No April Fools Joke, Here Comes the Temperature Plunge!

Just as the famous Yoshino cherry trees begin to blossom along the Tidal Basin, the North Atlantic Oscillation (NAO) is forecast to dive providing an ominous and reliable warning that a blocking pattern across the northern Atlantic Ocean is setting up as we head into early April.

The image above (courtesy NOAA) shows the NAO index plunging this week.  When the NAO is negative, high pressure builds over the north Atlantic Ocean causing the jet stream to dip into the eastern United States.  This dip in the jet stream typically initiates east coast storms followed by a blast of cold air.

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Update on Weekend Forecast

Sunday’s system is just not coming together with most of the forecast models bringing some light snow flurries or scattered snow showers. The weekend will be cold enough for snow, but the problem lies in that the coastal system that does form will occur well out to sea as the upper level pattern is not conducive for phasing of the northern and southern branches of the jet stream. However before we snow lovers pull our hair out and wring our fists in the air, some patience may pay off during the middle portion of February as the upper level pattern may actually become favorable for east coast Nor’easters.

Here’s the latest European and GFS models for Super Bowl Sunday:

European Model (ECMWF):

  • Coastal storm tries to form southeast of the North Carolina outer banks but the upper level trough swinging through is not phasing with the system meaning as of now confidence is medium that we will have some light flurries or snow showers around on Sunday as any coastal storm that forms will likely slip well out to sea.

ecmwf_slp_precip_ma_20

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Seeing Is Believing: Blocking & DC Area Blizzards

So with all the talk about the positive, neutral and negative phases of the North Atlantic Oscillation (NAO index), I thought I would pull up the 500 mb chart (colorful images) to show why it’s pretty unlikely to get crippling blizzards (20″ +) in the DC Area when the NAO index is in its positive phase.

Positive NAO (Progressive Jet Stream pulls storms that form right out to sea):

Positive NAO index indicative of fast southwest to northeast flow

Current upper level pattern:  Positive NAO index indicative of fast southwest to northeast flow.

Neutral to Negative Index of the North Atlantic Oscillation (NAO):  High pressure (Blocking) in the upper levels of the atmosphere (jet stream level/500mb) will prevent storms that do form off of the Carolina coastline from moving quickly out to sea.

Neutral to Negative NAO indicative of Blocking over the North Atlantic

500 mb chart for February 13th: Neutral to Negative NAO indicative of Blocking over the North Atlantic