Unsettled weather turns stormy by Sunday

Friday, December 19, 2014

The current unsettled weather pattern continues with periods of light snow ending by this afternoon. A transient ridge will move over the area Saturday, but will be quickly followed by storminess starting Sunday and lasting through Tuesday.

Atmospheric river loop from 20Dec2014Currently, what meteorologists are now calling an atmospheric river is inundating the Pacific Northwest, as shown in the infra-red satellite animation to the right. This is simply a relatively narrow and long fetch of concentrated moisture in the atmosphere. The Pineapple Express, for example, is the well-known moniker given to an atmospheric river originating around Hawaii.

The current atmospheric river with roots in the northern Pacific will move towards our area and begin producing precipitation around Saturday night. Even though we will be in strong and moist northwest flow, current model trends have the atmosphere warming during the day Sunday with very windy conditions, which throws a wrench into the previously very bullish forecast. My experience is that our forecast becomes far more uncertain, and generally leans pessimistic under a warming atmosphere. Our most recent experience with a similar scenario lead to our freezing rain event; we started out with moderate to heavy snowfall as temperatures rose in the morning, transitioning to freezing rain as the warming and upward motion associated with that warming ceased. I am not suggesting that we will see another freezing rain event as I’m STILL not sure what happened there, but wanted to highlight the uncertaintly present.

So, I am now very uncertain about snow amounts during the day. However, cooling does begin by later Sunday, and that increases confidence that heavy snows increase. Winds will stay very strong through the day Monday, continuing the moderate to heavy snowfall until the intensity drops off near the end of the day. Lighter snowfall, however, will continue overnight.

Snowfall from Sunday through Monday will likely be heavy, with what happens during the day Sunday being a bit of a wildcard. Travel will also be very difficult during this time period. At this point, I would guess maybe an inch or two by the Sunday morning report, with 5-10” reported Monday morning. Cooling increases later Monday, decreasing the density of the snow and increasing accumulations, possibly leading to another 4-8” by Tuesday morning.

Wednesday will be chilly with the 1-4” reported in the morning occurring during the day Tuesday. Models have trended significantly stronger with the ridging behind this storm, leading to warming conditions and weakening the storm earlier advertised for Christmas Day. The storm still looks to be significant though with snow increasing during the day Thursday and lasting through Friday morning, leaving another 6-12” by the morning report.

Four events to influence our weather over the next week

Tuesday, December 16, 2014

Now that the official and well-forecasted 19” storm total reported at mid-mountain is behind us (after the Steamboat ski area reported 2” this morning), attention turns to the unsettled pattern forecast to persist for at least the next week. Currently, light snow is falling and will continue through the early afternoon before ending. I expect accumulations of only an inch or two due to the brief duration of this first event.

Concurrently, a split trough has entered the west coast, and it looks like the southern part may produce precipitation over our area as soon as tomorrow and lasting through Friday. There is surprising disagreement between the American short-range NAM and the American medium-range AVN as to if this precipitation will occur for our area as the NAM is dry for us. I’m inclined to side with the moister AVN since that model tends to do produce a better forecast for our area in moist northwest flow with falling temperatures.

However, this model also tends to overpredict precipitation in the pre-frontal environment when southwest flow dominates, so I expect only minimal accumulations, if at all, during the day tomorrow. A weak front passes through near the end of the day, and that is when I expect snowfall to increase and become steadier.

Temperatures rise on Thursday during the day and this will lessen the snowfall. A trailing weak wave passes through Thursday night and should keep light snowfall going during the day Friday. At this point, I would expect around 4-8” between Wednesday and Friday afternoons.

A very brief break Friday night into Saturday morning before another splitting wave is forecast to bring another round of light snow starting Saturday afternoon and lasting through Sunday morning, likely leaving only several inches by noon Sunday.

Another very brief break that may only last Sunday afternoon before a period of moist northwest flow sets up over our area by Sunday night. Snows will be mostly light through Tuesday, though an enhancement may occur later Monday as some cool air moves over the area.

Models have been somewhat consistent in forecasting a major storm to cross the west coast midweek. This is another splitting storm, but this time the split occurs well off the coast and the dominant northern branch phases with some bitterly cold arctic air currently over northern Canada. The interaction between the Pacific and Canadian airmasses is forecast to lead to a major storm around Christmas for the northwestern US and Rockies. Current timing has heavy snows and plunging temperatures for us on Christmas Day, though of course that is subject to change as that forecast is a week away. In fact, the very latest European ECMWF has more interaction between the Pacific and arctic airmasses over the Gulf of Alaska early next week, and that might draw more cold air into the storm and change its trajectory.

Earlier model runs has this cold air remaining over our region for a while, but now, after forecasting a still cold post-Christmas weekend, they are forecasting warming temperatures by early in the next workweek.

Pattern change brings snow this weekend

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.

Continued dry and warm until mid-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.

Meager snowfall chances for the next week

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.

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30 April 2022

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