24 June Severe Weather
(Click on images to enlarge)
(Click on images to enlarge)
An upper level low visible on water vapor imagery was located near Pittsburg, PA at 12Z and forecast to move east northeast through the day, moving just south of KPOU. 12Z Soundings showed forecast cape around 900 J/kg here at Albany but around 1500 J/Kg on PIT sounding. Vigorous dynamics were expected with the upper level low which would provide lift to initiate thunderstorm development. Low level winds were not impressive so pulse storms were anticipated. WBZ was above 12,000ft MSL here at Albany so wet microburst rather than hail was anticipated.
This event was a classic very low shear and high CAPE environment. It is not often our forecast area goes from a "General Thunder" SPC scenario ay 06Z to a Slight Risk by 1630 UTC. The strength of the cold pool aloft was likely underforecasted by the guidance and was hard to decipher how large an event would occur. The convection tended to fire repeatedly off localized cold pools. A few past cases come to mind that were shown at past NROW's that occurred like this one in NY and New England. This case is worthy of a case study research project for NROW or a station meeting in the near future.
There were hints that the mid-level lapse rates would be steep enough (around 6.5 C/km) to support strong to severe convection despite the weak shear, high WBZ's and FZL's. The best lapse rates in the H700-500, H850-500 layers looked like they would be south of the region over eastern PA, NJ, and extreme southern NY. SPC did put a convective watch out here.
Using Frugis/Wasula hail study for severe worked well. -20C was at 24,000ft. Adding 9000ft gives 33,000ft. Warning decision was based on 50DBZ reaching 34,000ft. This happened frequently and usually in 2 volume scans: first volume scan showed shower, second volume scan had 50,000ft top.
For a pulse environment, the high adjustment "up" above the -20C height was excellent by the warning teams. A lot of melting occurs with the hail stones, and this adjustment was well applied to capture the large hail and wet microbursts. Another technique is to apply the -30C height and add about 5 kft AGL in pulse thunderstorms. For this event, the -30c height was about 29 kft AGL. Overall, adding 9 ft AGL to the -20C height was appropriate.
Above: Soundings from 1200 UTC 24 June at (left to right) Albany, NY (KALB), Buffalo, NY (KBUF), Upton, NY (KOKX) and Maniwaki, Canada (KWMW).
Above: Soundings from 0000 UTC 25 June at (left to right) Albany, NY (KALB), Buffalo, NY (KBUF), Upton, NY (KOKX) and Maniwaki, Canada (KWMW)
Above: Loops of the SPC Day 1 severe weather outlook (left), probability for hail (center) and probability of severe winds (right).
Above: Severe weather reports fro the SPC.
Above: Storm survey pictures.