Sunday, March 27, 2011

Rare Late March SNOW for Raleigh, NC?

A great distraction caused by the VCU win over Kansas and the UNC loss to KY almost caused me to miss something important going on in the weather!

Instability associated with a wave of low pressure is taking a track that could bring a rare late March snow to Central NC.   Cold air is in place and all that is needed is convection strong enough to precipitate enough to form snow.  The activity over southern TN and N AL needs to organize and maintain strength and track overnight to provide a snow event that will surprise anyone not reading this column.
The circled activity has surprise snowfall potential tomorrow morning.
Source: Univ. of Wisconsin SSEC

No significant accumulation is expected.

Tuesday, March 22, 2011

Big Weather Weekend for Eastern US

Even though Meteorological Spring runs March-May the season arrived yesterday for everyone else.  Here in Central NC and VA the sun was out, temperatures warm, daffodils blooming beautifully, and the TEMPTATION to get summer garden plants in the ground.

Two weeks ago a high pressure ridge setup in east central Asia which displaced Siberian air into North America.  Fortunately a stubborn heat ridge over Florida will blunt the worst of the cold here in the Southeast but areas north of Fredricksburg, VA are about to get buried in an early spring snowstorm.

Big storm complex is nosing SE.
Source: Univ of Wisconsin SSEC

The first storm will dump snow in the Northeast tomorrow.  The training front is windy and fairly dry but should be watched for severe thunderstorm development as it approaches NC and VA.

This weekend features the MAIN Event as Cold high pressure feeds another storm system and forces it to take a southern route.  How far south will dictate whether the snow line can reach Fredricksburg, VA and even press a bit south.  NC and the southeast will face a threat of severe thunderstorms.

GFS and other models depict deepening storm pushing east,

Following the passage of the storm I am very concerned about a frost threat, even a freeze.  My recommendation is to delay planting of summer vegetables until at least April 10.  The weather pattern is shifting to a warm west and cool stormy east.  It is best to allow the cool period to resolve itself then look at mid April to set possible planting dates.

Sunday, March 20, 2011

Winter 2010-11 Roundup Part 2

The question I am most often asked is: Why was everybody wrong about how cold it was going to get?   It is a very fair question because when a person hears the term La Nina and lives in the east the expectation is that most of the season will be mild and dry.  As discussed in part 1, La Nina had far less to do with the winter than the Arctic and North Atlantic Oscillations.

I have a theory based on the late 1970's Ice Age winters that hurricane paths have some correlation to the winter weather pattern.  Last summer's hurricanes roamed the east and Central Atlantic.  There were some impacts on Canada but otherwise out to sea.  Much like Summer 1978. Below is the summer 2010 tropical storm tracks:

Note the paths of long-lived major hurricanes in the West Central Atlantic.

The idea is that the storms carve up the high pressure ridges when tend to dominate the Atlantic by feeding King TUTT (Tropical Upper Tropospheric Trough).  The result is a tendency for low pressure in the Atlantic.  Add the tracks to westerly upper level winds in the tropics (West QBO) and favorable conditions for Negative NAO's and the result is a long lived low pressure region which then leads to cold arctic air flows from the land towards the system.
Image Source: Plymouth State Weather Center.
Upper level westerly winds in the tropics and easterlies in
the high latitudes combine to promote a low pressure area.

Fortunately by mid January the -NAO process began breaking down and pattern became milder (for NC and VA) but there were 2 very serious central US storms and historic Arctic outbreaks.

Now that Winter 2010-11 is now history we appear to continue to be set for a moderate summer which contains serious east coast hurricane threats.  My Summer 2011 preview will be coming soon.

Tuesday, March 15, 2011

Timing of late month cold snap just a little later

The appearance of a ridge in East Asia earlier this month was the beginning of a process that will come to fruition by mid next week.   While there is some model debate about how much cold will be delivered and how far south we can expect a period of cooler and stormier weather than we have seen in a while.

Another big development is the return of the negative NAO which dominated during the extraordinary cold period in December and January.  High pressure near NE Canada blocks the eastward progress of weather systems and sends them on a southerly detour.  Notice how the blocking high is expected to develop at the same time that NC and VA will be heating up!  The heat streams northward as part of the High Pressure building process which results in lower pressure, colder temperatures, and potential stormy weather in the places which were most recently warm.
NAO is once again predicted to go negative
Source: NOAA Climatic Prediction Center
I have little doubt that next week will be very exciting in the eastern US.  Stay tuned for details!

Wednesday, March 9, 2011

Winter 2010-11 Roundup Part 1

For the second winter in a row persistent cold was the story for Central NC and VA.  Who would have ever thought that a La Nina winter with one of the highest 90 day average SOI would be opposite of the historical correlation to US temperatures?
Would you believe that Florida experienced its #10 Coldest winter?
It was supposed to be toasty warm!  Source: NOAA ESRL

Here is how the states ranked in temperature.  Amazingly, only Maine and Utah were above normal!
2009-10 was a strong El Nino and FL hit its record coldest.
This year featured the opposite, strong La Nina.  FL still in top 10 coldest!
Did La Nina impact the east at all?  Pervasive dryness dominated the same regions which were most cold.  A dry winter is expected during a La Nina.
Note the divergence between highest precip (in NW)
and coldest temperature relative to norms in SE.
Finally,  here are some slides showing the dominance of cold in December in the east and a progression toward a more typical La Nina pattern by February. Part 2 will include ideas about what made this past winter unique.  I will also include highlights and lowlights or the season.

Sunday, March 6, 2011

East Asia Ridge portends late March Cold

A special thanks to Joe Bastardi  for directing my attention to recent developments in East Asia.  There is a relationship which he taught me years ago called "Cahir's Ridge", high pressure east of the Caspian Sea which pushes Siberian cold over the North Pole into North America.

Admittedly I had gotten a little weather complacent because of our new involvement with the American-Belorussian Relief Organization.  We are blessed to report that $950 has been donated toward the travel and other expenses to a little girl from the Chernobyl affected region to come to our home for 6 weeks of clean food and air, sanctuary from low level contamination, and medical assistance. 

The timing of the high pressure east of the Caspian is a relationship which would deliver the cold air to the eastern US between March 18-22.  Later this week a low pressure system is expected to replace that high, indicating that the cold surge will be short-lived.

Absolutely resist any temptation to plant your summer vegetables until after mid-April at least (Central NC and VA).  There may be a need to protect freeze sensitive lettuce and other spring cool season crops during the incoming cold snap.