WDTV Weather Blog

Welcome to the new WDTV Weather Blog:

***If you see a term in the blog that you are unfamiliar with, look at the bottom of the page for a description/definition!

SPRING IS IN THE AIR, BUT FOR HOW LONG?

High pressure and sunshine has lead to a nice afternoon here on Monday, but that same high pressure, dry air, and calm winds will lead to a cold night tonight.  Tuesday will be better than today with sunshine once again with a moderated air mass.  Translation:  The accumulation of sunshine Monday and Tuesday is greater than the heat lost at night.  Eurpoean 850 temperatures will be near 3C (38F) which will support highs in the upper 50s Tuesday afternoon.

Wednesday through Friday will feature a low pressure system moving east slowly.  The reason its moving slowly in our direction is because of blocking to the east and other storm systems behind the one in question that are forcing our next storm to the north (as opposed to the east).  This means that it will stay mild, and even get warmer, with southerly flow ahead of low pressure (due to counter clockwise flow around low pressure).  However, this will also bring incresed amounts of moisture in the form of clouds and rain.

With some upper level support moving overhead on Wednesday, scattered showers will be around.  With the center of the storm moving closer on Thursday, rain will be more prevalent (widespread), then the center of the storm moves over us on Friday and more rain will be around the state.

Colder air moves in later this weekend and there could be some snow in the higher elevations.

TERMS USED:

850 Millibar Temperature (850T):  The temperature of the air at approximately 5000 feet above the ground.  This temperature is used to determine the  "amount of heat" in an airmass because it is unaffected by heating from the sun.  This gives meteorologists an idea of what high temperatures will be like on a given day, when taking amount of sunshine, time of year, humidity, and other factors.

Decibels (dBz):  The units used to measure radar (precipitation) intensity.  For snow, 5-20 would be light, 20-30 moderate and 30+ would indicate heavy snow.  For rain, 20-40 is light, 40-55 is moderate, and 55+ indicates heavy rain.  When dBz get above 60 or 65, this usually indicates the presence of hail, with 80 dBz being about the highest possible returns to see on a radar.  Sometimes dBz get very high when ice is present becuase it is very reflective and more of the energy returns to the radar.

Dew Point:  The temperature at which the air becomes saturated (100% relative humidity).  This is a measure of ABSOLUTE humidity (the total amount of water in the air).  What it means:  Dew points represent the LOWEST the air temperature can go.  We achieve the dew point temperature under ideal conditions of clear skies and calm winds.

European Model (ECMWF):   A computer model that was designed by the European Centre for Medium Range Weather Forecasts.  Model data is limited because it is NOT free of charge.  The free data supplied consists of 850 temperatures, 500 milibar heights, 1000-500 meter thickness, plus 850mb and 700mb relative humidites.  Some would argue that this model is the most accurate and best to use.

Freezing Rain:  Ice that forms when rain falls through a SHALLOW layer of air that is below freezing.  When there is a shallow layer of sub-freezing air, the objects within that layer get below freezing.  The air above this layer is warm and supports liquid precipitation.  When the liquid comes in contact with a freezing surface, a coating of ice develops.  This is freezing rain.

GFS (Global Forecast System):  A computer model which extends out to 240 hours, producing various forecasts including amount of precipitation, wind speeds and directions at multiple levels, relative humidity, temperatures, and more.  This model has lower resolution than the NAM.

High Pressure (H):  High pressure is the highest/higher pressure than readings in its vicinity.  The reason it usually produces "nice" weather is that when the air spreads out (moves away from high pressure/toward low pressure), it causes the air above it to sink, which warms the air, and has a tendency to evaporate liquid water.  This has the effect of making clouds unable to form.

Liquid Equivalent Precipitation:  Computer models give precipitation in terms of how much liquid will be produced, regardless of if it falls in the form of snow, ice, or rain.  When it snows, the liquid equivalent must be multiplied by a factor (10 minimum, 15-20 common, or up to 50 in extreme cases) to determine what accumulation of snow is to be expected.

Low Pressure (L):  Low pressure is the lowest/lower pressure than readings in its vicinity.  The reason it is so often talked about is because wind CONVERGES (comes together) at the center of low pressure.  When air converges, it is coming in, and it can't go into the ground, so it can only go up!  When air rises, it cools, and the water (in gas form), becomes liquid, and eventually forms clouds and then rain/precipitation. 

NAM (North American Mesoscale model):  A computer model which produces forecasts out to 84 hours.  Variables forecasted include liquid amount of precipitation, wind speeds and directions at multiple levels, relative humidity, temperatures, and more.  This model has a better resolution than the GFS.

Nor'easter:  A low pressure system that generally moves northeast up the Atlantic coast, has persistent northeast winds (from the northeast), and attains its greatest power in the northeast part of the United States.

Precipital Water (PW):  The amont of water you would have if you took all of the water vapor (water in gas form) from a column of air over a particular location and condensed it all into liquid.

Sleet:  Ice pellets that form when liquid precipitation falls through a DEEP layer of sub-freezing air.  When liquid precipitation falls through a deep/long distance of below freezing air, it has enough time to freeze into ice before it reaches the ground. 

Ridge (high pressure):  The extension of high pressure in a particular direction.  When this occurs in the mid and upper levels of the atmosphere, we usually see sinking air downstream of the feature and rising air upstream.

(forecast) Sounding:  A vertical profile of temperature and humidity (dew point).  How fast temperatures decrease with height determines the stability of the air, and therefore if it is prone to  rising/sinking.  How close the temperature is to the dewpoint determines how much cloud cover is present at various levels.  This vertical profile is very useful to forecast thunderstorms as well.

Thickness:  The distance between the 1000 millibar pressure level and the 500 millibar pressure level.  This distance is relatively equal to the bottom half of the atmopsheric column above a point on the earth.  Smaller thicknesses mean colder temperatures and greater thicknesses mean warmer temperatures.  When the thickness equals 5400 meters, there is a 50/50 chance for rain or snow.  When they are lower, the chance of snow goes up, and when the thicknesses go up, there is a better chance for rain.

Trough (low pressure):  The extension of low pressure in a particular direction.  When this occurs in the mid and upper levels, we usually see rising air downstream of the feature and sinking air upstream.

Upsloping:  When air is forced to rise up and over a mountain range.  With the presence of rising air, clouds and precipitation usually result, and in the winter, this can lead to prolonged snow with widespread, heavy accumulations.

Vorticity:  The "spin" of the atmosphere around a point in space.  When air flows from higher to lower areas of vorticity, air spreads apart, allowing air to rise from lower elevations.  If air rises long enough and has moisture, clouds and precipitation form.

Warm Air Advection (WAA):  Warm air replacing colder air at a point in space.  Warm air tends to rise as it replaces cold air because cold air is more dense and the warm air cannot displace it from the surface.  Warm air eventually takes over at ground level, only  after the cold air has retreated.  Warm air advection commonly leads to clouds and precipitation due to it rising over colder/denser air.

Wet Bulb Zero (wb0):  The level at which the wet bulb temperature is 0 degrees Centigrade.  The wet bulb temperature is the temperature at 100% relative humidity.  This is useful when there is heavy or persistent precipitation.  As we get a lot of rain or snow, the moisture evaporates into the air, raises the dewpoint, lowers the temperature, and increased the humidity.  When we are at 100% relative humidity, the dewpoint temperature, the air temperature, and the wet bulb temperature are the same.  Wet bulb zero is useful for finding snow levels.