Wednesday, February 17, 2016

Todd, Is It Going To Snow Again?

If I had a nickel for each time I was asked that, I would be funding my own presidential campaign. LOL.

On Tuesday night, the local chapter of the National Weather Association/American Meteorological Society gathered for a meeting.  NWS Meteorologist John Lewis gave a fantastic presentation about the current El Nino and how it has influenced our weather. We are expecting a transition into a La Nina as we go further into this spring and summer and that will of course have impacts on our weather that will be discussed later.  He brought up a very interesting point, we have only had 29 nights at or below 32 degrees in Little Rock so far this meteorological winter (December 1 through February 17th).  That's tied with 2011-2012 for the fewest since 2000.   So where do we go from here?

I really like to look at the Arctic Oscillation Index (AO).  It's usually a great tool to see coming cold within the next 2 weeks.  It's not a long term tool like El Nino and La Nina outlooks.  There has been a near perfect correlation this winter between the positive phase bringing mild temperatures and the negative phase bringing colder temperatures.  The positive AO simply tells us the jet has retreated to the north not allowing the cold air to dive south, but the negative phase results in the transport of colder air to the south as blocking occurs and a more amplified pattern results.   As you would expect, we're in that positive phase now.  We were in most of December, then we went negative in January.  It's during the negative phase that we must watch for moisture as arctic air gets introduced into our region of the country.  Forecasting the moisture aspect is much more difficult until we can get within just a few short days of any event, but it's safe to say we need that AO in the negative for snow.  HOWEVER, I do remember a very positive AO in the past and snow around here.  It happened one December a few years ago with a cut-off low bringing light snow amounts of 1-2''.  Also, if my memory is correct, we had a positive AO many times last winter 2014-2015, but several rounds of arctic air.  That may have been due to another index known as the EPO (Eastern Pacific Oscillation).  Using the AO this winter has proven to be a great tool.  So what about the remainder portion of this month into March?  Check out the maps and charts below.

One more important note I'm now calling the "John Lewis rule" since he told me this several years ago and it makes perfect sense.  Watch when the AO makes a steep rise from negative to positive.  That's when we are more likely to see heavy rain/severe weather events in the winter and spring.

Here's the AO from the Operational GFS off of since the beginning of the month.  The blue line is the actual AO verification.  Notice it's going above the 0 line now into the positive and that correlates with the mild air coming.  Now look down the road at the latest model run with the bigger circles.  It's forecast to dip to neutral then negative by the last week of this month.  All the smaller circles are previous model runs.   Remember, the GFS is run 4 times a day and can be erratic in the long range.
The GFS Ensembles (bunch of models) shows some agreement with the AO going negative soon.  It should be noted the Euro has us going negative with the AO soon as well and I would show you that from, but they have a strict policy about distributing those products.
With the positive AO, we see ridging and a warm flow off the Pacific over the next few days.   The cold air and PV (polar vortex) locked up well into Canada.  The flow off the Pacific warms even further for areas east of the Rockies due to the downsloping effect.  That's why I think we'll be into the 70s and some places over OK, TX and MAYBE western AR could get close to 80.
Now as the AO goes negative, you can see the cold air in Canada getting a push southward as the pattern becomes more amplified by the middle of next week.  If you want cold air here, you really need ridging into western Canada and Alaska.  Downstream, you can see the trough by the middle of next week digging into the central and eastern United States.  This is a colder weather pattern for us.  But what about moisture?  Like I said above, with arctic air getting into the pattern, you must watch as these storm systems develop, but it's impossible to know this far in advance.  At least we have one ingredient right!?
We are tied with 2011-2012 for the fewest nights 32 degrees or colder this meteorological winter.

1 comment:

Wes said...

Great write-up! I do not believe winter is over yet in Texas either. More of the GFS ensembles are trending negative with today's run. It should be interesting as we head into March!

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