Valentine's Day Possible Winter Storm

It looks like the atmosphere is preparing itself for a winter storm that would hit around Valentine's Day.

Above is the GFS Ensemble 500 millibar height anomaly forecast for February 10th. We can see low pressure in the Southwest US, as well as high pressure stacked on top of it in the Pacific Northwest and Southwest Canadian regions. This stacking formation is an atmospheric blocking pattern called the Rex Block. When the Rex Block sets up, you have two centers of low pressure and high pressure that are essentially not moving unless the other does. As a result of Rex Blocks that form on the West Coast, one tends to find high pressure and zonally-oriented (plain west-to-east wind flow, no cold shots pushing south) wind flow. This is exhibited well in the forecast image above, where you can see strong high pressure in the East US as a byproduct of not only the solid Rex Block, but also due to the deep trough in the Bering Sea. This trough of low pressure means an index called the West Pacific Oscillation is positive. The positive WPO brings about low pressure in the Bering Sea and high pressure (and warm temperatures) in the East US.

Moving ahead to February 12th, we see that it is the low pressure system in the Southwest that has budged first. As a result, the high pressure system in the Pacific Northwest and Southwest Canada has backed off and begun to shift a tad west back towards the Pacific Ocean and Gulf of Alaska. Due to the concurrent weakening of the trough in the Bering Sea, the positive WPO regime weakens, and the high pressure system in the East US begins to move out to sea and weaken in the process. But back to the storm, that low pressure system that essentially blinked first in the Rex Block has to go somewhere. And the only option is for it to go east into the Plains. This is displayed well in the ensemble forecasts for this depression in 500 millibar heights to shift towards New Mexico and Texas. If we look at the rest of the Northern Hemisphere, we can get a feel for the factors that will play into the eventual track for this system. For one, low pressure is centered over southern Greenland, so this storm is unlikely to go up the coast....for now. Low pressure in the Bering Sea and high pressure in the East pretty much rules out a coastal track or even a track that brings the best snow to the Ohio Valley and central Great Lakes. It is very possible that the track would be up through the central and Northern Plains, a track that has fooled the Central and East US out of far too many storm systems this winter.

This is certainly bound to change, and I will update when it does. This is a look at what could happen come Valentine's Day.

Andrew