Second Storm Will Blast Star Valley Early Saturday

The second of two storms will bring an end to the brief respite from the cold of December.  One system brought significant precipitation to the western Wyoming mountains and valleys early Thursday.  Due to the unseasonably warm air accompanying the system, Star Valley experienced more rain than snow with little accumulation on the valley floor.  While total precipitation, as measured by the weather stations in the valley, ranged upwards of half an inch, only a inch or two of wet snow was observed.  This was not the case in higher elevations above about 7000 feet with around a foot observed in parts of the Wyoming Range and from 1-2 feet in the Tetons.  The following chart was produced by Riverton WFO showing the observed snowfall from the first storm.

Very strong winds were observed across the high country, with Mt Coffin in the Wyoming range reporting gusts to 134 mph Thursday morning.

Warm air continued to cover the area through Friday with afternoon temperatures  into the mid 40s under quite a bit of late December sunshine. 

Storm number 2 now moving through the Pacific Northwest will bring an end to the brief respite from winter cold.  Rain and snow will spread across Star Valley, beginning later Friday evening, becoming all snow after midnight.  The cold front will arrive prior to sunrise accompanied by strong, gusty winds and 2-3 hours of moderate to heavy snow.  The snow should decrease to showers/flurries by later Saturday morning, however. cold air will drop temperatures to well below freezing during the afternoon.  Given the rapid movement of the storms, snow totals should generally range from 3-6 inches with the greatest potential in the northern portion of the valley. 

High pressure then builds into the area by New Years Day and will dominate into the middle of the first week of the New Year.  This returns Star Valley to weather experienced much of December, cold nights with sunshine most days.  Overnight temperatures will drop below zero in colder valley locations, then struggle to reach the teens during the afternoon, given the low sun angle this time or year.

The next chance of significant snow should not occur until after midweek.

Snows Coming

The advertised pattern change is underway as the storm track which has bypassed Star Valley much of December will be providing a snowy end to the year.  As can be seen on this evenings water vapor imagery a moist westerly jet is moving eastward across the eastern Pacific toward the Pacific Northwest.

Water Vapor Imagery Monday Evening 12/26/2011

The effects will be noticed beginning with increasing cloud cover  holding temperatures well above the zero readings experienced much of the nights this month in Star Valley. Precipitation in the form of snow will commence during the daylight hours Wednesday  It appears that intensities will pick up by Wednesday night and continue through Thursday and possibly Friday.    Temperatures will warm to near and possibly slightly above freezing in Star Valley by Thursday.  While precipitation will likely remain as snow, the warmer air mass should lead to a much wetter snow than that experienced so far this month.   The Riverton Weather Forecast Office has indicated over a foot can be expected in the nearby mountains and possibly 6-12 inches in Star Valley with the heaviest amounts from Thayne to Alpine.

Snowfall forecast 12/28-29 2011

 With the strong westerly winds, the snow will fall mainly over and west of the mountains of Western Wyoming. Downslope flow will result in little snow over much of central and eastern portions of the state, however winds will be strong and gusty.

The greatest amounts are likely over and west of the Tetons where by this weekend, new snowfall could be measured in feet, possibly up to 2-3.

The month long stretch of below freezing days could come to an end at Thayne E.S. during the next couple of days.

Cold, Clear Christmas Weekend, Followed by Moderation and Increasing Snow Potential

A couple more cold nights this Christmas weekend, then Star Valley will see a change in the weather pattern the last week of 2011.  As an illustration of how cold a month it has been thus far, Thayne Elementary has dropped below zero on 18 of the first 23 days of December, hitting a low of -15 on Friday morning. At the official observing site in Afton it dipped to -17 Friday morning with the temperature only rising to 3 above on a sunny afternoon.  The strong inversion which has been dominant much of the month was very pronounced again on Friday. As an example Star Valley Ranch reached a high of 20 above around 10am. However the cold air over the valley deepened dropping the temperature to under 10 above by noon where it remained the rest of the afternoon.


 Across the Salt Range at the Box Y Ranch after a morning low of 5 below the afternoon maximum was 27 above.  The inversion in the narrow Grays River Valley is typically weaker than over the Salt and the mid winter sun can still provide good warming.

High pressure will assure Star Valley if not still cold, a beautifully clear Christmas weekend.  With the fresh 2-3 inches of snow on Thursday, a White Christmas will prevail.

The expected major pattern change still looks on schedule for the final week of 2011.  The storm track along with a moist westerly flow off the Pacific Ocean will become an increasing factor through next week.  If nothing else the cold nights should come to an end by mid-week if not before as clouds provide a blanket over the valley. 

300 mb Forecast Thursday Evening 12/29 2011

 While the core of the strong polar jet should remain somewhat north of Star Valley, the moist westerly flow should provide periods of snow in the mountains and some snow or even rain in the valleys beginning mid week.

The model forecast of precipitation  for this next week through Thursday suggest potential for significant snowfall in the mountains of Northwestern Wyoming, possibly as far south as Star Valley.

Model forecasted Precipitation Through Thursday evening 12/29/11

Fast Moving Storm to Affect Star Valley Wednesday followed by a Cold but Snow-Free Christmas Weekend.

A fast moving upper level low pressure trough located over southern British Columbia Tuesday evening is expected to move quickly southeastward and provide a temporary return to winter weather over Star Valley on Wednesday.  The water vapor imagery below shows not only this trough but a weaker one over Northern Baja California

Water Vapor Imagery Tuesday evening 12/20/11

This system will strengthen as it moves across southeast Idaho and western Wyoming during the day on Wednesday.  Snow is expected to begin during the morning and as the system continues on  southeast to Utah and Colorado, the snow will end from the north by Wednesday evening. The Riverton Weather Forecast Office has created a graphic with the expected associated snowfall amounts by Wednesday Night.

The greatest totals are expected in the higher elevations with 1-3 inches in Star Valley. 

High pressure then provides a return to the conditions that have been experienced through much of December.  Cold nights, with temperatures generally below zero, will prevail into the Christmas weekend. Minimums could  possibly dip to 10-15 below by Thursday morning in coldest valley locations. It still appears that from Thursday until Christmas Day fair conditions will prevail with the potential for mostly sunny days.

However, the longer range models are now indicating that the high pressure that has been protecting Wyoming from storms much of December will be weakening next week.  The jetstream is now forecast to start tracking from the Pacific eastward across Wyoming.

300 mb(30,000 ft) Forecast Tuesday 12/27/11

This change in pattern, if it materializes, will result in a more active pattern across Star Valley.  Since it is almost a week away, the model predictability limitations preclude specifics at this time- although the snow potential could increase substantially in the last week of 2011.

December to Remain Abnormally Dry

Persistent high pressure has been parked over the Star Valley area much of December.  Result has been minimal amounts of snow and rather cold temperatures. It appears now that high pressure will be around at least through Christmas Weekend.  The latest model forecast indicates that pressures will be  above normal over the eastern Pacific Ocean, which will in turn force the storm track far to the north into Western Canada. This graphic is centered on Christmas Day.

Christmas Day Height Anomaly 500 mb
Red indicates above normal heights, blue below

There will be a low pressure trough that will be passing off to the northeast of Star Valley on Wednesday, Some  snow is possible with this system, but again amounts should be generally light.

500 mb forecast Wednesday December 21 2011

The longer range forecast through the remainder of the month do not hold out much hope for a significant snow storm the rest of 2011. While travelers will favor this outcome, skiers will not be pleased.

8-14 Day Outlook Precipitation Valid Dec 25-31 2011

The strong temperature inversion across the valley much of December is likely to persist.  This will keep nights cold, near or below zero, and afternoon highs not likely reaching freezing.  At Thayne Elementary the high so far this month has only been 26.

Cold But Little Snow First Half Of December

So far December 2011 has been cold with little in the way of significant snow across Star Valley.  Both Thayne and Etna Elementary School Weather Stations have now been providing reliable weather data since the fall of 2010. Following are data from the school stations.

Thayne                   Avg Max Avg Min 
Dec 1-15 2011            19               -3
Dec 2010                    31              14
Jan   2011                   26                6
Feb  2011                    27               5

Etna                        Avg Max   Avg Min
Dec 1-15 2011             20                  0
Dec 2010                     31                15
Jan 2011                      26                  7
Feb 2011                     28                  5

The first 15 days of December's averages, thus far, are much colder than December 2010 as well as the normal temperatures for the month at the Beford Climatological station with a maximum of 28 and minimum of 6. Additionally, both stations are well below those observed last January and February.

The coldest morning so far this month was the 6th when Thayne dipped to -14 and Etna -10.  Minimums at Thayne ES have been below zero 12 mornings so far this month.

While still cold, the Star Valley Ranch station has  recorded average maximum/minimums of 26 and 7 thus far in December. The only morning below zero was on the 6th when it dipped to -4.

A strong temperature inversion has been in place much of the month which typically keeps the valley floor much colder than higher elevations.  Thus the higher readings at Star Valley Ranch which is 300-400 feet above the adjacent valley floor.

While the early Fall started out with several significant snows, high pressure has dominated across Star Valley so far in December with the storms tracking both north and south of much of southeast Idaho and Wyoming.

Snowfall so far this month at Star Valley Ranch has totaled just 7 inches, while the station south of Smoot has had just 8 inches.   The average snow for the month of December at the Bedford Climate station is over 30 inches.  Storms are expected to continue bypassing Star Valley for the next several days, so any appreciable snow is unlikely.  With the early fall storms the snow totals since Oct 1st are 58 inches  at Star Valley Ranch and 61 inches at the station south of Smoot.

Pattern Change Over The Western States

A significant change in the storm track is taking place over the Western States for the coming week.  Star  Valley has been protected from storms for much of December by high pressure.  While the skies have been generally clear the valley temperature inversions have been pronounced,. In Star Valley this has resulted in continued cold conditions overnight with lows below zero and afternoon high remaining in the teens.,  In the Cokeville area afternoon highs have not climbed out of the single digits over the weekend. Not the case above the valley floor, as the station at Star Valley Ranch warmed into the middle 20s and reminded well above zero at night. And just to the east at the Box Y Ranch along the Greys River maximums have been near freezing.

The high pressure which has been protecting much of the west has broken down and a new storm track is being established well to the south of Star Valley across the desert southwest.By Monday evening a large low pressure center will have developed over Southern California.

300 mb Forecast Monday Evening 12/12/2011

 While this system will bring considerable precipitation, rain in the valleys and snow in the higher elevations of the Southwest, it will have minimal effects on Star Valley for the next few days. In fact the increase in clouds with the southwesterly flow ahead of the low will tend to lessen the strong overnight cooling allowing temperatures to generally remain above zero for much of the week.  Additionally afternoon highs will approach freezing even on the valley floor as warmer air mixes down with an increase in the winds aloft.

With regards to snow, chances will remain low until mid week. Additional disturbances will be moving inland at that time, however model forecasts continue to take most of the effects to the south of southeastern Idaho and western Wyoming. Some light snow will be possible most days after Wednesday but any major snow event seems unlikely through next weekend.

Strong Temperature Inversions and High Pressure with Little Snow for the Next Week

High pressure has become well established over much of the Western U.S. and will dominate through the coming weekend. The storm track has shifted well to the north of Star Valley resulting in a rather stagnant weather pattern.  Strong inversions have developed in Star Valley under the generally clear and calm conditions and only limited daytime heating this time of year.   The results are that the coldest daytime temperatures are occurring on the valley floor with considerably warmer readings above the inversion in the higher elevations.  As an example while the afternoon temperature in Afton was only able to rise to 10 above  Thursday, after a morning low of -9, it warmed to near freezing at Salt River Summit  at an elevation some 1500 feet above the valley floor after a morning low of plus 7.

Visual evidence of this strong temperature inversion can be seen, particularly around Afton at the top of the haze/smoke layer.

Once these inversions become established this time of year, they will require a breakdown of the current high pressure to increase the atmospheric mixing to remove them. The earliest this seems likely to occur will be early next week when a weakening upper trough of low pressure is forecasted to cross the area.  Until then expect plentiful daytime sunshine, but continued cold conditions in the Valley.  Warmer conditions will continue to be observed in higher elevations  where in some cases the morning lows will be higher than the adjacent valley afternoon highs.

Snow flurries are likely this Wednesday night as a weak disturbance passes across the area, however no accumulation is expected. The next chance of any significant snow appears to be no sooner than the middle of next week.

How Cold Was It?

Overnight cloud cover thinned/dissipated toward sunrise allowing a rapid temperature drop.  In the Lower Salt River Valley clouds hung around longer and as a result minimums were only near zero.  However, from about Thayne southward it cleared and was much colder.  Following are the observed minimums in the valley this morning.

Auburn                -17
Afton                   -13
Thayne ES           -8
Osmond ES          -8
Smoot                   -5
Etna ES               -1
Star Valley Ranch  2
Alpine                   3

Nearby it was:


Pixley south of Cokeville   -21
Driggs ID                         -17

Cokeville                          -15
Jackson Airport                -13

In other areas clouds were not a factor and as a result it was much colder.


Old Faithful             -33
West Yellowstone    -35

and the coldest was a -41 at the Peter Sink Site which is located just south of Logan Summit Utah west of Bear Lake.  This high elevation sheltered site is typically one of the coldest around.

Bitter Cold Monday Morning

The cold northerly flow continues across Star Valley.  Fast moving disturbances embedded in the flow will bring periods of clouds and light snow.  One such system brought 1-2 inches Saturday night and another one is scheduled to drop southward and bring no more than an inch or two on Sunday.

Of more importance, this next system is going to be followed by very cold air, originating over Northwest Canada.  Although this is still not a true Arctic air mass as temperatures this morning over the Yukon  are generally above zero, the ingredients are coming into place to drop temperatures over Star Valley Sunday night to well below zero.  Skies should clear in the wake of Sundays system and the combination of  light winds and the cold air aloft will lead to ideal conditions for a very cold night.

It now appears that 10-15 below zero will be common in the valley locations by sunrise Monday, with the coldest locations dropping to 20 below or lower. The only areas that may be spared the extreme cold would be the communities above the valley floor such as Star Valley Ranch where it could go no lower than about 5 below.

Gradual moderation in temperatures will occur into the middle of the coming week, however they are expected to remain well below normal for the first week in December. The pattern remains unfavorable for a significant snowstorm in Star Valley, probably through the coming week.

Star Valley to Have an Extended Period of Below Normal Temperatures with Little Snow

A major adjustment is taking place in the storm track/jet stream over a large portion of the U.S. and northeastern Pacific Ocean.  The latest storm to move from the Pacific is affecting the southwestern states Wednesday night, bringing very strong and damaging winds to some sections of California.

Western Wyoming is now dominated by a cold northerly flow of drier air from Canada.  As high pressure builds north into the Gulf of Alaska, this will put Star Valley  under an extended period of possibly more than a week of reinforcing surges of cold air from Canada.  With the storms tracking across western Canada and then southeastward...most of the snow will occur east of the Continental Divide.

This first chart shows the forecasted abnormally high pressure centered over the Gulf Of Alaska.  The red contours in the chart are areas of higher than normally 500 mb heights, with blue contours indicating areas of below normal 500 mb heights.

500 mb Anomalies Sat 12/3/11

The models suggest that this pattern will persist as shown on the following chart displaying  the same fields as the one above except for Wednesday of next week.

500 mb Anomalies Wed 12/7/11

This pattern will be characterized by only brief periods of clouds and light snow as disturbances pass through Star Valley. Much of the time for the next week will see only limited amounts of cloud cover allowing for nighttime temperatures to drop below zero, and afternoons remaining well below freezing.

Fortunately there is not a pool of Arctic air yet available in Canada, so even though the jet stream pattern will be favorable, a bitter cold air mass from the north is not expected at least into early next week.  If the pattern persists, then the possibility of an arctic air mass invading Wyoming will increase.

Nevertheless conditions will be favorable for colder valley locations dipping to 10 below zero or lower Friday morning under clear skies.