Thursday, February 07, 2013

In A Phase

Modeling last night with Friday's coastal storm continues to show the fine line that the models are walking regarding the storm's development and pace of development.  The Euro and GFS vary by about 75 miles and a few hours in pace of intensification through 7 PM Friday...and this difference is pretty critical for us as a more rapid development in the coastal storm would provide us with a bit more snowfall than a slower, less wrapped up storm.

Again, this will be a huge storm for New England...and those graphics that were tossed around yesterday from the RPM model that were suggesting more than a foot of snow near Philadelphia have now been replaced by two inches locally.  Yeah, that's why you don't post RPM snowfall graphics on air two and a half days's not a reliable model outside of the short term.  In sweeps months like February, I guess anything goes?

Back to the storm...the GFS and Euro vary quite a bit on snowfall.  The GFS is suggesting an inch or two of backend snow on Friday night in the city, with four inches as close as Trenton and Yardley.  The Euro has the city right around six inches, with eight or nine inches near Trenton.

The GFS (above) is a bit farther east with the storm than the Euro (below), which means that the shield of heavier precipitation is a bit farther east on the GFS as well.  Since the steadier and heavier precipitation is a bit farther east on the GFS, we don't see as much precipitation falling locally on Friday and the transition to snow is a bit slower to take place than on the Euro (below).  Since the Euro is intensifying a bit quicker, tracking a bit closer to the coast, and is throwing more moisture back into the region, our mix of light wintry slop has transitioned over to snow faster and snow will fall a bit harder.  You can see more precipitation on the Euro graphic compared to the GFS...and with a slightly heavier thump, that transition from slop to snow takes place a bit sooner.

As I was mentioning, the differences in intensification are critical -- the slower to develop GFS is throwing less snow back into the city (a couple of inches), while the faster developing Euro is bringing more snow into town (three to six south of Philly, six to nine Northeast Philly on north).

Most of the snows that are expected will fall Friday evening with this storm -- however the Euro's faster transition and more rapid development may suggest the changeover take place during the afternoon and the brunt of thump would be in the evening hours.  The GFS would hold that transition off until the evening rush hour.

You might be wondering what the heck that chart is...well, it's a snow chart of the various SREF modeling shows how much snow the various short range model ensembling is projecting along with the mean (average) of the various runs -- from the outlier that thinks 14" is falling to the others that say nada.  For Philly, the mean is 4.6", for Trenton it's 6.8".  If you throw out the 14" run, that total drops by about a half inch in Philly.  Perspective doth apply.  A few inches is certainly in the cards, which is why we didn't want to put a top end on the accumulation graphics that went out last night because there was a good bit of uncertainty still on the board.

The forecast is not a clear cut or slam dunk locally, as is the case with many snowfalls!  That said, the trend is for at least some backend snowfall locally.   We'll look at the midday data later on and then offer up a revised forecast on snow this evening.