5:15 AM

500 MB Pattern

The upper trough that finally forced a cold front off the East coast on Thursday morning will continue to slide to the east of the Mid Atlantic by this afternoon.  The trough axis will be off the New Jersey coast  this morning and passing the end of the Long Island Sound by this afternoon.  This will put much of the eastern Great Lakes and Mid Atlantic under a northwesterly flow and strong upper level convergence, supporting high pressure at the surface.

Meanwhile, the strong upper low that has been feeding passing troughs over the Great Lakes with plenty of moisture, will weaken over the central Plains and eventually will open up as a strong shortwave by Saturday afternoon over the Mid West.  A strong disturbance diving into the Pacific Northwest will force the upper level disturbance to the east towards the Mid Atlantic and will keep the pattern relatively progressive.

On Saturday night, the disturbance will spawn an area of low pressure along a developing warm front.  This warm front will be the leading edge of much warmer and more humid air mass, which will lead to widespread showers and periods of steady moderate to heavy rainfall through Sunday morning.  A few elevated thunderstorms can also be expected as the warm front moves through.

A strong cold front will follow as the disturbance from the Pacific Northwest will dig a deep through over the Great Lakes, Ohio Valley, and Northeast.  The cold front will produce widespread showers and thunderstorms with heavy rainfall at times.  I do not believe these thunderstorms will reach widespread severe levels, however a few isolated thunderstorms may produce wind gusts over 55 mph as the storms tap into the energetic 850 MB low level jet stream.

Strong high pressure will follow Monday night through Thursday with dry conditions and temperatures averaging slightly below normal.

5:15 AM

Surface high pressure will continue to drift off the coast, which will lead to an established southwesterly wind over the forecast area.  A weak disturbance at 500 MB, represented by an even weaker cold front at the surface, will move through the forecast area this evening with an isolated showers possible, but otherwise dry conditions.

The important feature to key on for this afternoon’s 500 MB map is a shortwave over the central Plains.  This disturbance will play a significant roll in influencing the tropical disturbance currently north of the Caribbean Islands and approaching the Bahamas.  By the end of this week, the shortwave over the Plains will dig into the Southeast and intensify.  The orientation of that trough; positive, neutral, or negative will play a key role on where this disturbance will go by the end of the week.  The model guidance that produces a negative tilt to the upper level trough over the Southeast pulls what could be tropical storm or hurricane Danny into the Mid Atlantic and over the forecast area.  The combination of divergence at the upper levels, strong lifting at the mid levels, the latent heat release of the tropical system, and the abundant amount of moisture over the forecast area does support rainfall amounts that could easily exceed 2 inches throughout the forecast area.  However, should the trough have a neutral to positive tilt than the entire tropical system will be forced to the east and eventually turn northeast over the Atlantic.  This scenario would lead to little if any precipitation at all this weekend.  So we basically have an all or nothing scenario, which is great if you like stress and uncertainty.  If you don’t, then this weekend forecast is not for you!

There is little doubt that the trough over the Plains this afternoon is going to be a determining factor on the type of weekend we will have.  If the storm misses wide right, then most of the forecast area can expect clear skies yet choppy seas.  I am certain that beach conditions will not be safe for swimmers with at least a lingering rip tide threat.  I am going to remain middle of the road on this forecast until we get the two main players, the tropical disturbance and upper level trough, to a more advanced stage of development.  The trough over the Rockies this morning is barely visible in the 500 MB heights and the tropical disturbance still has not developed a closed surface low.

Regardless of the outcome on Saturday into Sunday morning, a strong cold front will move through the region on Sunday afternoon with showers and thunderstorms.  Of course, the tropical system may enhance rainfall along and ahead of this cold front, but that is another issue that will have to wait for more data.  Strong Canadian high pressure over the Great Lakes will build into the forecast area and the entire Mid Atlantic on Monday and Tuesday with clear skies, lower humidity, and cool below normal high temperatures.  By next week, temperatures in the afternoon will be hard press to break the mid 70′s and now where near the 80′s as a cool Canadian air mass takes hold.

7:51 AM

Taking a look at the pattern via surface maps, the potential and interest in the pattern through this weekend into next week would best be described as boring, especially the GFS.  However, if one was to look at 500 MB, they would come away with a totally different perspective on this pattern that is developing. 

Note some important difference compared to previous patterns.  One, the negitve EPO ridge is in game and developing nicely.  The cold air express is certainly on the way.  Two, the PV that is developing over North America is further south and east this time compared to early December, suggesting very cold conditions over the Northeast along with a storm track through the Mid Atlantic.  Three, the supposed death of the negative NAO has been greatly exaggerated.  There is clearly a weak 50/50 low/trough in place through the end of next week, with a weak ridge over Greenland.  Remember I said model guidance was going to kick the influence of that strong blocking out the door too fast.  The impact of that blocking is the atmosphere having an easier time setting up a weak negative or nuetral NAO pattern, which is exactly what the Northeast and Mid Atlantic wants when setting up a pattern for potential storms.  Finally, the model guidance whether GFS, ECMWF, Canadian, or GEFS has a lot of energy diving into the trough over the eastern CONUS with plenty of shortwaves moving through the forecast area.  So what does this mean for us?

Well, the overall storm track type we can expect are clippers and the possible interaction of southern branch disturbances.  Clippers tend to produce snowfall amounts of 1 to 3 or 3 to 6 inches, however this winter we’ve also seen them tend to blow up pretty quickly.  Just as northeastern Connecticut!  So that leads us to Saturday.

The model guidance has trended to a more southern track and a slightly colder solution.  The ECMWF would suggest a light to moderate snowfall north of Philadelphia through central New Jersey and all of the New York City metro.  In fact, the ECMWF also indicates that some of the southern branch energy that was expected to break off into a bowling ball over the Southwest hangs with the northern branch, and produces a more dynamic clipper for the forecast area.  The latest SREF guidance is also trending more south with this clipper and slowly but surely, so is the GFS, UKMET, and Canadian.  With a ridge building over the West and more energy in the deepening trough, I still am stead fast in believing we need to keep an eye on this clipper.  For locations from Philadelphia to southern New Jersey, I think the track of this clipper is still too far north, therefore some rain may get mixed in and the best lifting will remain to your north as well. 

There also appears to be growing potential for a potential storm for the middle of the week, although the details are sketch at this point.  What is clear is that a lot of PVA is going to be driving towards the coast ahead of an impressive Arctic blast for the eastern third of the nation.  The ECMWF is stronly hinting at some sort of coastal development or jump as a clipper races through the southern Great Lakes and a low begins to develop along the coast.  The GFS is also showing this, but I have doubts on the strength of the GFS 500 MB pattern, frankly looks a bit unrealistic to me at this time to have a deep closed 500 MB low diving towards the Mid Atlantic on Tuesday.  The point here is that we are entering a pattern where a lot of disturbances will move through with the potential for light to moderate snowfall over the forecast area and plenty of cold air to work with.  The amplitude of the pattern would suggest the potential for one or a few of these disturbances to crank up a bit to surprise some with a heavier than expected snowfall, likely along the immediate coast.  The weak negative to nuetral NAO pattern would allow low pressure system to move more slowly than in previous patterns. 

That’s the potential going foward.  How the details work out is unknown.  This could easily be a very snowy period for the forecast area or be a dry and cold period with most of the snow over southern New England.  The details will tell the story.

8:59 PM

If you put your ear to the ground, you could hear the fall of a million snow lovers faint to the ground after seeing the 12Z GFS, which basically if taken verbatum would produce a significant snowfall for most of the forecast area, save southern New Jersey (even that location would still get measureable snow if the 12Z guidance is correct).  However, before we get ready for an all snow event, we better reason a bit on what the GFS and other guidance is suggesting.  Let’s deal with the 12Z GFS right off the bat.

12Z GFS:  The basic idea of the GFS is this.  The strong shortwave, which is a closed 500 MB low over the central Great Lakes by Friday morning runs into one very impressive 50/50 low at 500 MB.  Sensing that there is no way that the disturbance can go much further east, the strong PVA dives towards the Mid Atlantic coast.  The short wave trough tilts to the negative towards the coast and a coastal low is spawned.  The cold air on the model is kept over the coastal plain as the developing coastal low bombs outs and pulls the deeper cold air towards the center of the low pressure system.  Heights crash.  Intense lifting is seen along the coastal plain, and everyone gets into the snow parade over the forecast area along the coast.  The position of the high pressure is mute in this scenario as the idea is the cold air behind the clipper reenforces and strengthens the marginal cold air in place left by the previous clipper on New Years Eve.  So can this solution happen? 

Verbatum, I think that the idea of no mixing of sleet and/or rain along the coast is a bit of wishful thinking especially with a high pressure system off the coast.  The strongest high over eastern North America is over Ontario, but too far away to have a direct impact on the forecast area at this point.  The key here is that the coastal low boms and tracks in such a way that cold air has no where else to go but towards the coastal plain, and any warm air advection is forced above 700 MB to not produce significant change in precipitation type.  Although even in this situation, you have to expect some sleet.  So by the naked eye, yes the solution looks like a significant snow storm for the forecast area.  However, reasoning of the set up should lead up to the idea of some mixing back and forth through the evolution of the storm.

As for precipitation amount, the 12Z GFS produces intense Omega at 850 MB as a new 850 MB low quickly develops and significant low level forcing develops as a result of the bombing low pressure system.  The guidance suggest a deepening rate of around 2 MB per hour between 18Z Friday and 06Z Saturday.

The 500 MB set up is possible.  Yes.  There is also support of this solution with the Ensemble guidance and the UKMET.  The NAM hints at the possiblity as well.

12Z ECMWF:

Then we have the ECMWF, which likely caused snow lovers to faint again.  The ECMWF and the 18Z GFS for that matter completely loses the storm.  Why?  Well, because of the other possible solution.  The ECMWF and GFS up to 96 hours or Friday morning is rather similar in the upper level set up.  A strong 50/50 is in place.  A strong disturbance is driving towards the Great Lakes.  Then BAM.  The ECMWF loses the storm.  Where did it go?  The answer is that the ECMWF decides to try to merge the disturbance with the 50/50 low.  Basically shearing the storm appart and not allowing any sort of redevelopment.  Why would the low need to redevelop if the low simply falls apart?  Well, I have some problems with this idea.  First, I could understand if the disturbance was weak and unorganized, but this disturbance is anything but.  Although the blocking is strong, I doubt that the entire upper level system will just fall apart and those lead to a non-issue of a storm for not only the forecast area but the entire Northeast.  The idea just sounds suspect to me.  So for now I am throwing the solution out.  However, if the ECMWF is trying to pick up on the idea that this disturbance is much weaker than what has been forecasted, then the idea of this storm being sheared apart is plausable. 

00Z Guidance:

The 00Z guidance is going to be very interesting to watch tonight.  Why?  Because the models will be getting a better read on the disturbance, which is currently over the coastal waters of British Columbia, really is.  If the disturbance is as strong as what has been advertised, then I think we end up seeing guidance similar to the 12Z GFS.  If this disturbance is a 98 lb weakling, then the 50/50 low will crush this disturbance like a tin can, thus leading to a non-event for the forecast area. 

From this point on, we will get a much better feel on not only the evolution of this storm, but the track and some boundary layer characteristics. 

So the next few days are going to be hectic for all of us, but hey we have the summer to rest!

10:22 AM

The up coming forecast for the middle and end of this week is going to be very difficult.  The overall idea here on all model guidance is that the negative NAO block will be established while a series of clippers move through the forecast area.  One clipper intensifies, and produces a strong Miller B storm along the Mid Atlantic or New England coast.  That’s the basic overview, which all models agree with.  The question is in the details like when and where does coastal cyclogenesis begin?  How strong is the disturbance that moves through the Great Lakes on Thursday/Friday?  What position will the 50/50 low be in?  I’m going to try to answer some of those questions today with the pretense that the forecast is likely to change.  Let’s start with Wednesday.

By Wednesday, the 50/50 low will be well on the way of developing.  A weak clipper with some reenforcing cold air will be moving through the forecast area.  The low should track from the Great Lakes through the southern Hudson Valley with a band of light to moderate snow along and to the north of the low track.  A few inches of accumulating snow will be possible with this clipper, especially over extreme northeastern Pennsylvania, the Hudson Valley, and Connecticut.  A few snow showers can also be expected over the entire forecast area as the cold front passes.  At the upper levels, the Greenland block will continue to build while the 50/50 low intensifies. 

As we move to Thursday, some interesting and important processes begin.  Strong convergence and confluence intensifies over southeastern Canada, specifically over Ontario and western Quebec.  An area of high pressure will be over the forecast area, producing clear skies.  However, what is important is that as the surface high over the forecast area departs, a stronger high pressure begins to build over Ontario.  This is extremely important for the up coming storm.  Why?  Well, if the stronger high was over the forecast area, then southwesterly winds would develop over the coastal plain, thus producing strong WAA ahead of the approaching low presure system.  Instead, the strong high, which will be strengthening through the entire forecast period, will be over Ontario.  This means, the overall flow of air is from the northwest and north, not the south.  This also means that the support of a developing CAD will be strong along the coastal plain. 

At this point, we enter a very difficult period in the forecast.  I’m still staying away from specifics for this time period, but I do have a general idea.  I’ll say this, I don’t agree with what the GFS is producing.  All model guidance agrees that a 50/50 upper low structure will develop.  The question lies in the position of the 50/50 low and the strength of the disturbance that dives through the northern Plains and towards the Mid Atlantic/New England coast.  The GFS representation produces a closed and intense low over the Great Lakes, which basically ignores the 50/50 low.  The GFS also wants to make the disturbance over the Plains stronger than the developing 50/50 low.  The ECMWF meanwhile has the disturbance over the Great Lakes as an open wave with the 50/50 low much stronger, thus forcing the disturbance south.  Redevelopment along the coast happens faster and overall a colder solution is the result. 

I like the ECMWF representation better because of the physically reasoning.  I can’t find much upper level support in jet dynamics for either model to support the intensification and development of the Great Lakes disturbance to the degree that the GFS has.  The GFS depiction appears to be a typical error of over amplifying a short wave disturbance in this time period.  More importantly, the idea of such a disturbance being so far north with a strengthing negative NAO block in this time period argues against the entire physical/synoptic set up.  Now, is it possible that the storm acts to some degree like the GFS is showing?  Yes, if the 50/50 low is displaced too far north than the storm track is likely.  I am siding with the ECMWF because the model makes more sense synoptically and also more consistent.  While the GFS has been flipping between the current 00Z depiction and the 18Z depiction, which is similar to the 00Z ECMWF by the way. 

The overall idea for Friday through Saturday is this.  I think the clipper will dive towards the Ohio Valley on Friday.  The upper level short wave will remain open and will not be as intense as the 50/50 low  to the north.  The strong convergence and confluence over Ontario/western Quebec will support strong high pressure over Ontario that will ridge down the coastal plain, producing a CAD effect along the coast.  The strong thermal gradient along the coast will support cyclogenesis somewhere between southern Virginia and the Delmarva coastal waters.  The forcing of the shortwave to the southeast due to the placement of the 50/50 low will support strong PVA towards the coastal plain and strong divergence and difluence with a developing jet maximum along the coast at 300 MB to 200 MB. 

A developing coastal low will quickly race to the northeast on Friday night and Saturday morning.  This low pressure system will take over as the primary and rapidly intensify along the coast or over the coastal waters of the northern Mid Atlantic and then somewhere along the coast or over the coastal waters of southern New England.  The track at this point is obviously unknown and will have a significant impact on precipitation type and amounts.  There is a significant potential for some locations to be dry slotted depending on the track of the storm and upper level features.  All of those details are obviously unknown as well as precipitation type.  What I am confident in is that locations south of Philadelphia will likely miss out on this event.  I am also confident in that much of New England will get a good amount of snow out of this storm.  The question lies in the impacts for the forecast area, namely the northern Mid Atlantic. 

So that’s where I am now on this storm.  With the way conditions are setting up, I issued an Alert Stage 2 for the entire forecast area, which means the atmosphere is setting up for a likely winter storm event.