It’s All About Timing

January 6 Snowfall -- Bethesda, MD

Although they don’t usually produce prodigious amounts of snow, “Alberta Clippers” can produce a few inches in spots.  Such was the case earlier this month when 2” to 4” accumulations were common across the region.  An “Alberta Clipper” is a fast moving area of low pressure that originates in the Canadian province of Alberta and typically moves Southeast across the Northeastern United States.  Another “Alberta Clipper” will pass through the Mid-Atlantic Region late tonight and tomorrow.  Forecasting any accumulations are especially difficult with the typically fast moving and light bands of precipitation characteristic of such a system.

The major difference between the “Alberta Clipper” on January 6 (that brought 2.4” to National Airport and 4.2” to Dulles Airport in nearby Virginia) and tomorrow’s is the amount of cold air.  Part of what made the snowfall earlier this month so tricky for area schools and commuters was that there was ample cold air already in place so the snow began to accumulate with the first flakes.  The snow also began at roughly the same time as the morning rushour and road crews simply cannot treat the roads if they get stuck in rushour traffic.

Tomorrow’s system will pass through at a similar time with precipitation expected to begin around daybreak in areas west of town first.  The storm should be east of the region by tomorrow evening and any associated precipitation should be over by then too.  However, there is much less cold air in place for tomorrow’s system to work with.  And it’s been relatively mild the last two days too with high temperatures reaching 50 degrees in spots.  That makes the timing and location of any precipitation just as tough to forecast as type. 

If there’s a light accumulation on rooftops and grassy surfaces then that shouldn’t have much of an impact on road conditions.  However, if there is a more significant accumulation closer to the DC-Baltimore corridor, that could create travel trouble depending on both air and ground temperatures.  If this “Alberta Clipper” passed through on the weekend as opposed to a weekday school officials wouldn’t be faced with the difficult decision of opening schools amid adverse weather conditions.

There is a high degree of variability inherent with any forecast of winter precipitation.  Whenever temperatures are going to be near freezing, the precipitation forecast often becomes more challenging.  As we saw earlier this month, it doesn’t take much snow to have a significant impact on the daily routine for many in the Nation’s Capital.  That’s why monitoring the latest forecasts from your favorite meteorologists – such as those I work with at WUSA 9 – is so important.