Warm-Season Lake-/Sea-Breeze Severe Weather in the Northeast

Warm-Season Lake-/Sea-Breeze Severe Weather in the Northeast

(Patrick Wilson)

(Focal Point: Tom Wasula, National Weather Service, Albany, NY)

Research Summary (1 November 2006–30 April 2007):

This CSTAR-II 6-month report describes research that started on June 2006 and is currently in progress on investigating severe weather triggered by lake-/sea-breezes during the warm season over the Northeast. The goal of this study is to investigate the dynamical and thermodynamical processes, and their modulation by physiographic effects, that are responsible for producing lake-/sea-breeze severe weather. It is hoped that this research will raise awareness and understanding for these phenomena. Several cases have been chosen for detailed analysis. Plots of various severe weather parameters, radar data, soundings, surface observations, satellite images, and other sources have been used to analyze these cases. The focal point for this research is Tom Wasula from the National Weather Service in Albany, NY.

Since the Eighth Northeast Regional Operational Workshop that was held in Albany, NY, on 1–2 November 2006, the case list has expanded to 15 as shown in Table 1. Five additional cases were added after helpful contributions from National Weather Service meteorologists at the aforementioned conference. The 15-case sample consists of four pure cases, 10 mixed cases, and one null case. Pure cases are defined as having lake-/sea-breeze convergence zones primarily responsible for initiating convection in the apparent absence of synoptic-scale forcing, while mixed cases have synoptic-scale forcing acting in conjunction with mesoscale forcing to generate convection. A null case features suppression of convection from lake-/sea-breezes. The high impact that severe weather can cause in the populated Northeast along with the challenge of predicting these events motivates our attempt to analyze these events to help to better understand their evolution and improve predictability.

Convective outlook reports from the Storm Prediction Center (SPC) were obtained to test how well these severe weather events were anticipated. Only cases from 2003–2006 could be verified. Thus, three of the four pure cases, 7 of the 10 mixed cases, and the only null case from the original 15-case sample were tested. The SPC issued a slight risk for two of the three pure cases but only a general thunderstorm outlook for the third case. For the seven mixed cases, the SPC issued a slight risk for two of them and a general thunderstorm risk for four others. In the remaining mixed case, no thunderstorms were anticipated. The null case was not handled well by the SPC as the suppression of convection by a sea breeze was not predicted.

One of the most challenging forecasting issues found in this research involves predicting pure lake-/sea-breeze cases. The lack of synoptic-scale forcing found in these events presents a difficult task for forecasters. One classic example occurred at 1800 UTC 2 August 2006 when storms first formed on an apparent lake breeze boundary along the southeastern shore of Lake Ontario. These storms intensified and moved eastward down the Mohawk Valley. As the convection fell apart over the Hudson Valley, an apparent outflow boundary triggered new storms in western MA that moved southeastward towards the RI coastline. Numerous wind damage reports were observed from this case. Figure 1 shows a 500-hPa plot of geopotential heights and relative vorticity at 1200 UTC and 1800 UTC on 2 August 2006. A strong ridge was anchored over the southeastern US with a warm westerly wind prevailing to the north. Heights only fell 1–2 dam between 1200 UTC and 1800 UTC over NY in response to a weakening shortwave trough over WI. Figure 2 shows an 850-hPa plot of geopotential heights and mixing ratios at the same times as in Fig. 1. A ridge was also present at 850 hPa over the Northeast with plenty of moisture available for convection, so there was no obvious synoptic signal of forcing for ascent at the surface or aloft. The other three pure cases in the sample also demonstrated these same synoptic patterns.

The addition of the five new cases and further data analysis on the entire sample bolstered and reaffirmed earlier conclusions made in the previous CSTAR-II report. Consequently, confidence in all the conclusions made from this study has been increased. The most crucial variable from this study appears to be the 925-hPa equivalent potential temperature (θe). Figure 3 shows the 925-hPa θefor a pure case from 2 August 2006 and a mixed case from 19 June 2002. All pure cases had θegreater than 335 K, while all mixed cases had θebetween 320 and 350 K. A θe-ridge axis seems to indicate the timing and placement of convection no matter what kind of case is occurring. Overall, pure cases have been found to most occur in a high-CAPE but low-shear environment, while mixed cases seem to favor an environment of lower CAPE but higher shear.

NWS Interactions:

Tom Wasula from the National Weather Service in Albany, NY has been in contact throughout the last six months through personal meetings and emails. Tom has offered operational input and provided feedback on the results obtained so far for this research.

Publications and Workshop Submissions:

This project has been presented at the 8th Northeast Regional Operational Workshop in Albany, NY, on 1–2 November 2006 and the 32ndAnnual Northeastern Storm Conference in Springfield, MA, on 9–11 March 2007. In addition, this research will be presented at the 23rd Conference on Weather Analysis and Forecasting in Park City, UT, on 25–29 June 2007. No formal publications have been produced yet on this research. A Master’s thesis based on this research will be written starting later this year.

TABLE 1: The final 15-case sample of lake-/sea-breeze cases from this research with the dates, the bodies of water responsible, and the classifications for each case.

a)

b)

Fig. 1: 500-hPa plot on 2 August 2006 of geopotential height contoured every 6 dam, wind barbs with a half barb as 5 kt and a full barb as 10 kt, and relative vorticity shaded according to the colorbar every 4 10−5 s−1for values above 14 10−5 s−1for (a) 1200 UTC and (b) 1800 UTC when convection fired east of Lake Ontario.

a)

b)

Fig. 2: 850-hPa plot on 2 August 2006 of geopotential height contoured every 6 dam, wind as before, and mixing ratio shaded according to the color bar every 2 g kg−1 for values above 6 g kg−1for (a) 1200 UTC and(b) 1800 UTC.

a)

b)

Fig. 3: 925-hPa plot of geopotential heights contoured in black every 3 dam, wind barbs as before, and θecontoured in blue (shaded according to the colorbar) every 5 K below (above) 320 K for 1800 UTC on (a)2 August 2006 (pure case) and (b) 19 June 2002(mixed case).

Warm-Season Lake-/Sea-Breeze Severe Weather in the Northeast

(Patrick Wilson)

(Focal Point: Tom Wasula, National Weather Service, Albany, NY)

Research Summary (1 May 2007–31 October 2007):

This 6-month CSTAR-II report describes research in progress on investigating severe weather triggered by lake/sea breezes during the warm season over the Northeast. The goal of this study is to investigate the dynamical and thermodynamical processes, and their modulation by physiographic effects, that are responsible for producing severe weather from lake and sea breezes. It is hoped that this research will raise awareness and understanding for these phenomena. Fifteen cases have been chosen for detailed analysis. Plots of various severe weather parameters, radar data, soundings, surface observations, satellite images, and other sources have been used to analyze these cases. The focal point for this research is Tom Wasula from the National Weather Service in Albany, NY.

Table 1 shows the finalized case list. The 15-case sample consists of four pure cases, ten mixed cases, and one null case. Pure cases are defined as having lake-/sea-breeze convergence zones primarily responsible for initiating convection in the apparent absence of synoptic-scale forcing, while mixed cases have synoptic-scale forcing acting in conjunction with mesoscale forcing to generate convection. A null case features suppression of convection from lake/sea breezes. A schematic of the tracks and formation areas of all the cases can be seen in Fig. 1. Two tornadoes occurred in two separate cases in the pink shaded area near Buffalo, NY. One case of each type was chosen for examination with the Rapid Update Cycle gridded dataset at 20- km resolution (RUC-20), and those three cases are highlighted in Table 1.

The RUC-20 dataset was able to replicate actual surface analyses to a reasonable approximation. Two example plots containing sea level pressure (SLP), and surface temperature, dewpoint, and wind are shown for 2 August 2006 (pure case) in Fig. 2a and for 19 June 2002 (mixed case) in Fig. 2b. A ridge axis from a high-pressure system extends from WV to Lake Ontario in Fig. 2a. This pattern allows for onshore westerly flow over eastern Lake Ontario to enhance the lake breeze. A hot and humid boundary layer is clearly depicted for the developing convection in this pure case. A high-pressure system in Fig. 2b can be seen just west of ME. The clockwise flow from this system causes onshore easterly flow across NJ and the Delmarva Peninsula. Note the 8°C temperature difference between western NJ and just off the NJ shore. This temperature difference sets up an important baroclinic zone for this mixed case.

Another component in this research has been a compilation of plots showing CAPE, convective inhibition (CIN), and 1000–700 hPa wind shear. The RUC-20 dataset was implemented to create these plots. Respective examples are shown for 2 August 2006 in Fig. 3a and for 19 June 2002 in Fig. 3b. While the pure case features CAPE well over 3000 J kg−1 in western NY where the convection would fire, the mixed case barely reveals 500 J kg−1 for DE and NJ. Convection in the pure case propagated downshear in good agreement with the vertical wind shear pattern. The mixed case needed support from an upper-level trough to provide synoptic lift and vertical wind shear aloft. These plots show potential for predicting what mode of convection may be expected with these cases.

NWS Interactions:

Tom Wasula from the National Weather Service in Albany, NY, has been in contact throughout the last six months through personal meetings and emails. Tom has offered operational input and provided feedback on the results obtained from this research. He has also provided copies of archived presentations that focus on previously documented lake-/sea-breeze severe weather cases in the Northeast.

Publications and Workshop Submissions:

This research has recently been presented at the 23rd American Meteorological Society Conference on Weather Analysis and Forecasting in Park City, UT, on 25–29 June 2007. This project will also be presented at the National Weather Association’s 32nd Annual Meeting in Reno, NV, on 13–18 October 2007 and the 9th Northeast Regional Operational Workshop in Albany, NY, on 7–8 November 2007. No formal publications have been produced yet on this research. A Master’s thesis is currently in preparation, and this thesis is planned to be completed before the next six-month CSTAR-II report.

TABLE 1: The final 15-case sample of lake-/sea-breeze cases from this research with the dates, the bodies of water responsible, and the classifications for each case. Cases that were chosen for analysis with the RUC-20 gridded dataset are highlighted.

Fig. 1: Schematic of the storm formation areas and tracks for all cases. This figure was presented at the 22nd American Meteorological Society Conference on Weather Analysis and Forecasting in Park City, UT, on 25–29 June 2007 and will continue to be referenced in upcoming conferences.

a)

b)

Fig. 2: Plot of SLP in solid black every 1 hPa, and surface temperature in solid red every 2°C, dewpoint in dashed green every 2°C, and wind barbs with a half barb as 5 kt and a full barb as 10 kt for (a) 1600 UTC 2 August 2006 (pure case) and (b) 1700 UTC 19 June 2002 (mixed case).

a)

b)

Fig. 3: Plot of CIN contoured every −100 J kg−1, 1000–700 hPa wind shear in kt according to the wind barbs, and CAPE shaded every 500 J kg−1 according to the colorbar for (a) 1600 UTC 2 August 2006 (pure case) and (b) 1700 UTC 19 June 2002 (mixed case).

Warm-Season Lake-/Sea-Breeze Severe Weather in the Northeast

(Patrick Wilson)

(Focal Point: Tom Wasula, National Weather Service, Albany, NY)

Research Summary (1 November 2007–30 April 2008):

This 6-month CSTAR-II report describes research in progress on investigating severe weather triggered by lake and sea breezes during the warm season over the Northeast. The goal of this study is to investigate the dynamical and thermodynamical processes, and their modulation by physiographic effects, that are responsible for producing severe weather from lake and sea breezes. It is hoped that this research will raise awareness and understanding of lake-/sea-breeze severe weather. Fifteen cases have been chosen for detailed analysis. Plots of various severe weather parameters, radar data, soundings, surface observations, satellite images, and other sources have been used to analyze these cases. The focal point for this research is Tom Wasula from the National Weather Service in Albany, NY.

Table 1 shows the finalized case list that was established in the previous 6-month report. The 15–case sample consists of four pure cases, ten mixed cases, and one null case. Pure cases are defined as having lake-/sea-breeze convergence zones primarily responsible for initiating convection in the apparent absence of synoptic-scale forcing, while mixed cases have synoptic-scale forcing acting in conjunction with mesoscale forcing to generate convection. A null case features suppression of convection from lake and sea breezes. One case of each type was chosen for examination with the Rapid Update Cycle gridded dataset at 20–km resolution (RUC-20), and those three cases are highlighted in Table 1.

The analyses of all the case studies have been completed since the previous six-month report. The Storm Prediction Center (SPC) preliminary storm reports were obtained from the three cases that were chosen for RUC-20 analysis to verify the observed impact and type of severe weather. On 2 August 2006 (pure case), severe convection resulted in 40 severe wind reports and five severe hail reports within the circled area depicted in Fig. 1. On 19 June 2002 (mixed case), severe convection resulted in 15 severe hail reports and one severe wind report within the circled area depicted in Fig. 2. Perhaps the biggest contribution to the synoptic-scale environment playing a role in differing types of severe weather was the 1000–500 hPa thickness. The RUC-20 analyzed a 1000–500 hPa thickness of approximately 580 dam at 1800 UTC 2 August 2006 as opposed to approximately 558 dam at 1800 UTC 19 June 2002. This 22-dam difference indicates that different synoptic-scale air masses were present for these two cases, which contributed in part to the differing modes of observed severe weather.

On 11 July 2006 (null case), however, RI, southeastern CT, and southeastern MA had no severe weather reports despite 21 severe wind reports and 33 severe hail reportsjust to the northwest as shown within the circled area of Fig. 3. Hail in diameter of ≥2 in. was observed in 5 of the 33 severe hail reports. The convection was being suppressed in RI, southeastern CT, and southeastern MA from onshore advection of marine air by a sea breeze. Furthermore, the 24-h Quantitative Precipitation Estimates from the Hydrometeorological Prediction Center (HPC) ending at 1200 UTC 12 July 2006 (to include the convective precipitation during the afternoon hours of 11 July 2006), shown in Fig. 4, reveals that the circled region of RI, southeastern CT, and southeastern MA virtually received no precipitation. This finding further suggests how strongly the advected marine air in the boundary layer from the sea breeze suppressed the severe convection.