December 3, 2008: Will anafrontal precipitation bring a white surprise later Thursday?

This is a quick post before bed - hey, I gotta get up early (!) - but here's something to consider that I thought of this morning, but didn't have a chance to sit down and post until this evening.  Looking back to our west, the cold front moving toward us finally has some prefrontal convection with it, but most of yesterday, overnight last night, and the first half of today (Wednesday) the precipitation was all falling behind the front.  Even as some rain showers develop ahead of the front as I write this, there's still snow falling anafrontal, or behind the front.  This raises an interesting thought that I ran by Danielle and Tim today, and the more I looked into it, the more I'm getting sold on it...and that idea is similar to the old Red Sox saying from the 2004 World Series: "Why not us?"  That is, why shouldn't New England also see snow behind the front tomorrow?  There are a few answers to that question, like the vorticity splits, the surface convergence splits and there is low ambient relative humidity.  But on the flip side, the frontal characteristics change little, and a close inspection of the model guidance shows that the models - and especially the NMM - are actually doing a good job of swinging the front through FIRST, then bringing the precip in.  So, if a meteorologist is surprised when more snow than rain falls later Thursday, shame on the meteorologist!  I actually fit into that shameful category until I really started to take a step back this morning and look at the radar, and then the 12Z run as it came in.  Something didn't line up - the precip was waaaayyyy too slow.  That's when it hit me.  So, a look at the 12Z NMM for someplace like Worcester, MA, shows only .03" in the first hour as rain, then .09" as snow, post-frontal.  Of course, then the 18Z run came in with only 1/1000th of an inch of rain, occurring exactly at the time of frontal passage.  So, how seriously do we really want to take this?  Well, there's no question the vorticity field is weak and split aloft, and the low level wind field is strongly downsloping behind the front as it blows out of the west.  Hence, the guidance is really keeping this a very light event.  The WRF is the most hefty with about .10" liquid equivalent, most falling as snow after 11 PM Thursday night.  Probably not a bad idea to put a chance of light snow/snow showers in the forecast for Thursday night with a dusting possible especially Worcester Hills north and west (falling earlier in Northern and Western New England - Thursday afternoon into eve, with some prefrontal possible even earlier), but this is something I will review in the 00Z and 06Z runs, prior to my 5 AM Thursday broadcast.

Farther out, the weekend scenario may be gaining clarity.  Good signals of a Norlun trough developing on Sunday, and perhaps the Southern coast of Maine will be targeted, which is a climatologically favored zone for such events.  Nonetheless, while that's the direction of the guidance right now - and the guidance is good enough at convective parameterization that it is picking up nicely on this potential right now (though one also must show concern for convective feedback overdoing the event) - let's still keep a close eye on the situation.  It won't take much to get the vort to amplify just a little quicker south of New England as it zips along, and make for a more interesting scenario, though the confluent middle and upper level flow over and just east of us really suggests it's Norlun or bust.  Let's keep an eye on it.

Noyesie

Comments

[Chip]

Hi Matt, I love reading your detailed written blogs and watching NECN's video blogs. No other news station in New England gets in depth about weather like you and the crew. I feel that you're the force behind this so please keep up the great work- it's very much appreciated. Anyways, I keep getting my hopes up looking at the computer models, but nothing has come together out of all the systems that have passed through in the past few weeks to favor snow. The storms always look so great 7-10 days out but we haven't lucked out with any moderate snowfall events yet. I loved hearing the "Norlun trough" talk because Sunday's event sure looks favorable for that on the models and I was waiting to hear you mention it as a possibility since it seems to be one of your more favorite winter weather phenomenons. It just doesn't seem to get enough attention by other meteorologists until the day before type of deal. Well, bring us snow soon! even tomorrow's dusting would be nice around here in the UMass Amherst area! I know you're just as or probably more eager for snow as I am. And I'd like to thank you again for taking the time to make a very nice response to my email awhile back. Maybe a lot of snow for the Merrimack Valley over the college winter break would be nice. Thanks Matt! Take care,
Chip