Monday, February 20, 2017
A storm off the West Coast is again pounding the California mountains, and energy ejecting out ahead of the storm will help form a southwest to northeast oriented front across the Great Basin. This front will bring clouds and the possibility of light showers to the Steamboat Springs area on Tuesday ahead of new storm cycle that that will persist into the weekend.
The front is forecast to move through northern Colorado early Wednesday, bringing cooler temperatures and a likely burst of heavy snow down to the Yampa Valley floor. Light to moderate snows should continue during the day and overnight as the front stalls near our region and subtle, hard-to-resolve waves of energy periodically enhance snowfall rates.
Snowfall forecast amounts are bolstered by atmospheric cooling, but tempered by the predominantly west-southwest mountain-top winds and the location of the upper level jet stream just north of the Colorado border. Right now, I expect 4-8” of snow by Thursday morning.
Meanwhile, additional energy from the northern latitudes kicks the West Coast storm inland later Wednesday across the Great Basin, and another push of significantly colder air with moderate to heavy snows is expected sometime on Thursday. Good snow should follow the front as the upper level jet stream pushes south of northern Colorado and maintains the large-scale upward motion that is conducive to precipitation.
Light to moderate snows will continue overnight Thursday and through much of Friday as cool, moist and unstable northwest flow lingers behind the departing storm. If the storm comes together as advertised by some models, we could see as much as 6-12” by Friday morning and an additional 3-6” during the day.
Additional energy and cold air from the northern latitudes travels down the east side of a building Bering Sea ridge into the Gulf of Alaska, forming a new storm and reinforcing a generally west-to-east frontal boundary across the Great Basin. Models disagree on how much energy loiters off the West Coast vs. how much moves inland and the amount of ridging ahead of the storm, but unsettled conditions with seasonably cool temperatures are expected for the weekend.
Some models have the storm off the West Coast drawing in subtropical moisture and setting the stage for another atmospheric river event. Though the timing is uncertain, models have additional northern latitude energy again dropping into the Gulf of Alaska, displacing the hopefully very moist storm westward early in the work week and bringing another round of likely significant precipitation to the area.
Friday, February 17, 2017
A storm from the central Pacific that is currently inundating California will split as it crosses the West Coast on Saturday. Some energy ejecting out ahead of the storm will bring the chance of light showers to the Steamboat Springs area tonight and again later Saturday, but the bulk of the storm will stay south of our area.
A trailing wave of energy strengthens the northern part of the split later Sunday and brings some cool air and the best chance of accumulating snowfall through Monday morning. Due to the split flow and additional upstream energy, the snowfall forecast for Monday morning is uncertain, but at this point I would expect 4-8” for the President’s Day report as there is some cool, moist and unstable northwest flow for a time.
Unsettled weather will persist after a brief clearing later Monday behind the cool front as waves in the relatively warm Pacific airmass move over the area on Tuesday and early Wednesday.
Meanwhile, a central Pacific ridge builds somewhere around the Bering Sea and the Gulf of Alaska, similar to patterns observed earlier in this winter season. Cold air from Siberia and the North Pole is forecast to slide southward along the east side of this ridge and begin a likely long-duration winter storm event starting later Wednesday and lasting into the weekend.
The specifics are highly uncertain as some of the cold air is forecast to move southwestward underneath the eastern Pacific ridge and merge with the branch of the Pacific jet stream that undercuts this ridge. If this pattern evolves similar to the pattern observed earlier in the season, which seems likely, then significant snows are possible as the moist Pacific jet stream once again battles the cold air from the northern latitudes over the western states. Current long-term forecasts have March arriving like a lion as this active period looks to last through at least the first week of March.
Monday, February 13, 2017
The disappearance of the Bering Sea ridge has allowed cold air from Siberia and the North Pole to surge into the central Pacific and re-energize the jet stream. A ridge of high pressure that has built off the West Coast in response to the strong Pacific jet stream will translate over the western U.S. during the work week, bringing warm and mostly sunny conditions through Thursday.
Starting around Friday, energy from the Pacific jet will force the ridge of high pressure eastward and bring showers to the western U.S. There are a number of waves of energy that will move through the area for the long President’s Day weekend, and the forecast timing, strength and duration of the precipitation will almost certainly change as we get closer to the weekend. Hopefully, better model agreement and more details will emerge for my late-week forecast.
Right now, the first wave is forecast to move over the Continental Divide on Friday, bringing mostly light snow showers to the Steamboat Springs area during the day.
Another Pacific wave splits as it crosses the West Coast on Saturday, and this may bring some more showers to the area mid-weekend if the parts of the storm are close enough to Colorado.
A third wave quickly follows the second, and this may bring a better chance of snows to the Steamboat Springs area around Monday before a brief break is advertised ahead of another stormy period that is forecast to close out the month.
Friday, February 10, 2017
A splitting wave is currently crossing the West Coast and will affect our weather this weekend. The southern portion of the split will form a closed low and dive south to Baja by Sunday as the northern portion of the split screams across the northern U.S. this weekend as an open wave.
Before the Steamboat Springs area feels the effects from the northern part of the storm, energy ejecting out of the southern part of the storm will bring rain showers to elevations below around 9000′ by this evening.
The precipitation will increase in intensity overnight as temperatures begin to cool due to the approach of the northern portion of the storm. Snow levels will drop overnight and into tomorrow morning before the Yampa Valley sees snow, but the timing of that switch is uncertain. Models have trended a bit faster with the arrival of cool air, and current forecasts have the switch to snow occurring early Saturday morning for the valley.
Temperatures will continue to cool through Sunday morning as the northern portion of the storm moves east of our area, but snows should end by later Saturday as moisture quickly erodes behind the diffuse cool front. Snow forecasts for the hill are tough due to the timing of the switch and the temperature-related density of snow, but right now I expect 3-6” of dense snow for the morning report and and additional 3-6” of lighter and fluffier snow during the day Saturday, most of which will occur before noon.
While the main part of this storm ends later Saturday, and some dry air works into northern Colorado by early Sunday, moisture from the Baja low is forecast to move northward and bring some clouds with the chance of light showers later Sunday as it combines with some left-over energy from the northern part of the storm.
Meanwhile, the Bering Sea ridge present these last few weeks is forecast to collapse, allowing cold air from Siberia and the North Pole to surge into the central Pacific. Initially, a ridge of high pressure will build off the West Coast as the Pacific jet stream is re-energized. This ridge is forecast to translate over the western U.S. during the work week, bringing warm and mostly sunny conditions. However, the re-energized Pacific jet will likely bring more stormy weather sometime around President’s Day weekend or soon thereafter.
Thursday, February 9, 2017
It’s hard to believe that we received so little snowfall from this storm, especially since all guidance pointed to significant accumulations.
I’ve prepared a loop of the Western U.S. IR satellite from 1pm Tuesday, 7 Feb 2016 MST through 7am Wed when numerical models indicated the Steamboat Springs area would have the heaviest snowfall. Colorado is outlined in red, and there is a red dot in northwest Colorado that represents the location of Steamboat Springs. My 6-12” forecast was woefully wrong as we only received about 2” during the afternoon on Tuesday and some rain early Wednesday morning.
I’ve annotated some of the images in the loop to roughly outline the Pineapple Express snaking through the Great Basin. I’ve also highlighted a large dry area that appeared later Tuesday in yellow.
Rather than Pineapple Express, meteorologists have started calling these wet events with a tropical and/or subtropical connection Atmospheric Rivers, as that descriptive term better captures the sometimes thin and wavy moisture feed.
Two features that likely doomed snowfall for the Steamboat Springs area were the large dry area highlighted in yellow that moved over our area late Tuesday afternoon, and the thinning Atmospheric River of moisture that passed just south of us Tuesday evening. The southern movement of the Atmospheric River was due to the passage of a correctly forecast weak cool front. There were periodic enhancements along the I-70 corridor that contributed to some 10” reports as the Atmospheric River moved south, but the big winner was Monarch that reported 17” and was the result of the Atmospheric River loitering over that area before it became diffuse and moved back north as a warm front overnight. The passage of this warm front brought the rain and gusty winds observed very early Wednesday morning.
But still, how could the models have gotten it so wrong? This Atmospheric River event was relatively narrow in width and I believe that made it’s measurement problematic. Upper air moisture is measured by balloons released every 12 hours, but their locations are several hundred miles apart or more. It becomes difficult to accurately measure and resolve a narrow band of moisture, and all of the numerical models were initialized with a broader extent of moisture than was actually present.
The result was that snowfall produced under the Atmospheric River was narrower and spottier in areal extent than forecast. This was exacerbated by the narrowing of the Atmospheric River in the latter half of the storm.