Significant rain and wind event expected

7:37 AM

I’ve been commenting to some of my friends in the field and on some of the weather boards that the cool season (now through next March) is going to significantly challenge forecasters and those that model hug are going to be in trouble.  Is this storm situation a preview of what to expect this winter?  Well, yes I think it is.  The Sub Tropical jet stream is going to have a lot of tricks up it’s sleeve over the next several months, and this situation is actually a great example.  I’m going to go through a lot of images and breaking this storm down.  Feel free to open the images up to view while reading.  Okay, let’s get to this!

Morning Surface Map, Penn State E-wall

This morning we have the stages being set for a battle of the air masses.  Strong Polar high pressure is building into the Mid Atlantic and down the Southeast coast.  I haven’t said this in a while, but we clearly have a Cold Air Damning (CAD) signature in the pressure field down the East coast.  The cold cold front that moved through yesterday morning is now over the South Carolina and cutting back through to southeastern Texas.  Waves of low pressure are developing along this front and plenty of rainfall is developing from the Gulf Coast through the Mid West.  The Polar air mass is running up against two different air masses this morning.  The first is the warmer semi-tropical air mass off the East coast and along the Gulf Coast.  The other air mass is elevated and harder to observe with just surface data.  Instead, we need the water

This Morning's Water Vapor NWS/NOAA

vapor satellite to see that the moisture from the Tropical Pacific is also interacting with this fresh Polar air mass at the mid levels or roughly 850 to 500 MB.  These are roughly where planes and other flying craft would roam in case you are wondering.

Now, before diving into this forecast.  Let’s look at the water vapor satellite image and see what it tells us.  For one, note the ridge off the coast.  This ridge is a source of heat and a focusing agent for moisture advection over the Gulf of Mexico.  What this tells me this morning is the trough over the Southeast is quickly deepening because the ridge axis over the Atlantic is becoming shaper with each passing frame.  Also, note the upper disturbance diving into the central Mississippi Valley this morning.  Obviously, most individuals will pick up on the large area of blue over the Mid West and Great Lakes.  However, I want to draw your attention to the smaller areas or streaks of blue diving into the southern Plains.  This tells us that this is not a compact singular disturbance carving out this trough.  What we have here is a series of small disturbance rotating through this trough and instigating the moisture streaming into the Gulf Coast through Mid West.  Thus the increasing coverage in rainfall.  Finally of course, if anyone had any doubt in the potential for rainfall with this set up over the next several days, we have the Sub Tropical jet stream sending boat loads of moisture into eastern North America.

500 MB pattern Thursday PM

Now we are going to fast forward to Thursday morning.  I have a few important features that I have labeled at 500 MB by Thursday morning.  Disturbance One is actually a series of broken small disturbances that will carve out a deepening trough over the eastern United States.  There will be weak to moderate levels of PVA or lifting that will focus along the Southeast coast on Thursday morning, which will allow a surface low to develop off the North Carolina coast.  This low pressure will move New Jersey coastal waters on Thursday night and continue to intensify as the upper level trough becomes more defined.  Bring your attention further north for a second and also notice that the height lines pack themselves over New England.  This is a clear indication of convergence of the upper level winds and thus strong high pressure at the surface over New England.  This high pressure system will enhance the pressure gradient over the forecast area and support strong winds along the coast, I will detail this more in a few minutes.

Back towards the Rockies, we have Disturbance 2.  This disturbance is destined to become a closed upper low over the Southeast, but what makes this disturbance so special?  Well, great question!  The disturbance itself is no more powerful than the first.  The difference is what is going on in the Pacific.  The upper low over the Gulf of Alaska will position itself in such a way to allow the ridge to build over western North America.  Think of this set up like a roller coaster.  The steeper the decline, the faster and

500 MB Saturday Night

further the coaster will race down that decline.  Well, in this case, the Polar jet stream  will be forced to intensify and drive this second disturbance into the Southeast.  This motion forces intensification of the disturbance itself and then BAM!  You have an upper low and a second coastal low!

By Saturday night, this upper low will be positioned over the Tennessee Valley.  Strong upper level disturbance will be present over the Mid Atlantic, which will allow the formation of what we call an inverted trough, which will lift through New Jersey and eastern Pennsylvania on Sunday morning through the afternoon hours.  Also note, that there is plenty of PVA that will have to rotate through the Mid Atlantic before exiting into the Atlantic, which will keep rain a threat through Monday morning.

Sunday AM Surface

Now going towards Sunday morning, I want to point out a few important details that is showing up in most model guidance.  Of course the main feature that really stands out here is the pressure gradient between the strong 1026 MB high pressure system over Maine and the surface low over the Mid Atlantic coastal waters.  Let’s not pretend that this storm on it’s own is strong, because a 1012 MB or 1010 MB low pressure system is not going to set records.  However, the pressure gradient between this high pressure system and the surface low will support strong easterly winds on Sunday morning as well, which should range from 20 to 35 mph on Sunday morning with gusts to 45 mph at times along the coast.

The other feature that shows up on most model guidance is this inverted trough, which I have in dash lines in red.  This inverted trough is going to be an annoying feature from Thursday through Monday morning.  This trough is a focusing agent for precipitation and will have the potential to produce very heavy rains to the north and east of the trough axis.  Now, this far out, I think it is rather foolish to pinpoint the position of this trough exactly.  We do know that rainfall amounts will be enhanced significantly ahead of it, and any location within the Philadelphia and New York City metropolitan area will be under the threat for this enhanced rainfall.

So here is the forecast in a nut shell, peanuts or cashews, doesn’t matter.  Although I do love peanut butter!

Through today expect clouds to increase as the moisture from the south and west races towards the Mid Atlantic.  The effects of the Polar air mass will be felt as temperatures will be much cooler than the previous day with highs in the upper 40′s to mid 50′s.

Tonight, overcast skies can be expected with an increasing wind from the north and northeast around 5 to 15 mph.  The first low pressure system will begin to develop off the North Carolina coast by this point.

On Thursday, rain will enter the forecast area from the Atlantic, first over southern New Jersey, by roughly 10 AM and then continue to expand in coverage north and west through the late morning and early afternoon hours.  Heavy rain can be expected, especially along the coast, through the overnight hours and into Friday morning.  Winds will be sustained around 15 to 30 mph over the interior and 20 to 40 mph along the coast with higher gusts exceeding 45 mph at times.

The rainfall will taper off in the afternoon, however showers will continue to be a threat as a weakness or trough remains off the New Jersey coast.  For those looking towards sporting events on Friday evening (college football, the Yankees, etc.), conditions are going to be raw with a persistent north to northeast wind, however the heavier rain will be at a brief end.

On Saturday, scattered showers will remain a threat in the morning hours, however the steady and heavy rain will hold off through the day.  A new low pressure system will finally take shape on Saturday night along with the inverted trough discussed earlier.  Heavy rain will return to the region on Saturday night and continue through Sunday afternoon.  Showers will linger on through Monday morning as the low pressure departs into the Canadian Maritimes, however the weak inverted trough will linger off the coast.  Winds will be strong from Saturday night through Sunday as well with sustained winds around 15 to 30 mph throughout the northern Mid Atlantic, with higher gusts exceeding 45 mph along the coast.

These two storms combined will be capable of producing 4 to 8 inches of rain by Monday morning over the Philadelphia and New York City metropolitan areas.  The heaviest rain is going to be found to the north and east of where ever this inverted trough sets up and over the mountain locations of eastern Pennsylvania where orographic lifting will be a concern.

From Thursday afternoon through Sunday night, there will be a significant potential for flash flooding, urban flooding, coastal flooding and erosion, and the threat for minor wind damage.  Wave heights will be elevated above 5 feet  through the period, possibly exceeding 8 feet on Sunday afternoon with the strong easterly pressure gradient.

Once this storm exits, high pressure will take hold for Monday afternoon through Tuesday with dry conditions, clearing skies, and a return to more seasonable temperatures.

Note, I’m going to do a live discussion on this storm around 8:30 PM in place of the evening discussion.  All are invited to come by and ask questions on this event.