Definitions of Weather Watch, Warnings and Advisories

Blizzard Warning

A Blizzard Warning means that the following conditions are occurring or expected within the next 12 to 18 hours.
1) Snow and/or blowing snow reducing visibility to 1/4 mile or less for 3 hours or longer

AND
2)  Sustained winds of 35 mph or greater or frequent gusts to 35 mph or greater.
There is no temperature requirement that must be met to achieve blizzard conditions.
 

Freeze Warning

A freeze warning is issued when significant, widespread freezing temperatures are expected.

A freeze warning is issued in the autumn until October 15th west of a line from 
Frederick, Maryland to Charlottesville, Virginia.  It is issued in the autumn until 
November 1st east of that line.

A freeze warning is issued in the spring when it is late enough to cause damage to 
new plants and crops.

Frost Warning

A frost warning is issued when significant, widespread frost is expected.

A frost warning is issued in the autumn until October 15th west of a line from 
Frederick, Maryland to Charlottesville, Virginia.  It is issued in the autumn 
until November 1st east of that line.

A frost warning is issued in the spring when it is late enough to cause damage
 to new plants and crops.

Wind Chill Advisory

A wind chill advisory is issued when wind chills of -15F to -29F are expected.

Wind Chill Warning

A wind chill warning is issued when wind chills of -30F or lower are expected.

Winter Storm Watch

A winter storm watch is issued when there is the potential for significant and hazardous  winter weather within 48 hours.  It does not mean that significant and hazardous winter weather will occur...it only means it is possible.

Significant and hazardous winter weather is defined as:
1)  Over 4 inches of snow (and/or sleet).
OR
2)  Glaze accumulation (freezing rain) of 1/4 inch or more
OR
3)  Enough ice accumulation to cause damage to trees or powerlines.
OR
4)  a life threatening or damaging combination of snow and/or ice accumulation with wind.

Winter Storm Warning

A winter storm warning is issued when significant and hazardous winter weather is occurring or imminent.

Significant and hazardous winter weather is defined as:
1)  Over 4 inches of snow (and/or sleet).
OR
2)  Glaze accumulation (freezing rain) of 1/4 inch or more
OR
3)  Enough ice accumulation to cause damage to trees or powerlines.
OR
4)  a life threatening or damaging combination of snow and/or ice 
accumulation with wind.

Winter Weather Advisory

A winter weather advisory is issued when snow, sleet, freezing rain, or
 a combination of precipitation types is expected to cause a significant 
inconvenience but not serious enough to warrant a warning.

A winter weather advisory is issued when one or more of the following
 conditions are met:
 
1)  2 inches of snow east of a line from Frederick Maryland to 
Charlottesville Virginia
     3 inches of snow west of a line from Frederick Maryland to 
Charlottesville Virginia.
OR
2)  For freezing rain or freezing drizzle, you issue the advisory when
 any accumulation of glaze is expected on streets or sidewalks.

Dense Fog Advisory

A dense fog advisory is issued when widespread fog is expected to reduce 
visibilities to 1/4 mile or less over a large area for an extended period of
 time (more than 3 hours).

High Wind Watch

A high wind watch is issued when the following conditions are possible:

1) sustained winds of 40 mph or higher for one hour or more

OR

2) wind gusts of 58 mph or higher for one hour or more.
 

High Wind Warning

A high wind warning is issued when the following conditions are occuring or imminent:

1) sustained winds of 40 mph or higher for one hour or more

OR

2) wind gusts of 58 mph or higher for one hour or more.

Hurricane Watch

A hurricane watch is issued when the following conditions are possible.

1) storm surge, coastal flooding, river flooding

AND

2) winds of 74 mph or higher

The watch is issued for areas that border the Potomac River from Washington
 DC southward and for areas that border the Chesapeake Bay.

The watch does not mean that hurricane conditions will occur.  It only means 
that these conditions are possible.

Hurricane Warning

A hurricane warning is issued when the following conditions are occurring
 or imminent.

1) storm surge, coastal flooding, river flooding

AND

2) winds of 74 mph or higher

The warning is issued for areas that border the Potomac River from 
Washington DC southward and for areas that border the Chesapeake Bay.
 
 

Inland Hurricane Wind Watch

This watch is issued when there is a possibility that hurricane force winds 
(74 mph or higher) will move inland from the coast beyond areas that are
 covered by the hurricane watch.

The watch does not mean that the hurricane force winds will occur.  It just 
means that they are possible.
 

Inland Hurricane Wind Warning

This watch is issued when hurricane force winds (74 mph or higher) are expected
 to  move inland from the coast beyond areas that are covered by the hurricane warning.
 

Coastal Flood Watch

A coastal flood watch is issued when moderate to severe coastal flooding is possible.

Coastal flood watches are issued for areas along the Tidal Potomac River and
 areas along the Chesapeake Bay.

Coastal Flood Warning

A coastal flood warning is issued when moderate to severe coastal flooding is 
occurring or imminent.

Coastal flood warnings are issued for areas along the Tidal Potomac River
  and areas along the Chesapeake Bay.

The excessive heat warning shall be issued when the following conditions are expected to:

a minimum heat index of 80°F or higher at night.

A maximum heat index of 115F or higher regardless of duration or nighttime minimum.

A flash flood warning is issued when flash flooding is imminent or occurring.

Flash Flood Watch

A flash flood watch is issued when conditions are favorable for flash flooding.  It does not mean that flash flooding will occur, but it is possible.

Flood Warning

A flood warning is issued when flooding is imminent or occurring.

Flood Watch

A flood watch is issued when conditions are favorable for flooding.  It does not mean flooding will occur, but it is possible.
 

Gale Warning

Gale warnings are issued for the Tidal Potomac River and the Chesapeake Bay when one or both of the following conditions is expected within 24 hours:

1)  sustained winds of 35 knots to 49 knots

OR

2) frequent gusts between 35 knots and 49 knots.
 

River Flood Warning

A river flood warning is issued when river flooding is occurring or imminent at one 
or more forecast points along a river.

River Flood Watch

A river flood watch is issued when river flooding is possible at one or more 
forecast points along a river.
 

Small Craft Advisory

Small craft advisories are issued for the Tidal Potomac River and the Chesapeake 
Bay when one or both of the following conditions is expected within 12 hours:

1)  sustained winds of 20 knots to 34 knots

OR

2) waves of 4 feet or higher

Storm Warning (Over Water Winds)

Storm warnings are issued for the Tidal Potomac River and the Chesapeake Bay 
when one or both of the following conditions is expected within 24 hours:

1)  sustained winds of 50 knots or higher

OR

2) frequent gusts of 50 knots or higher
 

Severe Thunderstorm Warning

A severe thunderstorm warning is issued when severe thunderstorms are occurring 
or imminent in the warning area.

Severe thunderstorms are defined as follows:

Winds of 58 mph or higher

AND/OR

Hail 3/4 of an inch in diameter or larger.

Severe Thunderstorm Watch

A severe thunderstorm watch is issued when severe thunderstorms are possible in 
and near the watch area.  It does not mean that they will occur. It only means they 
are possible.

Severe thunderstorms are defined as follows:

Winds of 58 mph or higher

AND/OR

Hail 3/4 of an inch in diameter or larger.

Tornado Warning

A tornado warning is issued when a tornado is imminent.  When a tornado warning
 is issued, seek safe shelter immediately.

Tornado Watch

A tornado watch is issued when severe thunderstorms and tornadoes are possible in 
and near the watch area.  It does not mean that they will occur. It only means they 
are possible.

Severe thunderstorms are defined as follows:

Winds of 58 mph or higher

AND/OR

Hail 3/4 of an inch in diameter or larger.

Tropical Storm Warning

A tropical storm watch is issued when the following conditions are imminent or occurring:

1) storm surge, coastal flooding, river flooding

AND

2) winds of 39 mph to 73 mph

The warning is issued for areas that border the Potomac River from Washington DC 
southward and for areas that border the Chesapeake Bay.
 

Tropical Storm Watch

A tropical storm watch is issued when the following conditions are possible.

1) storm surge, coastal flooding, river flooding

AND

2) winds of 39 mph to 73 mph

The watch is issued for areas that border the Potomac River from Washington DC
 southward and for areas that border the Chesapeake Bay.

The watch does not mean that tropical storm conditions will occur.  It only means
 that these conditions are possible.

Wind Advisory

A wind advisory is issued when the following conditions are expected for 3 hours or longer.

1) sustained winds of 31 to 39 mph

AND/OR

2) wind gusts of 46 to 57 mph.