9:14 AM

A stretch of great weather conditions continues today with strong high pressure over the Ohio Valley.  The high pressure system, producing a moderating west and northwesterly wind, will slowly move to the East coast over the next several days.  The shift in the high pressure system to the Tennessee Valley and eventually the southern Mid Atlantic coast will lead to a developing southwesterly wind and highs in the 50′s returning for several days for the first time since November.

However, trouble is brewing over the West that will eventually lead to a strong and slow moving area of low pressure producing rainfall for several days.  While there is some disagreement on the exact details and timing of the the surface low pressure system among model guidance, there is a strong agreement that a strong draw of moisture from the Gulf of Mexico and then the Atlantic will impact the region with several days of periods of moderate to heavy rainfall with otherwise overcast skies and drizzle.  Overall a very raw forecast period awaits the Philadelphia and New York City metropolitan area starting Thursday evening and continuing on through Sunday morning.

7:08 AM

A cold front is approaching the Philadelphia and New York City metropolitan area this morning with snow showers for most locations as the front progresses to the east.  This cold front will move through the northern Mid Atlantic today with scattered rain and snow showers.  Temperatures this afternoon will peak in the lower 40′s along the coast and upper 30′s over the interior ahead of the cold front, causing the precipitation with this cold front to turn to rain showers as the line reaches the coast.

The Polar air mass behind this cold front will set up a potential moderate winter storm for Friday.  High pressure behind the front will move from western Ontario to the St. Lawrence River Valley this evening through Thursday evening, producing scattered clouds, dry conditions, and supporting the building Polar air mass over the region.  As a result, high temperatures on both Wednesday and Thursday will range from the lower to mid 30′s throughout the region.  This air mass is not an Arctic air mass and temperatures will be marginal for frozen precipitation, but a clear thermal gradient from the southern Mid Atlantic to the northern Mid Atlantic will be established which will lead to a tricky forecast period Thursday night through Friday night.

While the Polar jet stream reintroduces cold air back into the northern Mid Atlantic, a Sub Tropical disturbance will enter the southern Plains and interact with a stationary frontal boundary.  This frontal boundary is the same front that is moving through the northern Mid Atlantic today.  An area of low pressure will develop along this front and travel eastward towards the Tennessee River Valley on Thursday night and then jumping to the southern Mid Atlantic coast on Friday morning.  Now, here is where this storm gets interesting!

As I discussed last night, the timing of several features will be key in determining precipitation type throughout the region and the intensity of that precipitation.  This is clearly a storm that will be tracking west to east through the Plain.

ECMWF-Thursday PM

The first key to keep an eye on is that high pressure system over Ontario.  The stronger the push of the low level and mid level cold air from this surface high pressure, the more likely the precipitation will be snow or sleet for many locations from Philadelphia to New York City.  The 500 MB pattern for the Polar jet stream does provide for convergence and confluence aloft, albeit in a rather unusual way compared to a normal pattern for snow production.  The cold air mass is introduced via a trough over Ontario that progresses towards the Canadian Maritimes.  The position of this trough and the ridge over central Canada leads to a pattern where surface high pressure is supports over Ontario, Canada.  The developing Sub Tropical disturbance over the Mississippi Valley and entering the Tennessee Valley enhances this convergence aloft, thus allowing the surface high pressure system to remain in place.  The position of this high pressure system gives support to keeping a low to mid level cold air mass in place.  This set up is support on the majority of model guidance.

So next key is the Sub Tropical disturbance.  The 500 MB Sub Tropical disturbance is rather strong, which has me concerned about precipitation types along the coastal plain.  A strong low pressure system over the Tennessee Valley may force an easterly wind along the coast depending on the location of the high and low.  This would lead to warming of the boundary layer and as a result the precipitation would start as rain on early Friday morning.  However, if the surface low over the Tennessee Valley remains relatively weak as a new coastal low forms over the Virginia/North Carolina coastal waters, then the combined influence of the high pressure system to the north and the developing coastal low to the south would enhance a northeasterly wind along the coastal plain and thus support a colder solution.  As the low pressure system intensifies off the Virginia/Delaware coast on Friday morning, precipitation will become heavy as the cold air mass drives towards the deepening area of low pressure.  This solution is seen on the Canadian guidance very well along with some support from the UKMET and ECMWF guidance.  If this solution is verified than a moderate to even a heavy snowfall is possible on Friday throughout the Philadelphia and New York City metropolitan area with the exception of extreme southern New Jersey where boundary layer temperatures would likely remain above freezing at all levels.

The primary lifting mechanism for precipitation will be isentropic lifting until the coastal low can become established.  As such, the thermal gradient ahead of this developing low pressure system will be key and likely will not be handled properly not only at 850 MB but at the surface as well until 24 to 48 hours ahead of the event.  Given the factors above, there is just as much chance as the majority of this precipitation to be all rain as it is to be all snow or a mix of snow, sleet, and rain.  Much of this forecast depends on the interaction and development of the high pressure over Ontario and the Sub Tropical disturbance exiting the Plains.  Changes in the forecast are likely and confidence is on the low side.

After this storm exits, the Canadian high pressure is expected to continue to build south, providing clearing skies and dry conditions on Saturday along with highs in the 30′s throughout the region.  The high pressure system will slide to the east of the region on Sunday, allowing for some modification of the air mass.

A very strong cold front will approach the region on Monday.  Strong warm air advection ahead of the cold front on Sunday night and Monday will support very warm conditions in the morning with some locations breaking 50 degrees ahead of the cold front passage.  However, as the cold front exits, temperatures will fall rapidly through the 40′s and into the 20′s and 30′s.  Heavy rainfall can be expected with this front with the potential for even some convective (thunderstorms) embedded in the heavy rainfall along with the potential for flash flooding.

8:02 AM

Last night, an impressive low pressure system cut through the eastern Great Lakes, producing moderate to heavy rain over much of the Philadelphia and New York City metropolitan areas.

The heavy rain has come to an end with the linger threat of a few scattered showers through the morning hours.  Temperatures are very warm this morning, exceeding forecasted temperatures for this period with many locations in the lower to mid 60′s throughout southern New Jersey.  The cold front is currently over central Pennsylvania and will move through later this morning.  Today’s highs are basically occurring now.  Temperatures will fall through the 50′s and 40′s this evening as winds veer to the west and northwest at around 10 to 20 mph, decreasing to 5 to 15 mph as the evening continues.

As the Polar air mass takes hold tonight, high pressure will provide clearing skies and dry conditions through Friday afternoon.  A 500 MB trough will be established over the eastern two-thirds of the nation as cold air pours into the United States.  A strong disturbance within this trough will dive towards the Gulf Coast on Friday night and lift northeast towards the Mid Atlantic coast on Saturday.  A coastal low will develop along the old cold front boundary and lift northeastward towards the coastal waters of New England.

My thoughts on the potential snowfall for Saturday night have not changed much.  While many point to many of the indicators that would support snowfall like 850 MB temperatures below 0°C, 1000-500 MB thickness below 540, and other typical indicators; the one factor that has not changed one bit in the model guidance is the relatively warm boundary layer temperatures.  Temperatures for this forecast period still remain above freezing for roughly 90 percent of the time period that precipitation moves through the northern Mid Atlantic.  Now, if the indicators discussed above were present in later this month or in January and February; I would be much more aggressive with potential snowfall.  The reason being that ocean temperatures would be cooler and the boundary layer would be naturally colder under these conditions.

What I expect on Saturday evening is a mix of rain and snow throughout the northern Mid Atlantic will gradually change over to all snow.  The lifting via the strong 500 MB disturbance and natural evaporational cooling will lead to temperatures throughout the atmosphere to cool.  Temperatures will gradually fall upper 30′s and lower 40′s to the mid to upper 30′s.  All guidance suggest that the surface freezing line remains over interior eastern Pennsylvania through at least 3 AM Sunday.  Towards early Sunday morning, temperatures will fall into the lower to mid 30′s throughout the northern Mid Atlantic, which will support some slight accumulations.

As for the areas of best lifting, the focus is clearly towards the New Jersey coast and Long Island.  Actually, the best dynamics will be found over the coastal waters.  Locations north and west of Philadelphia and New York City will have some precipitation from this coastal low, but will be generally light and scattered.

I plan on issuing a preliminary snow map tonight and will continue to update through Saturday morning if changes are needed.  Overall, I expect a trace to an inch of snow on cold surfaces (car tops and what not) along the coast as much of the snow will melt while falling on the ground.  Over the interior locations of northeastern Pennsylvania, higher elevations of Sussex County in New Jersey, and the Hudson Valley; one to three inches will be possible as what ever does fall will have the best chance to accumulate.

Once the low pressure system exits on early Sunday morning, cold high pressure will take hold by Sunday afternoon and remain in control through Tuesday with dry conditions and temperatures averaging near to slightly below normal through the period.

Towards the end of the week, another strong low pressure system will be making an impact on much of the Philadelphia and New York City metropolitan area.  There has been some slight yet important changes in the model guidance for this period compared to previous runs.  The Polar Vortex is been adjusted further to the east and north compared to previous runs of the past few days on the ECMWF and the result is that the trough axis throughout North America is a bit further east as well.  This is a case of a strong negative NAO block shifting a storm track, but rather the model guidance having a better handle on the negative EPO/negative AO pattern and thus the implications of that pattern.

On Wednesday, a strong disturbance with the Sub Tropical Jet stream will interact with the Polar jet stream.  How this disturbance interacts with the Polar jet stream will be key for the storm track.  The latest 00Z guidance strongly suggest the PVA being forced towards the southern Mid Atlantic coast, strongly suggesting coastal redevelopment.  Now, I think it would be wise to not go into details as far as boundary layer temperatures and the like, however this scenario would certainly support a significant snowfall for those in northeastern Pennsylvania through the Hudson Valley.  Further, unlike this Saturday, there is growing support for a colder boundary layer even along the coast depending on storm track and timing of the redevelopment.  So this storm should be watched carefully over the next several days.  I would suggest not to worry about the details of this storm until early next week, but instead focus on the 500 MB pattern.

As for long range thoughts, I still am wary of buying into the negative EPO collapsing as fast as it does on the GFS and ECMWF beyond 180 hours.  The idea continues to suggest that the strong Polar Vortex over central Canada retrogrades west and leads to a strong and deep trough along the West Coast.  As stated before, a combination of a negative EPO and negative AO pattern tends to last longer than model guidance suggest as the two features feed off each other.  So I don’t have a great deal of confidence in the forecast beyond day 7.

7:22 AM

Water Vapor- NWS/NOAA

A strong upper level disturbance developing over the southern Plains is building a ridge over the Southeast and southern Mid Atlantic this morning.  This ridge of high pressure will continue to build northward through the day as surface high pressure gains control over much of the Mid Atlantic.  The disturbance will eventually cut off from the Polar jet stream and become stationary over the Mississippi Valley through Wednesday.  As a result, high pressure will remain over New England through Thursday providing clear skies and temperatures slightly above normal in the 50′s for most locations.

The strong upper low will move towards the Ohio Valley on Thursday and become phased into the Polar jet stream once again.  A cold front will move through the New York City and Philadelphia metropolitan areas on Friday with periods of rain, heavy at times.  Currently, the guidance suggest a slow cold front passage in the morning.

The weekend once again is looking rather interesting for much of the Mid Atlantic as the GFS and ECMWF to varying degrees suggesting another coastal storm is possible for Sunday and Monday.  The idea is that the cold front that slowly exits on Friday will stall over the Gulf Stream and off the North Carolina coast.  A disturbance from the Polar jet stream drops south into the Tennessee Valley and then the fun begins as a new coastal low takes shape and parallels the New Jersey coast on Sunday night.

Now, my thoughts on this is that the pattern is still progressive at this point.  However, it is clear that a pattern change is on the way at the end of this forecast period.  Beyond Sunday, there is strengthening consensus of a pattern change.  When the pattern changes, storms usually are the result.  So a storm in this time period is not unreasonable.  However, I would caution in trusting the details of this potential storm.  I should also note that this storm lacks a cold source, at least the way the guidance projects this storm right now.  The majority of the Polar air is still over the northern Plains and Rockies at this point.  In order to get that cold air towards the East coast, a much more significant and slower storm would have to develop.

Extended Thoughts:

The flipping and flopping of model guidance for the pattern change is certainly flipping more towards cold than warm of late.  Of course, the support for the change in the pattern is not due to models saying yes, but what is going on at the stratosphere as temperatures at 30 MB, 50 MB, and 70 MB remain well above normal.  Even a decrease in temperatures in the next few days would still keep temperatures well above normal at this level and that type of warming will have an impact on the troposphere.  The strongest warming is occurring over the Aleutians and over the Sea of Japan, which strongly would suggest a negative EPO pattern is about to begin in the next two weeks, right about when December begins.  ECMWF forecasts by Thanksgiving suggest this warm anomaly will move towards much of Canada, which should lead to a negative Artic Oscillation for December and interesting weather events for much of the Continental United States.

5:25 AM

High pressure will produce clear to scattered cloud cover through this afternoon.  As the high pressure system shifts off the coast, clouds will increase tonight with broken to overcast cloud cover yet dry conditions.  On Friday, a warm front will drive through the coastal waters and stall around the Long Island Sound with overcast skies and scattered showers.  By Friday evening, widespread showers can be expected over the northern New Jersey coastal waters through the Long Island Sound, however only isolated showers with overcast conditions are expected to the south of Long Island and over the central and southern New Jersey coastal waters.  An area of low pressure will move from the Ohio Valley through the Hudson Valley on Saturday morning, which will force a cold front through the coastal waters.  Widespread showers can be expected through the day on Saturday followed by clearing conditions on Saturday evening.

Winds will be from the northwest around 5 to 15 mph through this afternoon.  Tonight, winds will back to the south around 5 to 15 mph and remain from the south around 10 to 20 mph through Friday afternoon.  On Friday night, winds will veer to the southwest around 5 to 15 mph and then to the northwest around 5 to 15 mph through Saturday.

Temperatures will rise into the lower to mid 60′s for afternoon highs today.  On Friday, temperatures will fall into the lower to mid 50′s for morning lows and rebound into the mid 60′s to lower 70′s for afternoon highs.  On Saturday, temperatures will fall into the upper 50′s to lower 60′s for morning lows and rebound into the mid to upper 60′s for afternoon highs.

Water temperatures will range from the mid to upper 60′s through the forecast period for the entire coastal waters.

Wave heights will range from 3 to 6 feet through this afternoon then subside to 2 to 4 feet tonight through Friday morning.  Wave will rebuild to 3 to 6 feet Friday night through Saturday.

A small craft advisory is in effect through this morning.