Showers/T'storms possible Friday; Arctic Blast/Snow Squalls/High Winds Behind the Cold Front

A strong cold front will be approaching southern New England Friday afternoon/evening.  Out ahead of the cold front the airmass is expected to become modestly unstable thanks to surface temperatures in the mid to upper 60's, dewpoints near 60F, and the possibility for mid-level lapse rates to approach 6.5 - 7 C/KM.  Current computer model forecast is suggesting that we could see several hundred joules of most-unstable (MUcape) develop across the region, with a few hundred to perhaps several hundred joules of mixed-layer cape (MLcape), and lifted index values as low as -2C.  These values would be suggestive of a modestly unstable airmass:

 
 With an amplifying trough approaching the region (and responsible for sliding the cold front east), winds aloft will begin to increase as well.  Computer forecast models indicate by mid afternoon we will see a strong mid-level jet begin to approach the region with 700mb winds expected to increase to as much as 50-60 knots and the region getting into the right entrance region of a potent 70-80 knot mid-level jet and the region also entering the right entrance region of an intensifying 120-130+knot upper-level jet.  This will really help to enhance upward motion and lift across the region:

While we will not see rain all day long, after some overnight and early morning shower activity there will be a break in the action.  In fact, it is very possible there are some brief breaks of sunshine at times.  If there are indeed any breaks of sunshine this would further work to destabilize the atmosphere locally.  However, given how the airmass will be fairly moist and very little in the way of capping in place, showers could develop at any time. 

As the cold front continues to approach and encounters a modestly unstable airmass in combination with strong winds aloft, the possibility will exist for a line of heavy downpours and thunderstorms to develop along and just ahead of the cold front.  Given the presence of modest instability and strong shear, a few thunderstorms would have the potential to become strong to marginally severe and have the potential to produce strong winds and small hail.  We will also have to watch for any cells which may take on some rotation which could locally enhance the possibility for stronger wind gusts and perhaps some larger hail.  While the threat for any strong to severe thunderstorms is on the low side as the ingredients in place are marginal, they are just high enough to raise some awareness. 

Once this cold front passes we will enter a cold air advection regime and much COLDER temperatures will filter in from the northwest.  While Saturday may warm-up, the cold air advection will really intensify as the afternoon progresses and temperatures will begin to fall rapidly and the winds will increase.  On Sunday, temperatures may struggle to reach the 40°F mark across many locations and we will be dealing with very strong wind gusts, possibly on the order of 60-65 mph!!  This will certainly yield the risk for power outages.  In addition to the strong winds, overnight Saturday into Sunday expected isolated to widely scattered snow squalls to occur across the region.  While not everyone will see snow, a few very localized areas could potentially pick up a couple inches of snow as the atmosphere will be quite unstable thanks to an area of very steep mid-level lapse rates (>7.5-8 C/KM) moving overhead:

Given the very steep mid-level lapse rates which would enhance instability values, we could be looking at the potential for thunder/lightning to occur with the heavier snow squalls as well.  If this does occur this would vastly increase snowfall rates locally and would enhance the potential for a localized area receiving a couple inches of snow.  The greater potential for this occurrence may be along and just north of the MA Pike, however, this all depends on the track of the shortwave energy which will be responsible for the developing snow squalls.   

Stormy Friday Followed by Arctic Cold/Snow Potential

By this point I'm sure everyone (or mostly everyone) is ready to turn the page on winter 2015-2016 and make the full transition to spring and spring-like weather.  With the calendar set to change to the month of April you would think this transition is well underway.  Well unfortunately mother nature has different plans as our weather pattern is set to go under a significant change and we will see a return to much colder weather (perhaps even some record cold) and even the threat for snow...yes snow.  These changes will begin on Friday.

An amplifying trough will begin to dig into the central United States over the next day and an associated area of low pressure will track well to our northwest.  With the trough digging into the central United States first this is allowing for some ridging to build here in the Northeast.  As we move into Thursday and Friday, the area of low pressure tracking to our northwest will allow for a warm front to pass through the region ushering in even warmer and more moist air.  For Thursday and Friday we are looking at temperatures pushing well into the 60's and perhaps even near 70F on Thursday in some spots!  By Friday dewpoints should be well into the 50's.  The graphic below is from Wednesday's morning run of the GFS forecast model looking at the 850mb (~5,000ft ASL) level.  The darker blues and purple shadings indicate much colder temperatures associated with the amplifying trough while the brighter green and yellow shadings indicate warmer temperatures at this level associated with ridging:

As the strong cold front progresses into the region on Friday it will encounter a modestly unstable airmass thanks to temperatures well into the 60's, dewpoint temperatures well into the 50's, and the potential for some steeper mid-level lapse rates to work into the region as well:

In addition to the potential for some modest instability, winds aloft will also be rather strong across the region in association with the approaching trough.  In fact, by early afternoon the nose of a rather strong 500mb mid-level jet is expected to push into the region (note the edge of those "red" shadings just to our west).  As this happens this will really enhance lift across the region ahead of the cold front.  Although not shown, it also appears the region may end up in the right front quadrant of a potent upper-level jet streak which would also enhance lift:

The combination of modest instability and strong wind shear ahead of the cold front will not only yield showers and heavier downpours (given presence of strong moisture) but we also may see a line of thunderstorms develop ahead of the cold front which could have the potential to produce strong wind gusts and perhaps even some small hail!

Now onto the passage of the cold front.  Once this Arctic front passes we will see a return to MUCH colder temperatures.  Ending the weekend and into the first part of next week we will see high temperatures from the upper 30's to lower 40's, brisk winds at times (especially Friday night/Saturday), low temperatures down into the teens and 20's and yes even some snow showers possible at times.  In fact, we will be watching the potential around Tuesday-ish for a potentially bigger snow threat.

As we move into April it will feel more like December as the developing weather pattern will allow for Arctic air straight from the Arctic to drain into the Northeast.  After Wednesday or so of next week we do moderate some, however, there doesn't appear to be any prolonged warmth through the first 10-14 days of April at least.

The image below also from this morning's GFS run shows 850mb temperatures over the region Sunday evening.  This is pretty chilly stuff for this time of year!

Winter Ain't Over Until It's Over

We have had quite the interesting stretch of weather throughout the month of March.  We've received a taste of spring and even early summer when we had several days make their way into the 70's with a few locations hitting the 80F mark.  We've also been reminded that winter is not yet over as we have had bursts of colder air and even snow.  As we move into the end of March and into the first week of April it appears that winter will yet again make a comeback.  There are several signals that the final week of March and first week of April will feature well below-average temperatures across much of the continental United States.

Below is a graphic from the Extended CPC Analogs Model (EXCAM) taken from Al Marinaro's website (models.wxmidwest.com).  The graphic is showing projected surface temperature anomalies across the continental United States for the week of March 31st to April 6th:

Long-range computer forecast models have been very consistent with the degree of below-average temperatures across the continental United States.  

If we look at Wednesday evening's projected 8-10 day 500mb pattern both the GFS and European forecast models develop a massive ridge in the Gulf of Alaska and into western Canada.  The response to this is for a deep trough to dig into the United States and open the flood gates for much colder air from central Canada to work into the United States:

If we look at several various teleconnection indices we can see there is definitely some merit for this type of pattern development across the United States.

Looking below we will discuss several of the more prominent global teleconnection indices and discuss why the projected pattern has a great deal of merit.

**(Note:  PNA = Pacific North-American, NAO = North Atlantic Oscillation, AO = Arctic Oscillation, EPO = Eastern Pacific Oscillation)**

Finally, given how there will be a source for cold air around this also means there could be some chances for snow, especially across the higher elevations of interior New England.  While it certainly is tougher to get snow and big snows as we move into April it certainly isn't unprecedented.  This doesn't mean we will see snow but the potential will be there.  April snows certainly have happened before.  Looking even longer range, hopefully after mid-April we should finally start to see a transition to spring and we can put winter in the rear view mirror!  

What's the deal with the Sunday/Monday Snow???

I would like to start off by saying this has been one of the most complicated, complex, and confusing forecasts I have at least ever attempted and I'm sure many who forecast as well, whether they be meteorologists, aspiring meteorologists, or hobbyists, I'm sure would feel the same way.  The degree of model uncertainty, the lack of run-to-run consistency, and the lack of model agreement has been jaw dropping.  While this is extremely frustrating for forecasters, the most frustrating aspect of the situation is trying to inform the public.  Unfortunately many in the public eye don't understand exactly how challenging weather forecasting can be and what goes into composing a forecast.  While its completely understandable how everyone would like exacts, unfortunately there are times in the forecasting industry where you can't really be exact and that's just the nature of the beast.  What I would like to do is begin with trying to explain and show why this forecast is so challenging and extremely low forecast and hopefully this will at least give the public an idea as to why it seems like meteorologists don't really know what will happen.

What I would like to show first is a graphic from one of the computer forecasts models called the "SREF" (short-range ensemble forecast).  The SREF is an incredibly useful tool as it is composed of various members which each suggest a potential solution as well as showing a mean of all the members.  When there is very little spread within the members (meaning there is great consistency with all solutions) this typically leads to a higher than normal forecast confidence.  On the other hand, where there is a greater deal of spread, this leads to a lower than normal forecast confidence.  The image below is from today's 15z run (meaning it was initialized at 11:00 AM EDT) and centered at Windsor Locks, CT (BDL).  This particular graphic is showing projected snowfall totals.  Each of the different colored lines represents the various members which compose of the SREF.  Notice how the range is from 0'' all the way up to 12'' and notice how far apart each of the members are:

 
This degree of spread this close to an event is UNREAL.  Typically in some situations it is a good idea to just take the mean of the spread but that may not really be ideal here.  If we look at each individual member, there seems to be a greater clustering between like 0'' to 4''.  The mean (the solid black line) is right up around 5-6''.  It is possible this is being skewed slightly due to several members spitting out as much as 8-12'' of snow.

Let's also look at the past runs of two of the American forecast models, the NAM and the GFS.  We will look at this mornings runs as well as this afternoon to illustrate the inconsistencies.  The displayed graphics are showing sea-level pressure (SLP) along with 3-HR precipitation accumulations.  Just look at each model has shifted in just 6-hours with regards to track and intensity of the precipitation (the yellows indicate moderate to heavy precipitation):

It is these differences, these inconsistencies, and these changes which have led to all the confusion and low confidence forecasts.  The major problem is all major sets of computer forecast guidance has continued to waiver back and forth and with that happening there is really no way to even decide which model to perhaps favor or lean on.

Hopefully this little bit of insight has perhaps clarified why this forecast is so challenging and why forecasts for this event keep on changing.  With this said, it is now time for me to make my very best educated guess, using as much science and knowledge as I have in order to compose some sort of forecast.  In composing this forecast I realize the likelihood of being wrong and busting (either too low or too high) is more likely than not and if that does occur all you can do is hope you learned as much as possible.

The forecast:

1) There are ingredients on the table which could allow for a bigger storm to happen.  We have a nice ridge out in the western United States with a deepening trough here in the east coast.  We have some shortwave energy rounding the base of the trough as well as a piece of energy pushing in from the northwest.  We also have a trough which appears to be trying to become negatively tilted (which is something we need for a storm to become stronger and also track closer to the coast):

2) This may or may not play a role, however, in my opinion I think it can but sea-surface temperatures off the coast are quite warm, in fact much above-average.  If the system tracks over these warmer waters, this will intensify the low-level temperature gradient and supply ample moisture.  This would certainly allow for rapid storm intensification and would mean some major snowfall amounts somewhere:

 With the latest set of guidance coming west, this would allow for some very intense lift to traverse the region which would mean a very intense banding of snowfall occurring across a narrow zone with high snowfall rates (up to 2'' per hour).  I will post a bufkit profile from Windsor Locks, CT (BDL) showing how even some models would suggest a period of intense snows back into central CT at least.  Bufkit profiles for other areas across southern New England are also suggestive of this.  The yellow contoured lines (which is what the "snow growth" arrow is pointing too) shows which level of the atmosphere is prime for the development of dendrites.  You would like this zone to between 12,000 and 18,000ft and that's exactly what we have here.  The omega values (where the omega arrow is pointing at) measures atmospheric lift.  Red values indicate upward motion (needed for generation of clouds and precipitation) and blue values indicate downward motion (inhibiting development of clouds and precipitation).  If you're looking for precip (and heavy precip) you want red values within the best snow growth zone the more negative the values, the more intense the precip will likely be (the red values are measured in negative values).  In this particular sounding we see -15 units of omega which would indicate at least a period of moderate to heavy snow:

 
Given what I have said above, I do think we will see a rather impressive coastal transpire and develop and it will be close enough to the coast to provide some impressive spring-time snowfall totals, especially the further east you go.  I do not have much confidence in this forecast, however, the ingredients are on the table for a major event and it will not take much for that to occur and seeing some of the latest models make a big shift west leads me to believe that potential is still very much in play.

I would expect snow to come down moderate to heavy at times, beginning perhaps early to mid Sunday evening (maybe some flurries earlier in the day) and intensifying as we move into the overnight hours.  During the peak intensity, some areas could see snowfall rates upwards of 2'' possible along with the potential for thundersnow which could locally enhance snowfall rates even further.  Winds could become quite gusty as well and could lead to some power outages.  The snow should last through late Monday morning so expect very hazardous travel for the Monday morning commute.

Again this is very low confidence and HOPEFULLY either later tonight and come tomorrow morning the models will all converge on one solution, one track and a more confident forecast can be made.

Sunday's Snow Threat: Do we go big or do we go home?

Sunday is set to be the official start to the spring season, however, mother nature may have other plans in store.  If you put away all the winter equipment (clothes, snow removal tools, etc) you may want to pull them back out because winter is not done yet!  After getting out first taste of spring and early summer like weather, winter is set to make a return.

For the past week, computer forecast models have been suggestive of the potential for a rather potent coastal storm to develop and perhaps impact southern New England.  While there has been disagreement (and there continues to be some) as far as exact track and strength, virtually all guidance has said there will be at least a storm.

At this time we're still dealing with a bit of uncertainty within the computer forecast models.  Some models such as the European forecast model is very aggressive and suggestive of a major snowstorm for the region, certainly the biggest for most locations of this season.  Other computer forecast models such as the American GFS forecast model aren't as aggressive and is suggestive of a much weaker snow event but still potentially producing several inches for some.  Over the next 24-36 hours these details will begin to become much more clear.  There also exists a bit of uncertainty with regards to the timing of snow with the GFS much faster (breaking out snow by mid to late Sunday AM) and the Euro much slower (breaking out precipitation much later on Sunday).  

Let's take a look at some 500mb graphics from both the GFS and European forecast models:

The below graphic is showing the projected 500mb pattern along with sea-level pressure from the 3/17/2016 12z run of the European model.  What we see is ridging out in the west with a trough digging into the eastern United States.  This is a very good look if you're hoping for a storm to track along or just off the coast.  Also note the VERY intense storm just off Cape Cod!!!  If this solution verified we would be looking at the potential for several inches to perhaps upwards of a foot-plus across some locations!

The below graphic is showing the projected 500mb pattern with vorticity during the evening on Sunday from the GFS forecast model.  Like the Euro, both models are showing ridging out west with troughing into the east.  However, the GFS doesn't have as significant as a storm as you can see below: (Note: if you look back at the Euro graphic you can see the Euro is showing a 978mb low east of Cape Cod while the GFS is 992mb...quite the difference!

As you can see there is still some spread involved with the exact strength of this system which isn't really unusual at this range, however, hopefully with tonight's set of model runs the models start coming to an agreement.

While we are getting closer to the short-range aspect of forecasting, we are still at the range where computer ensemble members can be very useful (ensembles are composed of different model members, each advertising a potential solution and the ensembles also take the mean (or average) of the solutions and this can be very useful in the medium range when determining storm potential.  Today's 12z GFS emsembles clearly show that the potential for a storm is certainly high and the possibility of a track east enough for all snow is quite high as well:

While its still too early to get into further details and specifics as star/end times and potential snowfall amounts, what we can derive at this juncture is that the potential for a rather significant storm is increasing and it appears that many may see their biggest snowfall accumulations of the season.  In addition to the potential of snowfall we may also deal with the possibility of strong winds and even some coastal flooding.  The snow may be on the wet/heavy side as well so tree damage and power outages would be possible as well, especially if we see strong winds.

The potential will continue to be monitored quite closely and further updates will be provided as needed.

Weekend Storm Potential

As we move ahead towards the weekend all eyes will be on the potential for a storm system to impact New England towards the end of the weekend.  At this juncture there is still quite a bit of model disagreement on strength and track (which is common at this range), however, computer forecast guidance has been consistent in showing some sort of storm developing and working up or near the coast.  Initially I felt that this system would either track over New England or track to our west meaning we would be dealing with a warmer scenario, however, there have been some interesting changes with the projected pattern which leads me to believe we certainly can't discount the potential for a more colder scenario. 

Looking at today's 12z run of the American GFS computer forecast model we will take a look at the projected 500mb pattern for Sunday morning and we will note several interesting features.  As we progress through the next several days we will have to watch how these features are handled by various computer forecast models as how they evolve will ultimately dictate the storms track as well as strength:

 

 

 

Today's run of the GFS is a pretty decent look if you're hoping for a colder track and for snow.  However, the GFS does track the storm well off-shore.  The GFS has a ridge building across the western-tier of the United States.  As this ridge builds northward into western Canada an amplifying trough will dig into the eastern United States in response.  The GFS also has a rather intense area of low pressure situated south of Greenland in what may act as a "50/50" block.  Having a 50/50 block is HUGE if you're hoping for a really significant event.  The GFS also shows a ridge building into Greenland which is suggestive of the North Atlantic Oscillation (NAO) becoming negative, another HUGE plus if you're hoping for an off the coast track. 

 

 

Obviously we are now into mid-March and with an increasing sun angle you have to worry about temperatures (moreso than say January), both at the surface and throughout the atmospheric column.  A very iffy temperature in profile may produce good snows in January, however, that same iffy profile probably won't work in March.  If we take a look at projected 850mb temperatures (temperatures ~5000ft ASL) we can see there is certainly plenty of cold air to work with aloft just to our northwest so if a track were to be to our south and east that would certainly allow for the colder air mass to advect into New England:

 

 



  At this juncture we are still too far out to really hammer out any details, however, as we move through the week the situation should begin to become much more clear as new data continues to arrive and hopefully computer forecast guidance starts having much better agreement.  

Is EL Nino on its Last Leg?

Unless we see a major surge of westerly trade winds across the equatorial Pacific and another surge of warmth within the sea-surface temperatures across the equatorial Pacific it is highly likely we have seen the current EL Nino reach its maximum strength.  If the EL Nino has indeed reached its peak, it will have tied the EL Nino of 1997-1998 for the strongest on record since 1950 and fall short of the EL Nino of 1876-1878 (this EL Nino reached its peak strength in the winter of 1877-1878) for strongest on record.  Looking at Oceanic Nino Index (ONI) data, we can see that both the EL Nino of 1997-1998 and the current EL Nino have peaked at +2.3 degrees Celsius.  Reanalysis data had the 1876-1878 EL Nino peak at +2.48 degrees Celsius:

When measuring the sea-surface temperatures and determining sea-surface temperature anomalies (anomaly meaning how far from average) across the equatorial Pacific, the basin is broken down into 4 different ENSO regions; region 1.2, region 3, region 3.4, and region 4.  Below is a visual reference:

Looking at the anomalies (in degrees Celsius) for each of these regions dating back to the end of September we can see the trends which have occurred since the end of September.  What we can take from this is that since about mid-January or so, the trend has been for cooling of the sea-surface temperatures overall with some periods of some very minor warming:

Despite the cooling we've seen the past several weeks, looking at the current region anomalies (As of March 9th in the above chart and looking at the current sea-surface temperature map we can see the EL Nino is still going strong:

Going forward with the EL Nino one thing we can look at are the 850mb trade winds across the equatorial Pacific ocean.  Below is a time-series graph showing the progression of the trade winds (the darker shadings indicate easterly trade winds and the brighter shadings indicating either very weak easterly trade winds or westerly trade winds).  One area to focus on is about 80W to 120W.  If in this region we are seeing darker shadings, this is indicating easterly trade winds which allows for colder waters off the coast of South America to push westward into the equatorial Pacific.  This would work to weaken the EL Nino state.  If between 120E to 120W we see brighter shadings, this indicates either very weak easterlies or westerly trade winds (especially once you get into the reds and pinks) and would allow for warmer waters to push towards the coast of South America which would help maintain or even strengthen the EL Nino.  Looking below after that substantial burst of westerlies centered around the international dateline there are no indications of strong westerlies and it appears that the trade winds may even be in the beginning phases of reversing back to easterlies.  If this is the case, this is a signal the EL Nino is indeed in the weakening process:

We can also look at the depth of the sea-surface temperature anomalies across the equatorial Pacific as well to give us some insight.  In order for an EL Nino to maintain or even strengthen you want to see above-average anomalies extending deep beyond the surface of the ocean.  This slows the process of upwelling and not only keeps sea-surface temperatures warm but can allow them to further warm.  If the depth of the above-average anomalies is shallow and confined close to the surface, this would allow for upwelling and mixing of the surface waters preventing warming and also leading to cooling.  Looking at the current update on March 9th you can see the depth of the above-average anomalies is not very deep.  In fact, if this graphic was looped, you would see the depth of the above-average anomalies shrinking overtime:

  

We can also look at the depth of the 25 degree Celsius thermocline as this can give us an idea of how deep the warmer waters extend.  Looking at the plot below we can see that the depth of the 25C thermocline is very shallow and in fact if this was looped you can see the 25C thermocline begin to shift upwards towards the surface.  Also notice how the 20C thermocline is beginning to near the surface as well:

What we can take from all of this is that it appears that the current EL Nino is well on its way to a weakening state.  Unless we were to see substantial changes which is very unlikely based on historical records, the current EL Nino will only tie 1997-1998 for strongest in modern records and fall shy of the all time record.  Moving forward towards the summer and beyond there are numerous indications that the EL Nino will eventually rapidly weaken and perhaps even enter La Nina territory by the end of the summer or by next Fall, but we'll save that for another post!