11:00 PM

In this morning’s post I discussed the threat of a stronger coastal low due to a more pronounced coastal front.  The guidance from 12Z and 18Z took a troubling turn in suggesting just that scenario.  One of which, that even suggest that the coastal low becomes the primary low SOUTH of the northern Mid Atlantic.  This would have significant implications for the entire forecast area given that a cold Arctic/Polar high pressure is positioned to the north of this storm on all guidance from Friday through Sunday.

The latest 00Z NAM continues to illustrate the threat of a 1040 MB high remaining over the St. Lawrence Valley and a new coastal low taking shape.  The primary low over the Mid West weakens from a strong area of low pressure of 996 MB to 1004 MB from Friday afternoon to Saturday morning.  Meanwhile the coastal low continues to deepen to 1010 MB.  The NAM still suggests that the primary is over the interior, how the threat of a sustained CAD can not be ignored.  One aspect that stands out very well is that thickness from the 1000-850 MB would support frozen precipitation.  The NAM also illustrates the transition from a moisture advection from the Gulf of Mexico to one from the Atlantic, which means a steady stream of precipitation will develop over the northern Mid Atlantic from Friday evening on through Saturday morning.  Even more interesting to me is the 500 MB and 300 MB developments in this guidance.  Now, I don’t take the NAM beyond 66 hours verbatim, as there is usually significant error involved, how there is support for PVA driving towards the coast as the old primary becomes stacked over the central Great Lakes.  Further, at 200 MB and 300 MB, there is a strong jet streak that develops from the Gulf of Mexico into the southern Mid Atlantic.  The position of this jet streak strongly supports a strengthening coastal low around the North Carolina/Virginia coastal waters.  Further, there is additional support for strong convergence and confluence over southeastern Canada and northern New England.

UKMET

Now, if the NAM was the only guidance showing this potential, I would simply disregard the NAM as being too aggressive with the CAD.  However, there is growing support of a stronger coastal low development on the SREF ensemble and mean guidance, Canadian, ECWMF, and most impressively on the UKMET from 12Z.

The UKMET, seen to the left, is the most troubling of the solutions.  The 12Z UKMET is the first guidance to have to very important changes.  The first is to portray the transition of the primary circulation of this low pressure system to the coast to the south of the northern Mid Atlantic.  The second change is to significant slow down the progression of this entire storm with persistent precipitation from Friday night through Sunday evening.

The significance of a transfer of the primary low to the coast is an important change in the UKMET model guidance and has been hinted at by the NAM for some time.  The development of a strong jet streak up to the Mid Atlantic coast on the NAM would give support to the UKMET solution.  So what impacts would this have on the precipitation type across the region.

To the right is a depiction of just such a solution.  On this map I have the primary low weakening rapidly, which is supported by all modelguidance for Friday evening.  The coastal low begins to take over at the triple point or the location where the occluded front meets the cold front and the coastal front.  Note, that in this solution I have the coastal front slightly further east than guidance suggest.  I made this adjustment on the idea that the snow pack influenced cold boundary layer will force the thermal gradient slightly further east than guidance suggest.  Since the secondary low will form along the developing coastal front, I think the development will like be forced further to the east.  Meanwhile, the cold Canadian high pressure will continue to enforce the cold air south into the coastal plain of the northern Mid Atlantic.  The ridge over the St. Lawrence Valley in combination with the Polar Vortex diving south will lead to a pattern slowing down, thus supporting a blocking mechanism for the old primary low.  The coastal is therefore forced to take over.  As the new primary low takes shape, cold air is drawn to the coast to try to fill the new coastal low to the south and east.  Strong moisture advection from the Atlantic and Gulf of Mexico will combine with strong isentropic lifting and synoptic lifting from PVA driving east to produce heavy precipitation over much of the northern Mid Atlantic.

I still believe the greatest threat from this storm will be over interior eastern Pennsylvania through the Hudson Valley where several inches of snow will fall along with the potential for significant icing.  The continued cooling of the 2M temperatures on most model guidance suggest that the guidance does not have a handle on the boundary layer that will be in place.  Further, if the primary low does take shape along the coast, the overall forecast will end up much colder and whiter than currently forecasted.

More thoughts on this potentially dangerous situation tomorrow morning!

7:05 AM

The model guidance in this period is in some disagreement on how to handle the upper level development of the pattern and therefore the surface features.  I think the GFS is an outlier here and leaned more on the ECMWF with some tweaks here and there.  

On Sunday afternoon, a disturbance in the southern branch will move from the Plains into the Ohio Valley with rain entering the forecast area by late on Sunday evening.  High pressure will linger over New England, but will be slowly exiting into the Atlantic towards Monday morning.  The collapsing negative NAO pattern will have enough influence to forcing the primary low over the southern Ohio Valley to redevelop towards the New Jersey coast on early Monday morning.  With a weak CAD in place and a redeveloping low off the coast, cold air will linger over the Hudson Valley and interior Connecticut to produce a mix of snow, sleet, and rain over these locations.  However, while I think that accumulating snow is possible, I think mixing issues will limit snowfall amounts.  Elsewhere, rain can be expected Sunday evening through Monday morning.

High pressure will build into the forecast area once again on Monday afternoon through Tuesday with partly cloudy skies and temperatures ranging from slightly below normal to near normal.  

The pattern evolution still suggests that a weak trough will influence the conditions over much of the Northeast and northern Mid Atlantic with an active and volatile storm track.  However, snow chances will be more likely over the interior with a cold rain the overall precipitation along the coast.  While much of the nation will moderate with above normal temperatures going into next week, the overall theme over the forecast area with near to slightly below normal temperatures.

6:36 AM

Next week will feature a storm track that will begin over the southern Plains and head towards the eastern Great Lakes.  This type of storm track is NOT a snow producer, but has the potential to develop an ice situation for some locations in the forecast area.

On Monday, the cold front will clear the coast with high pressure rapidly building in behind the departing front.  The developing negative NAO and 50/50 low will support high pressure to the north of the forecast area through Tuesday night, which will support near normal temperatures and clear to partly cloudy skies.

On Tuesday night, the first of two low pressure systems will move towards the eastern Great Lakes.  With high pressure positioned over Quebec, I think there is a slightly potential for cold air to be “trapped” over portions of the Hudson Valley and northeastern Pennsylvania in the form of a weak CAD ahead of the warm front.  There is a potential over the interior for freezing rain on Tuesday night into Wednesday morning.  The rest of the forecast area can expected scattered showers.  Temperatures will warm above freezing throughout the entire forecast area by late Wednesday morning producing scattered showers throughout the region.  

A stronger and wetter low pressure system will move towards the eastern Great Lakes on Wednesday night through Thursday morning with heavy rain throughout the forecast area.  With a lack of high pressure to the north, even the interior will have all rain from this low pressure system.  

Clearing conditions can be expected by Thursday afternoon with temperatures beginning to fall quickly in the late afternoon due to strong cold air advection.

9:04 AM

While the forecast is certainly going as planned, even the thoughts about the mid level forcing have worked out, I am noticing an interesting trend at 850 MB that bares watch.  Note over western Pennsylvania that the progression of the 850 MB freezing line has stalled from moving north and then strong cold air advection (CAA) starts to drive through Ohio and into Pittsburgh.  This has several implications for the forecast area.  

First and foremost, if the upper low at 850 MB is still weak and remains so, then the push of warm air to the north into the Hudson Valley is going to have some issues in verifying.  This also means that portions of northeastern Pennsylvania and the Hudson Valley north of White Plains, New York will have an increasing chance of remaining all snow for this storm and not mix, certainly not change over to rain.  Further, the CAD that has develop and looks to remain sustained over western New Jersey through much of eastern Pennsylvania will support much colder temperatures than forecasted.  This is exactly why I didn’t buy the idea of rain in these locations in the first place.  Never underestimate the power of a cold, dry air mass.  

As I monitor the storm through this afternoon, I am going to keep an eye on just how strong the push of warm is at 850 MB and also the temperatures at the surface.  The colder temperatures are in the mid and lower levels, the more likely that the back end of this storm is going to produce more frozen precipitation than thought earlier.

5:34 AM

Good Morning Everyone!

So far this morning the forecast is pretty much on track.  Precipitation has changed over to rain over the immediate southern New Jersey Coast or basically from Atlantic City southward through Cape May County.  A mix of snow, sleet, and freezing rain is falling over interior southern New Jersey into Philadelphia as of 5 AM.  Snow continues to fall over locations north of Philadelphia through central New Jersey and into New York City.  

The banding I was concerned about is starting to develop with heavy precipitation over parts of the forecast area.  Specifically, a heavier band of snow and sleet has formed over Philadelphia through Monmouth County.  This band of snow over Freehold is producing heavy snowfall, and I can report snowfall amounts over 2 inches of snow already.  As discussed in the forecast, the banding will intensify through the morning hours.  

A few interesting observations as well.  First, the CAD is holding strong (big surprise) as there is a defined ridge right over the forecast area, which is going to slow the progression of the low level warm air into the forecast area.  Winds are from the northeast with the exception of the immediate coast where winds have veered to the east.  Temperatures just away from the coast area in the mid to upper 20′s, so before precipitation changes over to rain, there will be a few hours of moderate to potentially heavy icing over the forecast area.