Thursday, February 7, 2013

One day snow forecast for NYC 2 - 20" ??

The epic nor'easter is only one day away from New York City with precipitation already spreading up to Virginia and the pressures are falling fast just offshore and yet the model guidance cannot give us any clue whether or not they'll get 2 or 20" of snow.  Twenty one members of model guidance shows an absolutely huge spread in possible snowfall amounts for JFK airport.  In the timeline plot below, the grayish curves show the snow fall rapidly increasing and then reaching a peak before the snow pack settles down as the time increases to the left.  But some of the model solutions show almost no snow at all.

A time trace of 21 model members of the Short Range Ensemble Forecast (SREF) system for JFK airport from the run starting at 2013 Feb 07 21 UTC.  Time runs from right to left.  The grey traces represent snow whose amounts can be determined from the right vertical axis.  The green to blue traces represent the liquid equivalent amount forecasts.  The horizontal blue line represents a static snow to liquid ratio of 12:1.
What in the world is giving this amazingly huge uncertainty for being only one day out?  One reason could be the wide spread in precipitation forecasted by the forecast ensembles.  The blue traces above also show a huge spread ranging 0.6" to nearly 4.0" of liquid equivalent.  The total precipitation map below shows how some model solutions almost leave New York City high and dry.

The total liquid equivalent precipitation forecast from each SREF member in the small squares and the mean in the large square.  The model run was at the same time as the figure above and valid for 2013-Feb-08 21 UTC.  Note some of the members forecast the heavy precipitation to fall south of NYC.  Image courtesy of the Penn State e-wall.

However that's not the full story.  Not only is the amount of precipitation unusually uncertain for NYC but the model members are unsure what side of the freezing line the temperatures should be.  Here is another multi-panel image from the Penn State e-wall site showing the expected precipitation type and the location of the surface and 850 mb freezing line for the afternoon on Feb 08 (21 UTC).

Same SREF model run time and forecast hour as above but now I show the expected precipitation type (blue for snow). The blue contours show the surface freezing line while the black contour shows the freezing line at 850 mb.  Notice the uncertainty of the freezing line around NYC.

Visualized another way is this SREF sounding plot below showing all the members plotted.  All the members show that surface temperatures should be cold and likely from 30 to 40 F.  That's unfortunately centered around freezing and the results are dramatic with respect to what kind of precipitation is expected to fall.  The left panel shows a huge splatter of expected precipitation types in the late afternoon based on a partial thickness technique.  Unfortunately changing the technique won't improve the uncertainty what will fall in the late afternoon for NYC.

Forecast SREF profile of temperature (red) and dew point (green) for JFK valid for 2013 Feb 08 21 UTC.  The left panel shows the expected precipitation type for each model run (red circles) based on the partial thickness technique.  

This forecast dilemma is not going to improve until we actually see the observations in the morning and see which side of the freezing line New York City will happen to reside.  I'm going to be paying special attention to the AMDAR aircraft sounding profiles as they take off from the NYC area airports.  Unfortunately I hear that flights are being canceled left and right in the NYC area just when we need frequent observations of temperature aloft.  That's why frequent radiosonde launches will be so critical for the NYC area tomorrow.  It's the only platform that can successfully get us the data we need in bad weather.  That is unless some group can fly foul weather drones.

Now for the Boston area, there is more confidence of higher liquid equivalent precipitation though a couple model members leave Boston almost as dry as New York City.   As for snowfall, there is also a similar uncertainty in temperatures but all the runs "safely" keep Boston below freezing throughout the atmosphere and snow, and lots of it, is a virtual certainty.

Same kind of time trace for the SREF as above but for Boston, MA.  Note the precipitation uncertainty is similar as fo

Same forecast model sounding for the SREF as above but for Boston, MA.  The low-level temperature uncertainty appears just as high for BOS as for JFK but the soundingis colder and all members forecast snow.

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