Ramblings of a Graduate Student

On Monday, I posted that the forecast indicated a wet period over the southern plains would develop later this week. Well, based on tonight’s Hydrometeorological Prediction Center’s forecast (above) it certainly appears that forecast is on track!  Hopefully the rain will clear out some of the pollen that has been driving my allergies crazy!

Answer to last night’s question:
The low pressure in southeast Colorado developed within what is known as the “Lee Trough”.  The reason then the pressure contours arched northward was because within the background pressure field, the lowest pressure was located to the north.  Thus, when drawing contours, the contours around the low, arched north toward the areas of the map where the background pressure was already that low.

For more information on what a “Lee Trough” is, please read the definition below from the American Meteorological Society’s Glossary of Meteorology.

Lee trough—(Same as dynamic trough.) A pressure trough formed on the lee side of a mountain range in situations where the wind is blowing with a substantial component across the mountain ridge; often seen on United States weather maps east of the Rocky Mountains, and sometimes east of the Appalachians, where it is less pronounced.

Its formation may be explained thermodynamically by the warming due to adiabatic compression of the sinking air on the lee side of the mountain range, or dynamically by generation of cyclonic circulation (cyclogenesis) by the horizontal convergence associated with vertical stretching of air columns passing over the ridge and descending the lee slope. Alternatively, the latter viewpoint is often expressed as the conservation of potential vorticity, where the vertical stretching of the columns is compensated by an increase in their relative vorticity. See lee cyclogenesis.

Last night at the time of the blog post, a squall line was developing across Kansas.  This squall line did not persist through the night as it dissipated.  A new round of thunderstorms developed in northwest Texas and spread northeastward overnight into the morning hours.  Damage was reported in portions of Oklahoma as this second round of convection moved through.  In fact, I had an opportunity to help with damage surveys today, but my schedule wouldn’t permit it.

The image above shows rainfall totals for today throughout Oklahoma.  As the squall line moved through Oklahoma, the forward progression slowed somewhat and so, in general, greater precipitation amounts were found in the eastern part of the state.

The mid-level low discussed in last night’s post has moved into far southeastern California and far western Arizona tonight.  Downstream (east in this instance) strong ascent (rising motion) in the atmosphere has resulted in widespread showers and thunderstorms across portions of the desert southwest.  In fact, severe supercell thunderstorms (with severe thunderstorm warnings issued for them) developed this evening across portions of far eastern New Mexico.

This mid-level low will continue to move east overnight and will be bringing widespread showers and thunderstorms to a large portion of the south-central United States tomorrow.

Today marks the end of what is often referred to as “meteorological winter”.  Typically seasons begin and end on the solstices and equinoxes, but meteorologists tend to break our seasons based on calendar months.  Below are the months contained in each “meteorological season”.

• Winter: December, January, and February
• Spring: March, April, and May
• Summer: June, July, and August
• Autumn: September, October, and November

The United States will end a very active meteorological winter that saw numerous blizzards, at least one ice storm (in Oklahoma), and snow in all 50 states.  Two mid-level lows (shortwave troughs) are moving through the southern plains (yellow x’s) as well as a strong mid-to-upper-level jet stream (cyan color to the southwest of the southernmost x).  Also, the strong nor’easter that has affected New England for the past few days is slowly moving east, out to sea.

These mid-level lows are aiding in the development of precipitation in the central plains and western Texas as indicated by the radar images below.  There is a chance that portions of western and central Oklahoma will see some snow mix in with the rain, but little-to-no accumulation is expected.

Last night I mentioned in my post that it looked more and more like the winter storm for Oklahoma was going to end up being a non-story.  For most of Oklahoma, this turned out to be the case.  Whereas there were places that received several inches of a wet fluffy snow, most places didn’t see any accumulations.  Accumulations that did occur were primarily confined to grassy surfaces and travel issues were minimal.

This was not the case in far eastern and southeast Oklahoma.  Nor was it the case for a large portion of Arkansas.  The forecast when most residents went to bed called for rain, possibly mixing with snow at times, throughout the night.  No accumulations were forecast for places like Fort Smith, Little Rock, Memphis, etc.  Talk about a surprise when residents in these areas awoke to anywhere from 2-4″ of snow on the ground with more snow on the way.  So what happened?  How did snow fall in southern Kansas and eastern Oklahoma, and miss central Oklahoma?

First, as all my former synoptic students should be able to tell you, we need to diagnose why the precipitation developing where it was.  Normally, I would make them first tell me where there is precipitation, but I’ll provide that answer this time.

Above is a water vapor image valid at 0845 UTC (3:45 AM CST).  Below is the same image, annotated by me.  In the image above there are two areas in the southern plains that have brighter colors (more moisture aloft).  And as explained in previous posts, more moisture aloft is a good indicator that there is some sort of upward vertical motion transporting the moisture upward.  We’ll start our journey in these two areas (circled in the image below).

So why is their upward motion (and precipitation falling) in the area circled in red?  The answer here lies in the transport of warm, moist air about 5,000 feet (1500 meters) above the ground.  In the image below, areas that are shaded in various shades of red indicate where warm air was being advected (blown around) toward cold air.  The darker the red, the stronger the advection.

As the warm (and more moist) air begins to interact with the colder air, the warm air begins to move over the top of the cold air.  This is a common form of atmospheric lift in winter (known as isentropic lift, or, in slang, “overrunning” because the warm air is overrunning the cold air).  This atmospheric lift  was enough to produce precipitation during the night in northcentral Texas, southeast Oklahoma, and much of Arkansas.

To the northwest of this area, we have another area of atmospheric lift occurring in southern Kansas and northern New Mexico (shaded in green in the water vapor image mentioned previously).  If we look at the warm air advection map (above) we would see there is none of this occurring at that level of the atmosphere and so that isn’t the source of lift.  However, similar to how a surface cold front can sometimes provide the focus for atmospheric lift, fronts aloft can do the same.  If we look at a portion of the atmosphere typically around 10,000 feet (3000 meters) above the ground we can see that a relatively strong front was draped across southern Kansas (the brighter the colors in the image below, the stronger the front).  It turns out that convergence along the front was strong enough to induce vertical motion and, ultimately, precipitation.  As you can see, there really wasn’t much of a focusing mechanism for precipitation across much of Oklahoma, which is why we didn’t see widespread heavy snow (or rain).  Just a drizzle, mist for much of the day.

So, this explains why there was precipitation where there was, but why did the forecast rain actually fall as snow in southeast Oklahoma and western Arkansas – especially with the surface temperature above 32F?  The answer lies in what the temperature did above the ground. Normally, when temperatures are above 32F we should expect rain, right?  Well, not always.

In places like Fort Smith, AR and Little Rock, AR, the warm air (above 32F) at the surface was very shallow – no more than about 1000 feet deep.  As the initial precipitation fell across the area, it fell in the form of light rain – at the surface.  It was actually snow aloft, but it melted back to rain as it fell through the warm layer just above the surface.  Most forecasters thought that the precipitation would be light enough that the snowflakes would have time to melt as the fell through the warm layer and thus little to no snow would fall throughout most of the night.

However, as heavier precipitation began to develop around midnight, the precipitation began falling so fast that it did not have time to completely melt as it fell through the warm layer near the ground.  Normally, this would continue while the heavy precipitation fell and then transition back to rain as the heavier precipitation moved away.  Unfortunately for meteorologists, the precipitation remained heavy for a longer time than expected.  This allowed more and more big, wet snowflakes to make it to the ground before completely melting.  This had two effects: 1) it allowed snow to begin to stick to the ground as it was falling – there wasn’t enough time for a snowflake to melt before the next snowflake landed and 2) as the snow fell into the warm layer, the warm layer began to cool – similar to how a drink cools as you put ice into it.  The cooling of the warm layer allowed for less melting of the snow which meant more snow reached the ground.  As more reached the ground, the surface temperatures began to cool, allowing more snow to stick, and we’re off to the races.  This heavy precipitation induced snowfall is why when the precipitation was less intense, there was a tendency for it to mix with or switch back to rain across southeast Oklahoma and much of Arkansas.

Now, in southern Kansas, the temperature was below 32F throughout the entire atmosphere and melting of the snowflake was not an issue.  Thus, the snow wasn’t as much of a surprise (in fact it was well forecast) as it was for places to the south and east.

A very active weather pattern is shaping up for the next 10 days or so beginning with a shortwave tough (upper-low) moving through the southern United States.  I debated whether to talk about this upper-low as it currently is moving through the south central US or discuss what will happen (partly as a result of this upper-low) along the east coast starting tomorrow into the weekend (another major snow storm for the mid-Atlantic states!).  I figured there would be a lot to talk about tomorrow with respect to the mid-Atlantic snow so I decided to talk about the southern plains tonight.

Below is a radar image taken tonight from the National Weather Service radar in southwest Oklahoma.  At the time this image was taken, southwest Oklahoma was experiencing light to occasionally moderate intensity rain – at the surface.  Why do I make this distinction?  Because above 6000 feet above the ground it was actually snowing!

The radar image above has what is known to meteorologists as a “bright band” signature.  The bright band is the dark green and yellow pixels that appear to make a circle around the radar (the cyan dot with KFDR label).   What is happening is that high in the cloud, the precipitation starts out as snowflakes in the cold air aloft.  As the snow falls and gets closer to the surface it encounters air that is above 0C (32F) that causes the snowflake to begin to melt.  This partially melted snowflake shows up on a radar image much more easily than a snowflake or a raindrop itself would show up.  This causes the level with the most partially melted snowflakes to show up as a bright band or bright circle around the radar site.  (Without getting in the math and physics of it all, everywhere along the the dark green and yellow circle is essentially at the same height.   It has to due with the curvature of the earth’s surface.)

Because the image below has a fairly easily identifiable bright band, I can say with a lot of confidence that it is snowing higher up in the clouds.  Now, the million dollar question, will any of those snowflakes make it to the ground?

Ah, the night before a forecast big storm. Meteorologists keep checking the data non-stop hoping the latest piece of information is available. When the numerical weather prediction models’ forecast transitions away from the forecaster’s desired forecast, a lot of “wishcasting” takes hold. Meteorologists will often tend to find any piece of information to sustain hope that what they want will ultimately develop. This internal struggle can be a challenge for a meteorologist. I know I struggle with it from time to time.

As residents throughout the southern plains head to bed this evening, most are still unsure what tomorrow’s weather will hold.  This is because forecasters are still struggling to understand the evolution of the winter storm.  It is apparent that somewhere will see heavy snow, somewhere will see sleet and freezing rain, and somewhere will only see rain.  The problem is pinpointing those exact locations is difficult even 24 hours in the future.

A couple of things struck me tonight with respect to the shortwave trough (upper-low) responsible for the difficult forecast…

Above is a satellite image depicting the moisture content of the middle-to-upper atmosphere (brighter colors means more moisture aloft).  This is important because the most efficient way to get moisture aloft is for there to be rising motion in the atmosphere.  Thus, wherever there is the brighter colors, we can infer rising motion.  Rising motion is important because it is a key (but not a sufficient) ingredient  in the development of precipitation.  Thus, with all that rising motion in the southwest United States, one would expect a lot of precipitation in the southern plains as the rising motion moves into the area.  This agrees quite well with the precipitation forecasts from the Hydrometeorological Prediction Center (HPC) shown over the last few days.

Below I’ve identified the center of the upper-low (red L) and what I suspect will be the eventual forecast path based on a combination of observational and model trends.  This is considerably farther north and west than what was forecast several days ago.  As I mentioned last night, a farther north/west track will result in more places being affected by the dry slot – including Oklahoma City and points south and east.  However, before the dry slot affects central and eastern Oklahoma, these places will spend an extended period of time in the warm conveyor belt which will bring warm, moist air.  This warm, moist air will result in heavy precipitation and a warming of the temperature aloft (and possibly at the surface).  This warming aloft will prevent snow crystals from forming in central Oklahoma which leaves sleet, freezing rain, or a cold rain as the resulting precipitation types.

As you can imagine, I’ve been pretty busy the past few days handling all the requests for information and discussion regarding tomorrow’s (possible) winter storm.  I didn’t have time to annotate any more graphics tonight, but I will leave you with something better.  Below is a recording of today’s HWT Map Discussion.  It may be a little too technical, but it can give a better glimpse as to what a forecaster must look at in situations like these.  Please feel free to ask questions and provide feedback regarding anything in the last few days worth of blogs and / or the video below!

27 January 2010 NOAA HWT Map Discussion from Patrick Marsh on Vimeo.