Another potent late fall-like system to affect the Northeast 9/18/12

Yet another rather intense weather system is expected to affect much of the Northeast and mid-Atlantic states tomorrow as an unseasonably strong cold front will once again interact with a fairly warm and very moist airmass.

A rapidly deepening trough will be working eastward through the eastern United States dragging a cold front along with it.  At the same time an area of surface low pressure is expected to develop across the TN Valley region and work northward along the cold front swinging a warm front northward.  Computer models are generally in agreement that this warm front will lift northward through the NYC area and then stall across central CT.  South of the warm front the airmass will become very moist with dewpoints potentially rising into the lower 70's as far north as the NYC area and even into portions of southern CT.  North of here dewpoints will be into the mid and upper 60's.  Where the lower 70's dewpoints occur this will yield to an airmass which will become marginally unstable.  Computer forecast models develop anywhere between 500-1000 J/KG of MLcape.

Wind fields aloft are expected to become quite strong for this time of year.  Computer models are generating a 45-55 knot low-level jet working into the region, however, computer models have slowed down on the timing of the core of this jet getting into southern New England.  The mid-level jet is expected to increase to 60-70 knots with a 80-90 knot ULJ moving nearly overhead as well.

The combination of modest instability along with very strong wind fields aloft will yield to the potential for some strong to even severe t'storms tomorrow and there will be two storm modes we may have to deal with.

1) With the northward movement of the warm front the potential will exist for isolated low-topped supercells to develop.  Very similar to what we saw occur last Saturday where two confirmed tornadoes were confirmed in the NYC area.  This potential is dependent on how buoyant the boundary layer can become and how much instability can develop.  Where dewpoints get into the lower 70's this would be enough to yield a very buoyant boundary layer and be enough to create modest instability.  The best potential for this will likely be from NJ up through NYC area and into extreme southwestern portions of CT.

2) Narrow low-topped squall line associated with the cold front.  Extreme forcing along the front and a moisture rich airmass is expected to lead to the development of a very narrow low-topped squall line.  While thunder/lightning may be limited this line could be strong enough to bring down some of those stronger winds aloft yielding to a threat of damaging winds.  This line would move through eastern NY/southern New England probably after 9-10 PM.

With the main piece of shortwave energy expected to move north-northeasterly this means eventually the developing line will begin to ourtun the stronger forcing and dynamics aloft.  We actually saw this occur last Saturday as well which was a main reason for the line rapidly weakening as it approached southern New England.  As this line moves through central/eastern PA into SE NY and towards southern New England it will once again weaken.

With such strong winds aloft tomorrow and just above the surface tomorrow will be quite breezy as well, even without shower/t'storm activity.  Winds could gust as high as 30-40 mph region wide with the strongest winds occurring across the coast and areas above 1500'.  In fact, for elevations above 1500-2000' winds at the peak of the event could gust as high as 60-70 mph...perhaps even upwards of 80 mph!  This will likely happen across the Berkshires and into the Green/White mountains of VT/NH!

Torrential rains will be likely tomorrow as well and this could lead to pockets of flash flooding.

Below is a graphic indicating areas of greatest wind and severe weather potential.

Saturday, September 8th, 2012 severe weather threat

Severe weather across this part of the country starts to become less uncommon as you move through August and into September, however, we are by no means immune to it!  A developing and strengthening storm system will provide us with the potential for some strong to severe thunderstorms on Saturday along with some torrential rains.

A sharpening and deepening trough will slide eastward dragging along a cold front with it.  At the same time an area of surface low pressure is expected to develop and rapidly intensify as it moves northward from the Tennessee Valley through western PA/NY and into southern Canada.  This will allow a warm front to move northward through southern New England allowing for an unseasonably warm and moist airmass to advect into the region.  Even with limited sunshine tomorrow temperatures should range from the upper 70's to lower 80's with dewpoints creeping perhaps into the lower 70's.  This combination of warm temperatures and high dewpoints will lead to at least a marginally unstable airmass.  some computer models are indicating the possibility of several breaks of sunshine tomorrow across the entire region.  if this does occur and we can get a period of strong solar heating than the possibility of moderate instability will exist.

A piece of powerful shortwave energy is expected to round the base of the deepening and strengthening trough.  With the shortwave interacting with the trough we will see winds aloft increase.  Computer models are forecasting a low-level jet in excess of 30-35 knots with a mid-level jet of 45-55 knots and an upper-level jet streak of 90-100 knots placing the region in the right entrance region of the upper level jet.  Computer models are also forecasting a great deal of helicity thanks in part to directional shear in the lowest 2km of the atmosphere and strong speed shear.

Given the presence of strong forcing/lift associated with the cold front and approaching trough and the potential for at least weak instability a squall line is expected to develop just out ahead of the cold front.  Given the presence of very strong winds aloft, strong atmospheric mixing, and no inversion in the boundary layer this squall line would have the potential to produce strong to even damaging winds from it's entire journey from NY/PA all the way to the New England coastline.

We will have to keep an eye out for any discrete storm development out ahead of the main line as well.  Given the presence of high helicity and long/curved hodographs as observed on skew-t soundings if enough instability materializes the potential will exist for isolated tornadoes as well.  This threat could be further enhanced across the Hudson Valley of NY extending into southern VT as well as the CT River Valley.  This potential will be monitored extremely closely.

If discrete development does develop it could happen as early as 3-4 PM with the main line moving through during the late evening and early overnight hours.  While instability will decrease with the loss of daytime heating storng dynamics and forcing should help to sustain the line some as it gradually weakens with eastward progression, however, still possessing the threat for strong winds.

Given the presence of high dewpoints and lots of low-level moisture torrential rains will be likely as well, potentially leading to pockets of flash flooding.

While the entire region will be at risk for strong to severe storms the area with the greatest potential will be from eastern PA/NY extending into southern VT and through central CT/MA.