Archive for January, 2010

Storm Staying South, But Snow Spreads North
January 30, 2010


The frustrations of forecasting winter weather in Maryland really apparent today!  The low pressure system responsible for today’s snow is staying pretty close to the forecast track; moving through Georgia toward the South Carolina coast and expected to be just off Hatteras by 7pm.  But the significant snow has spread farther north than expected.  The 3-6″ snow band that was forecast to stay well south of Baltimore is now developing north of the city, even though the storm is well south!  I don’t have the answer as to why the shift, but I suspect the  very cold air in place (temperatures in the teens), is producing huge water to snow ratios, turning just a little bit of moisture into much heavier snow than would normally occur.  With just .25″ of liquid precipitation we could see 5-6″ of snow today…a 20:1 ratio compared to a more typical ratio of 10:1 or so.  At any rate, we should see the snow tapering off in the early evening, but not before leaving 3-6″ of snow north of Baltimore and 6″+ from the city south,  with a foot of snow possible in parts of southern Maryland.  That is, unless the storm throws us another curve…stay tuned!

Tom Tasselmyer

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Big Snowstorm To Slip Just South Of Maryland
January 30, 2010

Richmond, VA may see their biggest snowfall in nearly 27 years if the storm in Louisiana Friday evening stays on its forecast track.  The last time Richmond had a 12″ snowfall was back in February of 1983, but the elements are in place for that part of Virginia to pick up 12-15″ of snow on Saturday.  The low pressure system is forecast to move from Louisiana east through Mississippi and Alabama tonight before drifting just a bit to the north and sliding through Georgia and South Carolina Saturday, emerging off the coast, south of Hatteras Saturday evening.  The slight drift to the north means parts of far southern Maryland could see 3-6″ of snow and 1-3″ of snow is possible from Baltimore south to southern Anne Arundel Co. and for much of Queen Anne’s, Caroline, and Talbot Counties on the eastern shore.  North of Baltimore it still looks like little or no accumulation unless, of course, the drift to the north we’ve seen today becomes more significant overnight.  As always…stay tuned!

Tom Tasselmyer

Heavy Snow Aimed At The Old Dominion
January 28, 2010

Computer models tracking the weekend snowstorm continue to indicate the heaviest snow will fall across central and southern Virginia.  The area from Richmond, VA to Raleigh, NC appears to be in the snowfall jackpot zone right now.  6-12″ of snow are likely Saturday in this area with some spots likely to go over one foot.  However, since we are still more than 24 hours from the heart of the event, a change in the track of the storm is still possible…so as always, stay tuned!

Tom Tasselmyer

Storm Likely To Stay South
January 28, 2010

The Wednesday evening run of the numerical weather prediction models has begun to show a consensus on the track of Saturday’s snowstorm.  Most models are now indicating strong, arctic high pressure building in from the midwest will suppress the storm farther south.  As a result, the low pressure system will track from southern Arizona to south central Texas to southern Georgia and then, instead of moving up the coast toward Maryland, will track off the coast and out to sea.  If this track holds, the heavviest band of snow will stay well south of Baltimore, falling on central and southern Virginia.   If the high is weaker, the track could shift back to the north, but as of 11:00pm Wednesday evening it looks like a near miss for the Baltimore area.  Stay tuned!

Tom Tasselmyer

More Weekend Storm Information
January 27, 2010

It is still early in the forecast cycle for the anticipated weekend storm. I thought I’d add some information to Tom’s previous entry.

The Wednesday afternoon satellite image shows a band of clouds from the Great Lakes to Texas. This is evidence of the developing cold front that will drop temperatures by the end of the week. Clouds streaming up across Mexico into the Rocky Mountains are signs of the developing storm for the weekend. The actual low pressure center is just off the Baja California coast. The forecast map below show expected features for this evening.

The forecast track for this storm is still not nailed down but forecasters have projected a probable precipitation path into the weekend.

This forecast has the bulk of the moisture with the storm passing south of Maryland.

The map above shows the forecast storm position Saturday morning. At this early stage this could change but it is the current thought on the situation. The Mid Atlantic region is north of the storm and this is all cold air.

One computer forecast model shows the situation for Saturday afternoon. The north edge of the precipitation just reaches into southern Maryland. The 0 and -10 degree isotherms several thousand feet up in the atmosphere are south of Baltimore so, roughly speaking, the area between northern North Carolina and Baltimore would receive snow from this storm, with the bulk of it to the south of the metro area.

None of this is cast in stone so stay tuned for updates. Tom’s map in the previous entry points out the variables in the forecast.

John Collins

Weekend Storm?
January 26, 2010

As cold air returns to the mid Atlantic region over the next few days, the steady supply of Pacific storms tracking across the nation will bring a threat of winter weather instead of just wet weather.  Whether or not the storm approaching the area from the south this weekend brings a significant snowfall will depend on a subtle difference in possible storm tracks.  It appears likely that the low pressure system will make it to northern Louisiana by Friday morning, but where the low moves from there is the key to our weather on Saturday.  Computer models are beginning to favor a more southerly storm track, which would produce a near miss for the Baltimore area, keeping the heaviest snow in central and southern Virginia.  However, if the storm veers a bit to the north, the heavier snow would shift into Maryland.  The strength and position of the jet stream to the north will play a major role in determining which track the low takes.  A strong northern jet stream would likely suppress the storm farther south.  A slightly weaker northern jet would allowthe storm to move our way.  Stay tuned.

Tom Tasselmyer

Rain Maker Moving Out…More Storms On The Way
January 25, 2010

The Sunday-Monday rainmaker is pulling out of the area.

The surface map above shows the situation late Monday morning with the first of two cold fronts on the western shore of the Bay and a secondary front in the mountains. Note the strong winds on some of the individual station reports.

The early afternoon satellite image shows the drier air moving in behind the front which by this time has moved to the Atlantic coast.

While the rain was significant it does not appear that any rainfall records have been broken. At BWI-Marshall Airport, the record rainfall on Sunday is 1.74 inch and the record for Monday is 1.83 inch. The preliminary rain total for this storm is .88″ over the two days.

The Doppler Radar Rainfall Estimate from the Dover AFB radar shows that 1 to 1.5 inches of rain (green shades) is estimated to have fallen over the northern DELMARVA Peninsula and parts of Harford, Baltimore and Carroll counties north of Baltimore City. The dark blue shading indicates rain total estimates in the .5 inch to 1 inch range. These estimates pretty well line up with preliminary rain gauge reports.

A strong, southerly flow of air ran ahead of the initial cold front. Gusty winds and unseasonably mild temperatures were widespread. The preliminary Monday high temperature report from BWI-Marshall is 66 degrees. A high of 59 degrees was recorded at the Science Center at the Inner Harbor. The average high temperature this time of year is 41 degrees.

As mentioned in previous entries, the weather pattern is quite active.

The Monday morning western satellite image shows that several storms are lined up in the north Pacific, taking aim on the U.S. West Coast. The forecast maps below show a cold front approaching the Mid Atlantic region on Thursday and a stronger system along the East Coast south of Maryland on Saturday.

The Thursday cold front is expected to be followed by an invasion of cold air, setting the stage for a possibility of snow in the Mid Atlantic region late Friday or Saturday. As with all longer range forecasts, a lot of fine tuning will be necessary and an accurate forecast regarding location, timing, extent and type of precipitation is a few days away.

John Collins

Coastal Storm Cranking Farther East
January 22, 2010

Satellite images this  morning show the low pressure system that developed over the coastal Carolinas Thursday night is really getting wound up tight, but this time it is too far east to play a major role in Baltimore’s weather.  At 7:00am Friday the central pressure was estimated to be around 994 millibars, which is about what the computer models were forecasting yesterday.  The models this morning are forecasting more intensification, down to under 984 millibars by 7:00pm, as the system pulls farther away from the coast.  Meanwhile, the next Pacific storm is headed this way and should emerge east of the Rockies Saturday morning.  This one is forecast to track toward the Great Lakes, drawing warmer air and possibly some heavy rain into the eastern U.S. Sunday and early Monday.  However, winter weather lovers should not despair…a cold front is expected to push off the eastern seaboard Monday afternoon opening the door to much colder air next week.  Perhaps the foundation for a snowy start to February?  Stay tuned.

Tom Tasselmyer

A Look Ahead … And … A Quick Look Back
January 21, 2010

The weather pattern has become quite active with Pacific storms hitting the west coast and moving straight across the country.

The late morning full disk satellite image shows three major storms over the northern half of the western hemisphere. One over the north Atlantic, one moving in to the Atlantic seaboard and a third coming onshore along the Pacific coast. These storms are essentially moving from west to east. This pattern tends to moderate temperatures to a degree but allows for stormier conditions.

The Friday morning forecast map above shows the east coast storm beginning to move off the Mid Atlantic and southeast coast with its’ expected mix of rain, sleet and snow. The Pacific coast storm has progressed into the Rocky Mountains.

The pattern will drag the storm out west across the Great Lakes and Mid Atlantic region late in the weekend or early next week. With extremely cold air bottled up in Canada the storm should be mostly rain for our region.

This situation speaks to Tom’s previous entry. You should check it out. It addresses the swings in patterns that have affected the region so far this winter.

The early portion of this winter season was a cold one with the jet stream diving deep into the southeast U.S. One storm moving around the periphery of this cold air mass gave the area the big December snow.

The path of the jet stream this time of year is more or less controlled by the “Arctic Oscillation”. It is a pressure field that sits over the Arctic region like a rock in a stream with the large scale northern Hemisphere air flow moving around. Its’ position shifts around during the winter months and that shift is responsible for things like cold waves and “January Thaws”. This year’s early winter was dominated by a “negative phase” of the oscillation and the polar jet stream dove deep into the United States, as Tom’s graphic in the previous entry demonstrates. The U.S. wasn’t the only region of the world affected by this “negative phase.

The analysis above shows that Northern Europe, Russia and Northern Asia were also hit by unusually cold air. News at the time was full of stories of extreme cold and snow in these regions. In contrast, the U.S. West Coast, N.E. Canada and Greenland, North Africa and Central Asia were warmer than usual.

At the moment the “Arctic Oscillation” is in a more positive phase and weather over the eastern U.S. is not quite as extreme as far as cold temperatures are concerned. The “Oscillation” will likely swing more negative over the next few weeks and another shot of very cold air is possible.

This year the shorter term oscillations of pressure patterns over the Arctic regions are laid over the longer term El Nino pattern over the equatorial region of the Pacific. This pattern tends to generate an active storm pattern across the southern U.S. The combination of the two patterns this year, on average, has made this winter season in the Mid Atlantic region much more active, and so far, colder.

John Collins

What a difference a month makes!
January 19, 2010

One month ago the 7th biggest snowstorm on record in Baltimore was blanketing the area with over 20″ of snow (21.1″ officially at BWI-Marshall).  Today, afternoon temperatures were flirting with 60°.  What a difference a month makes!  So, what changed over the last 30 days?  Simply put, the jet stream has flattened.  Typical of an El Nino pattern, the southern branch of the jet stream has flooded the continental U.S. with mild Pacific air.  Embedded in the southern branch of the jet a series of wet storms has been tracking from southern California to Texas to the gulf coast.  For snow lovers there is always the chance of a brief shot of cold air coinciding with one of the wet storms chugging east in the southern branch of the jet, but the key word is “patience”.  Long range forecasts from the global models indicate the pattern “may” change back to a colder set up around the beginning of February.  Stay tuned.

Tom Tasselmyer