Wednesday, January 9, 2013

Snow Prospects Dim For Next Several Days

The prospects for accumulating snow over much of the nation are unusually low for the next several days, and there is one big factor causing this: the Negative PNA.

The image above depicts the typical atmospheric set-up in a negative Pacific North American (PNA) phase. We see high pressure build in the Gulf of Alaska, causing a depression of pressure to form in the Rockies. This then brings cold air to the region, as well as the North Plains. However, high pressure then forms in the Southeast and East Coast due to the presence of low pressure in the West. This high pressure redirects the storm track to favor precipitation along the Gulf Coast or in the Northern Plains. We already saw this pattern earlier this winter, and now it's returning.

This 500 millibar height anomaly forecast for the next 6 to 10 days clearly shows a defined negative PNA pattern, with high pressure in the Gulf of Alaska, low pressure in the Rockies and high pressure in the Southeast. However, because the high pressure in the Gulf of Alaska is so unusually strong, the low pressure in the Rockies also strengthens and expands, meaning the cold would then push into much of the Plains and parts of the Midwest (i.e. Iowa, Missouri). The storm track would then also push east, meaning any storms that took the southern route would affect the Midwest and Ohio Valley. But Mother Nature prefers to leave the United States snowless, as model forecasts have the North Plains storm track on overdrive and the Gulf Coast/Ohio Valley storm track running on empty.

There is some hope, though:

This is the Day 10 forecast from the ECMWF model's ensembles, commonly called the EPS. The EPS shows the majority of the polar vortex collapsing into the Northeast in a fashion that would inspire frigid temperatures and a very windy period in the entire East US. A negative North Atlantic Oscillation (NAO) is present with high pressure over Greenland, and a positive PNA appears with high pressure on the West Coast. I'm not too keen on buying into this idea at the moment, as other forecasts have the PNA in negative territory for a while to come.

Please do not ask if your area will receive snow, this is a general overview, not a precise snowfall timing forecast.


1 comment:

Anonymous said...

Thank you Andrew! It's not the news that I would like to hear, but I will have to live with it. Your explanation - to this novice - makes sense. Keep up the good work.