NY NJ PA Weather Forecasts

Moisture advances as rain develops

7:43 AM

This morning, a cold front is sliding south through the Philadelphia metropolitan area with little in the way of rainfall yet a significant change in conditions as much cooler air builds into the forecast area.

Much of the region is overcast with a strengthening east to northeasterly wind around 5 to 15 mph.  However, no rain is falling despite the growing precipitation on the radar.

The reason for the lack of rain can be found on this morning’s Skew-T chart, which is basically a vertical diagram of the atmosphere.

The chart this morning shows a layer of dry air between roughly 850 and 700 MB that is in place and is expected to remain in place through this afternoon.  What this dry air does is cause the rain to evaporate before reaching the surface.  The evaporation calls cools, thus temperatures have been falling from the 60’s into the 50’s throughout the region.  I expect temperatures to end up in the lower to mid 50’s as the morning progresses.

Now, for the past several days we’ve breaking down the key parts of this major storm over the Plains and the warm and cold frontal structures that will bring periods of rain to the forecast area.  Of course, the introduction of our friend from the Bahamas has to be considered as well as an enhancement.  For those that work out, consider the Bahamas disturbance as a Nitric Oxide supplement, giving the advancing cold front an extra pump of moisture to lift before exiting the East coast.

This morning’s water vapor satellite image clearly shows a large amount of moisture being drawn northward from the Gulf of Mexico.  The upper low is currently over Iowa and will lift into the central Great Lakes this evening.  Meanwhile, energy currently over the northern Plains will dive south into the Tennessee Valley tonight while the trough tilts into a negative position.  As a result, the Bahamas disturbance, which has been inching wester the past few days, will be quickly drawn northward and will become entrained by the trough, adding to the moisture advection ahead of this cold front.  There is one detail that must be taken into account, note the dry air racing north and east through the southern Mississippi Valley and over parts of the Tennessee Valley.  The stronger the lifting ahead of this cold front, the stronger the dry air advection behind the cold front and thus the faster the rain ends once the cold front moves through.

So here is how I expect this forecast to work out.

Through this afternoon, temperatures will hold or fall slightly into the 50’s with overcast skies and a northeasterly wind around 5 to 15 mph, gusting to 25 mph at times.  The cold front will stall just south of Cape May, New Jersey through Delaware and off towards central Maryland.

Tonight, the cold front will rebound as a warm front with widespread rain developing throughout the Philadelphia and New York City metropolitan areas.  Parts of extreme southern New Jersey may miss out on this rain as the warm front will easily pass to the north of the region before lifting can mature to a state where steady rain is falling.  The best isentropic lifting will become focused tonight from the northern suburbs of Philadelphia to the central New Jersey coast on northward.  The focus for heavy rain will be found over northeastern Pennsylvania through the Hudson Valley and into Connecticut through the early morning hours as the isentropic lifting will last longest in these locations.

By mid Saturday morning, the warm front will clear the entire forecast area.  Conditions behind the warm front will be warm and actually drier as the steady rain will be pushed north.  Scattered showers will be a threat through the early afternoon, however a break in the steady rain can be expected.

On Saturday afternoon, the march of the heavy rain begins.  At this time, I expect the moisture and energy from the Bahamas disturbance to interact with this trough.  On the radar, we will likely observe precipitation rates increase from Virginia to east-central Pennsylvania and radar DBZ to increase as well.  At this point I am going to break this down by region.

Eastern Pennsylvania: The moderate to heavy rainfall from this cold front will begin by around 1 PM and continue on through 7 PM.  Rainfall amounts of 1 to 2 inches can be expected with locally higher amounts in embedded thunderstorms.

New Jersey: Scattered showers can be expected throughout the afternoon, however steady rain for the entire state will pick up after 4 PM and continue on through 11 PM.  Rainfall amounts will range from 1 to 2 inches with locally higher amounts in embedded thunderstorms.

New York City through the Hudson Valley: Showers a threat through the afternoon.  Steady rain expected to begin by around 7 PM and continue on through 12 AM.  Rainfall amounts will range from 1 to 2 inches with locally higher amounts in embedded thunderstorms.

Long Island and Connecticut: Showers through the mid to late afternoon hours.  Steady rain begins by around 8 PM over western locations 10 PM over eastern Long Island, and continues on through 2 AM Sunday over the western areas and 4 AM over eastern Long Island.  Rainfall amounts will range from 1 to 2 inches with locally higher amounts in embedded thunderstorms.

All locations can expect periods of significantly reduced visibility and significant ponding on the road ways as this wall of rain moves through the region.  Flash flooding will be a concern along with the threat of minor wind damage as winds increase from the south around 1o to 20 mph with higher gusts.

Note: I made the forecast this way this morning as I have been flooded with request on this front will impact Game 6 of the ALCS and travel conditions.  Given I have had requests from everyone from the Pocono Mountains to Hartford, Connecticut; I think this addresses everyone’s needs.

Once this cold front exits the northern Mid Atlantic on early Sunday morning, high pressure will build into the region and remain in control through Monday.  A minor disturbance will spawn a weak coastal low on Tuesday with the threat of widely scattered showers, concentrated more towards coastal locations.  High pressure will return in control on Thursday with dry conditions and temperatures slightly above normal in the lower to mid 60’s.

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