Wednesday, November 19, 2008

Quiet For Most, Now Let's Find That Cold Front

It's a quiet weather day for much of the country.

More heavy lake effect snow is expected today and over the next few days across portions of Michigan.

Marquette looks to pick up another 6-18 inches of snow depending on where you live in that region.

The terrain and exact orientation of the wind off the lake can bring a foot of snow to one spot and just an inch or two down to another.

Jacksonville, FL set a new record low last night of 28 degrees.

Denver set 2 new records on Tuesday. One was a high of 78 degrees. The other was the overnight low, which was only 47. That is a new record high low temperature for the date.

The map above will be very hard to read but I want to use it for today's lesson about cold fronts.

I believe the picture will get bigger if you click on it.

So if you were to take a weather analysis class, this is an exercise you would likely have.

It's called find and draw the cold front! This map is showing surface observations at major reporting stations around the US for 8 am Wednesday, Nov. 19.

It shows different weather parameters for each location, including temperature, dewpoint, wind direction and speed.

Actually reading and decoding the observation could be another entire lesson within itself, and we may do that at some point if there is any interest.

So if a meteorologist had this map, to find the cold front, he or she would...

  • Look for sharp temperature changes over a short distance (such as the pocket of 40s and 50s over southern Kansas versus the 30s not too far to the north over Nebraska, Iowa and points north.

  • Changes in moisture (or dewpoint)-- which is much harder on the map I provided you because much of the country is under a dry air mass. It's much easier to find a cold front using dewpoints in the warm season.

  • Shifts in wind direction -- look from Texas up to Kansas, winds are blowing from the south. But above that in to Nebraska and Iowa northward, winds are coming in from the north.

  • You can also look at pressure and pressure changes if you have maps that span a few hours, or you can also look at the clouds and precipitation patterns.

    So just in taking a quick glance at the surface map above from this morning, simply by checking the temperatures and wind, we could porbably draw in a cold front somewhere in the central plains states, likely across southern Nebraska and extending toward Michigan on the east and the Rockies on the west.

    Who knew finding the cold front could be so much fun!

    Now I should say if we pull a surface map from mid to late afternoon, it could be much easier to spot the cold front -- because we will be at maximum daytime heating and the atmosphere will be completely stirred up.

    The problem with the 8 am map above is overnight the atmosphere mixes out and settles down with no sunshine to act as the "oven burner" to cook things up.

    So although we clearly get an idea something is happening in the Nebraska/Iowa vicinity from the 8 am map above, if we looked at a 3 pm map this afternoon, the front may be a little more obvious.

    Then again -- it may not be any more clear than the map above.

    This front isn't terribly dynamic, meaning there is already some cool air in place over much of the country, so the clash of the air mass in front of and behind the cold front isn't as dramatic as it could be -- so the afternoon map may not be more obvious than the one above.

    That is why people get into weather -- it is always different from day to day, hour to hour and in many cases minute by minute.

    You just never know -- and you can never say for sure because there are so many variables involved that can shift ever so slightly and make all the difference in the world to the outcome.
  • No comments:

    Post a Comment