Another storm with great potential, but also many things that can go wrong. This time, the models are latching on to those things that can go wrong, and so we are looking at a light snow event and possibly a moderate ice event.
Low pressure originating in Mexico and moving along a suppressed southern jet into the gulf, will ride northeast over the big bend in Florida. Not too much question about the track of the surface low in the case, because a Great Lakes cutter will supply a wide field of southerly flow in the east. The track will depend only on where the southern low turns, and after that it is just a matter of following the wind vectors northward. At this point, it appears the low will track north toward Long Island. This will not be a very strong low, as the primary over the Great Lakes will retain most of its energy.
The weakness of the coastal low will have two affects not good for snow. The first is the inability to counter strong southerly flow from the primary storm. Warm air will flow north into the region, changing the precipitation type to rain early in the ballgame for most of SNE. To the west and north of the coastal low, however, some weak northerly flow will keep the surging warmth mostly at bay.
Light precipitation will start falling past midnight in the area, but with only trace accumulations expected through 4am. Warm air advection will begin pushing into the mid levels, promoting enhanced precipitation over the region. There will be some locally high water equivalents due to the instability characteristic of the precipitation shield. Count on mainly snow, with possibly some sleet mixed in through 7am as 1000 - 850mb thickness levels will stay generally under 3000gpm and surface temperatures will be around 25F. Accumulations will still be on the light side, but could pick up an inch of snow and sleet.
SW NH and NW MA will begin to feel the effects of warm air advection past 7am. Some freezing rain will begin to mix in, and will probably change to all freezing rain by 10am. 1000 - 850mb thickness should stay under 3100gpm, and surface temperatures around 30F, so not expecting a change to rain. The heaviest precipitation will fall between 7am and 12pm, with up to .5" water equivalent possible during this time. However, snow accumulations will have ratios under 10:1, and a change to freezing rain early on will limit substantial accumulations. Expect another inch of snow before a change to freezing rain. As for ice accretion during this time, amounts up to .25" are possible, which will likely warrant a winter weather advisory.
Variables:
Precip type isnt too much of a worry at this point, because confidence is high for a change to freezing rain. I havent mentioned it yet, so Ill mention now that the models have a fairly good consensus for this storm, especially ptype wise. However, some slight shifts in the strength of the coastal versus the primary lows could change snow accumulations by an inch or snow. It should be mentioned that the NWS forecast is somewhat robust in snowfall again and keeping main ptype as snow.
QPF will be the largest issue for wintry accumulations with the storm. As said, some will be instability induced with southerly and easterly flow. The models have not be so consistent in this respect. A major changer in total QPF will be the speed of the storm. This is going to be a fast mover, but with a good burst of WAA induced precip. The low hanging around a little longer could increase totals. At this point in full range is .2 to .8 inches of water equivalent.
First call:
Total snow and sleet accumulation: 1" - 2"
Total ice accretion: .2" to .3"
--- Snowday outlook ---
Close call now even for a delay because the timing has shifted over slightly, with precip not starting until later. If we can get an inch of snow on the ground by 530, or at least have it falling a good clip by then, a delay is very likely. After that it depends on the speed of the storm. At this point it appears the low will take a little longer pulling out, so we could see precipitation continuing through 1pm. In addition, surface temperatures are expected to stay at or below freezing all day long, supporting very icy conditions especially on untreated roads. All this into consideration gives me some confidence in at least the continued potential for a cancellation.
Whatever the verdict is this Friday, it is very important to drive with caution in this type of wintry condition.
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