Arctic surge likely to return near the end of next week
Thursday, December 12, 2013
Mountain slopes have warmed nicely while the valleys remain cold while trapped in strong temperature inversions. In fact, warming temperatures aloft counter-intuitively strengthen lower-level inversions as vertical mixing in the lower levels is suppressed in the stabilizing airmass. The cold air in the valley bottoms then stays cold as the surface warming is minimized by the shallow sun angle and reflective snow surface.
There are some weak disturbances influencing our area starting tomorrow, though there is very little chance of significant accumulations. A wave to our south crosses too far south Friday morning for any snow, while another wave from the northwest is forecast to split as it moves over us later Friday, perhaps producing some light snow showers. Yet a third wave is forecast for early Sunday morning and that may produce a bit of snow as well.
Waves graze our area in warm and dry northwest flow Tuesday and Wednesday, but only a slight moderation of temperatures are expected. The developing story is the likely return near the end of the work week of Big Blue, the meteorological euphemism for an arctic outbreak similar to what we observed last week.
Lots of details are still to be resolved by the numerical models, but agreement between the models is trending stronger with this major cold and likely snowy pattern change near the end of next week.
Light snow today and late Friday as mountain slopes warm and valleys stay cold
Tuesday, December 10, 2013
Very light snow is currently falling on the hill as a weak wave in northwest flow grazes our area today. Temperatures on the hill are running about 10F warmer than yesterday, and they will warm further, especially Wednesday and Thursday. Mountain valleys, however, will remain cold as temperature inversions maintained by the clearing skies expected tomorrow and Thursday allow for strong nighttime cooling of the snow surface.
Our next chance for snow occurs late Friday as another weak Pacific wave from the northwest is forecast to cross our area. Snowfall is expected to be light at this time, but the bigger effect may be that valley inversion are moderated as the storm passes.
Seasonal weather is expected to continue into the following week before another arctic surge may impact us near the end of the work week. This outbreak may be similar to the current one with sharply colder temperatures and moderate to heavy snows, though a 10 day forecast is likely to change as the event nears.
Temperatures dropping rapidly as storm moves through
Sunday, December 8, 2013
Most of the remaining part of the storm is currently moving through the area as temperatures up top have dropped from 0F at 5am to -7F at 8:30am as light snow continues. Temperatures will likely continue to fall or remain this cold until tomorrow morning when a trailing wave finally pushes the coldest part of the storm east of the area. Snow showers on the hill will likely continue until tomorrow afternoon, though additional accumulations after noon today will be minimal due to the very cold temperatures and drying airmass.
Another grazing wave in northwest flow will keep temperatures cold and start light snow showers on the hill again during Tuesday afternoon and evening. There should be some warming on the mountain slopes by Wednesday, but it will be most noticeable on Thursday as the trough that had been sitting over the western part of the US moves east. Mountain valleys, on the other hand, are likely to remain cold as strong inversions develop and persist.
Models currently forecast a weak storm for the following weekend. A flat ridge looks to build in the Gulf of Alaska which may shunt Pacific energy to our north and create mostly dry conditions after that.
More snow and slightly less cold this weekend before next arctic surge on Monday
Friday, December 6, 2013
The very cold temperatures these last few days will moderate a bit later today and into the weekend as another surge of colder air drops into the western part of the large trough over the western US. This forces more southwesterly flow over our area by late tonight which should bring some noticeable warming to the mountain slopes. Valleys will remain cold.
That energy to our west will begin to move over our area by Saturday afternoon and will produce light to moderate snowfall through Sunday. While the absolute moisture in the air is low due to the cold temperatures, we may see some significant accumulations if the temperatures warm enough for the snow crystals called dendrites to form. Dendrites are the easily recognizable pointy star-shaped crystal that leads to very low density fluffy snow. If temperatures are too cold, as they are today, the snow crystals that form are more needle-like and pack together much more efficiently, leading to higher snow densities and denser snow.
Due to the snow continuing Saturday night and most of Sunday, I may expect a fluffly 6-12” on the hill by Sunday afternoon. The flow does finally turn to the northwest during the day Sunday which is constructive for snowfall, but moisture should be decreasing then partly offsetting the favorable wind direction.
A final dry trailing wave looks to cross the area early on Monday bringing another surge of arctic air into the area, but no snow. If skies clear Monday night, Tuesday morning could see the coldest temperatures of this event in the valleys as the fresh snow very efficiently cools the low-lying surfaces. However, mountain slope temperatures will begin to moderate as the week progresses, becoming noticeably warmer by Wednesday.
Another storm approaches the west coast near the end of the work week, though current model trends have this storm splitting and weakening as it enters the US.
Frigid temperatures and snow for the next week
Wednesday, December 4, 2013
I had another 6” on my deck this morning, and it is snowing lightly. The tail end of the first part of the storm cycle is moving through our area, and snowfall will decrease by this evening after an additional 3-6” during the day. I was optimistic we would be getting near the upper range of my 14-28” forecast, but it appears we will be at the lower end. The mountain-top wind direction never really turned to the northwest for an extended period during the storm and snowfall rates were lighter than I originally anticipated.
A longwave trough extending through the western US and Canada will keep the bitterly cold temperatures in place for the next week. Periods of snow and reinforcing cold air surges will occur this entire period as energy moves down from the polar regions into this longwave trough.
One of these surges is currently timed for Friday but only very light snowfall is expected. Another series of waves moves over us this weekend continuing the cold and forcing higher snowfall rates. Even though there will not be a lot of moisture in this very cold air, snow densities of around 2-4% will create extremely light and fluffy snow leading to not insignficant accumulations.
A final trailing wave on Monday will likely bring the coldest temperatures of this arctic outbreak. Mountain slopes should begin warming by mid-week, though the valleys will remain locked in bone-chilling cold as inversions develop, strengthen and persist.
