot as the La Nina-ish Low AAM condition is receiving a big jolt from Mountain Torque.  My oversimplified explanation is that the buildup of air particularly at the Andes has led to an ever so slight decrease in the earth's rotation in exchange for a jolt of momentum into the atmosphere at the equator.  It will take until the first week of August for the changes to work its way through the global weather machine to NC and VA so just a little more patience is required.

Image Source: NOAA Earth Science Research Laboratory
Note: The negative torque in late June/early July was not
THE cause of the heat wave, but b/c of the season be suspected as
a contributor toward the crash in AAM.
I suspect that some of the long range forecasting problems of recent years has some basis in the best data occurring since the 1960's and especially the 70's.   The period of 1980-2000 was relatively mild compared to the cold Pacific (-PDO) warm Atlantic (+AMO) of this decade.  Our current ocean pattern was more reminiscent of the 1930's and 1950's which still hold most all-time heat records and lots of extreme weather. Therefore we are missing the data that might have led to better accuracy in  forecasting the extremes of recent years.

One good thing is that even stubborn conditions like the extended La Nina must give way to her brother El Nino if only to temporarily satisfy imbalances in the economics that drive the atmosphere.  This and several other influences point the way to a cold winter with a very cold one as a possibility.  Until the Pacific warms and the Atlantic cools the overall propensity toward La Nina and extreme weather can be expected.  

Sunday, July 1, 2012

Heavy Thunderstorms may be taking aim at Raleigh-Durham Overnight.

My last check of Radar before heading to bed is a cause for concern.  A fast moving line of heavy to severe thunderstorms is crossing the mountains and seems to be taking aim at the Triangle.  So far the organized system is maintaining its shape and intensity and could be a problem several hours from now if it continues to encounter a favorable environment.

A cause for concern is the vast area of temps in the Low 80's North and West of RDU that would raise the likelihood that the system will hold together through the night.   Oddly enough, areas south and east of the Triangle feature a broad swath of upper 60's to low 70's .  Here in Garner we seem to be in a buffer zone between the warm and cool areas as it is a nice 75 degrees outside.

Dean Grubbs
The Dean Report