Wednesday, January 28, 2004


From time to time on this site, when there is a lull in the weather, I’ll take the opportunity to educate you on some of the terms used in the discussions. This way I can help you better understand what other forecasters are talking about when they start using weather gobbly-gook, and put it in a more plain language format. Now don’t think I’m going to turn this into a weather nerd site and use all kind of techno-speak terms all the time. But this is an important glossary you should archive somewhere in case we get a mongo storm and I have to spit out all kinds of terms to explain what is going on. Or if I post a little analysis from the weather service to illustrate a point. So grab a cup of coffee or soda, and when you have some free time, review this glossary of terms. I’ll post two of these a year…. One for winter, one for summer.

1. Computer Models:

GFS: Global Forecast System (primary US Model… not the magazine) that produces
forecasts out to 372 hours (which is 2 weeks)
UKMET: A medium-range weather prediction model operated by the United Kingdom
METeorological Agency.
ECMWF: A medium-range forecast model operated by the European Center for Medium-Range Weather
Forecasting. We call it the European.

Other computer models I use but not as often…
Canadian: Quite simply, the Canadian computer model forecast

ETA: Another large scale forecast model run by the US NWS but for a shorter time frame, usually up
to 84 hours. What ETA stands for I don’t remember.
RUC: Rapid Update Cycle, a numerical model run that focuses on short-term (up to 12 h) forecasts and small-scale weather features.

2. Approximate location codes used by the NWS and featured in our discussions

NWS: National Weather Service
STC: State College
BWI: Baltimore Airport
BAL: Baltimore
AOO: Altoona
IAD: Dulles Airport
PHL: Philadelphia
RIC: Richmond
DCA: Washington, DC
NYC: New York City
BOS: Boston
LGA: La Guardia Airport

3. School-related abbreviations:

BCPS: Baltimore County Public School
HCPS: Howard County Public Schools
MSDE: Maryland State Department of Education
Chesco: Chester County area schools

4. Basic weather terms: (get ready, it's a long list)

HUMIDITY: A measure of the amount water vapor content in the air

DEWPOINT: The temperature to which air must be cooled in order to reach saturation, assuming air pressure and moisture content are constant

SLEET: Precipitation that falls already in a frozen state, often in tiny ice pellets

FREEZING RAIN: Precipitation that falls as a liquid but freezes on impact at the ground

LOW: A disturbed area of air that spins counter-clockwise and has a pressure lower than the surrounding air. A low is usually marked by moisture-laden clouds, wind and precipitation,

HIGH: A calm area of air that spins clockwise in the Northern Hemisphere and has a pressure higher than the surrounding air. A high is usually marked by clear, dry air with few clouds. In winter, highs can be very cold, in summer, very warm.

FRONT: A boundary between two air masses of different density, and thus (usually) of different temperature. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.

RIDGE: An elongated area of relatively high atmospheric pressure; the opposite of trough

TROUGH: An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

JET: The jet stream

WAVE: A low pressure system developing along the jet

SOUTHERN STREAM: The sub-tropical jet steam

NORTHERN STREAM: The polar jet stream

CYCLOGENESIS: Development or intensification of a low-pressure center

DRY SLOT: An area of much drier air that cycles into a departing low, sometimes cutting off precip and lowering snow amounts.

SIGNIFICANT: In terms of snow, the NWS says that term means 4 or more inches in a 12-hour period or 6+ inches in 24 hours. That is also the criteria for “heavy snow”

WINTER WEATHER ADVISORY: When a light to moderate amount of snow or ice will fall in a 12 to 24 hour period. Amounts are usually less than 4 inches of snow, or .25” or less of ice. Code is a WWA.

WINTER STORM WATCH: Issued when conditions are favorable for the development of hazardous winter weather elements, such as heavy snow or sleet, blizzard conditions, significant accumulations of freezing rain or drizzle, or any combination thereof. Watches are usually issued 12 to 48 hours in advance of an event.

WINTER STORM WARNING: Issued when a winter storm is imminent or very likely, including any occurrence or combination of heavy snow, wind-driven snow, sleet, and/or freezing rain/drizzle. Winter Storm Warnings are usually issued for up to a 12-hour duration, but can be extended out to 24 hours if the situation warrants.

BLIZZARD WARNING: Issued when winter storms with sustained winds or frequent gusts of 35 miles per hour or greater and considerable falling and/or blowing snow reducing visibility to less than 1/4 mile. These conditions are expected to last at least 3 hours.

5. Some fun terms: (these are actual weather service terms)

BUST: An inaccurate forecast or one that goes sour over time
POWDERHOUNDS: Vernacular for snow enthusiasts. You are a powderhound if you just want snow all the time, and with every storm. You are disappointed if it is not enough to at least cover the grass.
TURKEY TOWER - a narrow, individual cloud tower that develops and falls apart rapidly in a thunderstorm.
GUNGE: Anything in the atmosphere that restricts visibility for storm spotting, such as fog, haze, precipitation (steady rain or drizzle), widespread low clouds

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