In the winter, forecasting precipitation type is not only a major challenge in the Treasure Valley, it is also extremely important to public safety, especially during peak morning and evening commute times of 5a.m. – 8:00a.m. and 4p.m. – 6p.m. respectively. The challenges arise from the extremely varied terrain surrounding the valley as well as the fact that climate of the area has an average surface temperature of 32.5 degrees F from December through February – a combination of its northern latitude, but low elevation. http://www.wrh.noaa.gov/boi/climo/average%20winter%20temperatures%2 0at%20boise,%20all%20sites.txt Lastly, the ground surfaces and their temperatures must be taken into consideration for the threat of snow and ice accumulation. Positioned on the west side of Idaho, the Treasure Valley is home to over 400,000 people and include among other, the cities of Boise, Meridian, Nampa, Caldwell, and Ontario, Oregon. The valley has two rivers flowing through it, the Snake and the Boise, with an average elevation of 2,600 feet. The 25 mile wide valley is oriented northwest to southeast, sloping down to the northwest at approximately 15 feet/mile, bordered on both sides by mountains. To the south are the Owyhee Mountains that range from 6000-8,500 feet in peak elevations. To the north are the beginnings on the Boise and Central Idaho Mountains with a range also of 6,000-8,500 feet in peak elevations. To the west is predominantly hilly terrain up to the foot of the Blue Mountains range in Oregon 100 miles away. The majority of the population lives down valley and commutes via I-84 and the downtown connector I-184. With an average of 64,000 cars commuting on those 4 lane interstates each weekday (Idaho Transportation Dept.), the traffic is considered heavy and any slide off or accident will have detrimental impacts on the individuals involved and surrounding traffic. To add to the issue, the road surface from Meridian to Boise is made of concrete and the connector is mostly elevated. Concrete surfaces were installed to withstand the heavy traffic loads but seem to be slicker to motorists in rain and snow conditions. Since the light colored concrete has a higher albedo, it will not absorb as much short wave solar radiation than the darker colored asphalt. Because of that, the concrete surface will cool quicker and longer due to radiational cooling; allowing for snow and ice to stick and form on the roadways, sooner than on asphalt. People driving in to town in the mornings will go from asphalt road ways to concrete just before the connector making for an abrupt worsening of road conditions. Being that the majority of the connector is elevated provides even more efficient radiational cooling to the road surface, further compounded by evaporational cooling of the air where it crosses the Boise River right before town and an sweeping turn (where there happen to be many black marks on the road side barriers!) On average, the Treasure Valley will receive 10-15 days of snowfall per year. Commonly the snowfall amount is only 1-3” however that has proven to create huge headaches for motorists and highway maintenance personnel due to the road surfaces and amount of traffic on the roads, inhibiting prompt snow removal and/or pre snow de-icing agents. With that in mind, the timing of the storm also becomes very important for everyone involved. The Idaho Transportation Dept. has crews that will report to duty at 2:00a.m. to monitor road and weather conditions from the office and on the streets themselves. Another challenge however, de-icing agents cannot be applied to wet roads due to dilution of the solution, so freezing conditions must happen first. Many of our winter storms start as rain and turn to snow, not allowing much response time before the morning rush hour, which also coincides with the coldest time of day. Forecasting a winter storm in the Treasure Valley is challenging in many ways. Beginning with current conditions, there are limited surface observation stations to our west, especially Oregon, due the fact that it is a very desolate area. Many stations don’t report moisture, but at least winds and temperatures can be used to determine frontal position. Compounding the observation issues, there is no radar station directly west of Boise. Nearest stations are Medford and Portland, Oregon. That leaves a large radar gap, allowing rain and snow to move out of the radar-free area about 60-80 miles west of the Idaho boarder, depending on the height of the storm –winter precipitation being more difficult to detect due to the lower heights of winter storms. Satellite imagery is used to infer/track precipitation from the west coast. However, due to our latitude and short winter days, visible satellite is not available to the morning forecaster until 8-9am, too late to identify strengthening upper level lows that might be over the region. Another challenge many mountainous regions share is the lack of model resolution to pick up on terrain features, so they need to be taken with a certain degree of skepticism. Obvious model adjustments need to be made to make up for the elevation; the 850mb is treated like the surface map, and the 700mb is treated like the 850mb. The 500mb and up can be used as normal. A commonly known method of forecasting snow is to examine the 1000mb-500mb thicknesses. The general rule of thicknesses at and below 5400gpm will result in snow. However, that doesn’t hold true in this area. I have noticed that the 5400gpm thickness over our area will migrate north and south with unusual frequency, more so than what even the diurnal temperatures might provide. Therefore I consulted an expert at the local National Weather Service office for assistance. George Skari, Lead Forecaster at the Boise NWS, has forecasted in many winter weather regions from Buffalo, NY to Bismarck, ND. He stated that the rules that apply in the mid west and northeast, don’t do as well in the intermountain west. One of the guides the Boise office has found useful is the 850mb-700mb thickness level. 1530gpm and lower marks the best chance for snow in their opinion. The night I met with him, we were tracking a short wave low moving towards the region and upon inspection of the 850mb-700mb thickness, precipitation would likely be snow. The next morning Boise woke up to 1.5” of snow. The day was April 9th, and thanks to the season’s longer days and highs in the upper 50’s, the roadways were only wet – but the guideline verified! Mr. Skari indicated that the three main elements help dictate weather the Treasure Valley will get snow or rain – elevation, wind direction, and track of the surface low. Other than in inversion conditions, the higher the elevation the more likely snow will occur. More often than not, 3500-4000 feet is the snow elevation for the region, just above the Treasure Valley. Winds coming from the SE at the surface at from the SW at 700mb will dry and warm the air mass killing the chance for precipitation or turning it to rain 90% of the time. Both air masses are dry but the SW winds will be down sloping winds, warming and drying adiabatically after crossing over the Owyhee Mountains. The best set-up of winds for snow is from the WNW, especially POST frontal which is a little different than a conventional frontal passage precipitation event. Winds aligned from the surface to 500mb in the 280-310 degree direction are also very conducive to the development of snow. As you would expect, the best chance for snow is when the surface low is 100 miles south of the Treasure Valley, putting the valley in the cold sector of the low. The best set-up is a negatively tilted trough with the upper cut off low (cutoff ideal) positioned over north central Oregon. With the public’s safety in mind, winter forecasting in the Treasure Valley is very important. Knowing the road surface temperatures in a real time environment would be a huge help to understanding and the road surface conditions, but at this point there is no such system in place. Using the guidelines that Mr. Skari mentioned could help, but he also acknowledged that accurate snow forecasts are one of their biggest shortcomings and a more detailed study would be prudent. |