Archive for February, 2007

The Storm Is Here
February 25, 2007

Finally found some time to sit and jot down a few thoughts. The storm is not behaving exactly as originally thought but it is pretty much running on schedule.

Overnight temperatures (Saturday-Sunday) were generally above freezing by a few degrees (unexpected) and the air was very dry (expected). The morning sounding at Dulles Airport showed the temperature at 850mb at freezing.As the precipitation started between 9-10am in Baltimore, the atmosphere at the surface chilled to just below freezing, allowing for more persistent snow and generally light accumulations around the metro area. Morning snow totals in the Virginia and West Virginia mountains were in the 3-6 inch range.

The main storm center is headed for the Great Lakes but some of the storm’s “energy” is expected to contribute to the development of a low pressure area near the mouth of the Chesapeake Bay. This should prolong the chance for precipitation and allow some time for warmer air to move in and bring a changeover to mixed precipitation.
A tongue of dry air is being drawn into the storm and a break in the precipitation is not out of the question but at the same time (1:45pm) a band of heavy precipitation has developed over the DELMARVA, rain at the south end and heavy snow at the north end, reaching across the Bay into Baltimore and Annapolis. This storm is throwing us a few curve balls. I’m glad I mentioned a possible accumulation of 1-3 inches of snow. At 2:oopm the snow is 2 inches deep here on TV Hill and still going.
More later.
John Collins


Complex Winter Storm Heads East
February 24, 2007

A powerful storm that came in from the Pacific should make weather headlines for much of the nation this weekend. A pretty good sign of the power and complexity of the storm can be found on the composite map of watches, warnings and advisories issued by the National Weather Service. The color coded chart from late Friday evening shows no less than 32 separate kinds of severe or threatening weather conditions affecting the United States, many of them associated with the storm emerging on the western plains. The plethora of headlines nationwide includes a tornado warning in Oklahoma, blizzard warnings for parts of Colorado, Kansas and Nebraska, not to mention freezing spray warnings in Alaska, avalanche warnings, high wind watches, brisk wind advisories and fire weather watches! For our area, a winter storm watch has been posted. The storm is expected to intensify near Kansas City Saturday night and send a slug of moisture into cold air locked on the east side of the Appalachian mountains. The end result for Maryland could be a little snow with more sleet and freezing rain developing Sunday morning. Winter storm watches could be upgraded to warnings as the storm approaches. Updates throughout the weekend.

Tom Tasselmyer

The Thaw
February 21, 2007

With a high of 47F and a low of 32F, the average temperature at BWI-Marshall yesterday was 40F, and that ended a streak of 17 straight days with average temperatures below normal. Digging through the record books it looks to me like this was the longest stretch of below normal temperatures since late spring of 2005. From May 16, 2005 through June 3, 2005 we had 19 consecutive days with average temperatures below normal. The thaw is underway now, but don’t expect winter to give up easily. A few shots of cold air are still possible, as we will see on Friday, when a clipper passing north of here pushes a glancing blow of arctic air south into the mid Atlantic states on strong northwest winds. The 6-10 day temperature outlook from the Climate Prediction Center of the National Weather Service covers the period from Feb. 26th through March 2nd and it does show warmer than normal temperatures for much of the eastern U.S. But, the 8-14 day outlook, covering Feb. 28th through Mar. 6th, shows the warmer than normal temperatures are gone and more cold air begins to show up over the northern plains, great lakes and Ohio valley. That might signal another flip in the weather pattern and perhaps a cold and wintry start to March.

Tom Tasselmyer

February Temperature Update
February 19, 2007

A quick update and slightly different skew to Tom’s February temperature figures below. National Weather Service records indicate that as of February 17, the official reporting stations in Baltimore over the past 137 years position this February in 9th place for the lowest average temperature with 24.9 degrees.

As Tom mentioned, the situation will be changing this week as the jet stream will be repositioning itself. By mid week it will be oriented in a more west to east alignment, allowing milder air to filter into the region from the southwest. By next weekend, a major storm will be cranking up over the central U.S. and the jet stream will be running from a southwest to northeast direction. This should drive the temperatures into the above normal range, at least temporarily. Computer models keep the temperatures high next weekend but as that storm approaches, easterly winds off of the Atlantic may set up a “cold air damming” situation and pull the temperatures back to below normal as well as generate clouds, drizzle and light rain. It will take a few days to fine tune this aspect of the forecast and see if the models grab hold of this idea. Enjoy the warm-up.

John Collins

Frigid February
February 17, 2007

Only 14 times in the past 136 years, dating back to the start of official weather records for Baltimore in 1871, has the average temperature for the first half of February been below 30F. So the frigid February of 2007 is certainly one to take note of. In fact, with an average temperature of just 24.9F through the 16th, this has been the second coldest start to February ever. The coldest Februarys on record in Baltimore:

Year Avg. Temp.
1. 1934 24.3
(2007) (24.9) as of 2/16
2. 1979 25.6
3. 1895 26.2
4. 1905 27.3
1978 27.3

The cold has also been accompanied by plenty of brisk winds. Two days have had wind gusts over 40 mph, 10 of the first 16 days have had a peak wind gust of at least 30 mph and all but one day this month has had a gust of at least 20 mph.

The cold, however, has not produced very much snow, with just 4.2″ measured so far at our official National Weather Service station at BWI-Marshall.

Changes may be on the way, though, with computer models indicating a shift in the upper level winds from northwest to southwest for a good part of the final week of the month. The chart displayed here is a map of the forecast jet stream for Tuesday, February 20th, showing winds up around 37,000 feet. The purple shaded area is the jet stream core with 184 mph winds screaming over Texas, Arkansas and northern Mississippi, pushing a milder weather pattern toward the mid Atlantic and southeastern United States.

Tom Tasselmyer

Why the wintry mix and not just snow?
February 15, 2007

The storm that dropped anywhere from a trace up to 6″ of snow and ice on central and eastern Maryland Tuesday and Wednesday is pushing farther north with blizzard warnings posted for much of interior New England. For much of the storm temperatures around the Baltimore area were well below freezing, prompting many to ask why we didn’t have just snow. Why the wintry mix of icy precipitation? The weather balloon that was launched by the National Weather Service down at Sterling, VA near Dulles Airport on Wednesday morning tells the meteorological story. The graphic posted here shows the trace of the temperature and dew point as the balloon climbed up through the atmosphere. The diagonal red line is the freezing line. Any part of the balloon trace (the thin, squiggly black lines) that shows up on the right side of this diagonal red line indicates temperatures, at that level of the atmosphere, that are above freezing. You can see that most of the balloon trace, high in the atmosphere and near the ground, is left of the freezing line, indicating most of the atmosphere is cold enough for snow. However, I have highlighted a thin layer of the trace that shows up on the warm side of the freezing line. It is this thin, 3,130 foot layer, that caused the snowflakes that formed higher up to melt into raindrops as they fell, only to refreeze into ice pellets (sleet) as they hit the cold layer near the ground. These thin layers of warm or cold air will frequently make forecasting winter weather around here a real challenge, especially since there are only a few scattered weather balloons launched in the mid Atlantic region to pinpoint them for forecasters, and the closest one to Baltimore is the one launched a few times each day down in northern Virginia.

Tom Tasselmyer

Recipe For Wintry Mix
February 13, 2007

The stage seems to be set for a mix of just about every form of winter precipitation possible to show up in Maryland Tuesday. From snow to sleet to freezing rain to just rain. With our biggest snowstorms, cold high pressure is locked in over New England or eastern Canada, supplying the necessary arctic air to keep the precipitation all snow. Tonight’s weather map shows the cold high pressure cell out of position for snow lovers, centered over central and western Canada. The high is slowly trying to build east into upstate New York, while the initial storm takes shape over Texas and Oklahoma. If the high can slip into place over the northeast U.S. before the approaching storm gets here, we could have a longer period of snow with more accumulations than currently forecast. The current run of the computer models, however, seems to indicate the high will not be able to establish itself firmly enough and the warm air ahead of the storm will rush into the middle levels of the atmosphere over the mid Atlantic states, changing our snow over to sleet, freezing rain and eventually rain. This is a tricky bit of timing and computer models do not do a great job of handling these sort of fine details, but for now I expect a general 1-3″ snowfall for most of central and eastern Maryland Tuesday morning, but 3-5″ of snow will be possible where the cold air holds on longer, up in the northern sections of Baltimore, Carroll and Frederick Counties. By afternoon the warm air aloft should be flooding in with our precipitation changing over to the wintry mix. With the cold, dense air hugging the ground, surface temperatures may be reluctant to climb above freezing, producing a period of sleet and freezing rain with some ice accumulation possible into the evening hours. Later Tuesday night, as the atmosphere warms a few degrees, the precipitation should change to just rain, which will hopefully make for a wet, not icy, Wednesday morning commute. There is still a lot to be decided with this storm and the forecast could easily change. Stay tuned to WBAL-TV or for updates.

Tom Tasselmyer

More Storm Fine Tuning
February 12, 2007

Probably “messy” will turn out to be the best way to describe the developing storm. The various computer models are beginning to come around to the idea that warm air(+3 to +6 degrees celsius))will be drawn into the storm at the 850 mb level(several thousand feet above the surface)over the Baltimore area. This is expected to happen late in the day on Tuesday, right when the precipitation rate starts to increase. What was snow at that altitude becomes rain. Temperatures at the surface are expected to remain below freezing and the rain drops that fall into this cold layer will either become sleet or freeze on contact with cold surfaces.

The change from snow to a “wintry mix” will cut into potential snow accumulations and makes it very difficult to come up with a snow depth forecast. A guess at this early stage might be that 4 to 8 inches of snow could fall around the Baltimore area northward to the Maryland line with over a foot of snow to the northwest in Pennsylvania. A lot more rain and sleet will likely be in the mix south and east of Baltimore and snow accumulation potential rapidly drops off. My confidence in these numbers is quite low but I guess it is a starting point.

I have included two charts from the 18z Sunday NAM model run. The charts display data for the pre dawn hours on Wednesday and show the 6 hour precipitation totals/sea level pressure and the 850mb heights/temperatures/winds.

I hope this discussion gives you an inside look at the complex nature of trying to forecast a storm like this. The public responds to snow forecasts in this region with quite a bit of anxiety and storms like this one can be frustrating. Forecasters tend to hold off on giving precise numbers in these cases until confidence levels are reasonably high. We’re not quite there yet. Tom, Neal and Domenica will have a lot more information to work with as the storm matures so stay tuned.

John Collins

Forecast Fine Tuning
February 11, 2007

Just a quick note about the snow forecast for Tuesday. The fine tuning continues. Computer models have been pushing the storm a bit further north with recent runs. This would move the likelyhood for the heaviest snow just to the north and northwest of Baltimore. If the northward migration continues in future model runs, the odds for some sleet or rain mixing with the snow would increase for the Baltimore area southward. These changes in models are common for approaching storms and it is a big reason why we try to hold off a bit on predicting snow totals. There are many variables that impact a storm’s behavior and as time closes in on the event, a single, and fairly accurate, forecast is usually arrived at.

The graphic with this item shows the probabilities of snow concentration areas of at least 8 inches that NOAA’s National Center for Environmental Predicition developed early Sunday for the Monday-Tuesday storm. Notice the skew to the north and northwest. It will be interesting to see if this shifts any more by Monday morning. The storm could easily produce 6-12 inches of snow. The speed of the storm and the track of the low pressure center will determine where the heavy snow band winds up, how much snow is in it and whether any sleet or rain will mix in.
Stay tuned.

John Collins

Lake Snow Woes Continue
February 10, 2007

The Lake Effect Snow Machine is still working as the weekend begins. The NASA picture above shows some detail. Note that western portions of Lake Erie are now ice covered and that puts a damper on excessive snow generation downwind. The more open waters of Lake Ontario continue to provide the moisture and temperature contrast necessary to generate the unbelievable snows that have hit western New York.

A developing storm out west will contribute to a change in the wind fetch over the Great Lakes and the lake effect snow machine should start to shut down Sunday. That same developing storm is expected to take aim on the Eastern Seaboard and the Mid Atlantic region Monday and Tuesday. It is too early to be specific but this storm has the potential to generate a significant snow. We’ll just have to see how things come together. Stay tuned.

John Collins