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Weatherwise, I suppose we should start out with what's known today - a change in the pattern is on our doorstep that will once again open the Gulf of Mexico as a moisture supply for storms brewing across the Lower 48, and this means the stakes are higher with the forecasts. What makes part of the forecast so tricky - focused on Saturday night - are two distinct and impressive disturbances embedded within this flow, so the challenge is to accurately predict the independent and then interactive behavior of these two disturbances, all amidst a changing longwave, broad-scale pattern.
But let's take it one step at a time and focus on the parts of the forecast that have more clarity first. In the short range forecast, we're looking at a very well defined vorticity maximum (energy center) that's moving east across Southern Ontario and Lake Ontario and has exceptional radar presentation. The problem is that for as good as the radar looks, as of this writing I can find only one station - Kingston, Ontario - reporting any precipitation at all, and that's in the form of light sleet. Looking in the vorticity maximum's path over Northern New England and Southern Canada in the coming 12 hours, the surface dewpoints are running in the single digits below and above zero, and the top of Mount Washington has a dewpoint of -27 F and dropping with a northwest wind continuing as of this writing. That doesn't promise much of a future for the disturbance, even when you factor in orographic (topographic) lift in the mountains of Northern New England. With the southeast wind that will develop later in the day, some boundary layer moistening will take place but it won't be much where the greatest dynamic lift will be found over Northern New England, so I really can't see this thing cranking out more than a dusting or coating of snow for most Northern New England locales with perhaps some exceptionally orographically enhanced spots topping out at a fresh inch or two, with virtually nothing farther south through Central and Southern New England. Remember that the instigator of clouds and any precipitation is the onset of warming aloft. The surface warmth's progress is being more significantly retarded by the presence of such deep cold but most importantly dry air in place. This front means business, though, and crossed through places like Pittsburgh, PA, today with a jump of at least 10 degrees with its passage, and some spots have seen over 20 degree jumps pre and post frontal passage.
So, to know that front is going to continue moving northeast through Friday means somebody in the Northeastern U.S. should warm nicely. This seems like a lock from New Jersey to NYC to warm into the 60s and 50s, respectively. I do think Southern New England will find warming on Friday, but I'm suspicious of one subtle feature, in particular, which is the development of a very weak and small mesoscale (medium scale) surface low pressure center along the warm front late Thursday night into early Friday morning just south of New England. This is being hinted at on both American guidance products and while I do think it will turn the winds east or perhaps even northeast early Friday on Cape Cod and perhaps part of the South Shore of MA, the progress of boundary layer warm advection should be enough to turn the winds back around and get the mild air in, though on the Outer Cape there's a bit of a tossup thanks to the cool ocean waters that won't be so accommodating. The farther north one goes, the harder it will be to push this warm front through, largely because it's so far removed from the parent, primary low that shoots up into Canada, meaning the wind field remains light. Therefore, it seems likely that while most of Southern New England gets into the warm air Friday afternoon, the front should stall across Central New England, keeping cloudy and cooler conditions north of the boundary. Of course, this boundary will be important, too, because it will help to focus moisture in advance of the next low pressure wave, which will be moving northeast through the Ohio Valley and eventually will deliver a shot of steady precipitation Friday night. The rain/snow line Friday night is unlikely to be located at the frontal boundary in Central New England, nor near the 850 mb (about 6000 feet in elevation) freezing line. Both of these features can and sometimes do play big roles in a precipitation type forecast, but temperatures at 925 mb (about 2000 feet off the ground) are as far north as the Canadian border of VT and NH at the start of Friday evening. As the strengthening wave of low pressure moves along the front, however, the counter-clockwise flow of air encourages a northerly wind flow that drags cool air southward once again, prompting a change from rain to snow progressing from north to south, but by that point what's left of moisture has been mostly spent as rain, meaning not more than a couple of inches would be likely even in Northern New England.
Of course, the southward spill of cooler and drier air not only means a break in the action as we await the next shortwave on Saturday, but also means the atmosphere becomes more conducive for solid, rather than liquid, precipitation. Of course, settling the baroclinic zone (area of changing temperature) southward means we play with the storm track, which, as we know, loves to follow the temperature gradient. Here's where we really start running into trouble for the forecast, as there's a fairly significant difference in how to handle the thermal gradient, and therefore where to bring the next storm. Of course, there are differences aloft, too, that are important, focused both on the integrity of a lobe of southern stream vorticity (energy) streaming across Ohio on Saturday, to a shortwave trough's impact on the longwave pattern over the far eastern Canadian Maritimes during the same time period. For the former, the NMM tends to perform better thanks to its higher resolution, while the latter would tend to be better handled by the GFS for its superior deciphering of broader disturbances. Also significant - and perhaps the most so - is the vigorous shortwave trough digging across Ontario Saturday night - the farther north and wetter guidance is amplifying this, while the drier guidance is flattening it out and speeding it up. As for the low level temperatures, the NMM is superior in resolution and usually performs better in COLD advection, but the GFS has performed better in WARM advection for New England this winter and early spring. This gives the NMM a bit of an advantage for this specific scenario. It's really important to note that while the NMM (one American product) is still north and wet/white for much of Southern New England, and the GFS (another American product) is southward suppressed, there's no question that the GFS has been farther north with its precipitation. In fact, both products are now indicating a storm for Cape Cod, so confidence should be increasing on such a scenario from the Cape through most of the South Coast of New England. While there is still lots of uncertainty, it's important to look for where things are subtly coming into line, and right now that appears to be the South Coast for Saturday night into Sunday.
As for the dilemma of what to do farther north, there are a few thoughts here: 1) The southward GFS has performed better through the season, in general. 2) The northward NMM had a big score a few weeks ago in a similar situation where southern moisture was being input quickly, but has not performed well outside 48 hours in contrast to its counterpart guidance, 3) The Ensemble Mean (average of many different versions) of the GFS favors its operational solution, 4) The ECMWF (European) and GGEM (global Canadian) are both even farther south and drier, and these models tend to perform well this winter. So, this leaves us with a rather confusing mixing and matching when it comes to assessing the situation, but I think our nugget on South Coast weather should be something to hold onto for now. One huge point of contention to nail down is how to handle the shortwave trough (disturbance) digging over Ontario, as the speed and intensity of it will determine how much of a southerly wind blows over New England with moisture Saturday night and Sunday. I have to say that while the GGEM may be drier, it's looking a lot like the NMM with the digging of this shortwave, and should really make one pause before discounting a north and wetter/whiter scenario. Off air thinking from me on this is that we're still looking at a situation that basically is equivalent to flipping a coin when you weigh all the factors. I could come up with just as good of an argument for a miss as for a hit, but based on a) my initial intuition, b) the northward trend of the GFS, and c) the possible nugget we've found with the handling of the Ontario shortwave, if I had to lean, I'd lean toward farther north and wetter Saturday night and Sunday. Publicly, on-air, I've been laying out the uncertainty quite clearly while encouraging Southern New Englanders to pay close attention - not a real difference from what I'm saying here in terms of substance, but perhaps without such a strong leaning as I realize those of you who read this discussion know what I'm getting at when we talk about uncertainty, and the average TV viewer may not have that complete understanding of potential changes having huge impacts.
Either way, this storm is REALLY going to crank up in a hurry as it moves south and east of New England, probably sufficient to be characterized as a bomb, though I suppose it may fall a bit shy of the 24 mb central pressure drop in 24 hours that's needed, though right now I'd bet on it hitting it. In fact, this rapid intensification is part of the reason for a) a cold forecast during the storm Saturday night and Sunday, and b) the tough forecasting of the precipitation. For the former, the ageostrophic flow (wind flowing to the center of the storm) will encourage a cold north and northeast wind, and for the latter, the precipitation shield will blossom quickly as the storm deepens. Either way, the confluent flow to our east will keep the storm rather flat (probably a pretty well defined northern edge to significant precipitation amounts, though light precipitation will extend pretty far north owing to the northern stream disturbance coming through Ontario) and fast moving, at least until it finishes bombing with storm force winds across all of the Canadian Maritimes Sunday into Monday, when it slows significantly. This adds up to lots of cool northerly wind for New England on both of those days, though the real deep well of cold will be a bit too far north and east for me to forecast any type of arctic chill or anything. Next disturbance on tap is warm advection on Tuesday.
Still fun to watch - and still a gamble! Enjoy.
Matt
