Evolving numerical guidance now includes the possibility of heavy snow Thursday into Friday
Wednesday, April 15, 2015
As discussed in yesterday’s forecast blog, model guidance has since evolved and we now have the possibility of heavy snows tomorrow afternoon extending into Friday. The American GFS has finally joined the other models in forecasting a consolidated circulation center that is currently spinning to our west. Furthermore, all models are now forecasting a more northward track of the cutoff low across Colorado which greatly increases our chance for significant snow.
The cutoff low is forecast to move southward along the Colorado - Utah border before being nudged eastward by the kicker wave mentioned yesterday approaching the West Coast. Even though the flow aloft will turn easterly for our area, this normally dry scenario can sometimes produce impressive snowfall when a TROWAL (TROugh of Warm air ALoft) develops. A TROWAL develops when warm and moist air is lifted ahead of the storm and rotates counter-clockwise around the parent low. The lifting of this elevated unstable layer over the cooler air near at lower elevations can create impressive precipitation rates, and it appears we may benefit from that process.
I do not have a lot of confidence in this solution since a small change to the track of the storm will change the location of this TROWAL. But the possibility exists for moderate to heavy snows from tomorrow afternoon through Friday afternoon. We may still get the 2-6” originally forecast for the hill, or we may get as much as 10-20”. Furthermore, the valley will also see significant snowfall if the wetter solution verifies.
Lastly, for those headed west for mountain biking, the slower and farther north solution offered by this morning’s model guidance would have the Grand Junction and possibly Moab areas wet through Friday, with cool conditions lasting at least through the weekend.
Summer-like weather ends tonight
Tuesday, April 14, 2015
After a couple of summer-like days, a strong cold front from a storm currently in central Nevada will bring winter-like days back to our area starting late tonight or early Wednesday and lasting possibly into the weekend.
Winds will increase through the day today before the front blasts through the region by tomorrow morning. At this point, there is a fair bit of uncertainty as the storm becomes separated from the jet stream along the Canadian border and evolves into a cutoff low. Because cutoff lows are removed from the strong and relatively predictable jet stream, their evolution is notoriously difficult to predict. Some models have this low further splitting into a couple of circulation centers that move around our area while others have a more coherent system staying mostly south and west of our area.
The strong front will drop snow levels to the valley bottom, even though upward forcing over our area is relatively benign. But the very cold air aloft will destabilize the atmosphere, and may lead to some localized areas of moderate to heavy snow as small storms develop. Generally though, I would expect only modest accumulation on the hill for the entire event, perhaps in the 2-6” range between Wednesday and Friday afternoon.
While some models have clearing by Friday night, there is no well defined movement of the cutoff low which means we may feel its effects into the weekend. A kicker Pacific wave is forecast to approach the West Coast later Friday and will force the cutoff low east of our area either late Friday or sometime Saturday.
This Pacific wave will split as it makes landfall, and may drag some cool air over our area on Sunday leading to showers for most of the day. But the main effect from this wave will be to phase with cold air over the central Canadian plains and begin a pattern change that may bring an active weather pattern for the rest of the month.
Specifically, as the cold air moves southward over the Midwest, a West Coast ridge rapidly rebuilds and becomes unstable, allowing Pacific energy to enter the west coast in a coherent manner next week. This energy is forecast to eventually interact with the cold Canadian air as the storms move eastward, leading to the possibility of a couple of major precipitation events near the end of the next workweek and again about a week later just before the end of the month.
Due to the number of interactions between different airmasses, and the fact that any interaction will influence future interactions, the forecast is very uncertain. However, my confidence is increasing that an active weather pattern may return for the second half of the month.
Grazing storm for tonight, tomorrow briefly interrupts warm and sunny weather
Wednesday, April 8, 2015
The storm that brought two feet of snow to the Sierra Nevada mountains yesterday is on our doorstep. Clouds are increasing and there may be some showers late this afternoon ahead of the main push of cool air expected around late this evening or midnight. Snows should increase around then and last through the morning, with the heaviest snows staying just north of our area where the atmosphere is the most unstable and upward forcing is strongest. I would expect 1-4” for the morning report with maybe another inch or two during the morning hours.
Non-accumulating showers will likely continue through the afternoon before drier air filters into the region. Additional weaker grazing waves will pass well north of us on Friday morning and each of the weekend mornings, but warming temperatures and sun should overcome the slight cooling from these waves giving us some beautiful weather for closing weekend.
Another storm crossing the Baja peninsula around mid-weekend will eventually pass south of us by late Monday and bring significant precipitation to the Colorado plains and possibly the southern mountains, likely leaving us with dry weather. Concurrently, a major storm is forecast to cross the west coast around Monday, and there is a lot of model uncertainty with the evolution of this storm. While is is nearly certain that California and the western Great Basin will receive significant precipitation, our forecast is considerably murkier for next week.
Cold front later Wednesday should bring snow
Tuesday, March 31, 2015
A storm currently crossing the Pacific Northwest coast will bring a cold front through our area around Wednesday afternoon, temporarily ending the abnormally warm and dry conditions for a few days. The atmosphere will destabilize tomorrow and we will be prone to gusty winds with falling temperatures, with even the possibility of some thunder and lightening later in the afternoon or early evening. Some of these showers may produce an inch or two or snow on the hill which would be reported Thursday morning.
While most of the energy stays north of us for Wednesday, by Thursday a trailing wave brings a reinforcing surge of cold air into the area. There is a much better chance of snow as the storm is forecast to move more directly over the area by Thursday night. Winds do look like they will turn to the northwest for a period of time between Thursday night and Friday morning, and this will be the best time for snow. I expect 2-6” by the Friday morning report and another inch or two during the morning hours. While we are likely to see snowflakes even in the valley, the very warm temperatures of the past few weeks have warmed the ground surfaces and will preclude accumulating snow.
Skies should clear later Friday and temperatures should warm during the day after a chilly start. Another spectacular weekend is on tap as dry air in generally westerly flow invades the area.
Another Pacific Northwest wave approaches our area early next workweek and brings some cooler temperatures, but this storm will pass well north of our area and likely be responsible for some windy but dry conditions.
Temperatures will warm again behind this grazing wave, but there is a lot of model uncertainty heading into closing weekend. The American GFS has a strong trough crossing the West Coast around Friday and threatens significant snow during the latter half of the weekend. The European ECMWF has a similar trough slightly further west at the limit of its 10 day forecast, but the forecast beyond that has not been generated yet.
Storm on track for Tuesday and Wednesday
Monday, March 23, 2015
The storm advertised in last week’s blog is on track to deliver snow tonight and tomorrow night through Wednesday. There is a slight change in the forecast as the last wave timed for Wednesday night, though still cold, will be drier than originally predicted.
The atmosphere has destabilized ahead of the cold front currently moving through Utah, and any showers that form this afternoon will likely fall as liquid below 9500 feet or so. Upward forcing will increase just ahead of the front by late this afternoon, increasing showers and lowering snow levels until the front moves through around 6pm or so.
Rain will change to snow in the valley around sunset while moderate to sometimes heavy snow should fall on the hill. We should continue accumulating snow in the cool and moist northwest flow into the early morning hours before the snow decreases and becomes more showery during the day Tuesday. I would expect 3-6” on the Tuesday morning snow report.
Another wave bringing more snow is currently forecast to move across the area around sunset Tuesday, leaving another 3-6” on the hill by the Wednesday morning report. Conditions will remain unsettled and showery Wednesday before the last and coldest, but driest wave moves over the area Wednesday night. I expect accumulating snows to end by midnight, with 1-4” reported Thursday morning.
There may be some isolated snow showers in the cool and unstable airmass Thursday before a building ridge brings much warmer and dry weather for Friday into Saturday morning. Current forecasts have the northern portion of a splitting Pacific wave moving through the ridge by later Saturday, bringing some cooling, though showers are currently forecast to remain north of us.
The warmer southern portion of this split wave is then forecast to approach the area Sunday afternoon, increasing the chance of showers later in the day and into the evening and bringing another unsettled start to the next workweek.
