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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.

More snow tonight before warming and drying early next week

Saturday, November 16, 2013

Snow continues this afternoon between the two waves before the main cold front arrives around sunset, Sharply colder temperatures and a burst of heavy snow is likely with snow continuing through the night as cold and moist northwest flow traverses the region. Likely an additional 6-12” or more by Sunday morning on the hill, with showers tapering off during the morning and ending by the afternoon as the cold air mass stabilizes.

Temperature will warm significantly on Monday and stay warm for at least Tuesday. A very weak wave traverses the northern part of our state on Wednesday leading to clouds and possibly some showers in seasonably warm air.

A stronger group of wave moves across the area around beginning Thursday afternoon and continuing through the day Friday. The European model has in the past few model runs split the jet stream which would weaken the storms in our area as most of the energy is forecast south of us, while the American model still brings these waves across in a coherent manner. Stay tuned as model differences are resolved in later forecasts.

Two waves to produce significant precipitation by Sunday afternoon

Friday, November 15, 2013

Rain in the valleys and snow on the hill should begin by this afternoon as temperatures start to gradually cool. Probably 2-4” around mid-mountain by Saturday morning.

There may be a smal break earlier Saturday before the main event begins to affect our area by later in the day. Sharply colder air and a burst of heavy snow should cross the area around sunset, with snow continuing through the night as cold northwest flow engulfs the region. Likely an additional 6-12” by Sunday morning on the hill, with showers tapering off during the morning and ending by the afternoon as the cold air mass stabilizes.

Temperature will warm significantly on Monday and stay warm through mid-week. A very weak wave traverses the northern part of our state on Wednesday leading to clouds and possibly some showers in seasonably warm air.

A stronger group of wave moves across the area around beginning Thursday afternoon and continuing through the day Friday. This may produce significant precipitation as winds are forecast to turn to the northwest while cooling, though earlier model runs had this wave more split, so the forecast will evolve.

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