Tuesday, February 1, 2011

Weather University (WxU)
By "Dr. Ratio" and the Forecast Advisors

Welcome to Weather University!
Where the Advisors and Forecast team try to give you:

Weather made really simple

Lesson #4- Highs and Lows
5 July 2011

We have all learned to associate High Pressure with fair weather and Low Pressure with stormy weather, and now, forecasters throw in "ridges" and "troughs". What gives?

OK, before we start, remember this:
Cold air cannot hold moisture as well as warm air.

(Simpler still? Cold air is drier than warm air)

High Pressure- In the Northern hemisphere these are areas of the atmosphere that rotate clockwise and outward, pulling the air in the center down to the surface. This, then causes the pressure on the Earth to be greater there, thus 'High' pressure. Since these systems bring cooler, therefore drier, air from higher altitudes, they are associated with clear skies, and no rainfall.

Low Pressure- In the Northern hemisphere these are areas of the atmosphere that rotate counter-clockwise and inward, forcing the air in the center up from the surface. This, then causes the pressure on the Earth to be lesser there, thus 'Low' pressure. Since these systems bring warmer, therefore wetter, air from the ground and lower altitudes, they are associated with cloudy skies, and rainfall. The air, as it is forced up, becomes cooler, and therefore loses its moisture in the form of clouds and rain.

Ridge- Rather than a closed system, an elongated area of higher pressure.

Trough- Rather than a closed system, an elongated area of lower pressure.



Lesson #3- Floods
(with help from NOAA)
7 March 2011
Flood Statement-
Statement to inform the public that stream flooding does not pose a serious threat to life or property. May also be issued after a Warning to add information. Forewarning of possible flooding may come in the form of a Hazardous Weather Outlook.
Flood Watch-
Statement that there is a threat of flooding, but the occurrence is neither certain nor imminent.
Flood Warning-
Statement to inform the public that stream flooding DOES pose a serious threat to life or property.

Flash Flood-
A rapid and extreme flow or rapid water level rise of high water into a normally dry area, or a stream or creek above flood level. This can be causes, but not limited to: intense rainfall, dam failure, ice jam, or rapid snow melt. It is considered a Flash Flood if it occurs within six hours of the event.
Dr. Ratio



Lesson #2: Snow, Sleet, Freezing Rain, and Virga
(from NOAA.NWS)

2/10/2011

SNOW


Precipitation in the form of ice crystals, intricately branched, hexagonal form and often agglomerated (from the Latin to form into a ball) into snowflakes, formed directly from the freezing of the water vapor in the air.

Supposedly, no two snowflakes are alike, but they can be classified by basic shapes. See how many shapes have been described by going to CalTech's website.

Snow is abbreviated on the NWS National Digital Forecast Database as...

... S (for snow) and SW (for snow showers)



SLEET

Sleet is defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces.

Sleet is abbreviated on the NWS National Digital Forecast Database as...

...IP


FREEZING RAIN

Freezing rain is just that. It is liquid precipitation that strikes sub-freezing surfaces...and freezes.


Freezing Rain is abbreviated on the NWS National Digital Forecast Database as...

... ZR (freezing rain) and ZL (freezing drizzle)


VIRGA

Virga are small bands of precipitation falling from a cloud but evaporating before reaching the ground. They frequently show up on weather radar and are interpreted by beginning weather enthusiasts as snow or sleet, when in fact the precip becomes water vapor before it hits the ground.

The word virga comes from the Latin for twig or branch. The amateur meteorologists have created a
backronym from the word virga-"Variable Intensity Rain Gradient Aloft." (a "backronym" is defined as an acronym made from an existing word)


Lesson #1: Eastern U.S. Coastal Storms

2/4/2011: Regarding a potential coastal storm system, bringing a "Winter Wx" event (Wx = weather in Wx-speak), forecasters have to consider the The Three T’s: Track, Timing, Temperature.

Track
Where is the storm coming from?
- From the West, where it will dump most of its moisture on the other side of the Appalachians?
- Up the coast, where it will-
a) hug the coast, and bring warm temps from the South;
b) pull out to sea, and miss us completely;
c) or, do a ‘Goldilocks’ and track “just right” to bring snowfall to the Mid-
Atlantic?

Timing

- When is it getting here?
- How fast is it going?
- Will it be blocked by a Canadian High that will keep it hanging around?

Temperature
- What is the surface temp when it arrives?
- What is the air temp when it arrives?
- What is the temp of the air it is bringing?
- Does it arrive at night, when the temps are colder?

These are three fundamental variables we consider and carefully analyze as we develop a Winter Storm forecast for the Eastern U.S. But you all just want to know...we getting snow? When? Where? How much?

ANSWER: After our team is finished analyzing the 3 T's, we'll do our best to let you know.

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