Friday, March 22, 2013

Arctic Oscillation Pointing Towards Easter Precipitation

What a week!?  At this time last week I warned you about a "spring break gone wild."  Let's look back a little and then I'll tell you what could be way down the road.

Here's a tweet I sent out on March 15th...

After highs in the lower 80s last Friday and Saturday, a strong cold front pushed into central Arkansas Sunday.  Northern and central Arkansas sat in the 40s and 50s all day while southern areas warmed into the 60s and 70s.  Monday, we had severe thunderstorms across portions of central and southern Arkansas.  We thought Tuesday and Wednesday would be the pick days, then a cold rain with snow possible up north.  Never say never to Arkansas Weather!  Salem had the most snow coming in at 8'' and Little Rock had a little sleet, then a cold and miserable rain.

After a Saturday with more cold and rain chances, we'll see some drier, but COLD weather Sunday through Tuesday, then temperatures will begin to climb out of this hole we're in.  It's next weekend I have my eyes on.  Easter COULD be a bumpy ride and I'll explain why here.

At our last meeting of the local chapter of the American Meteorological Society/National Weather Association, Meteorologist John Lewis gave a fantastic presentation about the climate in Arkansas in 2012 and what to expect in 2013.  He mentioned something that really caught my attention.  He said a sudden rise in the arctic oscillation index usually coincides with heavy rain and/or severe weather.  It makes sense when you think about it.  Let me explain and I'll try to make this as simple as possible.  When the Arctic Oscillation index (AO) is negative, this means cold air is allowed to move south out of the polar regions and suppresses the storm track with it.  This is typically when we get cold air and wintry weather.  Once it goes positive, the cold air and storm track shifts to the north.  This index can usually only be forecast accurately within 2 weeks.  Let's look at the AO Now and where it's going.

Look how extremely negative it is and it coincides with the big northern Arkansas snow Thursday evening.
The Global Forecasting System Ensembles agree with a sudden rise going into Easter Weekend, then it drops off again.  We'll worry about that later.  Now let's look at what the two main long range computer models indicate Easter weekend as the GFS and Euro have a storm system, but differ in the details.
DON'T TRUST THE SPECIFICS!!!!!  This is Easter Sunday at 7PM.  Look at the area of low pressure over the Texas Panhandle.  High pressure is located off the east coast.  This is sending in a fetch of Gulf moisture ahead of that low.  This would be a classic set up for heavy rain and big thunderstorms.  However, like I said, DON'T TRUST THE SPECIFICS.  The only thing I want you to see here is that there is a storm and warmer/moist air is transported northward as the AO rises.
This is the European model Easter Sunday at 1PM.  This is a flatter look and mainly a heavy rain producer.  Again, don't trust the specifics, but you can see there is a front and storm system to deal with.  The cold air is north.  See that blue dashed line?  That's a good indicator to the rain/snow line and it has retreated well north as the AO rises.  All the green and yellow blobs are precipitation amounts over the previous 6 hours.

In summary, the next few days will be cold, but we come out of this hole with milder temperatures going into Easter weekend.  With the AO's rise, we see the cold air retreating and the data shows a storm system next weekend.  How it affects us is unknown due to the long range nature of forecasting.  This could be just rain or we could deal with some big storms too.  Stay tuned.

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