12:35 PM | *Strong-to-severe thunderstorms arrive this afternoon*

[Convective Available Potential Energy (CAPE) values (2500 or higher indicate extreme instability); courtesy NOAA)

Discussion

Strong-to-severe thunderstorms are moving eastward from areas to the west of the I-95 corridor into highly unstable air and should make an early-to-mid-afternoon (1:30-3:00PM) arrival in the DC and Philly metro regions and mid-to-late afternoon (3:00-4:30PM) across New York City. Any thunderstorm can produce heavy amounts of rainfall in a short period of time, frequent lightning, and there can be some potentially damaging wind gusts in some of these storms. The culprit of this activity is the combination of an approaching frontal system, very humid air, and energy in the upper atmosphere.

Severe weather indices are often quite useful to weather forecasters in the determination of atmospheric instability and several of these are raising red flags for the I-95 corridor for this afternoon and early evening. One such index is called Convective Available Potential Energy (CAPE) and readings above 2500 are considered to be “extreme” and suggestive of a very unstable atmosphere. Currently, CAPE values of 2500–3000 extend from SE PA to the DC metro region (circled area) indicating the atmosphere is very unstable. Shut your windows if necessary.

[Latest GOES visible satellite image with N-S frontal boundary zone; courtesy Penn State eWall]