Freezing rain for some south, sure...but one to two feet of snow in the North Country? Is this guy kidding?
No joke...a major winter storm is on the way. By the time you're done reading this, you'll know why I'm forecasting what I am, and we'll all sit back and see if nature can deliver. This is a rather encyclopedic post, so sit back, relax, and continue reading...
Let's start with the here and now which is the impressive cold front settling south. A steady feed of moisture continued accumulating snow behind the front but the upper level dynamics are shifting south and that snow band will weaken soon, meaning no snow farther south. Additionally, what was an impressive flash-freeze scenario in especially Northern VT this morning has turned to a more gradual freeze, which is far kinder on drivers but still will cause problems all evening and night, as the fresh rain in a moist airmass will not evaporate quickly, and black ice will develop overnight all the way into interior Southern New England. The front sags southward, pushed by the northern stream shortwave evident both on water vapor imagery and accurately progged by the guidance to move across New England tonight. This shortwave will be important for the upcoming winter weather event, as the confluent flow aloft behind it will secure high pressure into Eastern Canada, and this high has provided daytime highs below zero at its core, so it's loaded with cold, and will be tapped in the upcoming situation. To a large extent, the tap is already underway, and this has resulted in the shift to wintry weather in the north.
It's important to acknowledge that today's rain didn't really shape up to be all that impressive. Amounts ranging from a quarter to three quarters of an inch thus far, and though we'll increase those amounts somewhat ahead of the front, this was generally a light to moderate rain event for many. There's a break between southern stream shortwaves that will result in far less coverage of precipitation overnight, though some showers are likely to linger near the baroclinic zone in the above-freezing air along the South Coast. Meanwhile, surface observations as of this late-afternoon writing are showing clearing skies as the Canadian border, and this is one telegraph from nature. Often, before storms, there are various clues nature gives forecasters, and sometimes we're lucky enough to pick up on them, while other times we're not. In this case, the clearing sky should indicate that cold and dry air is moving in not only at the surface, but aloft as well. This cold air means business, and the NMM is likely doing the best job of resolving the depth and strength of the cold. Continuing along the lines of the models and thermal profiles, the GFS is a warmer scenario than the NMM both at the surface, and in the mid-levels. I don't believe it entirely in either case, but am willing to buy some of its mid-level warmth. Let's start with the surface - the NMM has done a better job with cold air domes that are shallow, largely because of its superior resolution, and this isn't a time to doubt its ability to reconcile surface airmasses. I'd buy it for surface/2m temp, which will be crucial in determining where to place the freezing line on Thursday, which hovers right around Route 495 in Eastern MA, then into Northern RI and Northern CT. This makes perfect sense, and though a northeast wind flow may result in enough downsloping for some interior valleys to warm a degree above freezing, most spots in this area will remain below freezing, and this was indicated on the overview map included in the broadcast video I embedded into the General Weather Summary today. As for the mid-level warmth, I was a bit torn - the GFS has always done far better with mid-level warmth, but the last borderline event a couple of weeks ago that produced significant snow in Northern New England was an interesting signal. While we did well on the forecast at NECN with that storm, I was unhappy with my performance in Northern Carroll, Coos and Northern Oxford Counties in NH and ME, respectively. This made up a small percentage of our viewers, and most of them were on guard for significant snow (not to mention most are snow lovers there) anyway, but I really do take note of just a county or two not going the way I expected. In that instance, the warm advection was lower - at 850 mb - but the NMM far outperformed the GFS but accurately holding the cold. So, while we're now looking at a warm layer around 800 to 700 mb, the difference between the two solutions is only about 1 degree Celsius, and that was enough for me to favor the NMM profiles by and large. The real-time, natural clue of clearing in the North Country as I write this only reinforces my feeling on this. This means a mostly snow event for the North Country, though some mid-level warming is likely to introduce sleet to Central and Eastern Maine. One interesting point is that there is excellent agreement - no matter what model you turn to (except the GGEM, whose rain/snow scheme is questionable in this borderline event) - that Vermont will be almost all snow north of Bennington and Windham Counties. It would be foolish to question the guidance in an area where there's good agreement both on the thermal profile, and the precipitation amounts (to within about two tenths). So, as I made the accumulation map, I calculated ratios as I always do, but was intrigued to find that I came up with a better snow to water ratio for Central VT than Northern, and even better still for Concord, NH, which ends up going to sleet. The reasoning is good dendritic crystal growth in NH, borderline in Central Vermont, but condensation occurring greatest in a layer too cold for dendrites and more favorable for needles in Northern VT. Nonetheless, this yields a 10:1 ratio in Burlington (11.5" of snow) and about a 13:1 in Rutland (23.5" of snow!). I have to admit I did a double-take when I crunched these numbers, but when you have science at the root of your work, and you're working from good agreement in the guidance, you can't really call yourself a Maverick, but rather a student of the science who is considering every parameter he can. Whether that means anything in the end result can only be known when the flakes have fallen.
Though lots of sleet is expected in Concord, it's still quite possible to sneak out 6" or 9" with the dendritic crystal growth during the day Thursday. I think the biggest problem here is what looks to be a pretty tight thermal gradient around 850 mb that will probably create a very sharp drop in amounts from north to south. I could completely picture a 9" Laconia, 6" Concord, 2" Manchester, trace or 0 MA/NH border event. But a jog in the freezing line just north or south would have huge implications in such a tightly packed accumulation forecast. This means the threat for a bust is largest here, but travel implications will be huge regardless, because event those in Southern NH/VT/ME who do change from snow would probably have a deep enough cold dome to see sleet, which is equally treacherous on roadways (but doesn't match up to the ruler quite as well!).
Farther south, freezing rain will be the biggest problem, and be found outside of Route 495. To take the guidance literally, this is a vicious situation for the interior, and there should be no doubt about it that a worst-case scenario will be very severe ice storm for interior MA and far Southern NH. Having said that, this event is so borderline, that even downsloping wind of the slightest degree - even though it's a northeast wind - could push some spots one degree above freezing. Nonetheless, there will be others who remain well below freezing all day and all night Thursday night, especially in the hills of Central/Western MA, and through most of the Route 2 corridor into the Monadnock Region and Southern NH away from the coast. We're forecast to crank out between two and three inches of QPF as freezing rain in these spots, but one point made by the NWS office in Taunton was a really sharp one - that heavy freezing rain won't accrete as quickly as moderate freezing rain. Though it sounds counter-intuitive, it makes sense because each pounding raindrop will knock its predecessor off the branch or wire before it has a chance to freeze. This doesn't save you from a major ice event where the temp is cold enough, but does help to minimize accretion where temperatures are quite marginal. Finally, if this looks like a major impact event before I go on air in the morning tomorrow (Thursday) I will be quite cautious on how to play the travel angle in interior Southern New England. Even with trees and wires buckling, modern road treatments do a good job in freezing rain events, though I have to admit that tomorrow's rain should fall hard enough to wash treatment away and then start freezing. Nonetheless, it's never a topic I'm comfortable with, because you take the variables of the weather, then introduce the unknowns of what chemicals, how much and how often - facts I don't get while making a forecast. Of course, that usually only applies to major interstates, not the secondary highways and side streets which are always horrendous in such situations, nor to the power/tree scenario.
Wind is a concern on the Cape as the storm center races by Friday morning - and I do mean races. This thing is trucking like a 1938 hurricane up the coast, with the 500 mb center going from VA Beach to Eastern Canada in 12 hours from 12Z Fri to 00Z Sat!! This will deliver a quick blow of 50+ mph wind to the Outer Cape (or east of the storm center, depending on if my track is correct, I suppose) from the south, may verify the NWS GYX coastal flood watch (good foresight - high astronomical tide) on the way by the Maine coast, then allow for a downsloping and drying wind Friday afternoon. Cold shot on Saturday will be much colder than MOS indicates, but operational guidance is picking up on it well, then warmer air Sunday and Monday where this air coming in produced highs in the 40s in Utah and CO yesterday.
Looks like warmer than normal temps overall through Christmas. Was looking at everything yesterday and the fun White Christmas chances graphic I make for the kids on NECN every day at 6:41 and 7:41 AM is probably going to see some changes for the worse (less chance of a white xmas) in Eastern and Southern New England with a strongly positive North Atlantic Oscillation, a strongly negative Pacific North American Oscillation, and a pattern shift that favors a warm ridge in the East. The latter statement is made because after several weeks (a full month) of a four-wave planetary pattern - four major troughs and ridges - we've contracted to a three wave pattern in the last 5 days. A three wave pattern cannot hold, Siberia is unbearably cold (which can be cold for us in the end, but not immediately), the Eastern Canada trough is moderating, and this all favors a return to a four or possibly five wave pattern, but with the trough in the West and Central US, not the East. The Ensembles back this up in the mean through day 10, and there's no reason to question that. With the kind of warmth en route, rain would be most favored in Southern New England. That said, I wouldn't drop the chances below 30% at this point in Eastern MA, because we are near the baroclinic zone later next week (which means while rain looks likely now, it doesn't take much to get snow instead) and then the pattern does appear to progress slightly by the 23rd or so. Wishful thinking? Perhaps, but these possibilities all must be factored into the process of determining probabilities for a White Christmas...