Monday, February 11, 2013

Differences Aloft Yield Big Differences In Final Result

We've been watching the "on again, off again" Wednesday night/Valentine's morning storm system in the models for the last few days, with the GFS showing relative consistency over the past several days in the potential development of a storm system near the region at midweek, which would bring rain and snow to the Delaware Valley.  The Euro shows this same storm but not as strong or robust, which means a much different result over the Delaware Valley in terms of impact.

We're talking the difference between six or more inches of snow near or over Philadelphia, if last night's run of the GFS is to be believed, or nothing at all if the Euro is to be believed.

The reason why the GFS continues to suggest the possibility of a snow event over Philadelphia has to do with a difference aloft in how a wave of energy interacts with a low pressure center over the Carolinas.  On the GFS, this piece of energy is stronger, has a sharper amount of "dig" to it, and helps intensify the low pressure center at the surface more rapidly.  You can see it outlined below.  Look at where the "5400" line tracks down almost into North Carolina on the GFS, whereas on the Euro it doesn't go south of Pennsylvania.  This difference of a stronger, more robust wave of energy aloft, helps drive the development of the storm and, more importantly, because there's a sharper amount of trough on the disturbance aloft the resulting track brings the low more northeast as it develops.  Longtime readers know I explain troughs with "V" and "U" terminology -- a V shaped trough is sharper and allows for a more northeast track whereas U troughs are flatter and result in a east or east-northeast storm track where the turn isn't as sharp because the trough isn't as sharp.

That difference in detail at 18,000 feet in the atmosphere explains the difference at the surface.  The GFS develops a stronger low pressure center that tracks a bit closer to the region and produces rain changing to snow (or mostly snow farther northwest) in the Delaware Valley whereas the Euro is less robust with the storm and scoots it out to sea, grazing South Jersey with nuisance light rain or a mix at the tail end. Precipitation on the GFS is heavy at times over the Philadelphia metro and changes from rain to snow, with the potential for several inches of snow.  Given the environment is marginal aloft, the stronger GFS storm is able to produce heavy enough precipitation to cause the changeover from rain to snow as precipitation pulls down colder air from aloft.  If the storm were say, a few millibars weaker and on a similar track the result would probably a couple (2, maybe 3 at the most) inches of wet snow or simply a rain/snow mix.

This is from Earl Barker's website, showing the potential for six inches of snow per their algorithm of the GFS model for Wednesday night across the Delaware Valley, with the heaviest snows right over the Philadelphia metro.  One of the pay-per-view companies puts out a map of the GFS and shows more than this falling near Philadelphia (eight plus) -- and that would be plausible given the heavier rates of precipitation that fall.

The Euro? Nothing...again, just light rain and snow showers.

Modeling is still trying to smooth out the differences on this feature, which is currently over the Western US and will track east over the next two to three days.  If it weren't for the GFS' relative consistency with showing precipitation overhead -- and it's been showing it more often than not, the Euro would probably yield a much more greater level of confidence from me.  However, the Euro itself has had some issues with consistency in handling this feature.

Putting all this aside, know that the potential for some snow is there for Wednesday night, after 5-7 PM. It's a bit of a goldliocks scenario of sorts since it requires the features to come together just right to 'bomb out' but it could yield potentially some snow close to the region if that "just right" indeed happens.

We'll have another update on this tonight.