Tuesday, May 2, 2017

Blocking Pattern Keeps Us Cool Through the Weekend, Could Warm Us Next Week

The forecast for the beginning of May is definitely cooler than what it should be.  Highs struggled to make it out of the 40's the past couple of days with persistent cloud cover and rain showers.  And while temperatures will be warming from where they have been, the numbers will still fall short of where they should be.

High pressure developing over Greenland will cause the jet stream to buckle heading into the upcoming weekend.  This will cause low pressure, currently forming over the south-central Plains (see previous post), to 'sit and spin' over the East Coast.  Meanwhile, a strong ridge of high pressure will build over the inter-mountain west with temperatures climbing near 80 degrees as far north as Montana.  Temperatures closer to home will not make it much past the low 60's for the weekend as cool north winds keep the chilly air around the Great Lakes and Midwest a little longer.

This pattern, however, breaks down late in the weekend with a second blocking pattern developing.  This jet stream block, known as an Omega Block, keeps low pressure over the East Coast and low pressure in the Southwest, while high pressure builds over the middle of the country.  If the low over the East Coast isn't quite as strong as some models suggest and doesn't push too far westward, temperatures locally could warm back into the low to mid 70's.  However, if the low out East is a little stronger it may push the warmth to the west keeping the continued chill over the Stateline.

So, what's the cause for this shift in our weather?  Part of it has to do with the NAO, or North Atlantic Oscillation.  'The what' you may be asking yourself.  The NAO is a essentially a fluctuation in pressures in the mid-levels of the atmosphere between the central Atlantic Ocean and Greenland.  There are two phases to the NAO, a positive and negative phase.  During the positive phase lower pressure develops near Greenland allowing the jet stream to position itself more from west to east.  During the negative phase of the NAO, the jet stream becomes a little more blocky with high pressure developing over Greenland.  This causes the jet stream to 'back up' so to speak, causing storm systems further west to slow down - like what's soon to develop over the East Coast. 

Now, going back to the potential warm up into next week...
Notice how the forecast (in red) for the NAO keeps it negative through the middle of the month.  If that were to hold true, it wouldn't hold much promise for our temperatures warming early next week.

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