Last week's prospectus looked at the transition to a wintry pattern that was slated for this week, and the potential for a coastal storm development around the middle of this upcoming week. The reality a week later is that we're ready for a trough that will continue to dig and amplify over the Eastern United States this week, dumping cold air across the Northeast. The intense shortwave energy riding the fast westerlies will continue to drop over New England, and the aforementioned strong storm will indeed develop, but it appears just far enough offshore to preclude major storm development for New England. This doesn't eliminate the chance for accumulating snow in the next seven days, however - rather, I think it will be very difficult for many areas NOT to accumulate snow between now and next Sunday's prospectus.
But let's start with the here and now, which features one installment of cold air to New England, with many more yet to come. Westerly low level winds have instigated lake effect snow squalls, and these will carry east, enhanced by upslope flow against the western slopes of the Green Mountains tonight and Monday. By later Monday, mean layer flow in the lowest several thousand feet of the atmosphere favors carrying some of the snow showers into the Berkshires, as well, though accumulation there should be limited, while the favored locales of the Southern Greens, on the other hand, could pick up a few inches by Monday night.
Also of interest is a pronounced wind shift dropping across the North Country Monday afternoon and evening, and the remainder of New England Monday night. This shift in surface wind represents a cold front, providing a formidable shot of Canadian cold for Tuesday and the midweek. The passage of this front in Northern New England Monday afternoon should bring some snow squalls - and a few of these will probably be intense in the Green Mountains. Lifted index values are positive, but rise about 3 or 4 with the passage of the front, and the pressure rises 3 to 4 mb in the North Country after its passage, while there's plenty of mid-level instability. These parameters appear to favor a weak windex event, though relative humidity parameters in the lowest levels are a bit dry for a decent event. Nonetheless, orographic lift should make up for this in favored locales.
Tricky forecasting on Tuesday, though not for all parameters. For example, temperature and wind seem pretty straight-forward with good cold advection underway. What's trickier, though, is the precipitation forecast. I hate looking at a situation like this, because I realize from a couple of days out that a big bust is possible right up to the event on Tuesday. And the event is this: The major storm we looked at as a potential last week will be coming together southeast of New England. Though that's not a big deal for us in and of itself, there are problems here, because there's a ton of vorticity moving overhead, and how that vorticity interacts, combined with diffluence aloft and a mid-level speed max at 500 mb, as well as the approach of the trough axis at the jet stream level, creates a bulls-eye of upward vertical motion in all of the guidance products. The tricky part is that there's not really a well defined surface feature to cling to here - perhaps a weak Norlun-type troughing that takes place, but it's really driven by a complex pattern aloft, and nailing down where that lift will take place is going to be very difficult. Nailing down just how ripe the conditions will be for precipitation will be equally if not even more tricky, but so important given the temperatures through the column will support snow. The NMM and GFS both do very little with this setup, but the higher resolution WRF and SUNYSB MM5 go to town with accumulating snow for Eastern MA - with the WRF starting in Southern ME then sweeping south down the coast, and the MM5 focusing on the South Shore and Cape Cod, where some ocean enhancement aids development of a snowburst. At this point, I can see good evidence aloft to support such a snow burst, and following the guidance shows Eastern New England will be the closest to this burst, but further detail may remain difficult even right up to the event!
Wednesday looks to be the peak of the cold as the storm really deepens to our east, dragging down a shot of dense, cold air. NMM temp forecasts of max temps in the teens high terrain north to lower or middle 30s south look reasonable and with the wind chill we can knock about another 10 degrees off. Though the cold may relent a bit toward the end of the week, the longwave trough holds tough, and this raises eyebrows by next weekend. The GFS and many of its Ensemble members are rather insistent upon a strong shortwave digging out of Central Canada, dropping south in the fast jet stream flow and amplifying over the Midwest, then shooting east across the Ohio Valley and south of New England. This prompts coastal storm development in an airmass that would be cold enough for a mostly snow event except for areas warmed by a coastal front. The timing would be later Saturday through Saturday night as a moderate to strong nor'easter. Of course, we know the GFS is not the be all and end all of medium range forecasting, though good agreement among the Operational and Ensemble members certainly is of interest and warrants respect. Those of you who've read my posts for a long time know that I always go with what the upper level pattern supports when there is uncertainty, especially in the medium range, and while the precipitation pattern on the GFS Ensembles may say a moderate to strong event, the 500 mb mean prog is not so supportive. In fact, the GFS Ensemble mean 500 mb height fields look very similar to the ECMWF and especially the GGEM and GGEM Ensembles, which indicate some digging of the upstream shortwave, but also a lingering trough axis east of New England. This keeps a faster and less amplified flow from the Ohio Valley through New England, meaning less deepening of the shortwave and resultant surface storm. This is the solution I think should be followed for now, which actually can mean a colder scenario for more of New England, because the chance of a coastal front is much less with what essentially becomes a clipper going near or just south of the South Coast of New England, dropping a swath of accumulating snow on its northern side, which could drop anywhere between two and six inches on the way by. Of course, at nearly a week out and with a changing and fast flow pattern, there's lots of room for change here, but I would favor the faster and less amplified solution for now.
That disturbance does reinforce the upper level trough over the Northeast, which means the cold continues coming on strong. The Friday after Thanksgiving brings another strong shortwave worth watching, then by the first week of December, another strong shortwave coming off the Pacific will try to dig over the Plains and Midwest, and may bring another scenario that can be watched for coastal development.
Please do offer your thoughts in the comments section.
See you back here throughout the week.
Matt
