Well advertised pattern change still forecast to occur midweek
Thursday, November 28, 2013
Some very light snow is a possibility during the day Sunday, but amounts will be minimal.
Attention turns to the pattern change around midweek that was forecast as early as a week ago. A storm currently moving across Alaska will rapidly intensify in the Gulf of Alaska the weekend. Some energy splits from this storm by Wednesday and drags cold air across the northern third of the west. The rest of the storm drops south along the west coast and turns the flow over our area to the southwest. Good snow by Tuesday night or Wednesday morning should occur along the frontal boundary in our proximity.
Snow should be relatively steady until intensifying later in the work week as a cold wave from the polar regions kicks the storm over California to the west and over our area. I would expect significant accumulations from Wednesday through Friday before very cold and dryer mid-winter like air will moves over our area by Friday and especially Saturday.
Strong inversion are likely to develop and persist, keepiing mountain valleys in the deep freeze even as the upper elevations begin to moderate by the end of the weekend.
Likely light snow this weekend followed by big storm potential as storm track shifts
Monday, November 25, 2013
As the departing storm soaks the Gulf Coast and the the east coast during Thanksgiving week, a ridge over the west coast produces generally warm temperatures and dry weather until the weekend. A wave to our northeast grazes the area on Tuesday bringing some slight cooling and clouds before temperatures warm and skies around mid-week.
A strong storm in the Pacific crashes into the west coast ridge around Thanksgiving Day allowing some precipitation to reach southern California by Friday. Coincidentally, a strong wave breaks off from the polar vortex and moves south across Alaska, intensifying in the Gulf of Alaska by Saturday. A piece of energy from this storm is currently forecast to move over our area sometime this weekend producing snow as the mountain-top flow turns to the northwest and moistens.
There is a lot of uncertainty with regards to how much interaction between this wave in northwest flow and the storm moving eastward from southern California. Previous forecasts had a dry forecast for the weekend, but current forecasts are more optimistic.
But more certain is that a storm cycle for the northwest and Rocky Mountains begins as the Gulf of Alaska low begins to move south and east. Likely heavy snow starts this weekend along the northwest coast and move inland before reaching our area around Tuesday. Current forecasts have this as a major and long-lasting pattern change that may provide significant snows and cold winter temperatures through at least early December.
Stormy period may follow quiet Thanksgiving week
Sunday, November 24, 2013
The departing upper level low currently near the Four Corners may produce some snow for our area this afternoon, though central and southern Colorado should do much better as they are closer to the storm. A ridge builds over the west coast early in the week creating drier northwest flow over our area.
Generally quite weather is forecast for Thanksgiving week as four disturbances to our north graze northern Colorado on Monday, Tuesday, Thursday and Saturday. The most promising wave is the weekend wave, but models currently have only very light precipitation over our area then.
These disturbances are a result of energy being diverted around the west coast ridge, but the ridge finally breaks down by the weekend before becoming reestablished off the coast. This should allow waves rotating around the polar vortex (a persistent large-scale cyclone near the pole surrounding the very cold air generated in the polar regions during the winter) to finally influence our area early in the week after Thanksgiving.
Incidentally, he forecast I made late last week for the east coast looks to verify as the cutoff low that has been influencing us for the last few days moves east. It looks to soak the Gulf Coast before turning into a nor’easter by Wednesday. And unfortunately for the eastern ski areas, heavy rain is currently forecast to proceed the cold air behind the storm, although a small eastward change in the storm track could change that forecast to all snow.
Pesky cutoff low influences northern Colorado for the weekend before nice weather returns to start Thanksgiving week
Friday, November 22, 2013
About 2.5” of heavy dense snow on my deck yesterday morning, and another 2” of very light powder this morning, although all of that fell by 6 pm Thursday.
The cutoff low currently to our south and west will wobble eastward through the southern Great Basin and then New Mexico this weekend. While there is a ribbon of dry air just to our north, it appears this will be pushed north of us as this cutoff low approaches. There may even be snow snow showers on Saturday and especially Sunday before clearing for the Thanksgiving week as a ridge builds over the west coast behind the departing storm.
Incidentally, it appears this storm will create issues along the Gulf Coast early in the week before possibly turning into a major Nor’easter. This will likely affecting the entire eastern seaboard right around the Thanksgiving holiday, especially if it phases with another storm forecast to be along the Canadian border around that time.
There may be some showers near the end of the week as a storm off the coast interacts with the ridge and begins to send some energy over our area, although the latest model run keeps the storm too far west and south for that. It appears we won’t get a lot of weather from this storm, but it may set the stage for colder air from Canada to begin to affect the northern Rockies by the week after Thanksgiving.
Evolving storm brings in colder temperatures and snow on Thursday
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.
