Archive for November, 2012

Sleet or Hail or Something Else? How to Tell the Difference
November 13, 2012

A lot of you may have been surprised today when you noticed the rain start to bounce. Turns out, some of that rain was actually ice called sleet that some people confused as hail.  What’s the difference?

Sleet: ice pellets made from frozen raindrops or melting snowflakes

Sleet forms as precipitation travels through a wide range of temperatures on its way to the ground, including layers of air that are both above and below freezing.  Sometimes snow falls into warmer temperatures on its way to the ground and partially melts. Other times, raindrops enter colder temperatures near the earth’s surface and partially freeze before reaching the ground. If enough sleet pellets fall and accumulate, it can resemble snow on the ground.

Hail: round stones of ice that tend to be clear, and usually form in thunderstorms. can happen anytime of year, even in warm weather

Within a thunderstorm, there are areas of rising and sinking air. This causes rain droplets to rise into colder temperatures.  If it’s cold enough, the rain will freeze into a hailstone and begin to fall back down.  If the thunderstorm is strong enough, it’ll lift the hailstone up main times before it can fall to the ground.  Each time the hail stone rises, it collects more layers of ice. Therefore, very large hail stones come from very strong thunderstorms.  Larger hail stones are obviously more dangerous and more damaging.

Freezing Rain: very cold rain that freezes instantly when it hits the ground, forming a thin layer of ice

It happens when rain travels through a very shallow layer of cold temperatures near the earth’s surface. There’s not enough time for it to freeze into sleet, but it does cool down enough to freeze instantly when it hits the ground, forming a thin layer of ice on anything it touches, like roads, sidewalks, tree branches and power lines. Not only does it make for slippery travels, if enough ice collects, it can even be heavy enough to break tree branches and bring down power lines.

Snow: crystals of ice that come in many shapes and sizes

Light, fluffy snow usually happens in very cold temperatures, and can pile up very quickly, making it hard to travel.  Heavy, wet snow forms in slightly warmer temperatures, and can cause problems when it weighs down on trees branches, power lines and roofs. Snow causes the roads to become very slippery, especially as it begins to melt and then refreezes into ice.

Graupel: white snow pellets that resemble soft, milky colored hail stones

Graupel is the least familiar of all types of frozen precipitation, it is often confused with hail because it looks very similar, except that it forms through a completely different process.  It usually forms when water droplets run into snow and instantly freeze. The easiest way to tell it apart from hail is that it is usually opaque and milky looking, while hail tends to be more clear.  Hail tends to be associated with warm weather thunderstorms, while graupel is more common in cold weather, when no thunderstorms are present.

Today’s icy mix was indeed sleet, as seen by this diagram:

Image

This is called a Skew-T diagram, and it contains weather data from a balloon launched in Sterling, Virginia this morning. The balloon carries an instrument called a radiosonde that measures temperature, moisture and winds at different levels in the atmosphere.

While temperatures were above freezing at the surface at the time of the sleet, in the low 40s to be exact, the temperature graph shows that there was a shallow layer of subfreezing temperatures just above the surface. That layer was cold enough for some rain to freeze into sleet.

– Meteorologist Ava Marie, 11/13/2012 1 pm

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Glancing Blow Wednesday
November 7, 2012

The nor’easter is out there but just far enough away so as to not be a major headache for the Chesapeake Bay region.

A beautiful satellite image of today’s storm, about 150 miles off the south New Jersey coast at midday.

The surface analysis above corresponds to the satellite image. The central pressure of the storm is approximately 29.53 inches. Winds close to the coast are gusting in the 20-30 mph range. Off shore steady winds are blowing in excess of 35 mph.

As was the thought yesterday, areas west of the Bay will be on the outer fringes of the precipitation shield with generally less than a half inch of rain likely. Some snow could develop although accumulation will likely be limited. Paved surfaces are still quite warm and melting on contact would prevail in most if not all areas around Baltimore. The northeast corner of Maryland and the DELMARVA Peninsula will be the most likely areas to experience any accumulation. The computer forecast below is an indication of the snow potential from this storm.

This storm will be a more significant problem for for New Jersey, New York and southern New England because of ongoing recovery efforts related to hurricane Sandy.

The upshot for the Chesapeake Bay region? We have been fortunate to be on the less severe side of two, close spaced, major storms. Can that kind of luck hold up into and through the winter?

Check out the latest forecast at www.wbaltv.com/weather

John Collins

New Storm Targets The Coast
November 6, 2012

A beautiful sunrise Tuesday morning but a little chilly with temperatures dipping into the 20s.

A new storm is developing and all signs point to a nor’easter.

Source: NOAA/NASA

The clouds associated with the developing storm are just south of the Chesapeake Bay region.

Surface Weather 5:21am, Nov 6, 2012

Source: NOAA

Low pressure will shift eastward to the southeast Atlantic coast, turn to the north and strengthen. Computer models seem to be settling on a solution that would position the storm center just far enough off the Mid Atlantic coast Late Wednesday that the Baltimore/Washington metro areas would be on the western edge of the precipitation shield. If this proves to be correct, the heaviest rains of up to one inch are most likely east of the Bay.

Cold air on the west side of the storm is likely to allow for a chance of light snow or snow mixing with rain as the storm pulls northeast of the area late Wednesday.

The RPM/WRF model from WSI is on the aggressive side of the equation as far a snow is concerned. Keep in mind that the forecast map above represents one hour of accumulation and positions the storm center relatively close to the coast. Other models track the storm a bit farther east with a slightly reduced precipitation impact.

The storm’s wind field will likely bring blustery conditions to the area. The DELMARVA Peninsula should experience the strongest winds, especially along the coast.

Temperatures since Sandy moved through the area have been colder than the seasonal average. There is light at the end of the tunnel in that regard. By the weekend, temperatures should moderate to above average with dry conditions.

Check out the forecast any time at www.wbaltv.com/weather

John Collins