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.
Weather turning unsettled mid-week ahead of next storm
Monday, November 18, 2013
A weak wave traverses our state on late Tuesday into Wednesday leading to some showers producing only a few inches on the hill and light rain in the valleys.
A strong wave entering the west coast on Wednesday splits, with the southern portion forming a cutoff low moving southward along the coast while the northern portion moves along the US border with Canada.
By Thursday, 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 just off the coast of southern California. The interaction of this ejecting energy and the cold air from the north could create significant snows lasting into Friday.
Current forecasts have a nice but chilly weekend for northern Colorado as the cutoff low moves across the desert southwest. 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.
There is a lot of cold air lurking to our north after next weekend, and some models want to build a ridge over the Gulf of Alaska which will shunt this cold air over our region. Timing is uncertain, but confidence is growing that an active wet and cold storm track will grace our area around the end of the month.
More snow starting mid-week
Sunday, November 17, 2013
Showers may continue for most of the day on the hill while the valleys should clear this afternoon.
Temperature will warm significantly on Monday and stay warm for at least Tuesday. A weak wave traverses our state on late Tuesday into Wednesday leading to some showers producing only a few inches on the hill and light rain in the valleys, although model runs are trending a bit stronger with this disturbance.
It appears model differences have been resolved for the storm later in the week as the American GFS model has trended strongly towards the European ECMWF model. A strong wave entering the west coast on Wednesday splits, with the southern portion forming a cutoff low moving southward along the coast while the northern portion moves along the US border with Canada. By Thursday, some of this cold air from the northern wave will be over our area as energy begins to eject from the cutoff low now forecast to be just off the coast of southern California.
Some models forecast significant precipitation form this interaction for Thursday and most of Friday, before skies clear by the weekend. There is uncertainty with regards to how much energy is split between the northern and southern streams, so the forecast is subject to change.
