Eyes on weekend storm as model inconsistency continues

Active weather continued throughout much of the Northeast states Thursday, with transient clouds and showery activity for much of the day as a disturbance in the levels of the atmosphere passed through. This disturbance was relatively weak and, as mentioned, transient in nature, limiting the potential for strong to severe thunderstorm coverage that may have otherwise existed, especially given the presence of instability.

With that said, pleasant conditions gradually returned to the majority of the Northeast states on Friday morning — but it will be short lived.

A potent shortwave trough will approach the region on Friday.

What is occurring back to the southwest, however, will be a harbinger of bad news — a large and strong mid level disturbance will organize over the Mississippi River Valley and begin to move eastwards towards the Mid Atlantic States. As this occurs, a dual-jet structure in the atmosphere's mid and upper levels will begin to organize, with lift in the atmosphere promoting the development of precipitation across the Mid Atlantic states on Friday, initially right near the front we were just discussing.

As the system begins to organize, a low pressure center will deepen at the surface, drawing in ample moisture in the atmosphere's low levels (that's where we are). This will lead to increasingly humid and moist air with the potential for widespread rainfall throughout the Mid Atlantic states on Friday. The storm system will continue to organize Friday afternoon and evening, beginning a slow movement northeastward towards the Eastern Seaboard. That's where uncertainty dramatically increases.

Forecast models today have continued to offer little to no help in regards to actual "guidance" with the track and intensity of the storm system from Friday into Saturday as it pertains to the Northeast States. As a powerful mid level shortwave swings eastwards towards the coast, the surface low pressure system should deepen, and precipitation should expand to the north, bringing moderate to heavy rainfall as far north as Southern Pennsylvania and Southern New Jersey. Confidence in these areas experiencing heavy rainfall with the potential for flooding on Friday evening into Saturday morning is quite high at this time.

A surface low pressure will organize on Friday, leading to heavy rain in the Northern Mid Atlantic states.

Further north, over Northeast Pennsylvania, the rest of New Jersey, the NYC Metro, Long Island and New England, confidence is extremely low in regards to precipitation intensity and breadth. Much of this will be modulated by the track of the surface low itself, and the exact interactions that occur in the atmosphere's mid and upper levels. There are a few things to consider that will impact the track of the storm.

  1. A relatively progressive pattern in the atmosphere across the country, coupled with a positively tilted shortwave (most of its energy is back to the west/southwest) moving towards the region, should ensure that the storm system will not be overly amplified or incredibly strong. In other words, these two factors should reduce the chances of a "high impact" or severe rain and wind event.
  2. However, a stout blocking ridge has appeared over Greenland on the latest forecast models and ensembles. You may be asking yourself "what in the world does that have to do with us?" The answer is quite complicated, but think of it this way: The high pressure in greenland acts as a "block" to slow down all other weather systems around it. So our approaching disturbance's progress is slowed quite considerably, leading to a slower solution overall.
  3. There is ample moisture in place which will be advecting northward amidst convection (thunderstorms). This should allow precipitation to move northward into at least the Northern Mid Atlantic states before the storm system tracks out to sea.

The combination of all three of these factors has led us to a slightly wetter and more impactful forecast from this morning — but a relatively progressive solution overall. We have laid out the impacts across separate regions below.

impacts

Mid Atlantic States including Southern PA and NJ (Colored Red): Expect periods of moderate to heavy rain on Friday, continuing into Friday night. The potential will exist for 2-5" of rainfall and localized flooding. This will be especially true in areas where thunderstorms repeatedly occur, near the frontal boundary. Also, an onshore fetch into area beaches could lead to some coastal flooding and beach erosion. Rain will linger into Saturday morning across Southeastern PA and Southern NJ.

Eastern Pennsylvania, Central New Jersey, South shore of Long Island (Colored Yellow): High uncertainty exists in regards to the degree of impacts from the storm system. But rain will gradually increase in coverage late Friday Night and continue into the morning hours of Saturday. The rain will be heavy and torrential at times with embedded thunderstorms likely, especially near the coast. There will be a  sharp cutoff across this region. In specific, we expect 1-2" of rainfall across parts of Central NJ and Eastern PA, tapering to less than that once one travels northward towards Northern NJ and NYC.

Southern New England (Colored Green): Considerable uncertainty still exists here as well, but the demarcation line between impactful rain and a light event is likely to be somewhere south of this region. The southern shores of CT, RI and MA may see periodic moderate rain at times on Saturday. But subsidence (sinking air) is likely to keep most of Southern New England dry, especially northwestern CT and MA into the interior. The potential for rain will extend up the coast into Boston, but heaviest impacts are likely to stay to the southeast. The storm will taper later Saturday.

All in all, the progressive nature of the pattern and positive tilt of the storm system overall are expected to be the factors that modulate the potential for heavy rain. Although the Greenland Blocking will slow the system down, leading to continued uncertainty right up to the event time, confidence is currently highest in heavy rain across the Mid-Atlantic, Southern PA and Southern NJ from Friday into Saturday. Rain is still likely in the NYC Metro and Long Island, but we currently favor the worst of the storm system passing to the south and east of those regions.

Stay tuned for further updates through tonight and tomorrow morning, and be sure to continue to check the dashboard for the latest technical discussion.

1 reply

Trackbacks & Pingbacks

Comments are closed.