06/09/10 7:53 AM

Over the past two days, weather conditions couldn’t have been much better.  Clear skies, low humidity, and high temperatures in the 70′s have been the trade mark of weather conditions since Monday.  However, the excellent weather conditions will take a brief leave by this afternoon.

The same disturbance that produced severe thunderstorms throughout the Plains is marching east.  However, the disturbance is falling under the influence of upper level confluence.  Basically what is happening is that the upper level winds of the Polar jet stream and Sub Tropical jet stream are interacting in such a way to create convergence at the upper levels.  When this happens, a disturbance usually becomes sheared apart, which is starting to happen this morning over western  Pennsylvania.

Showers will continue to march towards eastern Pennsylvania this morning with increasing clouds throughout the northern Mid Atlantic.  Due to the dry air in place at the surface (dew points in the 40′s leading to relative humidity in the 50% to 70% range), the initial wave of rainfall over central Pennsylvania will not be as impressive as seen on the radar due to some verga.  This is where rain falls from the clouds but evaporates before reaching the surface.  However, the verga effect will not last long and I expect showers to overspread the northern Mid Atlantic by this afternoon.

The surface low pressure system will redevelop along the New Jersey coast, roughly just east of Monmouth County of New Jersey and south of Queens County of New York, tonight which will lead to periods of rain for most locations east of the Delaware River and more scattered showers over eastern Pennsylvania.  The rain will exit by tomorrow morning with gradually improving weather conditions.

High pressure will take hold on Friday and Saturday with scattered clouds, but generally dry conditions.  A few isolated showers may develop in the afternoon on Saturday due to an approaching mid level disturbance, however overall weather conditions will be tranquil with temperatures in the 70′s an 80′s.

A warm front with a series of disturbances will slowly move through the northern Mid Atlantic on Saturday night through Sunday with periods of scattered showers and thunderstorms along with much warmer temperatures.  Weather conditions are expected to be rather warm with highs in the 80′s and potentially lower 90′s along with an increase in humidity.  The precipitation will be mainly convective leading to a higher threat of precipitation in the afternoon however the coverage will be move scattered in nature.  I do not expect severe thunderstorms to be a threat as the dynamics or lifting is really not that impressive.

A cold front will follow on Monday afternoon with another round of showers and thunderstorms followed by clearing skies by the evening and a return to a more pleasant Polar air mass.  However, the return of humidity will not be far off as high pressure takes hold on Tuesday.

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04/08/10  5:54 PM

When looking at the temperatures this evening, it’s pretty clear were the cold front is this evening.  Temperatures ahead of the cold front this evening are in the mid 70′s to mid 80′s throughout the northern Mid Atlantic even the mid to upper 80′s over portions of Mercer and Monmouth County of New Jersey.  The air mass is clearly unstable, supporting the development of strong to severe thunderstorms for the overnight hours.

The cold front, currently over central Pennsylvania will move into the Philadelphia metropolitan area by mid night with a line of showers and thunderstorms.  The cold front will slow down in moving east due to the orientation of upper level winds remaining nearly parallel to the surface cold front.  The cold front will slowly move through the New York City metropolitan area on early Friday morning through the morning rush hour before exiting by the late morning hours.  The strongest thunderstorms are expected between 3 AM and 8 AM and will be capable of wind gusts over 40 mph, heavy downpours, frequent lightning, flash flooding, minor wind damage, and reduced visibility at times below 2 miles.

High pressure will gain control by tomorrow afternoon with clearing skies and significantly cooler temperatures with highs in the 60′s.  High pressure will remain in control through this weekend with a dry, cool northwesterly flow leading to clear skies and temperatures back to near normal.  High temperatures this weekend will range from the upper 50′s to upper 60′s throughout the region.

A northwesterly flow will continue through early next week with weak disturbances diving southeast through the northern Mid Atlantic producing widely scattered to isolated showers and thunderstorms.  Otherwise, tranquil conditions can be expected with more spring like conditions and highs in the 60′s and 70′s.

9:30 AM

High pressure has exited the New Jersey coast this morning as a warm front slowly moves north towards the Hudson Valley and Connecticut this afternoon.

The warm front is currently north of Philadelphia cutting through Monmouth County and is just south of Long Island. Clear skies and dry conditions are being observed throughout the region due to the strong lifting and convection over the Tennessee Valley and Southeast, which is essentially robbing the moisture advection associated with the warm front and creating an area of sinking air over the northern Mid Atlantic.  Note the air of dry and red color on the water vapor satellite image to the left.

However, increasing clouds can be expected by this afternoon with showers entering the region late this evening and overnight.  The strong Sub Tropical disturbance will drive towards New Jersey by tomorrow afternoon with periods of rain, heavy at times.  The heaviest rain is expected over the Delaware River Valley through the Hudson Valley.  A half inch to an inch of rain is expected along the coast and through much of Long Island into eastern Connecticut.  Heavier rain, over an inch to as much as 3 inches of rain, is expected just to the west where mid and upper level dynamics will be the strongest.

The rain will exit by Tuesday morning with clearing skies by Tuesday afternoon.  Flash flooding will be a significant threat for portions of northern New Jersey and the Hudson Valley considering the state of the rivers throughout the region this morning.  Caution should be taken!

High pressure will return on Wednesday and Thursday with dry yet cooler conditions with highs generally in the 50′s and 60′s.  It should be noted that these temperatures are generally within normal ranges.

Another area of low pressure will move through the northern Mid Atlantic on Friday with periods rain and possibly some snow mixing in at the highest elevations.  Otherwise, a light to moderate rain is expected from late Thursday night through Friday afternoon.

High pressure will regain control with significantly colder conditions on Saturday and Sunday.   Temperatures will struggle on Saturday with highs in the 40′s for most locations however by Sunday afternoon temperatures will rebound into the 50′s and 60′s once agin.  Those with farming concerns should take heed on Saturday and Sunday morning as temperatures will be near to slightly below freezing on both mornings!

8:25 AM

A new coastal storm is starting to take shape off the North Carolina coast this morning and has produced periods of moderate to heavy rain along the immediate coast and a mix of snow and sleet over the northwestern interior.  In fact, moderate to heavy snow is growing in coverage and intensity this morning from the Hudson Valley on east through Connecticut.

As the coastal low intensifies this afternoon and evening, the precipitation shield will expand westward and become more intense with periods of heavy rainfall along the coast and significant accumulating snow and ice over northeastern Pennsylvania through the Hudson Valley and towards interior Connecticut.  However, unlike previous storms, the old primary low over Lake Erie this morning, has done an excellent job is eliminating any cold air source for much of the Mid Atlantic, thus allowing the rain/snow line to penetrate well into the interior.  As such, rain will likely mix with snow as far north as the northern Delaware River Valley and along the Connecticut coast.

As the coastal low moves east of the New Jersey coast this afternoon and evening, heavy rain is expected from Cape May County through Monmouth County of New Jersey and through much of the New York City metropolitan area including Long Island.  The potential for flash flooding, especially of storm drains, will be an issue this evening with significant ponding on the road ways expected.  Meanwhile, as the coastal low moves north, colder air will work from west to east over Pennsylvania, changing much of the rain/snow mix to all snow and supporting moderate to minor snowfall accumulations.  Heavy snow will fall further north with snowfall amounts pushing above a foot in some locations over New England.

A break in the precipitation is expected by tomorrow afternoon with overcast skies as the first coastal storm pushes north and east towards the Canadian Maritimes.  However, another storm is already starting to develop over the Plains.  Note the water vapor image above.  A strong Sub Tropical disturbance (disturbance 2) is already producing moderate to heavy snowfall over central Texas while a strong Polar disturbance is diving south from the northern Plains.  These two disturbances will eventually phase into a strong upper low over the Tennessee Valley and support rapid cyclogenesis or low pressure development off the North Carolina coast.  There is strong agreement in the model guidance of these series of events unfolding.  At this point, due to the strong cold air advection behind today’s storm, the northern Mid Atlantic is cold enough to support frozen precipitation if not all snow for any potential storm that develops, so the stage is set for Thursday for what could be a significant snowfall through Friday afternoon.

The problem is that the development and track of the 500 MB upper low is going to be a dominant determining factor in the impacts of the winter storm for Thursday evening through Saturday.  The model guidance generally agrees that the upper low will track towards the Mid Atlantic and intensify, corresponding to an intensifying low pressure system from 700 MB down to the surface.  However, there is strong disagreement on the small details in terms of rate of intensification and exact location, which will make all the difference between a snowfall that is plowable or just a few snow showers with the focus of the storm north and east.  It is my hope that these differences in the solution will be ironed out over the next 24 hours, however the fact that the SREF guidance, made up of 21 different members, has 21 different solutions, does not give me much confidence in the details of the forecast at this time.

What I am sure of is that Thursday afternoon through Saturday morning will have very windy conditions throughout the region with temperatures barely moving much from the lower to mid 30′s throughout the region.  In fact many locations may record temperatures remaining nearly steady for several hours through this time period.  Additionally, I expect some snowfall, the intensity and duration is again unknown, but there will be snow falling over the region.

Regardless of outcome, all guidance agrees that what ever does develop, will be exiting north and east by Saturday morning with variable cloud cover and drying conditions.  Weak high pressure will take hold on Sunday with temperatures averaging near normal.

Into early next week, there is another threat for yet another winter storm approaching from the Gulf Coast.  However, the details and track of this storm is strongly connected to what happens with the 500 MB feature of the storm on Friday, specifically where that upper low ends up.  If the upper low remains over New England, the storm track is suppressed south.  However, if the upper low ends up over the St. Lawrence River Valley, then those snow shovels will get yet another work out and a true test of endurance.

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2:40 PM

The strong Arctic disturbance is on the move this afternoon and is holding together rather well.  The latest radar images have light to moderate snow over much of the Ohio Valley this afternoon which is slowly progressing eastward towards western Pennsylvania.

My overall thoughts on this storm from the past few days have not changed this afternoon, but some interesting trends towards what I felt would be a key feature to this minor snow event is starting to show up in the real guidance.  One of those features is the strong upper low still over the Canadian Maritimes.  Model guidance had suggested this feature would start to lift north and east faster than what we are seeing this afternoon.  The reality is that this feature is going to have an influence on the strong Arctic disturbance (which is a well developed closed upper low currently) towards the Mid Atlantic coast.  What does this mean?

Well, basically this means the strongest lifting via 500 MB PVA is going to be towards the southern New Jersey coast, where a strong thermal gradient at the surface to 850 MB will begin to development.  This will spawn an area of low pressure to develop east of the Delaware Bay early tomorrow morning and progress northeastward towards southern New England by tomorrow afternoon.  I can now say the areas most likely to experience a moderate band of snow developing is from southeast Monmouth County down through Cape May County of New Jersey and the eastern portions of Suffolk County in New York.

As the surface low over the Ohio Valley weakens this evening, a new low will start to take shape, forming a closed circulation after Midnight tonight.  The transition of low pressure centers will cause the precipitation over the Ohio Valley to weaken, thus reducing the snow shield to scattered snow showers and flurries over eastern Pennsylvania.  As the precipitation shield reorganizes along the coast on Friday morning, a more steady light to moderate snow will develop.  However the fast nature of the upper level pattern will force this storm out quickly.  All snow will be over by 3 PM throughout the northern Mid Atlantic.  The peak impact of the snow will be from 6 AM to 10 AM for the Philadelphia and New York City metropolitan areas.

Another update will be made late tonight in case any changes have to be made.