Very active and wavy front likely to bring widespread winter storm of varying precipitation types to New England

In the short range, we're locked, loaded and ready to fire.  In the medium range, the moving target is becoming more clear, and thankfully in a direction that makes some sense.

Let's start at the beginning - a nice burst of warm advection snow especially across Northern New England has continued to push northeast.  Now, the impressive Gulf of Mexico tap and its associated thunderstorms and heavy rain are pushing east, along with the strengthening low.  Not surprising to see that the NMM has slowed its frontal progression, though I think the initial instinct of the model to slide the front through very quickly is a testament to the strengthening of the anticyclone upstream that will be so important by the medium range.  Though the NMM was, as it appeared to be, too hasty in sliding this anticyclone east and southeast, it does do the best job with handling low level cold airmasses thanks to its higher resolution, and this dense cold air still looks as though it will slide in quickly behind the front, which is getting a push from the shortwave that races across Quebec later Wednesday.  Confluent flow follows the shortwave, and this favors sliding the high east, then holding it and strengthening it given confluence aloft.  Prefrontal, most of New England will find the strongest wind late Tuesday night into Wednesday morning.  Of course, there is an inversion present at about 960 mb most of Tuesday night, which rises to about 925 mb Wednesday morning, but even given this limited mixing, gusts to 45 and 50 mph remain possible through the period owing to a strong low level jet in advance of the front produced by a pronounced pressure gradient.  As the front nears, the low level jet slides east with the pressure gradient, meaning that the actual frontal passage Wednesday afternoon should bring slackening though shifting wind to blow from the northwest.  I've been carrying a chance for thunder in the forecast with the frontal passage, though am tempted to drop it.  Total totals climb to near 50, which is impressive, but lift doesn't look sufficiently deep to reach cold enough air for multiple active ice nuclei for lightning generation, except perhaps near the immediate South Coast.  Also worth noting is the time of heaviest precipitation - likely to occur either side of 18Z in most of New England as the front settles through, though early in the morning Northern New England and late in the day South Coast/Cape Cod.  It's at this point that the front slows substantially as shortwave ridging in advance of the digging trough over the Southeastern U.S. combines with the longwave ridge holding steady across the Western Atlantic to stop the front dead in its tracks.  Of course, this sets up the pathway for the intense and moisture-loaded system pulling northward.

This system has been the cause of great forecast uncertainty in the medium range as the guidance has struggled with how to handle the shortwave energy when it rounds the base of the longwave trough.  Slowest and most pronounced in the 00Z runs from last evening was the GFS, and quickest was the GGEM (Canadian).  I looked at all available guidance before the early morning shows, and had 00Z Friday low centers ranging from Louisiana to Nova Scotia!  In these moments of great uncertainty, a forecaster must rely on what they know at the simplest level, and that's the upper level pattern.  The Westerlies seemed too fast to allow the upper low to both close off AND drift south of the jet stream so substantially, not to mention this is climatologically quite an anomaly to forecast.  Therefore, the GFS solution was discounted and weighted the ECMWF forecast slightly toward the GGEM and the bulk of its Ensemble members (ie: slightly farther east/north and slightly more progressive).  This seems to be the path worth taking, at least for now, as the 12Z guidance came around nicely, though the GFS is still too laggard in its northeast acceleration of the vorticity maximum.  It not only is not consistent with its own mid-level wind forecast and the confluent flow creating that increased wind, but lags behind its counterparts.  Therefore, still really don't trust the GFS solution in the 12Z run.  Instead, the ECMWF, GGEM, GGEM Ensemble members and to a large extent the NMM have come around nicely, and fall in decent agreement at the 500 mb level.

At first, this seems to have implications only in the "timing" of the storm.  A more in-depth critique of what this really means, though, is that there's more than just timing at stake.  Rather, a quicker ejection of the shortwave trough implies a quicker return to warm and moist advection across the stalled front and into Southern New England.  One has to ask: Does the precipitation ever really stop?  If the warm advection and isentropic lift begins sooner, this doesn't give the high time to build in like the GFS is trying to do, thereby keeping ascent near the front over Southern New England and meaning that, especially the farther south one is, precipitation doesn't really stop.  It does, however, begin to morph.  The cold air streaming southward from the high over the St. Lawrence Valley that strengthens to either side of 1030 mb will make inroads in the lower levels of the atmosphere.  With a failure to bring in colder air aloft, however, we're going back to the seemingly unimportant point of interest discussed at the start of this discussion - the NMM's desire to charge the front through in its earlier runs, and the meaning of that with relation to dense, cold air.  This dense, cold air should seep southward and hold its own interior of a coastal frontal boundary.  This would appear to be a rain/snow line, but the failure of deeper cold air to establish into Southern and even South-Central New England implies it will be freezing and not frozen precipitation for these interior locales - freezing rain.  Farther north, there's great agreement among the NMM and ECMWF that the 850 mb 0 C line runs from Casco Bay to Bennington, Vermont.  This, however, is not the rain/snow line, because of an above freezing layer indicated in the NMM thermal profiles between 700 and 800 mb that penetrates even farther northwest, which makes sense with the lack of a good mid-level cooling signature.  Having said that, I think the NMM is still slightly too far north and west with the 850 and 700 mb front as a result of pulling the 500 mb vort center just a touch too far inland, but it is in good agreement with both the ECMWF and the GGEM, and is close enough to use as a decent guide.  Even a slight adjustment southeast keeps most of these Central New England areas as sleet and holds the snow line from Rutland, VT, to the Northern Lakes/Mount Washington Valley of NH, to Rumford, ME.

If I'm right on this setup, we have an almost all rain scenario for most of coastal Southern New England (surprise, surprise), a substantial freezing rain scenario for interior Southern and South-Central New England, a substantial sleet event for much of Central New England including Concord, NH, and Portland, ME, and a one to two foot snow event for some of Northern New England, from Central/North Central VT to the Mount Washington Valley to the Maine Mountains.

Remember that those of you who read this blog get my raw thoughts - I wouldn't necessarily go on the air with a map of hard and fast predictions for exact locations of ptype and amounts, because there is still lingering uncertainty with the storm track, and the efficiency of precipitation production, though I expect the latter to be high with differential temperature advection underway - warm advection aloft and cold advection below - and the cold side of the mid-level front should form the necessary deformation band on the cold side of its location to verify the heavy snow band in the North.

So, meteorologists and weather-enthusiasts alike have our work cut out for us.  Watch the GFS to come in line and just a touch farther southeast than the current NMM solution, and both should be fairly close in guidance prior to the event.  The biggest difference will probably come with regard to surface temp and the location for the freezing line, but the north-northeast component of the surface wind thanks to ageostrophic flow will favor colder farther south, especially given what is a good cold air damming and cold air drainage surface pattern with the strengthening then maintaining anticyclone to our northeast.

I invite your thoughts in the comments section of this post!
Matt

Comments

[Charlie Courcy]

Matt: Your prediction for Dec. 24 is a high of 25 deg. and for Dec 25 a low of 32 deg. How is this possible - what happened to the missing 7 degs.?

[Supercell]

West Springfield right now looks like mostly rain and freezing rain...check into NECN, this website and WeatherNewEngland.com as the event draws closer, though.

[matt]

this storm is a very hard one to follow. please tell me what you think i will get in west springfield mass?

[scott]

"a one to two foot snow event for some of Northern New England, from Central/North Central VT to the Mount Washington Valley to the Maine Mountains."

Well, now I'm standing at attention!

Pretty amazing, given that some model runs over the wknd were taking this storm much further out to sea, and giving I-95 its 1st shot of plowable snow.