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7:52 AM

As with the theme for much of this winter so far, model guidance continues to appear a bit lost in the handling of short wave disturbances over North America.  For the time period of Thursday night through Saturday night, there is a spread of solutions within the operational and ensemble guidance which generally point to a poor handling of the disturbances circled to the left, at least as of the 00Z guidance.  There is a theme in one camp, which the 00Z NAM and 00Z ECMWF support and the 06Z GFS and ensembles trended towards, which was basically a complete miss of the northern Mid Atlantic as the low pressure system forms too far out.  The 00Z UKMET and the Canadian along with various individual ensemble guidance disagree to varying degrees.

So what is happening here?

The problem with the model guidance is simply a case of timing.  There is no question at this point that the Sub Tropical and Polar disturbances will phase into a very strong upper level low that will lead to a very strong and long lasting negative NAO (more on that in a minute).  The question is simply WHEN!

As noted in the water vapor image above, the two disturbances are now in a much more favorable area of data collection.  The 12Z guidance this morning will likely finally trend towards a solution that will be the final forecast as there will now be an accurate depiction in the initialization of the model guidance.  In the above Water Vapor image I illustrate two potential out comes or camps in this scenario.  The first in magenta is a case where the phase of these two disturbances happens faster than currently forecasted.  The surface low is closer to the coast and the storm track is found from the North Carolina coastal waters to eastern Massachusetts.  The second solution is supported by the ECMWF/NAM camp from 00Z which leads to a developing surface low over the eastern edge of the Gulf Stream and thus leads to a complete miss of the northern Mid Atlantic.  The bend in the storm tracks is due to the developing upper level low that pulls the storm back west.

The storm track in magenta is a worse case scenario that I want to present because I frankly am uncomfortable with this current atmospheric set up.  Much like forecasting a perfect storm track, the same is true for a complete miss.  If timing off just a little bit, those scattered clouds can easily transition to six plus inches of snow along the coast in a short period of time.  In this case, the low pressure system rapidly deepens off the New Jersey coast leading to strong winds, heavy precipitation, and rapidly falling temperatures.  Precipitation type issues will have to be considered in this situation initially but all precipitation would go over to snow as the thermal gradient tights off the coast.

My current forecast is the storm track in red given the strong model support and trend of the ECMWF, NAM, and recent shifts in the GFS guidance at 06Z.  However, I do want to stress that confidence for this period is very low.

Beyond this storm, regardless of the outcome, the upper level pattern will be locked into one of the most favorable set ups for potential winter storms in some time.  Initially, the strength of the negative NAO will cause a suppression of the Sub Tropical jet stream through the middle of the week while cold Arctic air dominates.  Don’t be surprised if some locations pick up a quick 1 to 3 or 2 to 4 inches by passing disturbances around the upper low to the north in this arctic air mass.

Beyond Wednesday, there is very strong agreement of a building PNA ridge, a moderate negative NAO, and a contention between those two ridges.  This strongly suggest a high latitude blocking set up that not only will be cold but also very slow to break down.  Further, the ensemble guidance strongly suggest a trough axis over the eastern Mississippi Valley in this time period.  There is some disagreement on the placement of some upper level low features over southeastern Canada, but the overall idea is a pattern that is cold, slow, and supporting a high potential for coastal low pressure development.  In fact, there is growing support of a coastal storm in this time period from Thursday on through next weekend.  Again, much like the current storm, timing of these disturbances will be everything.  However, the point is that this pattern is a very favorable pattern, and I don’t expect a break down in this pattern through at least January.

3:22 PM

This morning I discussed why I was hesitant to buy into a major or moderate snow producer for the northern Mid Atlantic on Saturday afternoon through early Sunday morning.  There were two main contentions in the guidance.  The first was a lack of amplification of the ridge over the eastern Rockies, needed to support an amplifying short wave along the East coast.  The second as the relatively warm boundary layer in place that although chilly for this time of year, would not support a widespread accumulation of snow.

The latest 12Z guidance from the GFS, ECMWF, and ensemble guidance now strongly suggest a flatter and faster low pressure system can be expected on Saturday evening to the east of Long Island.  While 850 MB temperatures will be more than cold enough to support snowfall at the mid levels, boundary layer temperatures will likely hover around the 35 to 39 degree range late on Saturday afternoon.  While the best conditions for accumulating snow fall in terms of boundary layer support will be found over the interior (naturally), the best dynamics for lifting and precipitation production will be focused along the immediate coastlines of New Jersey and much of Long Island.  With boundary layer temperatures in the mid 30′s at the coldest along the coast and no guidance supports temperatures below 32 degree through Sunday morning along the immediate coast, this system clearly is not shaping up to be a moderate snowfall producer.

I’m expecting rain and snow along the coast on Saturday afternoon, changing over to snow as the evening progresses as cold air bleeds south and east towards the developing low pressure system.  The relatively warm temperatures will lead to a high water content snowfall, which will make accumulation very difficult.  While typical cold services like car roofs and the like will see a dusting to even an inch or two out of this storm, most surfaces will simply remain wet.

Meanwhile, locations further west will not be impacted all that much from this storm.  I expect the interior to have a few snow showers and even a period of brief light snow, but most of the precipitation will be focused well to the east.  Locations like the Poconos and the Hudson Valley will likely not see much from this event.

In order to support a more productive snow potential several changes will have to unfold at the boundary layer and at 500 MB.  Changes to keep an eye on would be a stronger cold air advection on Saturday morning supporting below freezing temperatures throughout the coastal plain.  Such a situation when combined with overcast skies by late morning would lead to a colder scenario.  Other factors that should be monitored is a more amplified ridge/trough relation.  This disturbance will be entering a better area of data collection tonight, so if that is going to transpire, the next two model runs (00Z tonight and 12Z tomorrow) will have to start to advertise that.  Another factor to keep an eye on, and this is a completely out in left field potential, but if the Sub Tropical jet stream pumps more moisture northward with this low, leading to a more expansive precipitation shield, that could lead to higher snowfall potential for locations north and west of Philadelphia and New York City.  Again, that is not likely with the current MJO state, but I figure I throw it out there.

A few thoughts on the medium and long range guidance of late.  The GFS and ECMWF have backed off on the large upper low over the United States Rockies that would have lead to a huge Southeast ridge over the United States.  However, there is little continuity

Para GFS- 240 hours

between model runs nor between the models themselves on how this pattern will evolve beyond this Monday.  There are signals of a true 500 MB negative NAO blocking scheme setting up on both the GFS and ECMWF (more so on ensemble guidance) and if that is the case, temperatures will be anything but warm for the northern Mid Atlantic and the potential for frozen precipitation will increase.  The PARA GFS (taking over as the operational GFS 12/15) is aggressive in forcing the Polar Vortex (PV) towards the Great Lakes and eventually towards Quebec.  This scenario would lead to a locking negative EPO/negative NAO pattern, which will make many from the Northwest to the Northeast very happy if they like winter weather.

The ECMWF goes with a completely different look from the GFS and the 00Z ECMWF guidance from

00Z ECMWF 12/1 at 240 HR

last night.  The 00Z ECMWF made many very nervous with the ideas for December (you should have seen the emails!) as the ECMWF produces an impressive and dominating PV over the Yukon and Northwest territories.  The ridge from the negative EPO pattern is nearly crushed and a ridge is starting to form over the Gulf of Mexico.  If not for a coastal low off New England, much of the northern Mid Atlantic and Northeast would be well above normal.  The idea of this large upper low seemed suspect to me in this period so I thought it was best to wait to see where the 12 Z guidance as heading.

12Z ECMWF 12/1 240 hours

The 12Z ECMWF  this afternoon is showing signs of backing off on the dominant west trough and strong upper low.  The guidance showers signs of a negative NAO forming and has the PV, once consolidated and dominating, split into three directions.  What we are seeing is that the models are having a very difficult time handling the various competing signals throughout the northern hemisphere.  There is strong support for northern latitude blocking, which can be seen in the model guidance with the potential negative NAO.  However, the uncertainty of the MJO state in this time period and beyond leads to the models split between a more La Nina look to the 500 MB pattern and the El Nino look that was experienced in October.  The trend away from a deep trough over the West coast is a sign of this uncertainty in the next move of the MJO.  For now, the best course of action is to sit tight and not over react to each model run.  I still see plenty of signals pointing towards a trough in the East and an active storm track, especially for late December.  The process to get to that point though is likely to drive many meteorologist nuts.

7:20 AM

Tropical Depression Ida

Tropical Depression Ida is expected to exit the coast of eastern Honduras by this afternoon and slowly move towards the Gulf of Mexico through this weekend.  The NHC expects Ida to strengthen back into a tropical storm this weekend and move towards the eastern Gulf Coast by the middle of next week.

Aside from the obvious threat of Ida to the eastern Gulf Coast for next week, I feel there are serious implications that need to be monitored in the forecast of an approaching trough and cold front on Tuesday and Wednesday for the Mid Atlantic.

Tuesday Afternoon-GFS-Penn State E-wall

By Tuesday afternoon, an energetic trough within the Polar jet stream will be digging through the central Great Lakes.  Both the GFS and ECMWF strongly suggest this trough will become a closed low over the eastern Great Lakes and New England, which will spawn a strong coastal storm off of the New England coast.

Neither model guidance takes much consideration in the potential influence that Ida will have on this trough in terms of additional moisture and latent heat released energy.  Now, the reasoning for the lack of interaction and the resulting wandering of Ida around the Gulf of Mexico is that the pattern is too progressive to pick up Ida and entrain the tropical cyclone into the 500 MB trough.  The question here is timing.

However, take note that at several levels of the atmosphere that some interaction does attempt to take place.  At 500 MB, additional PVA is generated and a short wave over the Gulf Coast does start to develop.  Latter in the period, this shortwave minors outs.  At 700 MB, tropical moisture is clearly becoming entrained into the southern portions of the cold front, but because Ida is never clearly entrained into the trough, the moisture remains over the Southeast.

To be frank, I’m a bit uneasy about this situation that may unfold.  The 18 Z GFS along with 11/5 12Z UKMET and the 00Z UKMET both suggest that the threat is there for Ida to become entrained in this trough and become an impressive coastal storm for the Mid Atlantic for the later part of this week.  The potential here is all about timing.  I think this threat is real and will need to be monitored in upcoming model guidance.  If Ida does indeed reach the eastern Gulf of Mexico waters by Tuesday morning, then I think the potential for a second coastal storm following the initial storm over New England is a real threat.  Consider, that the first coastal storm could potentially slow down the Polar jet stream just enough to allow the remnants of Ida to move up the East coast for next weekend.

So for now my confidence is low on this forecast as there are still too many unknown variables here.  However, the threat of Ida in my opinion is greater than just a weak tropical storm wandering the Gulf of Mexico and may extend it’s influences up into the northern Mid Atlantic by the end of next week.

Additional Thoughts (7:50 AM): The new Canadian guidance also  now suggest the potential threat with a deepening low pressure system over the Southeast and a strong high pressure system over the eastern Great Lakes by Wednesday night.  The result is an increasing pressure gradient over the northern Mid Atlantic with moderate rainfall approaching Philadelphia on Wednesday evening.  The difference between the UKMET and Canadian from the other guidance is the focus of the base of the trough having the strongest PVA.  Again, this MAY be the result of the model guidance of the UKMET and Canadian including the potential increase in Latent Heat Energy into this equation.  The pattern is slightly progressive as the 500 MB trough is forced to hang back with the majority of the energy at the base of the trough over the Gulf Coast rather than the Great Lakes.

The potential impact of this storm may be significant by the end of next week as the remnants of Ida will have plenty of tropical moisture to work with, a strengthening thermal gradient over the Mid Atlantic to enhance lifting, and a strong high pressure system over the Great Lakes to enhance the pressure gradient.  In short, a potentially nasty heavy rain and wind storm and significantly less potential for near or above normal temperatures due to this storm.

Long range, the potential impacts of this storm may increase the rate of development of the negative NAO pattern I envision for December and may force changes in the large scale 500 MB to develop faster.  All of this is still rather “foggy” to put it in weather terms, Ida could simply miss the trough interaction and have a very limited impact on the entire forecast pattern.  However, given how poorly the GFS and ECMWF have handled features in and around the Gulf Coast this year (mostly with the Sub Tropical Jet Stream) I would caution on the idea of leaning too heavily on the GFS/ECMWF idea of a quiet end of next week.

5:15 AM

Water Vapor- Eastern United States

The upper low that had dominated the weather conditions over the Plains and Mississippi Valley for several days has been ejected to the east and is rapidly weakening.

The remnant short wave trough over the Ohio Valley is still pulling a great deal of moisture to the north into the Southeast, Mid Atlantic, and Northeast.  As a result, a band of moderate to at times heavy rain has been moving through the Philadelphia and New York City metropolitan areas.  However, with strong lifting also comes strong sinking air, which continues to mark east this morning towards the region.

Rain will continue through the morning hours followed by precipitation more scattered in nature through the evening hours.

The disturbance that will kick the remnant upper low to the east will also intensify over the western Great Lakes and will eventually form into a strong upper low.  Initially, the strong upper low will send an impressive cold front through the region on Monday afternoon and evening with rain, heavy at times, and a few strong thunderstorms.

500 MB Pattern on Wednesday Morning

By Tuesday, the strong upper low will start to creep towards New England and will be a dominant influence of the forecast area.  The pattern over North America and frankly much of the Northern Hemisphere is very amplified by this time period.  The position of the ridges and troughs throughout the region strongly suggest a very volatile winter in story.

The position of the upper low will produce convergence and confluence over the eastern Geat Lakes through the southern Mid Atlantic.  The upper level pattern will lead to a developing strong area of high pressure to remain in control through Saturday.  As a result, dry conditions and cool temperatures can be expected.

The upper level pattern over North America strongly suggest that the coolest air masses will remain over

GFS VS ECMWF

much of the Continental United States.  Note the maps to the right.  These maps represent the forecast through day.  The upper level blocking pattern over central Asia will force the coldest air masses to move towards North America.  Meanwhile, the strong west based negative NAO will keep cool air locked into the forecast area.  The coldest conditions in the United States however is expected to be over much of the West coast.  A pattern of this nature, clearly influenced by the positive state of the PDO, will keep the coldest air over the West coast, however the negative NAO block will keep the cool yet modified air masses to march to the east over the next forty-either hours.

8:45 AM

Water Vapor This Morning

The pattern this morning is is dominated by two key features.  A strong trough over the Great Lakes and an upper low over the central Plains.

The upper trough over the Great Lakes is still centered to the west of the region, which means a southwesterly flow remains over the Mid Atlantic.  As a result, the cold front that produce showers and a few thunderstorms early this morning, has stalled along the New Jersey coast this morning.

Note how the upper low and the trough over the Great Lakes interacts.  The upper low is pulling a significant amount of moisture northward, and the upper trough is training the moisture over the forecast area.

As the upper trough slides to the east this afternoon, the surface cold front will finally exit the coast.  With the trough to the east, high pressure will dominate the region with great conditions tonight through Saturday.

The upper low will eventually impact the Mid Atlantic by this Sunday as a short wave trough, which will produce rain on Sunday morning, possibly heavy at times.  As the energy from the upper low interacts with the Polar Jet stream, a deep trough is going to form over the eastern third of the nation bringing much cooler air to the Mid Atlantic, Northeast, and Great Lakes.