Severe Weather Threat on Saturday: Run-of-the-Mill or Significant?

00z NAM Forecast Sounding for NYC, Valid for 5pm on Saturday.

Saturday will be the last day of our heatwave, as a strong cold front approaches the area. With strong cold fronts and very warm, moist air out ahead of them, severe weather threats often ensue, and Saturday will be no different. However, although there are some factors that support severe weather, there are plenty that do not.

A quick technical explanation as to why will be aided in illustration by the sounding above. With a strong storm system traversing southern Canada, the best dynamics — and thus, best forcing — will be located well north of the area. This also means that the strongest mid-level winds and best shear will also be located to the northwest of NYC as well. Taking a look at this sounding, you can see that the wind fields are not that impressive — which does not give very fast forward motion, helping to reduce potential wind speeds. Slower wind fields also support weaker storms, as this prevents the updraft and downdraft from sufficiently separating. The wind speeds are still fast enough for some storm survival, but nothing that is too terribly impressive.

Another thing is that there is essentially no directional shear — the wind field is pretty unidirectional. This will make any tornado threat for our area essentially negligible, and further hinders the amount of updraft and downdraft separation.

Another negative for severe weather is the very tame mid-level lapse rates. The 850mb-500mb lapse rates are only 5.8 C/KM, which is quite unimpressive. We like to see values of around 7 C/KM or greater. Poor mid-level lapse rates tend to lead to squall lines that struggle to maintain themselves, especially as the storms begin to outrun the cold front and thus outrun any dynamic forcing for ascent that there is. Further northwest, poor mid-level lapse rates won’t be as much of a problem, since any squall line can maintain itself via lift from the front; but further east, the lifting from the front is bit further removed from it, and thus weaker.

We do have very steep low-level lapse rates, as from 0-1.5km (about 850mb), they are 9.6 C/KM, which is nearly dry-adibatic. This provides steep low-level instability — and combining this with high heat and humidity — provides good surface-based CAPE of nearly 2000 J/KG. This will help to provide some lift and good, healthy updrafts. Additionally, the steep low-level lapse rates help to provide lots of downward momentum, or negative buoyancy, for any downdrafts. This could help lead to some damaging winds in any strong downdraft, and I suspect that this will be the main severe weather threat. The relatively dry mid-levels could also lead to a threat of hail as well, but this would also be isolated and mainly relegated to the north, west, and northwest as well, since mid-level lift may not be strong enough to generate the very high cloud tops for hail in the NYC area.

Regardless — although the above paragraph does lend some credence to a wind threat, the fact that by evening, the main convective mode may be lines and clusters may mean that the mid-level lapse rates being poor will outweigh the low-level lapse rates being favorable, especially further east and away from the best forcing. So, although some isolated severe wind may be in the cards for Saturday — the best, widespread severe wind threat will definitely be well to the north, west, and northwest of the area, in our opinion. Things could change, and of course, we will keep you posted.