26 July 2012 Severe weather clipping our southern forecast area

Click here for a detailed analysis from the Storm Prediction Center

A possible widespread and significant severe weather outbreak was anticipated for much of the ALY forecast area during the afternoon and evening of 26 July, including a perceived heightened threat of tornadoes and widespread wind damage. There were several complexities to consider in forecasting the possible event.

The Storm Prediction Center had placed the southern half of the ALY CWA in a Moderate Risk for severe weather the previous day. The northern half of our area was placed under a slight risk. This line of thinking was also agreed on by our office, as well as surrounding NWS offices. The risk areas from SPC did not change much for the 06Z and 13Z outlooks on 26 July, with the moderate and slight risks being shifted slightly to the south. The moderate risk shifted to our southern six counties, while the slight risk encompassed the rest of the area except for the western Adirondacks.

The synoptic setup seemed favorable for severe weather for at least half to three-quarters of our CWA. The main features were a surface cyclone migrating eastward across the Great Lakes, along a mainly east-west oriented warm front that was forecast to set up across the northern portion of our CWA. Along and north of the warm front, a large batch of showers was expected to encompass much of the area during the morning of 26 July. South of the warm front, a strong surge of moisture, humidity, and warmth was expected with PWATs around 2 inches and surface dew points into the upper 60s to lower 70s. In the upper levels, a strong jet was forecast to move eastward across southern Ontario and Quebec, which would place much of the interior Northeast and northern mid-Atlantic region in the favorable broad right entrance region of the jet. Also, the wind field was forecast to strengthen substantially, with resulting deep layer 0-6 km shear in the 40-50 kt range and even 0-3 km shear near 40 kt. There were indications of a veering wind profile in the lowest 3 km with clockwise turning hodograph, with 925 mb winds having a SW direction, 850 mb at WSW, and 700 mb at west. So there was forecast to be ample deep layer and 0-3 km shear, which is unusual for late July. The strong magnitude of shear and expected low LCL heights resulted in the heightened perceived tornado threat.

The main questions from a forecasting perspective were twofold: Where would the warm front set up and how much sunshine and subsequent destabilization would occur just south of the frontal boundary? Farther south it was perceived there would be more than enough sunshine/instability, thus the moderate risk. The slight risk area has the potential for severe storms if enough instability would develop.

Various sources of guidance even on the morning of 26 July indicated convective initiation would occur by early afternoon in the form of discrete cells and/or clusters of storms ahead of a main line developing farther to the south and west. The 12Z local HIRESWRF and NAM12 depicted several rounds of thunderstorms developing across our area once the better forcing and instability would arrive in the afternoon. When real-time satellite imagery and observations indicated substantial breaks in the clouds once the morning showers dissipated, the anticipated threat of severe storms was still high. Conference calls were held during the morning of 26 July, and SPC continued the risk areas in the same locations.

The 12Z ALB sounding was not representative of what the expected air mass would be like later in the day, as it was on the cool northern side of the warm front with showers occurring and stable conditions at the time of launch. Upstream soundings at PIT were thought to be more representative, which depicted strong instability and shear with abundant low level moisture.

A special 18Z sounding from ALB was launched after substantial sunshine had occurred and surface temps had warmed to around 80 with dew points in the upper 60s. The sounding revealed that unstable conditions had developed with 1700 J/Kg of surface-based CAPE, however there were two weak inversions around 800 and 500 mb. The large scale forcing and the warm front/differential heating boundaries were expected to be enough lift to overcome the weak inversions. However, the one red flag that is hypothesized to be a big factor in limiting convective initiation was poor mid-level lapse rates from 850-500 mb and 700-500 mb of only 5.1 and 5.2 degC/Km respectively. These lapse rate values are extremely low for severe weather days, and since it was already 18Z there would likely not be enough steepening through the rest of the afternoon and possibly precluded whatever weak lift was available to initiate convection.

As the rest of the day unfolded, a severe and large squall line developed over Lake Erie and expanded as it moved onshore, encompassing several states from the southern tier of NY and PA southwestward through the Ohio and upper Mississippi Valley regions. The main QLCS (Quasi Linear Convective System) raced eastward across the southern tier of NY and northern/central PA mainly affecting the BUF, BGM, and CTP forecast areas. Several bowing structures and spin ups along inflow notches resulted in several tornado warnings across portions of BGM and CTP areas. The northern-most storm along the line formed into a well-developed supercell and impacted the Chemung Valley region with a possible strong tornado from around Corning to Elmira to just west of Binghamton. This feature then evolved into a long-lived bow echo that persisted across the southern Catskills, mid/lower Hudson Valley and southern Connecticut and NYC/Long Island area. This bow echo clipped southern portions of Ulster, Dutchess, and Litchfield counties causing several downed trees and left thousands of people without power.

As for the rest of the ALY CWA, deep convection never initiated, with just showers with some embedded isolated thunder through the evening. The conditional threat for discrete supercells did not materialize, as well as the QLCS tracking south resulting in a null Tornado Watch event.

Link for map of LSR from southern tier of NY: http://www.wrh.noaa.gov/sgx/kml/lsr.php?cwa=bgm&lsr=All&start=201207261100&end=201207262300

Link for map of SPC damage reports: http://www.spc.noaa.gov/climo/reports/120726_rpts.html

What was learned from this event:

1. This was a day where significant severe weather occurred over southern portions of the Northeast CONUS, but with the majority not occurring in our area. There was a conditional threat of violent storms initiating during the afternoon in east central New York and western New England, but did not materialize. Some possible factors include very poor mid level lapse rates as well as a secondary warm front setting up near the NY/PA border that may have prevented the best moisture/instability from making it northward into much of our area.

2. As it turns out, there ended up being two warm sectors (which was not anticipated), with a secondary warm front setting up near the NY/PA border that deliniated the more truly unstable air which spawned widespread severe weather in the form of a large scale QLCS with embedded tornaoes farther to the south over mainly the BGM and CTP forecast areas.

3. We continue to struggle with verifying Tornado Watch issuances on days when there is a perceived tornado threat. In contrast to 29 May 2012 when there were several severe events from wind and hail, but no tornadoes...26 July resulted in only limited severe weather over our extreme southern areas, but a conditional tornado threat that did not materialize due to lack of convection in the warm sector. With the magnitude of shear that was over the area, if any discrete cells had formed they would have likely taken on supercell characteristics quickly. In fact, even a very shallow convective shower over Saratoga county between 7-8 pm exhibited rotation despite only 10 kft tops.