Isolated Strong to Severe T'storms Possible Tomorrow

Looks like March will be coming in like a lion as a rather anomalous (for the time of year) meteorological setup will be in place.  A secondary warm front will be pushing into and through parts of southern New England overnight tonight into tomorrow morning.  Once the warm front passes, areas to the south of the warm will see temperatures climb well into the upper 50's and perhaps even lower 60's with dewpoints perhaps near 60F as well.  There are some uncertainties, however, with regards to how high temperatures away from the coast get tomorrow.  We will have numerous clouds to contend with which will help to keep temperatures back, however, the exact direction of the surface wind will play a crucial role as well.  If surface winds end up more out of the southwest, this could allow for surface temperatures to perhaps spike into the mid-60's while a more southerly component to the winds will keep temperatures closer to 60F.  This will be one of the key's when dealing with the thunderstorm potential late tomorrow afternoon and evening.

It is very, very unusual to ever talk about the possibility for strong to severe thunderstorms in February or even March (we dealt with some the other day) but that potential is looking like a possibility tomorrow afternoon.  An area of low pressure passing to our north and west will work to swing the above mentioned warm front through the region.  The system will also swing a cold front through the region overnight tomorrow and early Thursday morning.  Out ahead of this cold front the atmosphere is expected to become modestly unstable thanks to temperatures and dewpoints near 60F.  There will be one other player which will enhance the instability of the atmosphere and that will be the presence of rather steep mid-level lapse rates associated with an elevated-mixed layer plume.  For more info on an elevated-mixed layer check out the following link:

http://weatherwiz.blogspot.com/2015/06/what-is-elevated-mixed-layer.html

Computer forecast models are indicating that we will see a plume of 7 C/KM to 7.5 C/KM 700-500mb lapse rates push into this area (this indicates a temperature decrease of 7-7.5C per km between the 700-500mb level which is a pretty impressive decrease) and this will really help to destabilize the atmosphere:

The combination of near 60F temperature, dewpoints, and the steep mid-level lapse rates will yield rather modest instability for this time of year.  Computer forecast models are indicating perhaps as much as 500-1000 J/KG of mixed-layer cape (a measure of a potential storm's updraft).  Typically this values isn't high, however, we will have very strong wind shear in place (along with a few other factors) which can help to compensate for the lack of higher cape and enhance updraft strength:

Winds aloft are expected to be rather strong as well with 500mb winds in excess of 80-90 knots and 700mb winds in excess of 70-80 knots.  These values will help to contribute to bulk shear values in excess of 70 knots!  This is extremely impressive:

The combination of instability and wind shear aloft will help set the stage for the development of some showers and t'storms during the afternoon and evening hours as some shortwave energy moves overhead.  Given the mentioned parameters, some of these thunderstorms will have the potential to become strong to locally severe and have the potential to produce strong to damaging wind gusts, some hail, and an isolated tornado can't even be ruled out.

Looking at the 12z NAM computer forecast model bufkit profile for Waterbury, CT shows a profile which is certainly favorable for strong to severe t'storms so long we get a forcing/lifting mechanism for storms to develop:

The NAM is projecting between 200 and 300 J/KG of 0-6km cape.  Again while this value isn't overly impressive, when combined with the strong shear this could be compensated for.  One flag to watch for, however, is given how strong the shear is, we would have to develop enough low-level buoyancy for any updrafts which develop to be able to sustain the strong winds aloft.  Higher cape values help to lead to stronger updrafts so we will need some cape.  If there is not enough cape, any updrafts would likely be toppled over and not grow tall enough to utilize the strong winds aloft and the threat for strong to severe t'storms would be vastly reduced.  As for as isolated tornado potential...looking at the hodograph (circled) we see a curved look to it...this indicates very strong speed shear aloft (increase of winds with height).  However, notice how this hodograph just begins to curve right away...this means there is little directional shear (change of wind direction with height) and that is critical for the possibility of tornado formation.  If the hodograph sort of went out in a straight line and then curved this would indicate more of a change of wind direction with height.
On the bottom left we see helicity values well over 400 m2s2 which is VERY impressive.  Typically when talking about the possibility for a tornado you want to see values approach 150 m2s2 so we're way higher than that.  Due to the lack of directional shear this could limit the potential for an isolated tornado, however, this could enhance the threat for damaging straight line winds and even large hail.  Given the degree of the shear/helicity, however, an isolated tornado certainly can't be ruled out.

This is a very delicate forecast (as typically is when dealing with severe weather here in New England) and very unusual for this time of year which raises even more questions and uncertainties.  The situation will have to be evaluated through the day tomorrow as many of our questions will be answered on the extent of the threat depending on whether we see any morning showers/t'storms, how much cloud cover, and surface winds.

Wednesday, March 1st, 2017 low-topped thunderstorm potential

We have seen some very interesting weather over the course of the past month.  Near the beginning of the month we saw our biggest snowfall event of the winter, last week and the start of this weekend we saw record warmth with temperatures well into the 60's and even low 70's.  This past Saturday we had a low-topped line of thunderstorms approach ahead of a strong cold front which event prompted an EF-1 tornado in Conway, MA.  As we move through this week we are once again looking at the threat for thunderstorms on Wednesday and then back to winter on Friday as we may deal with snow squalls.  

The setup we are looking at for Wednesday is quite impressive for March 1st and one of the driving factors for this potential will be the advection of very steep mid-level lapse rates (strong decrease of temperature with height) associated with an elevated mixed-layer plume.  The question, however, is will the steepest mid-level lapse rates end up staying south of southern New England or will they advect in:

The importance of the steep mid-level lapse rates will be on the degree of instability which develops.  With a warm front expected to push at least into CT early Wednesday morning, south of this warm front temperatures should warm into the 60's with dewpoints into the mid 50's and possibly near 60F.  Much of this too will depend on the surface winds.  If surface winds remain more out of the southwest, surface temperatures are more likely to climb well into the 60's with dewpoints near 60F.  If winds remain more southerly or southeasterly, this will likely prevent temperatures and dewpoints from climbing that high.  With the possibility of temperatures into the 60's and dewpoints near 60F combined with the steep mid-level lapse rates, this could potentially yield several hundred J/KG of cape (measure of instability):

Normally these values aren't very impressive, however, we will have very strong lift and wind shear in place as well.  What cape does is measure an updrafts strength...so the more cape you have the stronger a storm's updraft can be if the cape is tapped into.  If you have strong wind shear and lift in place this can compensate for the lack of cape as strong lift and shear can also enhance a storm's updraft and this is the potential situation we would see more Wednesday.  

For the shear we are looking at a potentially 90+ knots mid-level jet at 500mb and 50-60+ knots at the 850mb level.  These values will help to enhance bulk shear and vertical shear values which will enhance updraft potential:

What can we expect?  These setups are always very, very tricky and getting the potential for any type of strong to severe thunderstorms in our area this time of year is nearly unheard of.  However, given the projected parameters and setup we will have to watch for a line of thunderstorms which will develop ahead of a cold front to our west.  If the above mentioned parameters pan out we will have to watch this line as it approaches southern New England.  Given degree of wind shear aloft strong to damaging wind gusts would be the main threat.  Also, given how winds do sort of turn with height we will have to watch for embedded areas of rotation as well.  

After this system passes we will begin a transition back towards winter and on Friday we have to watch for the possibility of heavy snow squalls with perhaps some thunder and lightning as well but we'll save this for a later post.  

Wednesday/Thursday Storm Potential

We have certainly entered an extremely active weather pattern and this looks to continue through at least the upcoming work week as we are watching yet again for the possibility of another storm.  I will say, however, I think the chances for anything major or significant or fairly low but nonetheless we will continue to watch.

What we are watching are two pieces of shortwave energy at the 500mb level.  One piece of energy is associated with the northern branch of the jet stream and is modeled to be quite potent and the second piece of shortwave energy is associated with the southern branch of the jet stream.  The million dollar question is whether or not there will be any interaction between these two pieces of energy (phasing) and if so, when does it happen and what effects are we looking at?

As of now I'm not completely sold we will see any significant impacts from this system and alot would have to change for that to happen.  In the posted image below I have highlighted the two areas of energy which will be our focus.  In the northern branch of the jet stream we have a very potent piece of shortwave energy with an associated closed-off 500mb low.  These two combinations are not good for us here in southern New England if you're looking for big snows.  Also, because of the strength of the shortwave energy this would make it very difficult for it to phase with the southern stream energy.  This would be more likely to happen if the southern stream energy speeds up and gets out ahead of the northern branch energy.  As it stand now this does not appear to happen and the northern energy virtually "kicks" this southern stream energy right out over the ocean.  Some of the latest data, however, did speed up this southern energy a bit and, therefore, so more of an interaction and potential for something:

Taking a look at the GFS ensembles we can see why we can't completely rule out some sort of impact.  While most of the members don't have much of anything, several do suggest the possibility of at least a moderate type of impact so while I don't think we will see anything major materialize I'm not ready to say no right now:

Messy Winter Storm to Impact CT 2/12/2017

Typically when writing up blog posts I love to get really scientific with them and I try to do my best so anyone who reads them can sort of try and understand the process of making a forecast and see just how complex and challenging forecasting can be.  I understand that most people don't understand the scientific background but my hopes is always to give the reader an understanding of why forecasting can be challenging.  Unfortunately, due to time constraints I don't have the time to really get into the scientific background here which is a shame because this is a very complex forecasting event.  With that said I will briefly describe what went into making my forecast.

I see I am on the lower side of some forecasts, however, I have reasoning.  One reason is I think this system really strengthens too late for us here in CT to really cash in on heavier snowfalls.  Computer forecast guidance suggests that the surface low, 925mb low, 850mb low, 700mb low, and 500mb lows all don't really strengthen and close off until the system is east of CT.  That's a red flag for big snows here.  Also, looking at some bufkit profiles across Windsor Locks, CT and Waterbury, CT while we will have excellent snowgrowth, snow rations will be extremely poor (maybe 8:1) given warmer surface temperatures.  There also isn't a tremendous amount of lift present within the snowgrowth zone which means poor flake size and that coupled with poor ratios will make big accumulations tough.  I will illustrate by just using the 12z NAM computer forecast model bufkit profile for Windsor Locks, CT:

Given track and late timing of strongest development we will also have some temperature issues across the state with temperatures warming enough along the southern third of the state to promote a mixture of sleet and even rain.  We could even see sleet perhaps through the central portion of the state with the extreme northern third of the state remaining cold enough for snow.

Timing...snow should begin early to mid-morning and then increase rapidly through the afternoon.  Snow will come down moderate to heavy at times with rates nearing 1'' per hour rates.  Expect a very slow travel commute tomorrow and travel is not recommended unless absolutely necessary.  The system winds down late evening with leftover snow showers.  Delays possible Monday morning.  Below is my map:

Light Snows Friday night into Saturday

Active times continue through the weekend as we are expecting yet another period of some snow.  However, this event will be nothing like we just dealt with and we will just see a several hour window of some light to perhaps moderate snow as some shortwave energy dives southeast out of Canada overnight tomorrow into Friday.  While there isn't a great deal of moisture available with this system or much in the way of lift, extremely cold temperatures aloft will yield very high snowfall ratios; potentially as high as 15:1 to 20:1.  This will make for very large and fluffy snowflakes.

All in all this will be a relatively weak system and despite the lack of moisture, the system will be associated with just enough lift to allow for some light to even moderate bursts of snow at times.  The timeframe for any snow will be between 1 AM and 7 AM Saturday.  Snowfall totals should amount to 1-3'', however, most locations may see around 1'' as opposed to 3'' as the 3'' amounts will be rather isolated and it's virtually impossible to pindown where these totals will be.

Snow Forecast on Track

Our first significant winter storm of the season is upon us and already several inches of snow has accumulated across much of the state and we still have several hours of heavy snow to go through as the system continues to rapidly strengthen.  Given the latest trends and how things are unfolding this morning I have expanded my 14-18'' zone and have also included some text stating that locally some spots could pick up as much as 18-24'' of snow:

Mesoanalysis data shows a prime track of the system for heavy banding to setup across much of CT.  The tracks of the surface low, 925mb low, and 850mb low will be just to the southeast of CT which will allow the strongest low-level convergence and upper level divergence to be right over CT.  Also, the lows will be strengthening as they pass to the southeast of CT:

With the track and development of the lows along with a very tight temperature gradient (this isn't shown) this is setting up some exceptional lift which will transpire over CT.  As this lift continues to increase we will see snowfall rates approach 2-3'' per hour and perhaps even as much as 4'' per hour, especially in any areas of thundersnow:

Heavy snow will continue until 1:00 PM-2:00 PM beore beginning to taper off.  Strong winds will continue as well, perhaps gusting to near 50 mph which could lead to isolated power outages.

Snowfall Forecast Update.

The consensus continues to strengthen for a significant winter storm to impact the state of CT beginning very late overnight and lasting through the first part of tomorrow.  There is no doubt this will be the biggest snow event of the 2016-2017 season thus far.  As we zero in on the event, our computer forecast models are beginning to zero in on where perhaps the axis of heaviest snowfall will occur and within this axis we could be looking at some very hefty snowfall totals, even despite the rather fast speed of the system.

We are looking at a several hour period of explosive lift tomorrow and indications are that some of the most intense lift and heaviest banding will setup somewhere over CT and thus CT could receive some of the highest totals within the event.  Latest GFS computer forecast is showing exceptional lift crossing over the state tomorrow indicated by the 900-500mb vertically averaged frontogeneis map below.  The black and gray shadings are showing phenomenal lift and forcing in which snowfall rates could approach 3-4'' per hour along with thunder and lightning:

The latest NAM and GFS computer forecast bufkit profiles (using Waterbury, CT as location) also show exceptional upward vertical motion penetrating the snow growth zone which only verifies the potential for snowfall rates approaching 3-4'' per hour.  There is something of interest to note, however, on the NAM profile.  The NAM has an area of blue contours in the lower portion of the atmosphere which is indicative of sinking motion or subsidence.  Given the degree of exceptional lift and forcing which will yield to a very potent band of snow, its quite likely there will be an area of subsidence which means there will be a zone where snowfall totals come in much lower than the forecast map will indicate...this is the nature with these types of storms and is very difficult to forecast:

What to expect:

• Snow begins between 3 AM and 5 AM with the intensities quickly picking up.
• Heaviest snows fall between 6 AM and noon.  This is when snowfall rates could approach 3-4'' per hour along with the potential for thunder and lightning.  If thunder/lightning occurs this could yield snowfall rates locally approaching 5'' per hour.  
• In addition to extremely heavy snow making for near zero visibility, winds will be quite strong as well with winds between 20-30 mph and gusts potentially upwards of 45-50 mph.  This will allow for blowing and drifting snow along with the potential for isolated power outages.  
• After noon the heaviest snow will wind down and we will be left with leftover snow showers throughout the remainder of the afternoon.
• Travel will be extremely treacherous and is not advised during the storm unless it is absolutely necessary.  
• Below is what I'm expecting...keep in mind that zone of highest totals will be for wherever the heaviest banding sets-up...this could shift tomorrow depending on several feature  

Below is what I'm thinking:

Major Snowstorm Likely Thursday

Computer forecast models are zeroing in on a major snowstorm for the overnight period of Wednesday lasting through the first half of Thursday.  While this will be a relatively quick hitting storm (with the heaviest of the snow lasting for only a several hour period) it will be rather intense with extremely heavy snowfall rates.  This will yield rapidly accumulating snowfall between about 5 AM Thursday morning and 11 AM Thursday morning.

I did a much more in depth scientific post earlier today so will refine from all that fun stuff here (the link to that post if interested is http://weatherwiz.blogspot.com/2017/02/thursday-snowstorm-looking-more-likely.html) I will post a few bufkit profile soundings using Waterbury, CT too illustrate.  Both the latest NAM and GFS computer forecast models are showing a several hour period of excellent snowgrowth with great lift penetrating this snowgrowth zone.  This will yield a several hour window where snowfall rates could approach 3'' hour at times:

In addition to moderate to heavy snowfall we will have to deal with strong winds as well.  Winds could become sustained at 20-30 mph with gusts potentially up to 50 mph across the state.  This could yield to pockets of isolated power outages.  Light snows should begin to break out during the overnight hours Wednesday (perhaps as early as 3-4 AM) and the intensity could quickly pick.  During this time we will see a several hour period of extremely heavy snows where rates could approach 3'' per hour.  After about 11 AM-noon the rates should begin to taper off and much of the accumulations will be done.

As for snowfall totals this map will have to be fined tune tomorrow evening.  The axis of heaviest snowfall will occur where the deformation snow band sets up and this is very difficult to forecast up until the event is pretty much starting.  With this deformation band means there will also be a zone of subsidence (sinking air which greatly reduces precipitation...in this case snow).  This would likely mean a zone of much lower snowfall totals.  As for now this is what I'm thinking: (Again...this will be fined tune tomorrow evening to reflect totals where highest banding may set up and areas which may receive less snow due to subsidence)

Thursday Snowstorm Looking More Likely

Computer forecast models are beginning to become into stronger agreement over a quick hitting but rather significant winter storm late Wednesday night lasting through the first part of Thursday.  Despite how close we are timewise, there continues to be some uncertainties with regards to the exact track and strength so that suggests this will be a rather delicate forecast with a high room for bust potential.

By early Thursday morning a shortwave trough and associated shortwave energy really begins to amplify (strengthen) rather quickly.  This is due to the very impressive 120+ knot 500mb mid-level jet streak which begins to round the base of the amplifying trough.  The response from here is for rapid cyclogenesis (development and strengthening of a surface low pressure system) to occur:

 
There are some differences between the computer forecast models, however, on exactly how this evolution evolves, the strength of these features, and timing and hopefully this will be resolved as newer data rolls out later this afternoon, however, the consensus is point towards at least some type of impact.

One other indicator for the potential for at least a heavy swath of snowfall is a very impressive thermal gradient which will exist between southern Canada and just south of New England.  Illustrating this looking at 850mb temperatures (~5,000ft) temperatures are forecasted to be around -20C in southeast Canada and near +8C south of New England.  This yields a rather impressive gradient which is something that can really enhance forcing and vertical lift:

Both the NAM and GFS computer forecast models are also showing a several hour period of excellent snowgrowth coupled with vigorous lift within the snowgrowth zone.  Taking a look at a bufkit profile from this morning's run of the NAM forecast model over Waterbury, CT we see a several hour period where the snowgrowth zone (purple and yellow contours) is between 12,000-18,000ft (prime location) with the -15C temperature contour running through this zone (helps to maximize snowgrowth) and excellent upward vertical lift (red contours with negative values) within this zone (as much as 20-30 units of omega!!!).  All this indicates that there will be a several hour period of moderate to extremely heavy snowfall where snowfall rates could approach 3'' per hour during the peak:

A few other things to watch are developments of closed off low pressure center at 925mb, 850mb, and perhapd 700mb.  This would indicate a very intense system and the track of the lows would have to be watched too.  All models developed at least a closed low up to 850mb but some tracks vary.  In order to maximize snowfall rates and intensities you want to be on the northwestern side of these tracks:

All in all guidance is coming together for a potential quick hitting but potent snow event.  This could potentially be the biggest event of the season for many.  I will have another update this afternoon including a snowfall map.