Archive for January, 2009

Nor’easter Ensemble Update
January 31, 2009

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The morning run of the computer models trying to get a handle on next week’s possible nor’easter continue to show a near miss for the Baltimore area.  As we posted last night, the ensembles are the mean result of a given model processed many times with slight variations introduced for each run.  These ensemble means are sometimes the best way to forecast a storm that can’t seem to settle on a single track.  Posted here, from top to bottom, are the ensemble means for 7am Tuesday from the Canadian, European and U.S. GFS (Global Forecast System) models.  Each model is still showing the center of the low well off the coast, in a position that would keep the signifcant snow east of Baltimore.  Still three days away, however…stay tuned.

Tom Tasselmyer

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Next Week’s Nor’easter
January 31, 2009

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For the past few days computer models have been advertising a strong low pressure storm system moving up the eastern seaboard from Monday evening through Tuesday of next week.  These models have been producing a storm track that ranges from well inland to well off the coast and each scenario presents widely different weather possibilities for Maryland.  A track over land would bring mainly rain to the Baltimore area, while a track up the coast could produce heavy snow.  A track farther east, well off the coast, would only glance the Baltimore area with light precipitation, or possibly miss us all together.  A useful tool in cases like these is the “ensemble” model output.  The ensemble is the mean result of the same model run with many variations.  By watching the ensemble mean you can get a good feel for the most likely storm track.  I’ve posted the ensemble means for 7am Tuesday, Feb. 3rd for three reliable medium range forecast models.  From top to bottom the posted images are from the Canadian model, the European model and the U.S. GFS (Global Forecast System) model.  Each models ensemble mean position for 7am Tuesday is off the coast with the Canadian and European a little too far east for major snows here.  The GFS position is tucked in closer to the coast, but not too far inland to dislodge the cold air.  If this run of the GFS ensemble mean verifies, significant winter weather would develop here Monday night and continue into Tuesday.  If the European or Canadian models verify we would probably see some light snow but no real problems.  Whatever is coming this way is still 3-4 days away as I write this, so there is plenty of time for big changes to this potential storm.  Check back here for more updates as the storm approaches.

Tom Tasselmyer

What’s Next?
January 30, 2009

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Fairly quiet weather going into the weekend. A cold front will be moving across the region on Friday but moisture is limited and if anything develops it should only be in the form of flurries, sprinkles or snow showers. In the satellite picture above, a lot of what looks like clouds west of Maryland is actually snow on the ground, deposited by the last storm.

The weekend looks uneventful.

By Monday a storm system is expected to move up the eastern seaboard.

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The forecast map above shows the storms possible location on Tuesday morning. Various computer models have this storm moving faster or slower and tracking farther east or farther west so it is too early to tell exactly how it will behave. Yesterday’s forecast map had this storm stronger and farther north and west for the same time period. An easterly track would favor snow and a westerly track would favor a wintry mix. It is something to watch for but it is way too early to have anxiety attacks over it.

Enjoy the weekend!

John Collins

Thursday View
January 29, 2009

The image below is a NASA satellite image of the region at 11:45am Wednesday.

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The image clearly shows snow on the ground deposited by the most recent storm. Western Pennsylvania and West Virginia are covered by a cloud deck but the gray/white shaded areas of the rest of Pennsylvania and the northern half of Maryland show the distribution of snow.

John Collins

Almost Over
January 28, 2009

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The two day winter storm is pulling away from the region this evening. Temperatures finally warmed into the mid-upper 30s in northern Maryland, bringing and end to the freezing rain and freezing drizzle. The last of the rainfall will be leaving with the storm. Refreezing of snow and ice melt will occur overnight so untreated sidewalks and  streets will likely be slippery for the morning commute.

Below is a National Weather Service map and list of snow and ice totals from around the area.

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Tuesday Storm Update
January 27, 2009

The winter storm everyone is talking about is still on track and the expected impact on the region remains unchanged. This is not likely to be one of those blockbuster storms but the combination of snow, sleet, freezing rain and then rain, in that sequence, will disrupt highway travel for the next 24 hours. The bulk of the Tuesday precipitation will be in the form of snow in the 1-3 inch range. The most disruptive part of the storm will likely be the overnight transition to sleet and freezing rain.

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The morning satellite image shows the main storm is still developing in the Plains states but a disturbance running out ahead of the storm is generating precipitation in the east. In Maryland temperatures are cold enough that snow is the result.

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By evening, storm development is shifting into Tennessee and Kentucky with a warm front reaching into the Carolinas. The Mid Atlantic region is still trapped in cold air at the surface at this time. The purple dashed line reaching from southern Maryland into northern West Virginia depicts the rain/snow dividing line. The situation isn’t that clear cut though. Warmer air (above freezing) several thousand feet up in the atmosphere will begin pushing northward while subfreezing temperatures remain locked in at the surface. This means that an area around and south of the rain/snow line will see a wintry mix of sleet and then freezing rain as precipitation falls into a shallow layer of cold air at the surface. This south to north transition will be in progress during the overnight hours.

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The chart above depicts the areas west of the Bay as having a 40% chance of receiving at least 4 inches of snow. This indicates that computer models are pointing toward a snow accumulation roughly in the 1-3 inch range.

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The map above shows the Winter Storm Warning areas for the most significant snow/sleet/ice accumulations in dark blue. The light blue shaded counties are under a Winter Weather Advisory for lesser accumulations.

96fwbgusBy Wednesday morning the storm center will be shifting toward the upper Ohio River valley. Cold air at the surface remain locked in over Maryland with the warm front hung up to the south. The rain/snow line has moved into Pennsylvania but with continued cold temperatures around freezing at the surface, freezing rain is likely to persist during the morning, eventually changing over to rain from south to north. How quickly this will occur is the uncertain factor in the forecast.

day1_pice_gt_25The chart above indicates that much of central and northern Maryland has a 40% probability of receiving at least a .25 inch accumulation of ice in freezing rain and this is why the Winter Storm Warning is in effect. Areas north, northeast and northwest of Baltimore will be the slowest to warm above freezing and thus stand the best chance to receive the heaviest ice accumulations in this storm.

Additional information added at 12:00 noon, 1/27/09

I have mentioned that the difference in temperatures a few thousand feet up in the atmosphere and the surface will be critical as to the type of precipitation we receive. At 7am this morning the surface temperature at BWI-Marshall was 28 degrees and at 5,000 feet it was roughly 20 degrees (all Fahrenheit). All readings below freezing so snow was in the air. Forecast models indicate that by 1:00am Wednesday temperatures at 5,000 feet will have risen to around 32 F over Baltimore but remain several degrees below freezing at the surface. This is a recipe for sleet from Baltimore southward.

By 7:00am Wednesday, computer models forecast that the temperature at 5,000 feet over Baltimore will rise above the freezing point but that temperatures at the surface will be at or just below 32 F. This is a recipe for sleet or freezing rain, depending on the vertical depth of the freezing temperatures. If the freezing air mass is fairly deep, rain drops will freeze into sleet before hitting the ground. If the freezing air mass is fairly shallow, say under 1,000 feet, rain drops will hit the ground as a liquid but freeze on contact with cold surfaces.

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The map above shows one computer’s forecast for surface temperatures at 7:00am Wednesday. The darker shaded blue areas show the cold air locked in place over Maryland and northern Virginia. This is called “cold air damming” and frequently occurs in winter storms moving through this area.  The lightest blue shading on the map indicates temperatures at roughly 32 F.  The green, yellow and orange shadings show temperatures above freezing. Notice that the mountains in West Virginia are warmer. The surface elevation is high enough that it is in the warmer air. The forecast for the stubborn pocket of cold air to still be in the area around the Bay Wednesday morning is the reason icy conditions are expected.

Check in frequently with WBAL TV-11, wbaltv.com and Insta-Weather Plus on 11-2, Comcast Digital 208 and Verizon FiOS 460 for updates.

John Collins

Storm Brewing
January 26, 2009

Temperatures have been falling from the 50s the area experienced on Friday and now a storm is brewing that will be moving into this colder air.

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The morning satellite image shows extensive cloud cover over the eastern U.S. The latest storm is still developing in the Rockies and will begin to take shape in the southern Plaines in the next 24 hours. From there it is expected to move to the Mid Atlantic region. The timeline and forecast track is shown on the map below.

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This track may allow the storm to eventually take on the characteristics of a nor’easter but if that happened it would likely be after the storm moved across the Mid Atlantic region.

This storm will be moving into cold air and will probably start up with snow across much of the area.

day2_psnow_gt_04The map above shows the probabilities of at least 4 inches of snow accumulating Tuesday through Wednesday morning. The Baltimore-Washington area northward falls in the 40% range. The track of this storm will allow it to take in plenty of moisture from the Gulf of Mexico and eventually the Atlantic Ocean. 

A storm of this nature will typically draw warmer air in ahead of it but as often happens in the Chesapeake Bay area, cold air will remain trapped east of the mountains for an extended period of time and warmer air will filter in at a slower rate. The area of temperature transition is the area that tends to receive sleet and freezing rain. Because this can happen at a slower rate, icy conditions tend to develop.

day2_pice_gt_25The map above shows the probability of significant ice accumulation Tuesday through Wednesday morning.

The storm really hasn’t taken shape yet and a lot of fine tuning will be necessary before a more specific forecast for the area can be issued. This storm does have the potential to produce snowy and icy conditions on Tuesday and Wednesday. Even small amounts of snow and ice in this region snarls traffic patterns so the storm deserves watching. Several inches of snow accumulation is likely Tuesday and Tuesday evening before sleet and freezing rain enter the picture.

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The National Weather Service map above shows the areas under a Winter Storm Watch highlighted in blue. The Watch is in effect Tuesday evening through Wednesday afternoon.

Tom and Sandra will be tracking the storm’s development on WBAL TV-11 and on WBAL TV-11 Insta-Weather Plus on digital TV-11-2, Comcast Digital 208, and Verizon FiOS 460. And of course all of the storm information will be continuously updated on wbaltv.com.

John Collins

Nor’easter Anniversary
January 25, 2009

Today is the anniversary of a big nor’easter that plowed through the area not too long ago.

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This was one of five storms in the Baltimore January weather record books that snow totals exceeded 10 inches.

John Collins

Baltimore’s all-time coldest
January 23, 2009

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Today, January 22nd, is the anniversary of one of Baltimore’s most significant winter weather records.  On January 22, 1984 the temperature dropped to -7° at BWI, tying the record for the coldest temperature ever officially measured here.  That frigid benchmark has been reached five times since 1870 at our official National Weather Service stations, including three times in January and twice in February.

Tom Tasselmyer

Not As Cold
January 22, 2009

The coldest air is retreating as a more zonal weather pattern develops across the U.S.

latestfull2NASA Satellite Image Thursday, Jan 22, 2009, Late Morning

The satellite image above shows the relatively quiet weather over the western hemisphere.

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The morning weather map show that the only significant storms are over U.S./Canadian border just east of the Rockies and off the north Baja California coast. The more zonal weather pattern is expected to push the northern of the two storms along a more northerly track over the next couple of days, targeting the Great Lakes and eastern Canada, allowing our weather to remain relatively quiet.

Take note of the intense high pressure building up over the Yukon and Northwest Territories of Canada. Cold temperatures will intensify in this region over the next week or so and long range models indicate that the dense air will eventually surge southeastward by the end of January. For the Mid Atlantic region this means a return to colder, more unsettled weather.

Speaking of cold and unsettled. A check of the Baltimore record books shows that today’s record low temperature was -7 degrees, set back in 1984. This is one of three times the temperature reached -7 in January, the coldest on record. Also, on this date in 1987, 12.3 inches of snow fell. That is the record for the date and one of five January snowfalls in excess of 10 inches to fall in Baltimore.

John Collins