Wednesday, November 20, 2013
A strong wave entering the west coast on Wednesday splits the jet stream into a northern polar jet and southern subtropical jet, with the southern branch forming a cutoff low moving southward along the coast while the northern portion moves along the US border with Canada. The polar jet is named after the cold polar air mass to its north, while the subtropical jet is named after its much warmer and moister air mass to the south.
By Thursday noon, cold air from the northern wave will be over our area as energy begins to eject to the northeast from the cutoff low now forecast to be along the coast of southern California. As the polar jet sags south over our area, an overrunning situation occurs where the warm and moist subtropical jet aloft moves over the cold air near the ground brought south by the polar jet.
The timing of this interaction and exactly where it occurs will determine our snow amounts. Until the cold air arrives sometime Thursday morning or early afternoon, precipitation will be spotty rain in the valleys and snow above 9000 feet or so. There should be a period of moderate to heavy snows when the cold air arives as the warm air is lifted over the front.
Additionally, fronts like these tend to become stationary for a period of time around our area as the southern push of the cold air is balanced by the northern push of the warmer air. Given the above uncertainty, I might expect 6-12” on the hill by Friday morning, with latter periods producing light and fluffy low water content powder.
Current forecasts have a nice but chilly weekend for northern Colorado as the cutoff low moves across the desert southwest and a ridge builds behind the departing storm. These cutoffs are notoriously difficult to forecast as their movement depends upon upstream energy that is difficult to measure in the data sparse Pacific. If the cutoff low moves further north than forecast, the nice weekend forecast may be in jeopardy.
It looks like a break in this storm cycle occurs during next week. There is energy in the Pacific that should begin to affect the west coast possibly as early as the following weekend, but there is much model uncertainty as to the strength of the incoming energy and the strength and position of the west coast ridge.