Weather Dictionary
Terms
Powder Snow vs. Wet Snow
Powder Snow:
- Occurs mainly in the Rockies
- Snow Cloud Temps are between 3 and 10 degrees
- Low Cloud Temps induce a dendrite snow crystal
- They pack together loosely making air-filled, light snow
- If 25 inches of dry snow fell into a garbage can two feet in diameter, this type of snow would melt into 1.95 gallons of water
Wet Snow:
- Snow Cloud Temps are between 14 and 32 degrees
- Relatively Higher Cloud Temps induce needle and column snow crystals
- They pile up on top of each other and compact close together making heavy snow
- If 25 inches of wet snow fell into a garbage can two feet in diameter, this type of snow would melt into 4.9 gallons of water
The moral of this is to be careful when shoveling wet snow, that's the type that is good for making snowballs and snowmen but also the type for causing heartattacks. Dry snow is good for skiing.
Sleet
Sleet is a frozen raindrop. They fall intially as snowflakes then partially melt through areas of warm air aloft then through shallow areas of cold air near the surface they freeze.
Denver Cyclone
A small cyclonic circulation that develops a short distance north and east of Denver. It occurs when south to southwest winds curl around a low ridge south of the city. Its winds are only about 10 miles per hour, but the rotational motion encourages the growth of thunderstorms in the vicinity.
The Aleutian Low
Storm system originating from the Aleutian Islands between Siberia and Alaska. Cold air from the Arctic and warm air from the Pacific converge and forms this type of low. These vigorous cyclonic disturbances sweep southeast across the Alaskan panhandle into British Columbia and the Pacifc Northwest of the United States. Sometimes they come in a series of 3 or 4 at 24-36 hour intervals.
Thunderstorms
Localized storms that produce lightning and therefore thunder. A typical thunderstorm is only about 15 miles across and lasts only 30 minutes. The National Weather Service estimates that 100,000 thunderstomrs occur in the United States each year. 10,000 of those are classified severe meaning hail of at least 3/4 inch in diameter and/or winds of 58 mph. Here's a rundown of watches. A Severe Thunderstorm Watch can be issued for 6 hours normally meaning that the potential for severe thunderstorms exists in AND close to the watch area. A Severe Thunderstorm Warning means that a severe thunderstorm is occuring or will be occuring and you should take shelter immediately for the possibility for winds greater then 58 mph and/or hail 3/4 inch in diameter. Thunderstorms can occur anywhere in the U.S. with the hotbed of activity being on the Florida Peninsula where windstreams from the Gulf and Atlantic converge. Another hotspot is the Gulf Coast and Southeast, the Great Lakes states and the Rockies. The Northeast U.S. doesn't get many thunderstorms. Thunderstorms mostly occur in the spring and summer but can occur in the winter especially in the Pacific Northwest. And they occur during the afternoon and evening mostly through the heating and instability of the day however squall line thunderstorms can occur any time of the day and night also thunderstorms in the Great Plains often can sustain themselves well after midnight due to evaporative cooling in the upper levels and mixing between moist and dry air. Tommorrow the focus will be lightning.
Lightning
A visible discharge of electricity produced by a thunderstorm. Its a 100-million volt or more electrical spark within a cloud, from one cloud to another, or from a cloud to the ground. It can reach 50,000 degrees, hotter then the surface of the sun! The most dangerous places to be when lightning is occuring is under or near tall trees, in or near water, or in an open field. The safest place to be is inside a building and in a car. Remember, rubber-sole shoes or a car's rubber tires do not provide enough insulation to protect a person from lightning. Its the car's metal body that carrys the lightning to the ground. Tommorrow the focus will be downbursts.
Downbursts
Wind blasting down from a thunderstorm or shower. Speeds faster than 100 mph have been recorded. These are often mistaken for tornados because they can sound the same. Downbursts usually occur when the thunderstorm is in its final stages and is dieing. The storm collapses in on itself and fans out on the surface causing a microbust. Tommorrow the focus will be hail.
Hail
Balls of ice that grow in thunderstorm updrafts. Most hailstones are smaller than a quarter but can be as big as basketballs and fall at speeds around 100 mph. The layers of a hailstone signify its times its been carried up and down in vertical currents within a thunderhead. The most frequent place for hail, or "hail alley" would be in the High Plains east of the Rockies. The most hail prone city in America is Cheyenne, WY recieving an average of 9 to 10 hailstones a year. The Pacific Coast recieves the fewest number of hailstorms and the Arctic and Tropics rarely have hail. Tommorrow the focus will be tornados.
Tornado
Strong, rotating column of air extending from the base of a cloud to the ground. These produce the fastest wind speeds on Earth. The strongest tornados can rip apart well built homes and the weakest tornados can tear mobile homes and cars up. As far as storms go, tornados are small. A mile wide tornado would be large. The late Dr. T. Theodore Fujita created a tornado damage scale. An F0 is winds up to 72 mph causing light damage. An F1 is winds 73-112 mph causing moderate damage. An F2 is winds 113-157 mph causing considerable damage. An F3 is winds from 158-206 mph causing severe damage. An F4 with winds from 207-260 mph causes devestating damage. An F5 is winds above 261 mph causing incredible damage. Tornados can hit anywhere on Earth but are most common in the United States. The most tornado-prone region is between the Rockies and the Appalachians with the southern section more at risk. The most tornado prone city in America is Huntsville, Alabama. And lakes and mountains do not protect tornados from occuring. The prime tornado season for tornados is in February through April in the southeast and into the southern Great Lakes and also in California and Washington. May is prime for the Plains States, southern mid-Atlantic, Washington and Nevada. Florida, much of the west, Quebec, British Columbia have there tornados most often in June. July for Alberta, Manitoba, Ontario and the Northeast. August for the northeast. And even November for the Ark-la-miss. The Storm Prediction Center in Norman, OK issues tornado watches while the local National Weather Service offices issue tornado warnings. A tornado watch is issued when the ingredients are ripe for tornado development during the next 6 hours in AND close to the watch area. A tornado warning is issued when a tornado has been indicated on doppler radar and/or has been sighted. Tommorrow the focus will be floods.
Floods
A rise in water level over land. Rivers are more subject to flooding then lakes or arms of oceans. Intense rainfall can cause flash flooding to creeks and streams. Flash flooding is the number one weather related killer in the United States. People don't understand the magnitude of water. A gallon of water weighs about 8.5 pounds. A large bathtub holds 1 cubic yard of water weighing 0.75 tons, floods are nothing to play with. Floods kill 100 people annually causing a billion dollars damage in the United States. Prepare before floods come. Keep the gas tank full in your car. Store drinking water. Stock up on food that needs little refrigeration. Keep first-aid supplies on hand. Keep a NOAA weather radio, a battery radio or tv, emergency cooking equipment and flashlights in working order. Follow all evacuation orders or recommendations. Here's the straight up on the watchs and warnings. A Flash Flood Watch means that you need to be on the alert for signs of flooding and be ready to evacuate at a moments notice. If you live in a flood prone area in the watch area, you may need to evacuate when the watch is issued. A Flash Flood Warning means you may have only seconds to make it to safety. That flooding is occuring in your area and you need to seek higher ground. Another point. You should NEVER drive through an area of standing water, you do not know how high that water is and you could become stuck and then have your car float away. If your car conks out in flood water get out and wade to safety. Escaping is the only reason to wade in flood water. After a flood: Throw out food that came in contact with the water. Boil drinking water. Pump out wells and have the water tested before drinking it. Don't visit flood areas. Dry and check electrical equipment that's become wet. Use flashlights to examine buildings. Report broken utility lines and downed power lines. Tommorrow the focus will be straight-line winds.
Straight Line-Wind
Movement of air. Often accompaning thunderstorms. Houses susceptible to wind damage have open doors letting air seep through the house. In that situation, roofs can blow off so be sure to seal up the cracks during strong winds. A misconception is that a tornado will completely destroy your home if it goes through it. That's not always the case only in those F3-F5 storms. Large trees shield the house but can also be blown onto the house. Houses in the middle of a group of houses suffer less wind damage but have a better likelyhood of getting windblown pieces of other houses that have been torn apart. Here is a windscale: 40-75 mph winds: few roof shingles blow off, tv antennas bend, lightweight awnings, canopies damaged. 75-100 mph: flying debris breaks windows, large sections of shingles blow off roof corners and eaves, chiminey's collapse. 110-130 mph: Large sections of gable roos may tear away, carport roofs lift off, sections of roofs and porches are damaged. 130-160 mph: entire roofs blow away, roofs stay undamaged only if extraordinary anchorage precautions have been taken. 160-200 mph: two-story houses in near collapse, exterior walls on single-story dwellings collapse with only well-supported interior walls standing. 200 mph: little remains intact, debris scatteres down the path of the tornado. Be familiar with watches and warnings. There are Wind Advisories, High Wind Watches and Warnings all issued and determined by your local National Weather Service office. That concludes the weeks look at severe weather.
Spring Tides
Tides that occur at full and new moons and that generally bring the month's highest high tides and lowest low tides.
Heat Index
The apparent temperature when heat and humidity are factored. A measure of how dangerous conditions will be if you overexert yourself.
Monsoon
Moist air moves in from the Pacific it gets lifted into towering thunderheads in the daytime heating and explodes into thunderstorms that provide flash flooding, vivid lightning and strong winds. Its the main source of rainfall in the Southwest in a year and this pattern can stretch all the way to Idaho, Montana, and Wyoming!
Defending Yourself Against Severe Weather
Signs of a Possible Tornado:
Dark, often greenish sky
Wall cloud hanging from the bottom of a larger cloud
Large Hail
A loud roar, like a freight train
What to Do Before A Storm:
Know the county or parish you live in so you can know where a storm is and where it is in relation to you
Get a weather radio and keep it on alert so when a watch and warning is issued a tone will go off
Follow the weather reports
What to Do When A Storm Threatens:
Stay away from windows
If you're in a car when there is a tornado, don't try to outrun it! Leave your car and find shelter in a ditch or under a bridge
Leave the mobile home and find a safer shelter
Guide To Advisories And WarningsWinter Storm Watch/Warning/Outlook
- A Winter Storm Outlook (issued as a Special Weather Statement) is issued when there is a significant threat of a winter storm
beyond the period for which watches are usually issued. This is usually 48 hours or more in the future.
- A Winter Storm Watch is issued when conditions are favorable for hazardous winter weather conditions to develop, but
occurrence is still uncertain (12-48 hours).
- A Winter Storm Warning is issued when hazardous or significant winter weather is occurring, imminent or has a very high probability of
occurrence. This is normally the 1st forecast period (0-12 hours), but the warning can be extended into the 2nd period (12-24 hours).
Blizzard Warning
- Blizzard Watches are not issued.
- Criteria: sustained wind or frequent gusts to 35 mph or more and considerable falling and/or blowing snow reducing
visibility to less than 1/4 mile.
Winter Weather Advisory
Tornado Warning
- Issued when there is a radar signature indicative of a tornado, or when a
reliable spotter report is received.
Severe Thunderstorm Warning
- Issued when there is a radar signature indicative of a severe
thunderstorm, or when a reliable spotter report is received.
- Criteria: radar signature, or spotter reports, indicate 1) Winds of 58
mph or greater 2) Hail the size of dimes or larger 3) Or any thunderstorm producing
wind or hail damage.
- The National Weather Service does not issue warnings for lightning.
The Fujita Scale for Wind Damage -
F0- Weak - 40-72 mph
F1- Weak - 73-112 mph
F2- Strong - 113-157 mph
F3- Strong - 158-206 mph
F4- Violent - 207-260 mph
F5- Violent - 261-318 mph
Products Not Associated With Precipitation
Frost Warning/Freeze Warning
- A Frost Warning is issued when a frost of sufficient strength to end the
growing season is expected. This product is usually issued within 24 hours of an event.
- A Freeze Warning is issued when temperatures are expected to fall to 32
degrees or below over a large area for a climatologically significant period
of time. This product is usually issued within 24 hours of an event.
Wind Chill Warning/Advisory
- A Wind Chill Warning is issued when wind chill values are forecast to fall
to levels that will cause health concerns. This product is usually issued
within 12 hours of an event.
- A Wind Chill Advisory is issued when wind chill values are forecast to
fall to levels that will be of concern to people but less so than a warning. This product is usually issued within 12 hours of an event.
High Wind Outlook/Watch/Warning
- A High Wind Outlook is issued when there is a significant threat of
non-thunderstorm high winds beyond the period for which watches are usually issued. This is usually 48
hours or more in the future.
- A High Wind Watch is issued when non-thunderstorm high winds are expected to develop during
the 2nd-4th periods of the forecast (12-48 hours). An outlook is issued when high winds
are expected beyond the 3rd period.
- A High Wind Warning is issued when non-thunderstorm high winds are expected during the 1st,
and occasionally the 2nd period of the forecast. (0-24 hours)
- Criteria: Sustained non-thunderstorm winds of at least 40 mph for 1 hour or more, or
frequent gusts to 58 mph or greater. An Inland High Wind Watch for Hurricane Force
Winds can also be issued, but only when sustained winds of 74 mph or greater
are expected.
Wind Advisory
- Issued during the 1st period, but can occasionally be extended into the
second period (0-24 hours).
- Criteria: Sustained wind of 31-39 mph for at least 1 hour, or frequent gusts
46-57 mph.
Wind Advisory For Area Lakes
- Issued when conditions are expected to be hazardous for boaters on area
lakes. Can be thought of as an inland version of a Small Craft Advisory. This
product is only issued for the 1st period of the forecast (0-12 hours).
- Criteria: Sustained wind of 25 mph, or frequent gusts to 40-45 mph.
Excessive Heat Outlook/Watch/Warning
- An Excessive Heat Outlook is issued from the 3rd period of the forecast
outward (36 hours or more). An Excessive Heat Warning is issued mainly for the 2nd period of the
forecast (12-24 hours). An Excessive Heat Warning is issued only for the 1st
period of the forecast (0-12 hours).
Heat Advisory
- A Heat Advisory is issued only for the 1st period (0-12 hours).
Dense Fog Advisory
- Usually issued during the 1st period (0-12 hours).
- Criteria: fog reducing visibility to 1/4 mile or less over a large area.
Hydrometeorological Products
Flood Potential Outlook
- Issued beyond the period when a Flash Flood or Flood Watch is issued. This
is usually the 3rd period or beyond 36 hours or more).
Flash Flood/Flood Watch
- Issued when hydrometeorological conditions that could result in flooding
are expected in the 1st or 2nd period of the forecast (12-24 hours).
Flash Flood Warning
- A Flash Flood Warning is issued for flooding which occurs within 6 hours
of the onset of precipitation.
- Criteria: Issued when rapid increases in water levels along creeks,
streams, and non main-stem rivers is imminent that create a serious threat
to life and property. This is an event that is more than just standing water
on roads, water is usually quite deep or is moving rapidly. A flash flood
typically describes an event in which flood waters rise to dangerous levels
in a short period of time. Flash flood warnings are also issued for dam
failures.
Flood Warning.
- A Flood Warning is issued for flooding which occurs beyond 6 hours of the
onset of precipitation.
- Criteria: Flooding does not occur rapidly and lasts longer than 6 hours,
though it is still not associated with main-stem rivers. Because this type
of flooding can be widespread and flood waters do not subside quickly, it
can be considered a possible threat to life and/or property.
Precipitation Types
Lake Effect Snow
Noreasters
Alberta Clipper
MRS Weather All-Time Hurricane List
Hurricane Andrew 1992
Hurricane Betsy 1965
Hurricane Camille 1969
Hurricane David 1979
Hurricane Eloise 1975
Hurricane Floyd 1999
Hurricane Gilbert 1988
Hurricane Hugo 1989
Hurricane Inez 1966
Hurricane Joan 1988
Hurricane Kate 1985
Hurricane Lenny 1999
Hurricane Mitch 1998
Hurricane Opal 1995
Understanding Tropical Storm/Hurricane Probabilities
The National Hurricane Center puts out these storm probabilities to try to give emergency managers advance warning on where a storm could realitistically make landfall. Probabilities are defined on where the storm can make landfall within 65 miles of the cities on the probability chart. They are issued 4 times a day or at three hour intervals depending on the need for them to be issued. Here's how to use the numbers. First, if you live between two of the cities listed, you can estimate your probability by averaging the two numbers. Or, compare your probability with those locations around you and if you have the highest value, the chances of you getting the landfall are greatest. Finally, you should be sensitive to the fact that numbers change with every advisory as the margin of error gets less. When a storm is 36 to 72 hours, the probabilties are low. Its not till 36 hours when they become more reasonable. The maximum probability that a storm will directly hit you in 72 hours is only 10 percent. At 48 hours, the max is between 13-18 percent. At 36 hours, the max is 20-25 percent. At 24 hours, the max is 35-45 percent. In less then 24 hours, they can increase to 60-70 percent and ultimately 100 percent. So keep that in mind just in case you live in a low lying area prone to tidal flooding, you may want to leave early if those probabilities are high especially if you have a lot of traffic around. Also, don't confuse these numbers for precipation or intensity. This is strictly the chance that a hurricane or tropical storm will directly hit your area.
Hurricane Terms
Eye: Center of a tropical cyclone where winds are nearly calm. It's often a clear area in the middle of a storm but sometimes is covered up by clouds over the storm's top. In the strongest of hurricanes, storms may have two eyes!
Eye Wall: Wall of clouds, usually extending 40,000 feet or higher, around a hurricane's eye, which contains the storm's strongest winds.
Hurricane: Tropical cyclone with top sustained winds of 74 mph or greater.
Hurricane Warning: Issued when hurricane conditions are possible within 24 hours or are imminent.
Hurricane Watch: Issued when hurricane conditions are possible in the next 24-36 hours.
National Hurricane Center: Based in Miami, this center has the responsibility for forecasting tropical storms and hurricanes in the Atlantic Ocean, Caribbean Sea, Gulf of Mexico and the Pacific Ocean east of the International Date Line.
Spiral Band: Line of thunderstorms with wind gusts that spiral into the eye wall. Tornados can be spawned from this but they are generally weak and mostly found in the northeastern quadrant of the storm due to the vertical motion. Its the leading edge of the strongest part of the storm.
Storm Surge: The rise in sea level caused by a storm. A combination of low atmospheric pressure in the storm's eye and by water pushed by wind. The height depends on the strength of the storm and the nature of the ocea floor offshore. When the surge pushes into bays or rivers, it can pile up water higher then on open beaches. Storm surge is the biggest killer in hurricanes bigger then the wind!
Tropical Depression: Tropical cyclone with maximum sustained winds of less then 39 mph.
Tropical Storm: Tropical cyclone with 39 to 74 mph winds.
Tropical Storm Warning: Tropical Storm or gale force conditions are possible in the next 24 hours or are imminent.
Tropical Storm Watch: Tropical Storm or gale force conditions are possible within 24-36 hours.
Typhoon: Hurricane in the North Pacific west of the International Date Line.
Saffir-Simpson Scale
| Catagory Damage | Barometric Pressure | Winds | Storm Surge |
| 1 Minimal | +28.94 in | 74-95 mph | 4-5 feet |
| 2 Moderate | 28.50-28.91 in | 96-110 mph | 6-8 feet |
| 3 Extensive | 27.91-28.47 in | 111-130 mph | 9-12 feet |
| 4 Extreme | 27.17-27.88 in | 131-155 mph | 13-18 feet |
| 5 Catastrophic | -27.17 in | 155 mph+ | 18 + feet |
Hurricane Self-Defense
Before Moving to the Coast:
Learn about the area's hurricane and winter storm history
Don't believe stories about offshore reefs or protective weather patterns
Find out whether the area has building codes that are enforced
Make sure your home meets building codes
Before Hurricane Season
Put an up to date list of belongings in a safe-deposit box or another place away from hurricane danger
Copy important records and make plenty of copies and put them in a "grab and run" bag so you won't be searching for them when evacuating is necessary
Cut dead trees or branches that can be blown into houses in high winds
Arrange a method of quickly covering windows and glass doors
Have that plywood pre-cut so it can be needed long before a storm threatens
Stock up on emergency supplies of drinking water, food not needing refrigeration, cooking, medications, flashlights, batteries, a weather radio and a battery powered radio or TV
If the area is prone to evacuations, learn the best route and decide where you will go
When A Hurricane Watch is issued
Check to make sure emergency supplies are on hand
Bring in lawn furniture, trash cnas and other objects that will become guided missiles in high winds
If you live on a barrier island or place with limited evacuation routes, avoid the rush. Leave before the official order is given
Keep up with news of the storm and be ready to evacuate with little notice
Make sure all materials and tools needed to cover windows and glass doors are handy. If evacuation seems likely, cover windows and doors
When A Hurricane Warning Is Issued
If evacuation is ordered or suggested, finish boarding up doors and windows and other protective measures and turn off the power
Make sure emergency supplies and irreplacable photos and documents are in the car before leaving
If evacuation isn't required, finish covering windows and glass doors and make sure objects that could be blown around are inside
When the wind begins blowing stay away from windows even those that are covered. If wind is damaging the house, retreat into a small interior room such as a bathroom or closet
Leave doors closed and seal yourself from the outside because wind can get into the house and take the roof off
During the eye of the storm, do not venture out. It may be clear but the second part of the storm can approach rapidly and you may be stuck in that eyewall
MRS Weather
