12/10/14: The Inversion

High pressure continues to work to the east.  On the western side of the high a moderate pressure gradient has developed meaning another day with a noticeable south wind.

7:00 AM Surface Analysis
7:00 AM Surface Analysis

The south wind is advecting not only warmer air into the area, but also some moisture.  That increase in moisture over the cold frozen and at least partially snow covered ground is an excellent recipe for low clouds and fog.  The fog being liquid droplets in turn creates ideal conditions for that water vapor through the process of deposition to make for frosty, ice covered roads and in some instances, for hoar frost to form on the trees.

Because the ground radiates heat more efficiently than the air, the lowest levels of the atmosphere are almost always cooler than the air above the ground each morning during the coolest time of the day.  When the temperature warms as you go higher in the atmosphere, this is referenced as an inversion.  In the warm season, the sun will heat the ground, through conduction then convection the lower atmosphere gets heated, the thermals rise and the lower atmosphere mixes and becomes warmer than the air aloft.  But in the winter time, with a frozen ground and often snow cover, plus the very low sun angle, this process works very inefficiently.  In fact, much of the winter, the temperatures near the surface are colder than the air aloft.  So an inversion is almost always present.

Because the lowest levels remain unmixed with the air aloft, low clouds and fog can get “trapped” and remain in place for days on end.  This is one of the scenarios that makes this region a very cloudy place in November and December.  In January, cold dry high pressure from Canada is often the catalyst for moving the “gloom” out and bringing sunshine but also very cold air to the area.  This week with a south wind and a very strong inversion in place, the high temperature potential will very likely be limited.  Every day around 6:00 AM and 6:00 PM a weather balloon is launched at many locations round the United States.  The only location in North Dakota where this is done is in Bismarck.  Presented below is the “sounding” the graphical representation of the temperature, moisture and wind data from Bismarck’s balloon launch 6:00 AM today.

Bismarck 12/10/2014 Sounding from 12Z (6:00 AM)
Bismarck 12/10/2014 Sounding from 12Z (6:00 AM)

The red line represents the temperatures.  If you look to the right and in blue you will notice some numbers.  Those are the temperatures in degrees Celsius.  In Bismarck at the surface where the red line begins you will notice the temperatures was near -8° C (18° F) (bottom of graphic).  As the balloon rose into the atmosphere the temperature immediately got much warmer.  At between 900 and 800 millibars (mb) the temperatures climbed to 9° Celsius which is 48° F!  The temperature seemed to peak at 850 mb which is near 5000 feet above mean sea level.  Considering that the balloon was launched at 1660 feet above sea level, that means the temperature was nearly 50° around 3000 feet above the ground in Bismarck.  In fact, most of the upper Midwest would have temperatures at least in the 40s just a few thousand feet away, straight up.

Without snow or a frozen ground and a slightly higher sun angle, the mixing of the lower atmosphere already written about, would mix that air to the surface and bring that warmer air aloft to the ground, plus, with compressional warming increase that temperatures by at least another 10° C (18° F), meaning if it was, as an example mid October, with this atmosphere I would forecast a high near 70° today.  But with the strong inversion, limited mixing, some snow on the ground, the highs in eastern North Dakota will probably only reached 30 to 35 degrees and western North Dakota 40s with some localized low 50s in the higher elevations (above the inversion more and also lack of snow).

It is this very warm air aloft, which will only get warmer the next few days that suggests, as written in my Monday blog that record high potential exists this week, IF we can get rid of the low level cloudiness.  The day with the highest potential looks to be on Friday, possibly Saturday in eastern North Dakota and Thursday and Friday in western North Dakota.

The wind today will be quite brisk in the 10-25 mph range.  Same potential the next couple of days, if not even stronger, so many of these “warmer” days will come with a breeze.  The low clouds will likely stick around today in the eastern one-third of North Dakota so the warmest readings in the west.  A little bit more potential for sunshine statewide tomorrow and Friday, but with this very strong inversion in place, and forecasting trapped low level moisture in these scenarios being very difficult, no guarantees, but there is still 40s potential even in eastern North Dakota the next few days (especially Friday and Saturday), but only western North Dakota is expected to be that warm today.

Daryl Ritchison