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To block or not to block, that is the question

12/20/12  4:53 PM

Screen Shot 2012-12-20 at 4.30.02 PMThe 12Z models are all pretty much in and we continue to see a split in the guidance with one family (GFS, CMC, NAM, and most Ensembles) suggesting the storm tracks will be under the influence of a powerful upper level low over Ontario and Quebec.  The other family (mainly the ECMWF and UKMET) show the upper level low collapsing and thus a track cutting up towards central New York from the Tennessee River Valley.

Let’s back track first on how we get to this point.  The storm currently bombing out over Chicago will lift towards the St. Lawrence River Valley over the next 24 hours.  As this low pressure system lifts northeast, a large area of very heavy rainfall will move through the New York City and Philadelphia metropolitan areas between late tonight and tomorrow morning.  One interesting point is that temperatures are much colder than what was expected with high temperatures expecting to be in the upper 40′s to lower 50′s, but instead are struggling 10 degrees colder in the upper 30′s to lower 40′s.  What does this tell us?  It tells us that the blocking 500 MB pattern is stronger than what is being modeled and more importantly the low level cold air is more stubborn to leave than modeled.  Let’s consider this in future forecasts.

At any rate, the idea here is this low lifts northeast and drives an area of very heavy rain through the northern Mid Atlantic tomorrow morning.  Given the state of the atmosphere, I would not be surprised if some sleet and snow mixes in with this rain as the cold front moves through.  Temperatures will crash into the 30′s as the rain exits late tomorrow morning.

This low pressure system will form a powerful upper level low over southeastern Canada that will interact with an area of high pressure at 500 MB over the Hudson Bay.  When this occurs, the primary question in the forecast for Christmas morning and Decemeber 27th is whether this upper level low and upper level high pressure system remain in tact to force a storm track to the south and east of the Philadelphia and New York City metropolitan areas.

Given the observations I’ve been following with the stratosphere and persistence of lower heights around the northern Atlantic, I think the idea of the upper level low collapsing rapidly is poorly supported.  This is why the ensemble guidance on the ECMWF is further east this afternoon, not as favorable as other models for a snow storm, but still it’s there.

So I stand where I was this morning.  We really can’t take these models verbatim (nor should you ever really) until we actually set up the blocking pattern on Saturday morning.  By then we should be able to provide the models with more than enough data to derive a solution that is physically reasonable.  So in short it comes down to block or no block.  Going into details on what to expect is nothing but blowing smoke until we answer that fundamental question.