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7:27 AM

The pleasant weather conditions from yesterday are about to come to an end.  So is my confidence in model guidance, by the way.  However, before I get to that, let’s take a look at the precipitation that is charging towards the forecast area. 

High pressure currently centered over eastern Connecticut and Rhode Island, will continue to slow slide to the east, which is supporting a well developed easterly fetch off the Atlantic.  Meanwhile, a warm front is approaching from the Ohio Valley.  Note how the precipitation drives through southwestern Pennsylvania and Virginia, but comes to an abrupt stop over central Pennsylvania through coastal Virginia, almost like an invisible wall is up to protect the forecast area.  In a way, one could describe the high pressure system like a wall.  Dry, sinking air from the surface high pressure and the exiting trough over the Canadian Maritimes is suppressing the advance of cloud cover and precipitation this morning.  However, as the day progresses, high pressure will begin to lose influence and clouds will increase at first with very high clouds (seen this morning over much of eastern Pennsylvania) and then with mid level and low clouds by this afternoon and evening.  Rainfall will hold off until early on Thursday morning as the best isentropic lifting with the warm front will remain to the west of the forecast area.  

The forecast for Thursday is still a tricky forecast for me, and I think the model guidance is missing some key details.  I am suspicious of the exact placement of the warm front by Thursday afternoon.  The air mass behind this warm front is humid and very unstable, and I think will have a big influence on the development of precipitation over the forecast area.  Meanwhile, the marine air mass along the coast will continue to have some support from the exiting high pressure system through at least the early afternoon on Thursday.  The low level jet stream at 850 MB will be increasing to 50 KT from the south and southeast through Thursday afternoon, while winds at 500 MB will be from the west.  Note the potential for strong directional and speed shear over the forecast area.  So let’s put everything together.  The entire forecast area will be under the influence of strong wind shear vertically via direction and speed shear.  The low level jet stream will bring plenty of moisture into the forecast area from the Gulf of Mexico and the tropical Atlantic (note the moisture building over the coastal waters of Florida).  The marine air mass will be slow to give way along the coast, meanwhile locations like the Delaware River Valley and much of eastern Pennsylvania will be under the influence of a very warm, humid, and unstable air mass.  Just these influences alone would suggest periods of heavy rain throughout the forecast area.  However, a wild card here is if any breaks in the clouds develop behind this warm front is significantly destabilize the atmosphere, thus producing strong to severe thunderstorms.  These thunderstorms would then progress east and northeast towards the coast, become elevated over the weakening marine air mass, and produce very heavy downpours.  Oh, and did I forget to mention the strong lifting at 500 MB from the upper level trough.  So clearly, Thursday afternoon will exhibit two distinct precipitation types over the forecast area, convective and isentropic precipitation.  

The fun does not end on Thursday though.  I have some doubt on the accuracy of the model guidance for Friday through the beginning of next week.  While I agree that a trough will be present over the Northeast, I have doubts on the strength of this trough that is seen on the ECMWF and GFS model guidance.  I took a blend of these guidance with the GEFS guidance, however my confidence is low on the position and strength of low pressure system through this period.  The main problem is that the model guidance is over-amplifying the trough in the East due to a disturbance exiting out of the Southwest which interacts with the trough.  Model guidance for what ever reason always has a difficult time accurately forecasting the development of upper level patterns with energy coming out of the Southwest.  At any rate, I still expect a threat of showers and thunderstorms each day from Friday through Tuesday.  There is some thought that the trough axis will move over the forecast area, which would bring a cool, cloudy, and damp marine air mass into the region by Monday afternoon and Tuesday.  For now, I am keeping the same forecast that I felt comfortable with before the latest runs of model guidance, and will go with a cooler forecast for Tuesday to illustrate the back door front potential.

10:32 AM

Weak high pressure is exiting off this coast through this afternoon while a very weak cold front is pushing south through New England and the eastern Great Lakes this morning.  This cold front will be a key player through much of this coming weak, as I will explain shortly.

For today, expect clear skies and very comfortable temperatures for this afternoon and this evening.  The cold front that will stall over the forecast area this evening with some increasing clouds, however no precipitation is expected.  Yet.

A well developed through and upper low over the Rockies and Plains will send a series of disturbances along the stationary front through the week.  The first such disturbance will move through the region on Monday afternoon through Tuesday with scattered showers and thunderstorms.  There is some potential for a few of the thunderstorms will become severe as the majority of the region will be in “warm sector” of these disturbances, which means the atmosphere will be unstable and moisture will be plentiful.  

Another aspect to keep an eye on through the week will be the building moisture off the Southeast coast southwestward through the western Caribbean Sea.  This area of moisture will have the potential to be entrained into the disturbances cutting through the Ohio Valley and Mid Atlantic through the week, which would provide thunderstorms with plenty of latent heat release energy to push thunderstorms into severe levels.

The stationary front is expected to push into the Atlantic by Friday afternoon with high pressure building into the Mid Atlantic by Saturday with clear skies.  Temperatures through the period will average near normal however dew points in the 60′s will produce humid conditions at times.

6:49 AM

The one obvious observation that stands out on this morning’s IR satellite is that high pressure is in strong control over much of the Mid Atlantic and Northeast this morning.  However, another low pressure system and cold front is slowly moving out of the Plains and moving towards the Great Lakes, which will bring showers and thunderstorms to the forecast area by tomorrow evening.

High pressure will move right over the forecast area today with light winds veering from the northwest to southeast, clear skies, and near normal temperatures.  The weak southeasterly winds will keep temperatures slightly cooler along the coast with highs in the lower to mid 50′s and mid to upper 50′s across the rest of the forecast area.

The high pressure system will begin to move east and north of the region tonight through tomorrow morning with an increasing south and southwesterly wind and increasing clouds.  The weak warm air advection by tomorrow morning will keep lows in the mid to upper 30′s over the interior and around the lower 40′s along the coast.  

The low pressure system will have two fronts impacting the forecast area.  The warm front currently over the western Ohio Valley and western Great Lakes will approach the forecast area by tomorrow afternoon with increasing clouds and scattered showers.  By the time the warm front reaches the forecast area, much of the warm air advection behind the front will weaken as the low occludes.  However, a few isolated thunderstorms will still be possible.  The occluded front follows on Wednesday night with showers and a few thunderstorms as well.  The upper level dynamics associated with the occluded front are not expected to be nearly as strong as what was seen on Sunday.  In fact, much of the moisture and energy may be refocused over the southern Mid Atlantic where a new low pressure system will develop at the triple point (location where a warm front, cold front, and occluded front meets), which should limit the coverage of showers over the northern half of the forecast area.

The forecast area will be between low pressure systems, one exiting the coast and another over the Mississippi Valley, on Thursday with partly cloudy skies and a few isolated showers in the early morning.  Overall, most locations can expect dry conditions with temperatures near normal for highs in the mid to upper 50′s.

8:30 PM

An area of showers over western Pennsylvania and throughout much of the Tennessee Valley.  The showers are supported by weak isentropic lifting associated with a warm front moving towards the Mid Atlantic.  

Clouds will continue to increase over the forecast area, however the majority of the rainfall will hold off until later tomorrow morning through the evening hours.  The lifting associated with the showers over Philadelphia will weaken over eastern Pennsylvania and the low level dry air already in place over southern Pennsylvania and southern New Jersey will lead to periods of virga rather than precipitation over night and early tomorrow morning.

The main low pressure system that will support the strongest lifting over the forecast area is currently developing over eastern Texas under an intense area of strong to severe thunderstorms.  These types of lows can be tricky to forecast as the latent heat energy that is released can cause significant error in the model guidance.  However, given the amount of precipitation already over the Mississippi Valley and throughout the Southeast, the atmosphere will have plenty of moisture to work with through tomorrow evening to produce wide spread showers over both the Philadelphia and New York City metros.  

After this low pressure system exits the forecast area, the region will be in a mild regime as high pressure builds north from the Tennessee Valley.  Temperatures on Friday will be very comfortable and pleasant with highs in the upper 50′s to mid 60′s.