Strong to severe thunderstorms possible this afternoon

1:00pm Update: The NWS Storm Prediction Center has issued a Severe Thunderstorm Watch for Northern New Jersey and Southeast New York until 8:00pm tonight. Thunderstorms are expected to develop over the next few hours and shift southeastward through New Jersey and New York. The combination of developing instability, a nearby frontal zone, and a disturbance aloft will aid in organization of storms. Thunderstorms will be capable of producing frequent lightning, hail, and strong winds.

Severe Thunderstorm Watch graphic from the Storm Prediction Center. Blue highlighted counties are included in the watch.

The Storm Prediction Center has placed much of the area in a Slight Risk for severe weather on Thursday, including much nearly all of New Jersey. The Slight Risk area runs to about New York City and Western Long Island northward through Southeast New York and near the Connecticut border. A Slight Risk is issued by the Storm Prediction Center when organized severe thunderstorms are possible, in this case there is a 15 percent chance of gusty winds or hail within 25 miles of any point within the Slight Risk area.

Setup Overview

A nearby frontal boundary, developing instability, and favorable shear in the mid and low levels of the atmosphere are expected to combine to support the development of some thundesrtorms later this afternoon — those are things we know. The Storm Prediction Center’s Day 1 Outlook places the area in a “Slight Risk” for severe thunderstorms (the green area outlined below). The SPC indicates a 15% chance of severe-criteria wind gusts or hail stones within 25 miles of any point within that green outlined area. A “Slight Risk” means a few things:

– An increased number of organized severe storms

– Low in coverage, relative to area

– Stay weather alert

Day 1 Outlook from the Storm Prediction Center on May 22nd, 2014.

The main culprit for the severe weather potential this afternoon will be a warm frontal boundary and associated convergence zone, which will be draped near the area later this afternoon. This is the same boundary which the early morning thunderstorms rode along — beginning in Northeast Pennsylvania and eventually shifting southeast toward New Jersey. These frontal boundaries serve as both temperature/thermal boundaries, and also areas of locally focused lift for precipitation.

Northeast US surface analysis from the morning of May 22nd, 2014.

Detailed Breakdown/Timeline

As a mid level disturbance moves toward the area today, the warm frontal boundary and convergence zone will shift near the area during the late morning to afternoon hours. The combination of the two, when juxtaposed with daytime heating and some instability, will lead to sufficient lift to develop thunderstorms during the afternoon hours. The instability development, as always, will be contingent upon daytime heating and some sunshine. Forecast models which show higher instability parameters across interior areas show sunshine breaking out during the early afternoon hours in those locations.

Accordingly, we expect the development of some thundesrtorms by late morning into early afternoon over Northeast Pennsylvania, eventually shifting southeastward toward New Jersey and New York City. Even after these initial storms, additional thunderstorms are likely to develop through Eastern PA and shift toward New Jersey during the afternoon hours. These storms will be capable of producing very heavy rain, frequent lightning as well as some hail and gusty winds.

Close to the area beaches and shores, as well as over Long Island and Connecticut, cooler marine air will become entrained during the late morning to afternoon hours. As a result of the south/southeasterly winds off cooler ocean waters, the severe weather threat is likely to be very low to minimal across these areas. Notice how, on instability forecasts this afternoon, there is a rapid drop-off east of New York City where the cooler marine air is present.

NAM model forecast CAPE (instability) later Thursday afternoon.

In summary, as storms develop over Northeast PA later today they are expected to move southeastward through much of New Jersey and parts of New York City. The storms will be capable of producing heavy rain, frequent lightning and even some hail and gusty winds. Eventually, they will weaken and lessen in areal coverage as daytime heating is lost and the convergence zone sinks southward.

We encourage you to stay tuned throughout the day for updates and information including the issuance of any watches or warnings. This post will turn into a Live Blog during the early afternoon hours.