Ramblings of a Graduate Student

Above is a computer generated forecast for Tuesday morning (6 AM CST) 9 February 2010.  This forecast comes from the Global Forecast System (GFS) model and is used by meteorologists around the world to help predict the weather.  It is run 4 times a day (00Z, 06Z, 12Z, 18Z — 6 PM CST, 12 AM CST, 6 AM CST, 12 PM CST) and goes 384 hours into the future.  Now, as the forecast goes farther into the future the less accurate it tends to be.  This is why you will hardly ever see a meteorologists make a 384 hour prediction.  Often times something that is seen in the later parts of a forecast run (200+ hours into the future) will change from run to run.  This means that a major snow storm forecast at hour 288 might not be there on the next run.

If the forecasts are so unreliable that far into the future, then why do the models run that far into the future?  Well, the short answer is that while the exact details may change, often a pattern will emerge and you can get a big picture idea of what may happen in the future.  Take the above forecast image (valid 216 hours into the future).  The green on the map is areas where the model is predicting precipitation and areas to the north of the blue ’0′ line are places that are forecast to be below freezing.  Based on this forecast, a meteorologist might make a prediction that there is a chance of snow in northeast Oklahoma, SW Missouri, and NW Arkansas.  However, being so far into the future, that is not a good idea.

What a good meteorologist should take from this image (and the entire forecast) is that the model is forecasting cold air to come southward out of Canada into the southern plains sometime next week.  Furthermore, the model is predicting some precipitation mechanism to develop in the southern plains / Gulf Coast around the same time.  Now, will this occur at 12Z (6 AM CST) on 9 February?  It’s too far out into the future to know for sure.   If several model forecasts in a row try to develop a storm on 9 February 2010, then a meteorologist would have more confidence that will happen.  If the model continues to change the date and time of when this might occur, a good forecaster will refrain from making a specific prediction about where and when, but understand that ingredients are coming together for a possible winter storm.

As for what the models are currently forecasting?  Well, I wouldn’t put too much stock in the exact timing of things, however, several models are forecasting cold air to move south from Canada.  At the same time, the models continue to have cyclones (lows) move across northern Mexico or southern Texas into the southeast United States.  The cyclones will provide a source of lift for the atmosphere and help to bring moisture from the Gulf of Mexico northward into the southern United States.  Add in the cold air from Canada, and the models are hinting at the potential for more winter storms in the coming weeks.  It’s far too early to tell where and when and how much.  However, it isn’t too soon to begin paying attention to forecasts for next week and beyond.  A lot can (and probably will) change in the coming days, so stay tuned!

Continuing along with the cold air theme of the past few days, tonight we focus on the Tampa, FL area.  The cold front highlighted in last night’s post has moved through most of the Florida peninsula and the cold air is continuing to filter into the state.  At the same time, a mid-level low has organized in the Gulf of Mexico and is beginning to move across Florida.  This mid-level storm is resulting in modest lift atop the cold air at the surface, and this is lift is just enough to produce light showers in the Tampa area (as is indicated by the brighter colors on the radar image below).

Also on the image are current (05 UTC or midnight) temperatures for the Florida peninsula taken from a computer analysis of site-specific data.  The red line farthest north is the 40F line, indicating that any place north of this line is in the 30s.  The temperature contours (isotherms) are done every 4F, with the farthest south line being 52F.  An interesting observation when looking at the temperature contours is the V shape that the contours have.  This V shape is a result of the fact that the temperatures at a given latitude are cooler over the land than over the Gulf of Mexico.  The reason for this is that water heats and cools slower than the land.  At night, the water tends to be warmer than the land and during the day, the water tends to be cooler than the land.  (This temperature contrast is what leads to sea-breezes during the day [cooler air from over the water rushing inland] and land-breezes at night [ coolder air from over the land rushing out to sea].)  Because the water is warmer than land, areas near the coast will stay warmer than those farther inland.  So, all things being equal, the farther inland you go, the greater the likelihood you will be colder than areas closer to the water.

So why is all of this important?  The last time snow accumulated in Tampa, FL was 19 January 1977 when 0.2 inches of snow was recorded.  Overnight, into the morning hours tomorrow, there is a slight chance of snow falling in the Tampa Bay and the inland areas.  As the cold air continues to filter south, and the mid-level storm continues to induce showers over the area, it is possible for the inland areas to get cold enough to allow the precipitation to mix with, or even change over to, snow/sleet.  Areas along and near the coast will most likely remain too warm for snow/sleet to occur.   Either way, it will be interesting to watch this weather event unfold.

Well, the arctic surge is certainly on its way southward.  The radar image below  captures the first cold front as it moved into the Oklahoma City metro.  The higher reflectivity values (brighter colors; in this case the blues and green) are associated with the density gradient across the cold front.  In other words, these brighter colors are the result of the cold, dense air to the north of the front being advected southward, into the northward advecting warm, (relatively) buoyant air south of the front.

Below is the same image as above, except I’ve “drawn” the front in blue, along with the wind directions north and south of the front.

Note:  I would post a more in-depth explanation of the radar image, however, we lost power about 40 minutes ago and I’m working via a laptop and cell-phone internet.  Losing power when the temperature is supposed to go down to single digits doesn’t seem like a good thing…

Well, as the majority of the blog posts so far have indicated, most of the country is in the midst of an extended cold snap.  Unfortunately, it appears that the cold snap is about to get colder.  Today’s weather graphic is once again taken from the National Weather Service (NWS).  It is an image containing all valid NWS watches, warnings, and advisories that are currently in effect.   A good portion of the eastern two-thirds of the country has some sort of winter weather product in effect.  These range from a very small Blizzard Warning in eastern Montana (bright red) to a winter storm warning for much of western Iowa to a Hard Freeze Warning for a good portion of the southeast to a Freeze Warning throughout much of the Florida peninsula.

The good news is that the NWP models meteorologists use to help predict the weather are suggesting that Old Man Winter should lessen his icy grip on the US sometime next week.  We certainly won’t see summer like weather, but places that aren’t used to spending days below freezing should return to more normal winter temperatures.  Remember, a lot can happen between now and then, and the forecasts may change, but it should offer hope to those who are tired of the cold.

(Image courtesy of the National Weather Service.)

I hope you enjoyed the deviation from the cold air with yesterday’s post on severe thunderstorms and tornadoes.  Today, we’re back to highlighting the impending cold air outbreak.  Below is a graphic created from the Global Forecast System (GFS) numerical weather prediction model (NWP).  It is taken from the 12UTC (6 AM CST) run¹ from 4 January 2010 and is valid² at 12 UTC (6 AM CST) Friday, 8 January 2010.  In other words, this is a good approximation as to what the NWP model is predicting for overnight low temperatures Thursday night/Friday morning (for locations in central time zone).  The temperatures contoured are predicted 10 meters above ground level temperatures, which means locations where the winds are calm and the sky is clear, the surface temperature will most likely be even colder.

A couple of things to note about this forecast.

1. The temperatures are in degrees Celsius.  For those who don’t like math³, -30C = -22F ; -20C = -4F ;
   -10C = 14F ; 0C = 32F ; 10C = 50F ; 20C = 68F ; 30F = 86F.
2. The green color fills are the liquid equivalents for any precipitation that has fallen during the previous 6 hours⁴.
3. Almost everywhere is below 0C (32F).  The only places where this isn’t the case is the west coast, extreme southern Texas, and the peninsula of Florida (which will most likely experience below freezing temperatures the following night).

(Image Courtesy of the NWS National Centers for Environmental Prediction Central Operations)