Light to Moderate Snow Across CT Tuesday Afternoon into Evening

Embedded pieces of shortwave energy in a fast jet stream flow aloft are set to dive south of CT during the afternoon and evening hours Tuesday.  In response to this energy an area of surface low pressure will develop and will produce a several hour period of light to moderate snows:

  While we aren't expecting a great deal of snow, the timing of the snowfall (afternoon through the evening hours) will yield a rather slow afternoon rush hour commute which will lead to numerous delays.  Extra care should be taken if out traveling.  There are some differences, however, in our computer forecast models on some features of the system and with that we see some differences as to the extent of the water liquid equivalent which will fall.  While the NAM, GFS, and European computer forecast models all agree that a weak area of surface low pressure will develop and pass just to the south of CT, there is disagreement with a potential feature called an inverted trough.  Typically, in the northern hemisphere, troughs are characterized by a southerly "bulge" or dip in the jet stream or height contours.  An inverted trough is characterized by a northerly bulge in the jet stream or height contours associated with a low pressure system or when height contours have a westward kink in them.  The NAM computer forecast model is a bit more more prominent than the GFS computer forecast model and much more prominent than the Euro computer forecast model and thus indicates a higher likelihood for a longer duration of moderate snowfall:

 With this system being on the weaker side we will not see a fully mature system develop and with this the degree of mid-level lift will not be all that intense.  The majority of the lift will be due to low-level processes with the main contributor coming from surface and low-level convergence.  One factor that will preclude the potential for heavy snowfall and higher accumulations will be the lack of solid snow growth.  Taking a look at the 12z NAM and GFS computer forecast model bufkit profiles for over Waterbury, CT we can see that the dendritic snowgrowth zone is not between the optimal height levels of 12,000' and 18,000'.  In fact, the zone is between about 8,000' and 13,000'.  While the NAM computer forecast model shows some pretty decent upward vertical motion (suggestive of good lift) and this vertical motion intersecting the dendritic snowgrowth zone (the GFS isn't as impressive with the degree of upward vertical motion) along with the -15C temperature contour dissecting this zone, the fact that the dendritic snowgrowth zone is so shallow will preclude the giant snowflakes which accumulate quickly from developing in the upper atmosphere.  This means we will see snowflakes which are mainly on the smaller side.  Snowgrowth is optimized when the dendritic snowgrowth zone is positioned between 12,000' and 18,000' and strong upward vertical motion is present within this zone with the -15C isotherm dissecting through this zone:

What can we expect tomorrow?  Light snows should begin to breakout sometime during the early afternoon hours with intensity beginning to vary between light and moderate through the rest of the afternoon hours into mid-evening.  While snowfall rates should not be all that heavy (at most 0.5'/hr) the timing of the snows will still yield to a slow afternoon and rush hour commute leading to delays.  Snowfall amounts should generally range between 1-2'' state wide (perhaps some mixing with sleet or rain along the immediate shoreline if the system tracks a little further north) if the inverted trough feature becomes as established as the NAM suggests and/or the system tracks further northward, some areas could pick up as much as 3-4'' of snow.  This will have to be watched closely over the next 24-hours.  Below is my snowfall map:

Nor'easter to Impact Connecticut Tonight into Tomorrow

Quite the complex forecast for later this evening through the overnight hours as a potent nor'easter will bring some heavy rains, including some snow/sleet and perhaps freezing rain for interior CT and especially the extreme western portion of the state and NW hills.  In addition to rain and wintry precipitation, there will be a period of strong to perhaps even damaging wind gusts, especially along the CT coast along with some coastal flooding as well.

As of 7:00 AM EST this morning, a potent piece of shortwave energy was located down across the southeastern United States and was beginning to work northeastward along the eastern seaboard.  This piece of shortwave is quite potent and that is indicated by the closed-off nature of the low (indicated by the highlighted circles (height level of the 500mb level):

Leading up towards today, computer forecast models somewhat varied as to what exactly would happen with this piece of energy as it moved up the east coast towards southern New England.  A few computer forecast models actually began to weaken the energy allowing it to not be closed off anymore (called an open wave).  Over the past day or so, computer forecast models have actually come into better agreement that this will not actually happen and the energy will remain closed off (albeit begin to slowly weaken) as it passes just to the south and east of southern New England.  This will in fact work to keep the system a bit stronger for a longer duration and enhance the likelihood for heavier precipitation to build into CT along with potential for stronger winds as well.

One of the biggest concerns and questions with regards to this system will be the temperature of the thermal profile throughout the atmospheric column.  Even as off midday Monday, there are still some disagreements within the computer forecast models as to the strength of the warm air advection which will occur.  This issue, however, is likely more of a concern for areas north and west of I-84.  This also includes much of interior Farifled county, all of Litchfield county, and Hartford county for points north and west of Hartford.  Too illustrate, let's take a look at a bufkit profile for Windsor Locks, CT from this mornings runs of the NAM and GFS computer forecast models.  The NAM bufkit is plotted for midnight tonight while the GFS is plotted for 11:00 PM.  Notice how there are several MAJOR differences with the thermal profile throughout the atmospheric column:

This right here could be the difference between say about 2-3'' of snow for parts of northern CT and the difference between very little to no accumulations and mostly rain.  When looking at the two models and the differences, however, we can try and use some meteorological knowledge to decipher which model may be hedging towards the right direction.  For one thing, it is not very common to see the NAM forecast model actually being not only warmer but a solid deal warmer than the GFS computer forecast model but that is the case here.  There are some signals, however, which may suggest that the GFS computer model may sniff this one out and we will explore that below.

With the system still relatively strong at 500mb the response will be for the strengthening of the lows at 925mb, 850mb, and 700mb as the system works just south of CT.  Looking at the 850mb level we see the low beginning to close off east of NJ later this evening.  As long as this strengthening occurs and continues to occur dynamic cooling will be present within the middle and lower levels of the atmosphere and this will help to keep temperatures cooler and support more in the way of wintry precipitation, especially initially for inland and western CT.  However, with the track of the low expected to be close to the CT coast this will eventually usher in milder air and a transition to rain will occur.  It is certainly possible, however, that extreme NW CT could remain mostly all snow for the duration:

Associated with this system will be some very intense lift and this is evident by looking at 700mb vertical velocity values.  The shadings of yellows and reds indicate some very strong lift will move over the state during the late evening and early overnight hours and this will coincide with the heaviest of the precipitation:

This degree of lift is something we need to watch as well for the northwestern part of the state where there is more of a likelihood for accumulating snow and perhaps several inches.  A bufkit sounding from this mornings run of the GFS forecast model for a station in the NW corner of MA (right along MA border) shows excellent snowgrowth (you want the purple/yellow contours to be between 12,000' and 18,000') and decent upward vertical motion (red contours) intersecting this zone which indicates sufficient lift into this zone to produce dendrites (the type of snowflakes that accumulate the most quickly).  The issue of course will be whether the thermal profile remains cold enough to support all snow here and if so several inches of snow across the NW corner is definitely plausible:

As we head into the evening hours we have seen some light pockets of snow/sleet mix work into the state and this will be the theme over the next 3-4 hours before steadier and more heavy precipitation begins to work into the state.  Expect the heaviest of the precipitation to occur anywhere between 10:00 PM tonight and 4:00 AM tomorrow morning.  In addition, winds will continue to remain gust through the rest of the evening and overnight hours with winds of 25-35 mph and gusts upwards of 45-50 mph inland and as high as 55-60 mph along the immediate coastline.  This could yield pockets of isolated power outages.  The system departs off to our northeast tomorrow, however, we will be left with residual clouds and spotty showers.  Below is what I am thinking in terms of snow/sleet accumulation: