Showers/Thunderstorms Possible Ahead of Cold Front Friday, April 22nd, 2016

Our next weather maker looks to occur on Friday as a shortwave trough and associated piece of shortwave energy drives a cold front east through the region.  As the cold front approaches from the west, there are some indications the front could be accompanied by some showers and perhaps even some thunderstorms.  

When dealing with the potential for thunderstorms, the biggest question we will face is how much instability develops ahead of the approaching cold front.  With some cloud cover and increasing clouds, temperatures are only expected to reach the lower to perhaps mid-60's (cooler along the coast and higher elevations) and dewpoints are expected to climb into the lower to mid-50's.  This combination of temperatures/dewpoints isn't favorable for the development of high amounts of instability, however, with the approaching shortwave and shortwave trough, an area of steeper mid-level lapse rates is expected to move overhead thanks in part to cold air advection in the mid-levels of the atmosphere.  This will work to yield some modest instability values ahead of the cold front.  The GFS computer forecast model is projecting a plume of 700-500mb lapse rates exceeding 6.5 C/KM to 7 C/KM moving overhead early to mid Friday afternoon:

The GFS computer model at this time is forecasting surface-based cape values on the order of 500 J/KG with most-unstable cape values between 500-1000 J/KG.  These values are indicative of some very modest instability and while they aren't overly impressive, when combined with some wind shear aloft, these values may be sufficient enough to not only produce thunderstorms but perhaps a few stronger to marginally severe thunderstorms:

With the approaching shortwave trough, we will also see an increase in the winds aloft.  While the winds aloft aren't expected to be overly impressive, they should be sufficient enough to further enhance the degree of lift across the region, thus increasing the potential for showers and thunderstorms to develop ahead of the cold front.  While the upper-level jet around 200-300mb isn't forecasted to be overly strong, there will be some enhanced support from the mid-level jet at 500mb.  In fact, parts of southern New England, particularly south of the MA Pike will briefly enter the left-front quadrant of a 50-60 knots mid-level jet streak passing just to the south of CT.  This is a favorable quadrant for enhanced lift.  Bulk shear values from the surface to 700mb are forecasted to be around 25-35 knots with bulk shear values from the surface to 500mb forecasted to be 30-40 knots.  These values are sufficient enough to yield the possibility for organized thunderstorms if any develop:

What can we potentially expect on Friday?  For Friday we can expect winds to increase from the southwest as the morning progresses and the front approaches.  We'll see a mixture of sun and clouds as well with clouds increasing heading into the afternoon.  Expect showers to develop as well during the early afternoon hours.  If enough instability materializes, we could see a line of heavier downpours and thunderstorms develop ahead of the cold front.  Given the potential for fairly steep mid-level lapse rates and adequate shear aloft, some of the thunderstorms could become strong to marginally severe and pose a threat for hail and strong winds.  This potential though is highly dependent on the degree of instability that materializes.  Showers/thunderstorms should clear by early evening and temperatures at night will drop into the 30's with afternoon temperatures on Saturday and Sunday right at seasonal levels.  

Thursday, April 7th, 2016 Rain Storm

April has gotten off to a rather active start weather wise and this will continue with yet another storm system impacting the region on Thursday.  However, this time we will see the area of low pressure track well to our west meaning warmer and more moist air advecting into the region and rain as opposed to snow.

Looking at this morning's run of the GFS forecast model, we see that the trough will be amplifying (deepening) well off to our west.  This will allow for heights and subsequent ridging to build into southern New England and also allow for the developing area of low pressure to track to our west:

As the system approaches southern New England it will begin to strengthen and we will see an increase in the winds aloft and also an increase in warm air advection as well as moisture advection.  The strengthening of the winds aloft and approaching trough/cold front will also work to rapidly increase the degree of lift (upward vertical motion) across the region.  The combination of these factors will lead to widespread rainfall, some of the rain which will be extremely heavy at times:

Computer forecast models strengthen the upper-level jet (250mb winds) and mid-level jets (500mb and 700mb (not shown)) and low-level jets right over the region.  The increase in upper/mid level winds will work to strongly increase the degree of lifting aloft and the increase in the low-level jet from the south will transport a great deal of moisture into the region.  This is evident by precipitable water values (PWATS) exceeding 1'':

The result will be widespread rainfall on Thursday lasting well through the evening and into the overnight hours.  Expect rainfall to be quite heavy at times.  Rainfall amounts of 1-2'' should be fairly common with even a few of the favored upslope locations perhaps getting as much as 3'' of rainfall.  Localized flooding will be a possibility as well.

Once the trough and cold front pass we will once again see another strong shot of cold-air advection across the region and some more cold temperatures for the weekend.

More Accumulating Snow in the Forecast

We are going from one accumulating snow event right into another as yet another strong piece of shortwave energy and associative low pressure is projected to dig southeast across the Great Lakes and track either right over CT or just south of Connecticut.  The exact track will be critical as to how much snow falls and where the cutoff is between accumulating snowfall and a mixture of precipitation.

As the shortwave energy tracks over the Great Lakes it will acquire moisture and the low pressure will begin to strengthen rapidly due to a very tight baroclinic zone (very tight temperature gradient in the lower levels of the atmosphere).  This tight baroclinic zone will contribute to a swath of very intense precipitation, whether it be snow or rain:

Both the NAM and GFS forecast models (as well as others) suggest we will see some pretty intense 700mb vertical velocities with the low pressure as it tracks across the area.  As these intense vertical velocities move overhead this will yield a several hour period of intense precipitation (rain or snow) associated with one or several bands of precipitation:

With the possibility for very strong lift associated with bands of precipitation this also means there will be subsequent areas of sinking or or subsidence.  This will actually prevent precipitation from falling to the ground across some areas.  We can actually see this by looking at bufkit.  Looking at this morning's run of the GFS at Windsor Locks, CT notice those blue contours?  These indicate areas of sinking motion.  What we can take from this is if we do indeed see some intense bands there will be areas of sinking motion and that would cut away at potential snowfall totals for several areas:

Keeping with bufkit we will also illustrate just how strong the lifting may be and how this is indicative of the potential for a several period of extremely heavy precipitation.  We will look at Windsor Locks, CT and Worcester, MA to show this.  The following parameters are overlayed; snowgrowth (the purple and yellow contours), omega (with red contours indicating upward motion - the more contours and more negative the value the stronger the lift and blue contours indicating sinking motion - the more contours and more positive the value the stronger the sinking motion), and temperature (the faint yellow/brown contour).  For snowgrowth you want this zone to be between 12,000ft and 18,000ft and want the -15C contour to cross this zone.  When these conditions are met you stand a great chance of generating a great deal of dendrites and near perfect snow growth.  In the case of this system that criteria will be met.  Now to maximize and realize this snow growth you want very intense lift to run through this zone.  Notice how the NAM is showing some very intense upward vertical motion in this zone while the GFS is a little less aggressive  (at least at Windsor Locks):

With this all said what can we expect?  For one, the AM commute tomorrow is going to be quite hazardous as we are looking at the timing of precipitation from around the pre-dawn hours lasting until late morning or even early afternoon (later start and end times the further east you go).  Expect a very slow commute with numerous delays.  Areas across southern CT through southern RI and into SE MA will begin as mostly rain, however, may see a transition over to snow towards the end of the event as temperature begin to quickly crash.  Across inland locations as much as 3-5'' of snow will be possible with higher amounts possible across the higher elevations.  Initially, snow will have trouble sticking to the pavement, however, with heavier intensities of snowfall allowing to further cool the surface, snow may begin to stick to paved surfaces.  At the height, snowfall rates could approach 1.5'' to even 2'' per hour.  Below is a map of what I expect:

Vigorous Shortwave Energy to Bring Snow Squalls to Region Overnight

If you're a weather enthusiast you are going to enjoy the timeframe from say about 3:00 AM to 9:00 AM Sunday morning.  If you aren't a weather enthusiast well hopefully after reading this post you'll have a whole new appreciation for weather and just how crazy it can be at times.

For a good while our long-range computer forecast models have been extremely consistent at hinting at a major, major cooldown just after the first of the month.  In fact, some of the cold project has been near or at record cold type stuff.  Too put this into perspective, the high temperatures for Monday and Tuesday (at least) will be as much as 15-20F below average!!!  That is pretty remarkable.

Let us take a look at the big picture and then break everything down on what we can expect overnight tonight into Sunday morning.  Looking at this morning's run of the GFS forecast model we can see a vigorous piece of shortwave energy associated with an amplifying trough digging out of southern Canada into the Great Lakes.  The image on the left hand side is the forecast position of the shortwave about 8:00 PM EDT this evening and the image on the right is the forecast position of the shortwave about 5:00 AM EDT Sunday morning.  In addition to focusing on the track of the shortwave, note the back contours and the numbers associated with them.  These numbers are the height of the atmosphere in meters where the 500mb level is located.  These numbers are VERY low for this time of year and indicate that the 500mb level is very low in the atmosphere (we will actually discuss this a bit more further down):

What does a shortwave (and in this case a vigorous one) have to do with our weather and what we can expect?  Well several atmospheric variables (ingredients) will be in place for the generation of numerous snow squalls across the region from say about 3:00 AM EDT to 9:00 AM EDT.  Some of these snow squalls are going to be extremely heavy, be associated with high snowfall rates, and be accompanied by strong winds and perhaps even thunder and lightning!  We will explore these variables below.

The first variable we will look at are the forecasted mid-level lapse rates (change of temperature with height).  The steeper the lapse rates (indicating a stronger decrease of temperature with height) the more unstable the airmass (great for enhancing upward vertical motion and also increasing thunder potential).  Computer forecast models are forecasting mid-level lapse rates to be anywhere from 7.5 C/KM  to as high as 8.5 C/KM (meaning a 7.5C to 8.5C decrease in temperature per kilometer of the atmosphere).  Those values are very impressive, especially for our region:

The next variable we will look at is the degree of lift.  Now, it's one thing to have lift and strong lift but if there is no moisture to lift nothing will happen.  While we will not look at it, there will be plenty of moisture available with this system.  Computer forecast models are forecasting a period of VERY intense upward vertical motion moving across portions of southern New England during the overnight period.  This intense lift will be responsible for the development of snow squalls and also be responsible for any of the heavier snow squalls with very heavy snow fall rates.  We will also explore this more further down:

We will now look at a few bufkit forecast soundings for a few select locations across the region.  We will do this to further explore the degree of lift and also look at something very cool regarding the tropopause and the stratosphere.

I've selected three locations (Windsor Locks, CT, Worcester, MA, and Falmouth, MA to illustrate further the intense lifting being advertised by computer forecast models.  The images are from bufkit and overlayed is the area of best snow growth zone (the purple and yellow contours) and omega values (with the red contours indicating upward vertical motion and blue contours indicating sinking or downward vertical motion).  Since the red contours indicate upward vertical motion, the more red contours you have and the more negative the value the stronger the upward vertical motion.  Given how we are looking at values as high as 40-45 -ub/s that is indicative of some exceptional vertical motion!  Given how these values also intersect with the area of best snowgrowth this is a very good indication that we will not only see great snowgrowth but very large and fluffy flakes...the kind that accumulates very quickly:

One very cool aspect of this system will be how low the stratosphere will be or how close it will be to us here at ground level.  Typically around these parts we see the stratosphere up around 30,000-35,000ft or so.  Well given the extreme low heights associated with the shortwave, the stratosphere is actually going to descend to right around or perhaps just below the 500mb level and in this case that may be as low as 15,000ft to 18,000ft AGL!!!  Given how this will in fact occur well below the upper jet stream this is known as a tropopause fold and is something that doesn't happen very often.  Sometimes when these occur you can get a period of exceptionally strong wind gusts (>75-80mph).  Below is an example of how low the tropopause will be (note: the stratosphere begins just above the tropopause):

All in all what can we expect?  As the shortwave approaches and lift increases we can expect numerous snow showers and snow squalls to develop across the region between 3:00 AM EDT and 9:00 AM EDT.  Given the degree of lift and instability that will be present, some of these snow showers and snow squalls will produce periods of very heavy snows with snowfall rates perhaps exceeding 2-3'' per hour, especially if there is any thunder or lightning involved!  Now the one thing to remember is we are not talking about a widespread precipitation shield but snow squalls (think of it as those heavy passing downpours we would see in the summer).  This mean not everyone may see snow and not everyone will see accumulations.  For those who do see accumulations, as much as 1-3'' will be possible, however, there is even opportunity for a few locations to perhaps see as much as 5''...6''...even 7'' of snow!!!  If that occurs it will be extremely isolated but someone may very well receive this.  Don't worry though, despite the cold tomorrow this will melt very quickly with a much higher sun angle and length of day now in place.

Once the shortwave passes we are looking at very strong winds with damaging wind gusts across the region.  We are looking as sustained winds of 25-35 mph with wind gusts as high as 55-65 mph possible.  Winds of this magnitude will be enough to cause isolated to scattered tree damage and power outages.  Sunday is definitely going to feel more like late January/early February than early April!

After tonight's event our attention turns to overnight tomorrow into Monday where in fact more snow will be possible but we'll touch upon this later!  At this time it appears a track will favor more in the way of rain but we will watch.