The Storm Prediction Center has issued a moderate risk for severe weather for parts of New York state this afternoon, and several severe weather watches have been issued for the area today. There is also a risk for severe weather across parts of the Southern Plains (Texas & Oklahoma), but since they're more used to severe weather than the northeastern United States, this post will focus on the latter.
The storms that develop in the northeast today will have all modes of severe weather attached to them -- large hail (the size of quarters or larger), damaging winds (60+ MPH), and tornadoes. A cold front is pushing east across parts of the eastern US right now, and strong storms are forming in the unstable air out ahead of the front. Given the amount of shear and instability in place, the storms are expected to quickly become severe.
The types of storms that will develop today (lines and individual supercells) will be especially hazardous, so the SPC has issued a moderate risk for parts of Pennsylvania and a large chunk of New York. Don't let the slight risk zone fool you -- equally strong storms can form in the slight risk zone, too.
Here's the outlook from the SPC. The green shading indicates a risk of general non-severe storms, a yellow shading indicates a slight risk for severe storms, and a red shading indicates a moderate risk of severe storms.
Here's the tornado threat this afternoon. The highest threat is in the green/brown shaded areas where the low-level shear will be the greatest this afternoon, and especially because of the presence of supercells in the area. A 5% risk of tornadoes means that there is a 5% chance of tornadoes within 25 miles of any point in the shaded area. Concern starts to grow at 2%, so anything over that is worth watching intently.
Here's the damaging wind threat this afternoon, which is the reason for the moderate risk. As with the tornado threat, this means that there is x% chance of damaging winds within 25 miles of any point within the shaded area. This means that the storms that develop today are highly likely to produce damaging winds in excess of 60 MPH.
Here's the hail threat, with the map following the same rules as the tornado and wind threat maps.
For reference, here was the radar image at 1:10 PM EDT showing the storms moving into the area.
Here were the severe weather watches in effect as of 130PM EDT. Keep in mind that this may not be current in an hour or two. Check the SPC for the latest updates. Blue indicates a severe thunderstorm watch, and red indicates a tornado watch.
I won't be able to keep up a severe weather liveblog like I normally would in this instance, but you can follow me on Facebook for quick updates on the strongest storms and when any bigger cities are under the gun.
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12:30 PM PT: About an hour and a half ago, the NWS received photographic evidence of hail 3.50" in diameter in North Bolton, NY. That's just a bit smaller than a grapefruit.