Overnight Saturday into Sunday Snow Potential (3/18-3/19/17)

Can a weather forecast ever be easy?  Unfortunately in this field this answer to this question is more than likely not.  What makes weather forecasting rather unique (along with fun and stressful at the same time) is it is an imperfect science.  This is something that the general public doesn't understand I think and why they question how forecasts can be so widely ranged between forecasters and why there is always uncertainty.  Anyways, with this little rant out of the way we are faced with quite another complex situation tomorrow.  Unfortunately, due to time constraints I don't have ample enough time to really get into great depth of detail to fully explain the situation.

An amplifying "strengthening" trough will be digging into the northeastern United States during the day tomorrow and in response an area of surface low pressure will form somewhere south of Long Island (where exactly is still in question):

The biggest question at this stage is where exactly does the surface low develop?  There are substantial differences between the NAM computer forecast model and the GFS forecast model as shown below:

Both these images are valid for 2:00 AM Sunday morning from this morning's (12z) runs of each model.  You can certainly see there are substantial differences here.  The NAM barely grazes the southern coast of CT while the GFS brings what would be moderate snows all the way to the CT/MA border.  For what it's worth the European model (which I can't show) is very south and gives CT virtually nothing with the exception of some flurries.

One thing that has to be watched is the potential for what is called a norlun trough.  Typically a trough is oriented from a northerly position to a southerly position (this includes northwest to southeast, northeast to southwest).  A norlun, however, is a trough which orients from the southeast to the northwest with the main surface low to the south (a rather vague description here but again on a time constraint).  If this feature does setup it could act to draw in stronger moisture to part of the state along with lift which could yield the potential for some higher totals.

For now this is my forecast but limited time to view data and such so confidence not all that high  Hopefully I'll have an opportunity to better look at things later on:

Tuesday, March 14th, 2017 Blizzard Update

A rather impressive mid-March blizzard is set to strike the state of CT tomorrow.  While my forecasts have certainly been much lower with respect to everyone else (all my reasoning why is in my previous blog posts) there are several issues which I am worried about which is why I am much more conservative.  I've done enough scientific talk in the previous posts so I will refrain from that here and this post will just be a forecast.  However, I will briefly explain some of my concerns once again.
As we close on the event there actually continues to be some increasing spread within the computer forecast models which is rather unusual.  Typically they begin to converge.  There continues to be questions with exactly where the surface low pressure tracks and where the 850mb and 700mb lows track and this will be pivotal in determining where the heaviest banding occurs, where the highest snowfall totals occur, and if anyone sees any mixing.  I am also worried with dry air descending through the troposphere into the snowgrowth zone which would inhibit better snowflake production.  Very strong winds throughout the troposphere could also tear snowflakes apart as they descend towards the surface yielding smaller-sized flakes and poor ratios.  The speed is the system and how brief the strongest lift remains overhead is also a concern.  For the forecast:

• Snow begins to break out between 3 AM and 5 AM tomorrow morning.  
• The heaviest of the snow falls between the hours of 9:00 AM and 1:00 PM.  
• The likelihood at this time appears for the heaviest banding to perhaps hit extreme western CT during these hours.  Within these hours snowfall rates could approach 3'' per hour, however, could even briefly approach 4'' per hour.  The key for getting upwards or exceeding 18'' of snow would be for these rates to persist for a good 3-4 hours.  
• Winds will also be quite strong as well throughout the state with winds sustained between 25-35 mph with gusts upwards of 60 mph.  This will not only lead to blowing and drifting snow but lead to isolated pockets of tree damage and power outages.
• The combination of heavy snow and strong winds will yield blizzard conditions at times as well.
• Given degree of lift, thunder can't be ruled out. 
• There is a concern for mixing of sleet or even changing to rain across the southeastern part of the state (in fact some computer forecast models hint that mixing may even occur into north-central CT) so totals may be held back here. 
• After the heaviest snow moves out we will continue to see light to periods of moderate snow through the remainder of the afternoon (mix/rain across southeastern CT)
• Coastal flooding is also possible across southwestern CT along Long Island Sound.  

Tuesday, March 14th, 2017 Storm Discussion/Forecast

Computer forecast models continue to be in rather strong agreement that a coastal storm will provide major to significant impacts (in terms of accumulating snowfall and strong winds) across CT throughout the day on Tuesday.  While the agreement is strong on the impacts, there are still some uncertainties with regards to some of the details but they are becoming a bit more clearer as we close in on Tuesday.

One thing that is a bit uncertain at this time is the exact track this system will take and this holds true for the surface low as well as low pressure centers aloft as well.  If the surface low tracks further west and passes just southeast of CT this could allow for some milder air to work into portions of the state introducing the potential for mixed precipitation (sleet, freezing rain, rain).  A track just southeast of this position allows for colder air to remain in place and the likelihood for any mixing is extremely low.  As for the low pressure centers aloft, some guidance has suggested that the lows at 850mb and 700mb could track over or even just west of CT.  This track for the 850mb low would bring in warmer air up around 5,000' and increase the likelihood for mixed precipitation across portions of the state and this track for the 700mb low would actually allow for some drier air to work in around 10,000' and reduce snowfall intensity and rates.  If you're looking for lots of snow from this event you want all these lows to track just to the southeast of CT.

With this system becoming so strong there will be rather exceptional upward vertical motion which develops on the northwestern side of the low.  This will really work to enhance the development of snow and allow for a period of extremely heavy snowfall in which snowfall rates could approach 2-3'' per hour and even some 4'' per hour rates can't be ruled out.  The black and gray shadings of 800-600mb averaged frontogenesis suggest exceptional lift will push over a portion of the state on Tuesday:

Looking at bufkit profiles from this morning's run of the GFS computer forecast model for Waterbury, CT and Windsor Locks, CT we can also see the rather intense upward vertical motion as the GFS indicates as much as -40 to -50 units of omega within the dendritic snow growth zone!  This is very impressive and hints at the potential for those snowfall rates to approach 3'' per hour where the heaviest banding sets up:

The cards are certainly right on the table for a rather major/significant storm but the question is just how significant?  Well there are some factors we will be dealing with which could prevent this from being a situation where we are looking at 12-18'+ statewide.  One factor will be the speed of the storm.  It's not exactly a quick hitting storm (will last a solid 9-12 hours or so) but its not a very slow moving system and that will prevent the likelihood for any extreme snowfall totals (say several locations picking up 24"+ of snow).  If this was a longer duration storm the cards would be on the table for a swatch of 20-30'' of snow.

There is also some concern of some drier air which may sink into the middle and even lower levels of the atmosphere as the storm progresses and this shows up well on some forecast soundings.  For example, we will look at a sounding from the NAM computer forecast model for Waterbury, CT for 2:00 PM Tuesday afternoon.  Those three blue circles on the bottom left indicate that moderate to heavy snow will be falling.  However, if we look at the profile of the atmosphere we see a quite a bit of dry air (the further spaced apart the red (temperature line) line and green (dewpoint line) the drier the air is at that level):

What would this mean?  If the degree of dry air does indeed verify this could impact the snowfall rates in a way where the intensity is on the lighter side and the snow growth may not be all that stellar (smaller-sized flakes).  This will have to be watched to see if 1) We do indeed see drier air filter in and 2) If so, how quickly does that happen as this could really affect snowfall totals.

Another potential issue is subsidence (sinking motion which reduces intensity of precipitation or prevents it from reaching the ground) in and around the 850mb level (~5,000').  While there is no question with regards to the strength of upward vertical motion at 700mb (~10,000') this subsidence in the lower levels would likely reduce the intensity of the snowfall along with the snowfall rates.  Given the nature of this storm and how intense it will be there is guaranteed to be a zone of subsidence that sets up somewhere, it's just a question of where and that is not something that is very easy to forecast days out:

Winds with this system are expected to be rather strong as well and this is not good to keep big fluffy snowflakes intact as they fall towards the surface.  With winds both aloft and at the surface expected to be very strong, this could lead to snowflakes being ripped apart as they descend towards the ground.  This would yield smaller flakes and ones that are tougher to accumulate quickly as opposed to those bigger fluffier snowflakes.  These are a few things to take into account when making this forecast and to closely monitor over the next few days.

What can we expect?

• Snow begins to break out between 3 AM and 5 AM Tuesday morning and begin to quickly pickup in intensity.  
• The heaviest of the snowfall occurs between 9:00 AM and 1:00 PM.  This is also the time when the heaviest banding will setup somewhere within the state and this is where snowfall rates could approach 3'' per hour and perhaps even upwards of 4'' per hour.
• Given the timing of this system travel is not advised Tuesday morning.  Unlike with what happened Friday, the ground this time is much colder and we are looking at heavier snowfall rates and longer duration so the snow will stick to the roads.  
• Winds will become an issue as well as winds will be sustained between 20-30 mph with gusts upwards of 50-60 mph, especially down by the coast.  This will cause pockets of power outages and downed trees
• The storm begins ti push out during the early afternoon hours with some light snows sticking around into early evening.  
• Don't be shocked either if you hear some thunder given the degree of lift associated with the system

Before we get to the map just want to point something out with regards to that zone of 12-20'' totals.  Yes that is a rather big range, however, as the details with the banding become much more clear that range can be narrowed down.  Also this axis could shift as well depending on where the banding setsup and that is very, very difficult to pin down at this stage.  Based on some of the questions mentioned above, I think a widespread 8-12'' can be expected across the state, however, totals will be much higher in that area of banding.  There will also be an area of perhaps less than 8'' where the subsidence zone setusup:

Tracking Tuesday's Storm Potential

Computer forecast models have been in rather strong agreement on a potential major storm system to impact southern New England this upcoming Tuesday.  The agreement between the operational models and their ensembles have been rather impressive and this leads to a higher than average confidence in at least some sort of impact on Tuesday.  However, there are still numerous questions with regards to the details which is expected for this time frame.  Over the next few days as the details become more clear we will have a much better idea on how significant of an impact we are looking at.

What is the driver for this storm?  We are tracking two pieces of shortwave energy which computer forecast models are in strong agreement that these piece of energy will phase and the result will be explosive cyclogenesis just off the east coast:

The explosiveness of the cyclogenesis will be aided by an intensifying 500mb level jet streak of 100+ knots which develops and rounds the base of the upper trough.  This will allow for further amplification (strengthening) of the trough and enhance the rapid cyclogenesis of the surface low pressure:

Looking at each GFS computer forecast model ensemble from the 12z run of March 10th, we can see very strong consistency between each member for not only an impact from this system but the potential for rather significant impacts:

Over the next few days we will be watching several key features very closely and how the computer forecast models handle these features.  One major question as of right now is with regards to the exact track of the storm as this will vastly determine the significance of the impacts.  A storm track closer to the coast will increase the likelihood if ushering in warmer air into parts of the region which could result in reduced snowfall accumulations and risk for mixing and/or rain.  Of course there is still a shot this system goes completely out to sea, however, at this stage I think that sort of scenario is a bit unlikely.  One thing to watch for when determining how close to the coast a system will track is the behavior of the North American Oscillation over the next few days.  When the North American Oscillation transitions from a positive state (characterized by below-average 500mb height anomalies in and around Greenland) to a negative state (characterized by above-average height anomalies in and around Greenland) this yields what is called a "block" and this makes it rather difficult for a storm coming up the coast to go out to sea.  When the North Atlantic Oscillation transitions from a negative state to a positive state as a system is working up the coast this increases the likelihood the storm could scoot further off to the north and east.  Currently we have a North Atlantic Oscillation which is slightly positive and is expected to become more positive:

What this suggests is unless some other processes (such as a vast negative tilt to the system) occur this could mean a higher likelihood for a track a track which is more westerly as opposed to easterly.  This could mean mixing issues for a portion of southern New England.  

Outside of the track of the surface low, the tracks and development of the 925mb, 850mb, 700mb, and 500mb lows will be rather critical as well. Taking a look at the projected track of the 850mb low (as forecasted right now) the track is right over CT.  This sort of track would allow for warm air advection into a portion of southern New England which would lead to mixing issues:

The projected track of the 700mb low also raises some red flags.  The development and track is pretty much west and then over southern New England which is a huge signal for dry slotting:

These are some of the features we will be watching over the next few days but Tuesday certainly has potential to be quite the potent storm system which as potential to drop major snowfall accumulations.  Further updates will be posted as the situation becomes much more clear

Friday Snow Update

Not much changes from my previous forecast with the exception of decreasing snowfall totals a bit across the northern third of the state and increasing totals across the southeastern part of the state.  The highest totals should be confined to the southern third of the state as computer forecast models are converging on the idea that the strongest upward vertical motions and subsequent area of heaviest snows will occur here.  This is shown by looking at projected 700mb vertical velocity values from both the latest runs of the NAM and GFS computer forecast models:

Looking at a bufkit profile over Waterbury, CT from the latest GFS and NAM runs also shows the intense lift which will be across the southern part of the state:

Looking at bufkit profile over Windsor Locks, CT we see some vast differences in the profile and these differences strongly indicate the northern part of the state will see much less in the way of snow.  Both the NAM and GFS advertise  a brief period of stronger upward vertical motion which suggest the window here for more moderate snow is very limited:

  What can we expect tomorrow:

• Light snows begin to break out between 12:00 AM and 2:00 AM
• The most likelihood period for moderate to heavy snows (especially southern CT) will be between 4:00 AM and 10:00 AM tomorrow morning.  During this time snow rates could approach 1.5''/hr.  Perhaps upwards of 2''/hr for a brief time in the heaviest of bands.  
• Expect numerous delays on the roads tomorrow morning given the timing.  Travel is not advised tomorrow morning unless absolutely necessary.  
• The snow will begin winding down between 11:00 AM and noon with some light snow showers around for a few hours after that.
• Once the system passes a strong cold front pushes through and we turn windier and much colder.  
• Winds could gust upwards of 35-45 mph which could cause isolated pockets of power outages. 

Below is my updated map:

Friday, March 10th, 2017 Snow Event

Our stint of mild weather will be short-lived as winter weather is set to return to the state, both in terms of potential snowfall and very cold temperatures.  Overnight tomorrow into Friday morning a rather potent piece of shortwave energy and associated trough will dig just to the south of CT.  In response an area of surface low pressure will develop and strengthen with the aid of a rather impressive 120+ knot mid-level jet streak rounding the base of the trough.  There are some questions, however, as to whether or not the trough (as it digs south of CT) will capture the developing area of surface low pressure and allow it to strengthen rather quickly.  This sort of scenario would increase snowfall total potential across the state:

At this time we are still faced with several questions which makes this forecast rather difficult.  It isn't exactly clear how this system will evolve as there is a solid difference (for this time range) in spread between the computer forecast models.  Computer forecast models have been wobbling back and forth with regards to exactly how far north the system would track and they have been inconsistent as well with regards to whether or not the amplifying trough will capture the surface low in time to allow for heavy snow and upwards of several inches of snowfall accumulation across the state and there have been some questions with regards to whether or not there will be sufficient inflow into the system to generate across moisture for the system to work with.  All of these factors will determine how strong of a system this is and exactly how much snow will fall.

At this time, however, there are some signals that we could be looking at a greater potential for more in the way of a stronger system as opposed to a weaker system.  The first signal is what was discussed above with regards to whether or not the trough would capture the surface low.  Given how models do agree on amplification on the trough as it approaches our area it is a bit difficult to believe it would not capture the surface low and allow for an extended period of at least moderate snows across the state:

Another signal for at least a narrow zone of rather heavy snowfall is a very impressive baroclinic zone (tight temperature gradient = rapid change of temperature over a short distance) in the lowest 10,000' of the atmosphere.  What this will do is help to generate a zone of very intense upward vertical motion which would indicate a narrow zone of heavy snowfall.  It is within this zone the highest totals would occur:

Taking a look at bufkit profile for Waterbury, CT from the latest runs of the NAM and GFS computer forecast models, we see both models showing rather decent snow growth with the dendritic snowgrowtyh zone up around 12,000' to 15,000' along with the -15C isotherm bisecting this zone and a good deal of upward vertical motion (indicated by the red contours)...all of these are ingredients you want for "good" snow growth within the clouds...this is what gives those big fluffy flakes which accumulate.  The degree of upward vertical motion as well indicates the potential for at least a period of moderate to perhaps even heavy snows:

What can we expect?

The timing of the snow should be during the overnight hours of Thursday into Friday lasting until mid-morning or so, later if what was mentioned above with regards to the trough capturing the surface low.  Snow should begin to break out as early as 12:00 AM or so.  The heaviest of the snows should occur between about 1:00 AM and 6:00 AM but this is low confidence given what was described above and may last longer.   Surface temperatures will be rather marginal, especially across southeastern CT, however, temperatures aloft will be cold enough to support generally all snow, especially considering the timing is at night.  SE CT could see a mix which could hold totals down a bit there.  Once the snow does clear out a strong cold front passes through and we turn windy and much colder for the rest of the weekend.  Below is what I am expecting right now: