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Saturday, January 5, 2013

When Will The Snow Come Back?

Snow has dropped off the radar for a while now, and questions are arising- when will the snow be back?

The problem is something called zonal flow. The term zonal flow refers to when the jet stream is displaced north and moves in a monotonous west-to-east line. This monotonous jet stream formation then stirs up above normal temperatures in the East and Central US, and commonly stays until a new pattern comes along.  My weekly forecast for the next 7 days is shown above (see Week 1 and Week 2 explanations by clicking here), and I am expecting a zonal flow formation to set up during this time period. This comes as the Madden Julian Oscillation (MJO) moves into Phases 5-7, which are best known for being favorable for warm weather. So we can pretty much rule out the next 7 days for any snowstorms across the nation.

In Week 2, we see a significant change occur in the jet stream pattern. High pressure has formed in the Gulf of Alaska, leading to low pressure holding its ground in the West US and the Rocky Mountains. The jet stream accommodates this predicament by pulling the strongest upper level winds south, and then pushing them far north as high pressure builds across the East Coast. This is a good example of the negative phase of the Pacific North American (PNA) index. In the negative phase, low pressure builds over the West US and, as physics dictates, high pressure then forms in the East US to balance out the low pressure. The negative PNA pattern brings about cold weather in the West and warm weather in the East. This pattern is probably the most favorable for the Plains, and that is where I expect to see the next snowstorm.

I anticipate low pressure to enter the Southwest, eject into the Southern Plains and move northeast on a track known as a Great Lakes Cutter. This cutter gets its name for going through the Great Lakes region, hence Great Lakes Cutter. This system should move through Oklahoma to Wisconsin, giving snow to the Central Plains and Upper Midwest. Given the high moisture content of the storm system, heavy, flooding rains would be expected across the lower Midwest, Gulf Coast and Ohio Valley regions. I expect this storm to hit between the 11th and the 13th of January.

This negative PNA pattern continues for several days, with the trough responsible for below normal anomalies in the West beginning to slide east very slowly. When the trough breaks free, it should move northeast into Southern Canada, where a meeting with a piece of the polar vortex could be in order. A strong cold front could follow this trough, and that could very well be what kicks off the brutally-cold Late January into February. This would happen around the 20th of January.

So it's looking dry for the next week thanks to the infamous zonal flow pattern. A negative PNA pattern will then emerge in the wake of the zonal flow, and I expect heavy snow to hit the central Plains and Upper Midwest around the middle of the month, with flooding rains in the Lower Midwest and eastern Great Lakes into the Ohio Valley. Beyond that, things get unusually murky thanks to the incoming Arctic outbreak that will start in the West and bleed East.

Andrew

Weekly Forecasts: Warmth Precedes Sharp Cold Outbreak

Warmth is slated to precede a sharp cold outbreak that will occur in late January and into February. Above is my weekly forecast for January 6 to January 13th. I expect a zonal flow pattern to set up, with the jet stream displaced north and warm air overrunning much of the nation. This will certainly decimate any remaining snow pack, thus warming temperatures even further for areas around the previously snow-covered regions. The Madden-Julian Oscillation will be headed to Phases 5-6 during this timeframe, both of which are favorable for warmer than normal temperatures across much of the nation. As time goes on, the strength of the MJO signal in phases 5-6 is forecast to increase, only increasing the strength of warm air that will be present in the East US.

In Week 2, I expect low pressure to build across the West US and Rocky Mountains, and high pressure will then build on the East Coast in response to such an anomalous low pressure system. Colder than normal conditions will be displaced south, as the Arctic Oscillation goes negative and cold air presses south in preparation for the late January arctic blast. As the week progresses, I expect the low pressure system to move northeast into southern Canada, where it will stick around for awhile and bend the jet stream to provide cold air for much of the Plains. Eventually, it will bleed south and penetrate the entire nation later on in the month, when temperature anomalies as low as 10 degrees below normal could be found- and then some. The North American Oscillation should keep negative through Week 2, allowing for this bleeding cold air to at least try and push east. However, the still-unfavorable MJO and still-negative Pacific North American index (PNA) will inspire a battle to form between semi-warm air and frigid cold air. The frigid air will inevitably win out, but it could be delayed from hitting the East Coast thanks to these other teleconnections.

Andrew