Thursday, December 11, 2014
The current stretch of warm and dry weather looks to change by mid-weekend as a splitting storm currently pummeling California moves eastward. Another nice day on Friday will be followed by increasing cloudiness on Saturday as the storm enters the Great Basin. Precipitation for us may start as early as Saturday afternoon, or may hold off till later Saturday night or early Sunday morning.
The strongest part of the storm will pass to our south, but we should do well in the cool and moist northwest flow behind the main energy center of the storm during the day Sunday. Furthermore, models now have a portion of the northern part of the split storm hanging back over southern Idaho or southwestern Wyoming and enhancing snowfall again by late Sunday night or Monday morning.
Forecasting snow amounts is difficult due to the strongly evolving nature of this storm, but currently I would expect only light snowfall in the 1-4” range to be reported Sunday morning. Periods of moderate to sometimes heavy snowfall will likely occur during the day Sunday and overnight, leading to accumulations of 5-10” by Monday morning. And additional 1-4” will likely fall during the day Monday to be reported Tuesday morning.
After a break Tuesday, the progressive forecast from the American GFS model last week wins out over the forecast from the European ECMWF. Interestingly, this is the second time the this winter the GFS showed superior skill in the medium range, and is something to take note of moving forward.
The progressive forecast moves another splitting storm over our area by midweek. Again, there will be uncertainty with respect to snowfall amounts, but the storm will likely peak around later Wednesday before exiting the area on Thursday. And the storm train will continue with another similar wave timed for the following weekend.
Monday, December 8, 2014
The current warm and dry pattern will persist through this week. The weak storm for tomorrow mentioned in last week’s forecast will indeed remain to our north and will bring only high clouds to the area.
The next chance of any weather will be mid-weekend as a very strong and warm Pacific storm brings copious moisture to California. Clouds should increase over our area on Saturday, but the storm is forecast to split as it enters the Great Basin, as indicated in last week’s ensemble forecast.
When meteorologists look for clarity in the longer term, the ensemble forecast provides an indication of possible future states of the atmosphere. Basically, a model is initialized with slightly different initial conditions, which can be considered to be the result of small measurement errors. A number of model runs produce an ensemble forecast, and the hope is that the future state of the atmosphere will fall within the range of predicted solutions. Furthermore, the amount of spread between the solutions is representative of the uncertainty of the forecast.
So, even though the operational models last week indicated a big storm for this coming weekend, the ensemble members indicated a possible split in the storm that grew more likely as the week progressed. And, in fact, the operational models are now predicting a split storm that will produce only fair amounts of snow for our area.
There is some cool air associated with the storm, and winds should briefly turn to the northwest behind the front, but this does not look like a big snow producer. Current forecasts have precipitation starting later Saturday and peaking overnight, with snows tapering off during the day Sunday. If I had to guess today, I would expect 3-6” by Sunday afternoon.
There is considerable model disagreement after next weekend as the European ECMWF keeps energy off the west coast and builds a ridge over our area, while the American GFS moves this energy over our area by midweek.
Tuesday, December 2, 2014
Warm weather and weak storms will doom our snowfall potential through at least next week. The storm for tomorrow has weakened from earlier forecasts and while clouds will increase later today ahead of the wave and remain for tomorrow, I only expect an inch or two by Thursday noon, with rain possible in the valley during the day Wednesday.
Another disappointingly weak wave moves over the area on late Thursday night and Friday, with one model predicting no precipitation and another maybe an inch or two by the end of the day.
A quick-moving and shallow ridge for early Saturday will be followed by another weak storm which will increase clouds later Saturday into Sunday, but current forecasts have this weak storm splitting and weakening further as it enters the west coast, leaving our area precipitation free.
Yet another insignificant and likely precipitation-free wave is timed for around Tuesday, with a building ridge bringing even warmer and still dry weather to our area for later in the workweek.
Saturday, November 29, 2014
After some partly sunny days, moisture should increase later tonight and tomorrow ahead of a weak storm that will peak Sunday night or early Monday morning. Areas north of Steamboat will be favored, but only an inch or two is expected to be reported on the hill by Monday morning.
Light snows will end by noon Monday and clouds will decrease through the afternoon, leaving partly cloudy skies for the rest of the day and Tuesday.
A storm currently off the coast of California will move inland and affect our weather for Wednesday and Thursday. Models have trended weaker and warmer with this storm, but it still looks like we will receive not insignificant amounts of snow starting Wednesday afternoon and extending into Thursday. The most favorable time for snow will occur Wednesday night and Thursday morning as there is a subtle wind shift to northwest flow around then, but amounts will be limited by warm temperatures and light wind speeds. Based on the latest model runs, we may see 3-6” between noon Wednesday and noon Thursday.
Several weak and continued warm waves are forecast to follow this storm and pass over the area from Friday through the weekend, keeping the threat of light precipitation present.
Wednesday, November 26, 2014
Boy, forecasting is hard. And humbling. And what did I say about changing a forecast in the last post!?
Originally, I had thought the final wave in the storm cycle timed for Tuesday would struggle to produce snow in the warming atmosphere forecasted by the models. In fact, the temperature up top rose from about 3F first thing Tuesday morning to about 19F by the early evening. However, during the morning hours and until the early afternoon, snow was falling both up top and here in the valley at about 0.5”/hr as temperatures warmed, and that prompted me to revise my guarded forecast to be more optimistic.
I even joked how I don’t like to revise forecasts for the very reason that it tends to break BOTH the original forecast and the revision!
I heard reports from the hill today that by about 2pm or so yesterday, freezing rain had started on the upper mountain and quickly crusted over the high quality powder. The valley saw freezing rain by around 4:30pm, and then periods of mixed precipitation and freezing rain through the evening hours. Furthermore, the Tower SNOTEL site indicated about an inch of precipitation fell from late afternoon yesterday through early this morning without any increase in the snowpack depth, indicating rain or freezing rain was impacting the measurement there as well.
Freezing rain occurs when an elevated warm and moist layer above freezing sits above a cool layer of air below freezing. So, there must have been this layer somewhere above 10,000′, though I can’t find it in any of the upstream observations. Animations of satellite pictures do show the north-northwest to south-southeast jet stream moving eastward and allowing warmer air to enter the area as forecast, but I did not expect it to be above freezing above 10,000′. Furthermore, the average temperature profile of the atmosphere over our area was cold enough for snow, but in hindsight a thin warm layer would not significantly change the average temperature over a much deeper layer.
The end result was I missed forecasting freezing rain on the hill because of this thin warm layer, which meant the inch of liquid that should have produced a foot of snow stayed as rain. And produced a layer of ice 1/4” thick up top. In fact, the ice crust grew thicker with elevation as there was not enough time for any of the rain to turn to sleet before freezing on the snow surface near the top. At this point, I’m finding it hard to find the evidence I missed that would have created a better forecast.
While rain on the upper hill is not likely in the winter here, it does happen, albeit very infrequently. Freezing rain, on the other hand, is a much rarer occurrence.