Grand Junction

Grand Junction, CO is located in the Grand Valley high desert at 4,800 ft above sea level. It's surrounded by mountains and is subject to valley breezes. Since it sits in the high desert, precipitation is minimal, with snowfall melting quickly in winter. Temperatures are milder than what one would expect in Colorado, with the summer range of 60s-90s and the winter range of 20s-50s for norms. However, for our first forecasting period, a potent Arctic air mass with record snows came through and cast all the norms to the winds. And those winds proved to be one of our biggest challenges for this city - down-valley winds and temperature inversions. However, the effects of a strong arctic cold front behind a high-precipitation polar cold front proved to be the catalyst for our lesson learned. MOS could not foresee the snowpack of record proportions nor how it would keep the temperatures so frigid.

This Lesson Learned had to do with how trapped arctic air in a valley, especially when sitting over snowpack, can bust the MOS forecast high temperature by as much as 12 degrees Fahrenheit. The trapped arctic air creates a temperature inversion, wherein the colder air at the surface lies under a layer of warmer air (the environment warms with increasing altitude, causing air parcels to sink back down to their equilibrium level). This creates a stable environment. The sounding below illustrates the temperature inversion through a deep layer of the atmosphere at 12Z on 12/1/06.

This Skew-T/Log P diagram (sounding) from the University of Wyoming shows the deep temperature inversion in place at 00Z on 12/1/06 (5:00 pm MST 11/30/06). The signature of the inversion (environment warming with altitude) is shown by the right-hand line moving to the right (warmer temperatures) as it climbs up the chart (climbs in altitude). This temperature inversion occurs fairly deep into the troposphere, as witnessed by the large jog over to approximately 660 mb.

Since GJT saw a record 3.9 inches of snow, the arctic air sitting over this snowpack remained colder, causing high temperatures to be very low. MOS for 11/29 did not account for the snowpack, nor for the arctic air to become trapped in the valley, causing one model to forecast a high 13 degrees above what actually occurred (NGM forecast 34°F for 11/30, actual was 21°F). The other models were too high, as well, though not by this much: ETA forecast 31°F and GFS forecast 27°F. The dynamic models are known to move low-level cold air from a cA air mass (continental Arctic) out more quickly than it typically does, and so does not tend to account for it. However, a forecaster could look at the high temperatures behind the front (see surface analysis below) to see what kind of effect the arctic air mass would have.

This surface analysis plot from the Storm Prediction Center shows the temperatures behind the frontal boundary on 11/28 (image valid at 15Z 11/28/06). Note in the upper right corner the temperature of 18°F at Grand Junction (in front of the stationary frontal boundary) and the temperature of 1°F to its north behind the stationary frontal boundary. This is an indication of the strength of the arctic air mass. Of note is that a cold front had just passed through GJT, which was the catalyst for the 3.9" of snow. That frontal boundary can still be seen down in southern Arizona-New Mexico.

This same scenario affected the low temperatures, as well. MOS predicted 7/7/5 (NGM/ETA/GFS) lows for the forecast period 11/30-12/1, and the actual low temperature came in at 3°F. Again, the cold arctic air was trapped in the valley (under the deep temperature inversion) and this air was sitting over the snowpack. The actuals for 06Z 11/30 - 06Z 12/1 can be found on the Climate Report and METARS for the forecasting period. Of interest, and which spurred much discourse on the boards, was the fact that in spite of the deep temperature inversion and the snowpack, the down-valley winds were able to kick in with some momentum at 15 knots, which ordinarily would not occur under those conditions. It was determined that the down-valley winds were a result of the terrain, which enabled them to overcome the inversion and snowpack variables.

This meteogram from the University of Wyoming shows the stats for the forecast period 06Z 11/30/06 through 06Z 12/1/06. The 3°F low occurred between 13Z-14Z (note the 4 at 13Z and the 6 at 14Z). The high of 21°F occurred between 21Z-22Z (note the 20 at 21Z and the 19 at 22Z). The down-valley winds occurred in the 16Z-19Z timeframe, where the wind barbs show a southeast wind at 10 knots.

The next day (forecasting period 06Z 12/1/06 - 06Z 12/2/06) MOS did the same thing, forecasting roughly 10 degrees too high for the high temperature. MOS went 34/38/32 (NGM/ETA/GFS) for the high, and the actual came in at 26ºF (which can be seen on the Climate Report and METARS for 12/1). For the low temperature, however, MOS tended to have more of a handle on its forecast, predicting 12/17/9 (NGM/ETA/GFS) with actual low temperature of 13°F. This is illustrated on the meteogram below.

This meteogram from the University of Wyoming shows the stats for the forecast period 06Z 12/1 - 06Z 12/2. Note the high of 25°F at 22Z (actual 26°F between 22Z-23Z) and the low of 13°F (actual) at 06Z 12/1. Except for around the time of the high temperature, skies were overcast, as opposed to the clear skies of the day before. This allowed for a warmer low temperature.

The climatology for Grand Junction indicates that "spells of cold winter weather are sometimes prolonged due to cold air becoming trapped in the valley." However, MOS derives its data directly from the dynamic models, which do not recognize this variable. In addition, climatology also states that precipitation in this high desert location tends to be sparse, and what snow falls is usually light and melts quickly. Therefore, no models of any kind could account for the snowpack (remember the 3.9" of snow) that kept the low-level air, already arctic-chilled, in place in the boundary layer (temperature inversion). This unusual double-whammy no doubt kept the heaters running non-stop in this high desert city.

Actually, this was as fun as Jackson was for me, though extremely challenging. We came into this forecasting period with a very potent Arctic cold front, actually a double whammy of cP and cA back to back. We'd just digested the climatology of a high desert valley protected by mountains that had minimal precipitation and usually not harsh temperature changes. So what happens? a hum-dinger comes through, busting temperatures way below normals and dumping a record 3.9 inches of snow. This gave us the opportunity to see how snowpack affected MOS predictions for highs and lows - considerably. But our lesson was just how much MOS would be off.

Just to be fair, I'll play upstream again - this same cold front brought a whopper snowstorm to the midwest, including southern portions of Kansas City. Record inches of snow fell in a band from Oklahoma to the Great Lakes. The snow amount gradient was so small it was difficult for the local weatherman to forecast - several inches difference occurred in areas as small as 5-20 miles. Happily, it brought 11" to my farm, and the temperatures stayed low the next few days due to the snowpack. Obviously, I finally got my cold frontal system that packed a punch upstream as well as downstream. I enjoyed it thoroughly.

No Lesson Learned required for this week, which was fine since this last week of our forecasting contest, in Grand Junction, was relatively quiet. However, there was still something to be learned, and I've mentioned it below in my Reflections - for me, the lesson learned was how quickly a high desert climate will warm up once a record snowfall snowpack melts and trapped arctic air moves out. I underestimated how quickly that would occur.

We came into the contest on a quiet wave in Orlando, FL and went out with a quiet wave after the biggest storm event of the semester. We saw all kinds of weather that ran the gamut from hot sunny days to severe thunderstorms (squall line) to record snowfall and plummeting arctic temperatures. The quiet weather proved to be more of a challenge to forecast than one would think - at least more than I thought it would. This last week of Grand Junction, the snowpack was still around and kept the temperatures down. Then suddenly they started to climb. I'd been doing alright with the cold, and was at 23 in the contest before the temperatures began to warm. What I forgot to account for was the climate - without the snowpack, the arctic air could move out and the regular high desert climate would come back into play. Thus, temperatures warmed considerably once that snow melted down. This brought me to 74 the first warmer day, and 122 the next. That was rather disappointing, but once again it reminded me how important climate is to a forecast.

Below are two visible satellite images that correspond to the 25ºF high over the snowpack on 12/5.


This visible satellite image from the College of DuPage site shows the snowpack still over the entire western part of Colorado. This image, valid at 2015Z 12/5, corresponds to the high temperature of 25ºF, roughly 20 degrees below the normal high due to the snowpack and arctic air mass in place.	This visible satellite image from the College of DuPage site shows the snowpack at 2145Z 12/5. Once again, this corresponds to the high temperature of 25ºF.

The arctic air mass combined with the snowpack kept the high and low temperatures considerably below normals. Note the day's temperature spread and clear skies (forecast period only) on the meteogram below.

This meteogram from the University of Wyoming surface observations site shows the low of 8°F at 14Z and the high of 27°F at 21-22Z. The actual low of 7°F and actual high of 29°F occurred in the vicinity of these hours. Note the clear skies and light winds. What this meteogram can't show is the snowpack which served to keep these temperatures considerably below normals.