21 January 2016

Historic Storm Aims for the Mid-Atlantic. Why it’s so hard to predict it!

Posted by Dan Satterfield


Ocean storm surge models are showing a high end moderate to major coastal flood event along the Mid-Atlantic coastline.

Ocean storm surge models are showing a high end moderate to major coastal flood event along the Mid-Atlantic coastline.

There is little doubt that a major and perhaps historic blizzard will hit DC, Baltimore, and up toward Philadelphia Friday afternoon, lingering into late Saturday night. Winds will be very high along the Maryland, Delaware, and New Jersey coasts, with gusts well above 60 mph, and significant coastal flooding. There’s very high confidence in this forecast based on a close agreement among all the numerical weather models, but the details in some areas are very tricky!

Snow is always hard to predict, and in this storm, here are just a few of the factors that we have to consider:

  1. Track of the surface low and the upper level low. A 20 mile difference can make for significant changes to a forecast map of snowfall. 
  2. Surface temps. If it is above freezing, it will be a wet snow, and this may bring down more trees with increased power issues. 
  3. Ground temperatures. The ground is quite cold in the region because of the intense cold snap. This made a big difference last night when a dusting of snow led to a real mess along the beltway and in DC. Not a lot fell, but it all accumulated and made roads very icy and slick. 
  4. Temperatures in the lowest 3 km of the troposphere. The surface temperature makes little difference as to what falls from the sky. Meteorologists look at the temperature in the bottom 3 kilometers to forecast rain vs ice or freezing rain. A good rule of thumb is if the temperature at the 850 millibar pressure level (around 3 km) is below 0 celsius, then snow is most likely.

    Model forecast of 40 knot sustained winds will bring gusts to 60 knots, and significant beach erosion.

    Model forecast of 40-45 knot sustained winds, will bring gusts to 60 knots, and significant beach erosion.

  5. The rain to snow ratio. On average you see about ten inches of snow with an inch of rain but this varies greatly from a 5 to 1 ratio to 20 to one! Knowing what this ratio will be is vital. If your local weather person says you will see half an inch of rain and you get .25 inches, you wouldn’t likely notice but that could mean the difference between 3 and 7 inches of snow! You will notice that! 
  6. Ocean and Bay temperatures. A major mid-Atlantic nor’easter will draw in milder ocean air, and the amount of precipitation, along with the temperatures as it falls, are closely related to the water temps. The water temps. off the Jersey, Delaware, and Maryland coasts are unusually warm, and this will provide more moisture to the storm, increasing snow totals, but it will also likely push the rain snow line farther inland (and farther north) than usual. So, if climate deniers like Ted Cruz or Oklahoma Senator Inhofe want to parade a snowball on the Senate floor, and proclaim global warming is a hoax they will richly deserve the ridicule they will receive from anyone with the critical thinking skills greater than a kumquat.

sewells_point_tidegauge_8638610I left out issues dealing with ice nucleation (and how it is located compared to the greatest lift in the atmosphere), and model biases and differences between the numerical models in how they forecast snow amounts are also serious concerns. With this storm, the highest confidence is where the heaviest snow will be, with the lowest confidence in Delaware and the Eastern Shore of Maryland where a dramatic NW/SE snow gradient may be found (where I work!). We also know that little snow will fall along the ocean in Tidewater Virginia. The coastal flood event will stretch from the Maryland shore to Boston, and in some areas, the sea-level has risen around 15mm or half an inch, since Hurricane Sandy.

All of this is worth keeping in mind over the next few days…