weather.gov 
 
 
 
National Weather Service Burlington, VT Twitter Page National Weather Service Burlington, VT Facebook Page
Local forecast by
"City, St" or zip code
  
 Current Hazards
    
 Current Conditions
    
 Forecasts
    
 Model Data
    
 Climate
    
 Weather Safety
    
 Miscellaneous
    
 Contact Us
"Route 7 Runner": The May 26, 2010 Severe Weather Event Across the Champlain Valley


Part II: The role of an elevated mixed layer in thunderstorm severity during the evening of 26 May 2010
Part II of this event summary analyzes mid-tropospheric lapse rates and trajectories associated with the severe weather event occurring during the evening hours on 26 May 2010 across the Champlain Valley of Vermont southward across western New England and across Long Island.

Severe weather events in the North Country can occasionally be influenced by an elevated mixed layer - a displaced layer of relatively hot, dry air aloft originating from the Intermountain West or Mexican Plateau (Banacos and Ekster 2010). Such layers play several important roles: (1) they keep a "capping inversion" in place, allowing for a greater buildup of convective available potential energy (CAPE) prior to the initiation of thunderstorms, (2) thunderstorms tend to be taller with more intense updrafts as a result of the greater instability, and (3) convection in EML environs are associated with greater dry air aloft, which contributes to greater downdraft potential and cold pool generation once storms do develop. The EML is also often associated with heat waves, and several daily high temperatures for May 26th were set across New York and New England several hours before the start of the severe weather.
Click to enlarge
Using NOAA's HYSPLIT model (http://ready.arl.noaa.gov/HYSPLIT.php), we can trace back the air mass within which the first severe storms occurred along the eastern shores of Lake Champlain in the West Milton/Georgia, Vermont area around 00 UTC on 27 May 2010. Figure 1-EML below is a backward trajectory going back 120 hours (5 days).

As can be seen, the trajectory approaches the Champlain Valley in an anticyclonic arc from the North-Northwest, around an anomalously strong upper ridge which was centered across the northern Great Lakes region. The ridge extended north across Ontario, and the trajectory can be seen passing across James Bay around 26/06 UTC.
The trajectory moved along the western periphery of the upper ridge through the upper Mississippi River Valley on the 25th, after completing a complete anticyclonic loop across southern Missouri during the 24th. Prior to that, the air moved across the southern Plains on the 23rd and can be placed back to the high terrain of north-central Mexico on the 22nd. Anticyclonically curved air motion is a common trait of elevated mixed layers which are able to advect large distances; such motion is generally associated with descending air motion which suppresses widespread convective storms and maintains the integrity of the elevated mixed layer over time.

The lower portion of the figure shows a time-height cross section of the backward trajectory. It is notable that the parcel is sinking during its traverse around the upper ridge axis from about 24/18 UTC through 26/00 UTC. There is the start of rising parcel motion shown in the trajectory time-height cross section as the air moves into the Champlain Valley during the evening hours on May 26th. Note also in the top figure that the trajectory reaches or slightly passes an inflection point as it moves into the Champlain Valley; the 700mb flow was becoming slightly cyclonic due to the presence of an upper low over the Canadian Maritimes during the evening of the 26th.

Figure 1-EML also denotes soundings along/near the path of the trajectory, which we will examine next.
Click to enlarge
It appears from the soundings that strong insolational heating resulted in a deep mixed-layer over the Mexican Plateau, which is manifest as an elevated mixed layer in the Monterrey, Mexico at 12 UTC on 22 May 2010 (Fig. 2-EML) as the air moves northeastward to lower elevation. The Monterrey sounding shows steep lapse rates in the 800-550mb layer and a strong capping inversion between 850 and 800 mb.

Note also the strong drying apparent at 800mb associated with the interface between the local moist boundary layer and the dry EML which has moved in. Lapse rates are as steep as dry adiabatic (~9.8 C/km) in the 650-550 mb layer.
Click to enlarge
The sounding mosaic in Figure 3-EML shows the general evolution of the EML as it moves northeastward across the central United States and across Ontario.

While the EML changes and becomes somewhat less defined over time, the Maniwaki sounding at 12 UTC on May 26th (Figure 3f-EML) does show a layer of steep lapse rates from 750-550 mb (greater than 8 C/km), and the characteristic increase in relative humidity with height through this layer. The northerly winds allow this plume of steep lapse rates to advect southward into northern New York and Vermont during the day on May 26th.
Click to enlarge
The Albany, NY (ALB) sounding at 00 UTC on 27 May 2010 (Fig. 4-EML) reveals very steep lapse rates from the surface through 600mb, associated with the remnant EML and the hot, well-mixed boundary layer with surface temperatures still in the low 90s during the early evening hours. Resultant surface-based CAPE is nearly 4000 J/kg, which is very unstable for the Northeastern United States. Note also that the equilibrium level on the sounding is near 150mb, and allowed for very "deep" convection to occur.

In addition to the steep lapse rates, the surface to 6 km shear was around 25 kts, which was sufficient to promote thunderstorm organization during the evening hours. The development of the convection occurred on the "cool" edge of the EML plume and the western edge of the stronger deep-layer northerly shear associated with a vertically stacked low over the Canadian Maritimes (this will be more apparent in Fig 5-EML below).
Click to enlarge
To further detail the movement and evolution of the EML over time, RUC initial hour analyses were used to create a 5-day loop of 700-500mb lapse rates juxtaposed with 700mb geopotential heights, winds, and temperatures. The initial panel is displayed in Fig. 5-EML, and shows the generation of steep lapse rates 8-10 C/km across the higher terrain of the southern Rockies and Mexican Plateau. The 700mb trough moving across the Great Basin aids in ejecting the hot air northeastward as an elevated mixed layer on May 23rd (see loop here). Meanwhile, a building ridge across the Ohio Valley northward across the Great Lakes into Ontario allows for strong gradient flow to advect the EML plume northward in anticyclonic flow across the lower Missouri Valley into the upper Mississippi river valley on the 24th. While the EML slowly loses definition with time, it is evident that a portion of it remains within the anticyclonic circulation as it moves around the ridge into southern Quebec and New York and Vermont on the 26th. Convective initiation occurs near the eastern edge (cool side) of the EML plume, and along a gradient of increasing 700mb winds associated with the deep low over the Canadian Maritimes on the 26th. Typically, convection will occur on the northern or eastern edge of the EML where convective inhibition not as significant.
The presence of an EML in the northeastern United States is relatively rare. One reason is the distance (>3000 km) from the primary source region of the Intermountain West. Another precluding factor is convective overturning in the form of thunderstorms which typically process and eradicate the EML over the central United States (where the highest concentration of severe weather occurs in the United States). However, in the days leading up to May 26th, we find a lot of anticyclonically curved flow which generally leads to synoptic-scale decent limiting the areal coverage of thunderstorms. Thus, the anticylonically curved flow contributes to maintenance of the EML plume. We can infer the absence of widespread convection near the EML plume by looking at the 24-hour precipitation (12-12 UTC) overlaid with an intermediate snapshot of RUC 700-500mb lapse rates > 7 C/km (which we will loosely define as the edge of the EML plume).
Click to enlarge
The multi-sensor precipitation analysis for the CONUS is displayed in Fig. 6-EML. Looking specifically at precipitation amounts ending at 12 UTC on May 23rd (Fig. 6b-EML) and May 24th (Fig. 6c-EML), we see a general absence of rainfall across the southern and central Great Plains northeastward into the western Great Lakes region, allowing the EML to move without any significant changes. Precipitation becomes more widespread for the 24 hour period ending at 12 UTC on May 25th (Fig. 6d-EML), especially across Oklahoma, Kansas, and Nebraska. While this decreases the areal extent of the EML plume, there is a smaller, relatively unmodified portion of the EML across the upper Mississippi valley and Great Lakes region that has become entrenched in the 700mb ridge circulation. A portion of this remaining EML plume appears to move across Ontario and into the North Country on the 26th. The precipitation associated with the severe, forward-propagating MCS can be seen on the final panel (Fig. 6f-EML) across eastern New York and New England, and occurs along the eastern periphery of the EML.
In conclusion, the presence of an elevated mixed layer (EML) likely contributed to the overall severity and longevity of the forward-propagating mesoscale convective system (MCS) on the evening of 26 May 2010. The steep mid-tropospheric lapse rates resulted in large CAPE values (up to 4000 J/kg observed per 00 UTC/27th ALB sounding). Likewise, the dry air aloft aided in the development of a strong mesoscale cold pool that aided in the accelerated motion of the MCS. In the days leading up to the event, we showed that southwesterly winds associated with a mid-level trough across the Intermountain West helped eject the EML from the Mexican Plateau, and that the EML plume was able to move northeastward across the southern and central Plains and upper Midwest in the absence of widespread convective overturning, which would have otherwise destroyed the EML. The trajectory analysis suggests anticyclonically curved flow along the EML path which likely suppressed convection until it began to encounter cyclonically curved flow and a highly moist and unstable environment for thunderstorm formation from Montreal, Quebec into the Champlain Valley at 23 UTC on 26 May 2010. Because the air mass is usually capped with EML events, the initiation of convection may be later than normal (around 7 pm in this case) or nocturnal (e.g., past events on 7/15/1995 or 7/5/1999) as a source of lift is necessary to get deep moist convection started. Patience becomes an important attribute for forecasters when severe weather is possible in an EML environment.


Acknowledgements

The NOAA Air Resources Laboratory (ARL) READY website (http://ready.arl.noaa.gov/HYSPLIT.php) was used to generate the HYSPLIT model trajectory shown in Figure 1-EML.


Reference

Banacos, P. C., and M. L. Ekster, 2010: The association of the elevated mixed layer with significant severe weather events in the Northeastern United States. Wea. Forecasting, In Press.


<<< Back to Part I
Figure 1-EML. HYSPLIT backward trajectory from near 3 km AGL at location of first severe weather in the Champlain Valley of Vermont. Observed soundings indicating an EML near/along the trajectory are displayed (3-letter ID and date/time).
Figure 2-EML. Observed sounding from Monterrey, Mexico (MMY) at 12 UTC on 22 May 2010.
Figure 3-EML. Observed soundings near trajectory shown in Fig. 1-EML. The soundings are from (a) Del Rio, TX, at 00 UTC 23 May 2010, (b) Norman, OK, at 12 UTC 23 May 2010, (c) Springfield, MO, at 00 UTC 24 May 2010, (d) Minneapolis, MN, at 00 UTC 25 May 2010, (e) Moosonee, ON, at 00 UTC 26 May 2010, and (f) Maniwaki, QB, at 12 UTC 26 May 2010.
Figure 4-EML. The observed sounding from Albany, NY (ALB) at 00 UTC on 27 May 2010. Surface-based CAPE was 3948 J/kg with a 700-500mb lapse rate of 7.4 C/km.
Figure 5-EML. RUC 0-h analysis at 01 UTC on 22 May 2010 showing 700-500mb lapse rate (color filled, every 0.5 C/km), 700mb geopotential heights (yellow, every 30 dm), temperatures (red, every 2C), and wind barbs (kts). A loop of these variables is available in 6-hour intervals from 01 UTC on 22 May through 07 UTC 27 May 2010 here.
Figure 6-EML. NCEP Multi-sensor 24-hour precipitation analysis ending at (a) 12 UTC/22 May 2010, (b) 12 UTC/23 May 2010, (c) 12 UTC/24 May 2010, (d) 12 UTC/25 May 2010, (e) 12 UTC/26 May 2010, and (f) 12 UTC/27 May 2010. Precipitation shown in millimeters with color scale at left. Also shown in each panel is the approximate outline of 01z RUC > 7 C/km 700-500mb lapse rate associated with the EML layer (“EML” label placed at location of steepest lapse rates).


National Weather Service
Burlington
1200 Airport Drive
S. Burlington VT 05403
(802)862-2475

Webmaster: Webmaster
Page last modified: May 30, 2010
About Us
Disclaimer
Credits
Career Opportunities
Glossary
Privacy Policy