VISIT: Meteorological Interpretation Blog

he VISIT lesson “Forecasting Convective Downburst Potential Using GOES Sounder Derived Products” presents current applications of a suite of GOES sounder-derived products. The lesson has been recently revised to include updated imagery examples, and new case studies of downburst events that occurred over the United States Great Plains during June and August 2009. The cases demonstrated the effectiveness of coordinated use of the GOES Microburst products in evolving convective storm environments.  The objective of the lesson is to provide better understanding of techniques for predicting the risk of convective downbursts utilizing GOES sounder derived data.  The guide for the lesson is available at the following web address:  http://www.cira.colostate.edu/ramm/visit/downburst.html .  Also revised is the audio playback version of the lesson, also available on the student guide page.  An example of one of the case studies of a significant downburst event is summarized below. 

During the afternoon of 26 August 2009, strong convective storms developed along a weak, slow-moving cold front as it was tracking eastward over Oklahoma. Although there was very little temperature contrast across the front, the front acted as a convergence zone and a trigger for deep, moist convection. The pre-convective environment downstream of the cold front over western Oklahoma was dominated by vertical mixing that fostered the development and evolution of a convective boundary layer. Elevated Geostationary Operational Environmental Satellite (GOES) imager brightness temperature difference (BTD) values (yellow to orange shading) and Microburst Windspeed Potential Index (MWPI) values in the vicinity of downburst occurrence over western Oklahoma served as evidence of the presence of a well-developed mixed layer. Strong downbursts that were recorded by Oklahoma Mesonet stations between 0000 and 0100 UTC 27 August resulted from a combination of precipitation loading and sub-cloud evaporation of precipitation. These downbursts occurred in proximity to moderate to high microburst risk values as indicated in the 2200 UTC GOES microburst products.

The images above are a Geostationary Operational Environmental Satellite (GOES) imager microburst product at 2200 UTC 26 August 2009(top) and a corresponding GOES sounder Microburst Windspeed Potential Index (MWPI) product (bottom), with the location of Oklahoma mesonet observations (i.e BESS, WEAT, etc.) of downburst wind gusts plotted on the MWPI image. Both product images displayed convective storms developing along the cold front over western Oklahoma. Convective storm activity increased in coverage near the cold front during the following three hours. Downburst wind gusts between 41 and 56 knots were recorded by the Oklahoma Mesonet stations plotted in the MWPI image above between 0000 and 0100 UTC 27 August.
The following are significant downbursts recorded by the Oklahoma Mesonet during this event:
Station - Time (UTC) - Wind Gust (knots)
Bessie - 0005- 50
Kingfisher - 0020 - 43
Weatherford - 0030 - 41
El Reno - 0040 - 50
Medford- 0055 – 56 Also important to note the general increase in MWPI values from southwest (BESS) to northeast (MEDF) associated with a progression from hybrid to stronger wet type downbursts. Downbursts over western Oklahoma were predominantly “hybrid” type, while over north-central Oklahoma (MEDF, BREC), downbursts were “wet” type associated with heavy rainfall.

The VISIT lesson “Forecasting Convective Downburst Potential Using GOES Sounder Derived Products” presents current applications of a suite of GOES sounder-derived products. A recent concern pertaining to the GOES sounder products is the current temporal and spatial resolution (60 minutes, 10 km). The GOES-R Advanced Baseline Imager (ABI) has promising capability as a sounder with greatly improved temporal and spatial resolution as compared to the existing GOES (8-P) sounders. However, until GOES-R soundings and associated derived products are operational, a need has been established for a higher resolution GOES-derived microburst risk product. Accordingly, a multispectral GOES imager product has been developed and experimentally implemented to assess downburst potential over the western United States with improved temporal and spatial resolution. The availability of the split-window channel in the GOES-11 imager allows for the inference of boundary layer moisture content. The experimental product is available on the web:

http://www.star.nesdis.noaa.gov/smcd/opdb/kpryor/mburst/mbimg.html.

(From SPC MCD 1327 - Concerning Tornado Watch #509) 

Wednesday night, June 11, 2008, at just after 6:30 PM, some 93 boys, ages 13 to 18, along with 25 adult BSA staff members, were fighting for their lives as a deadly tornado roared through the Little Sioux Boy Scout Camp. Four of the Boy Scouts ended up losing their lives in this fight against Mother Nature while attending what was to be a weeklong leadership training camp.  Over 40 others were also injured and were either rescued and/or attended to by other staff and scouts who used their emergency/first aid training to the best of their abilities.  You couldn’t ask for a better bunch in a situation like this…and even then there were some terrible results.

The synoptic/mesoscale set-up had a cold front extending from a surface low pressure center over eastern South Dakota, south through eastern Nebraska and into central portions of Kansas.  A warm front stretched from the surface low eastward across southern Minnesota (not far north of the Iowa border).  In between these two features lay the warm (and moist) sector with surface temperatures ranging from the mid 70s (north) to nearly 90 (south) and dew points in the upper 60s to lower 70s.  There was also a slowly moving (westward) mesoscale boundary (through Iowa).  Mixed layer CAPE ranged from around 1500 j/kg (north) to nearly 3000 j/kg (south) by this time with lapse rates above 700mb  were running between 7 and 8 deg C/km (there was just a bit of a cap present near and above 600mb that kept the lid on long enough to make things explosive).  Effective shear was running between 40 (far south) and 60 (north in Minnesota) kts with low level storm relative helicities ranging from 200 (south into Kansas) to nearly 500 m²/s² (north into southern Minnesota and northern Iowa).

Movement of the cold front was to the east…with storms initiating ahead of the cold front along a (pre-frontal) trough from southeastern South Dakota into northeastern Nebraska by 4 PM LDT…back-building south southwestward into south central Nebraska by 5 PM (southwest Kansas also had storms going up by 5 PM).

Many of these storms became tornadic within in the first hour since initiation – first starting over portions of northwestern Iowa and southwestern Minnesota…then with reports following, down the line, into eastern Nebraska and western Iowa soon after.  Kansas then finished off the evening with continued reports up to just after midnight.  All told there were at least 53 separate reports of tornadoes (some reports, however, may be of the same storm) covering the four states of Minnesota, Iowa, Nebraska, and Kansas.  There were five fatalities total (the four in Iowa and one in Kansas) and many, many more injured.       

Please give the kids a second (and third) thought.  

(Image courtesy of Eric Thaler, SOO WFO DEN/BOU.  Data source - NOAA/NWS; Map - FEMA) 

Thursday, May 22, 2008 was truly a day the will live in infamy for many folks in and around the communities of western Weld County (and north eastern Larimer county), Colorado.  While the city of Windsor, Colorado sustained the most damage (total amounts still at large), many other towns were also affected by this large early season tornado (Platteville, Gilcrest, Milliken, western Greeley - where there was one fatality -, Timnath, and points just northeast of Fort Collins).  Albany County, Wyoming (including the city of Laramie) was also affected and damaged by this same storm early in the afternoon.  The area around Dacono, Colorado also took on some damage just after noon on the 22nd as a tornado, connected with a separate severe storm, bounced west of town.  This second storm ended up following a near parallel track to the first storm - only was displaced further to the west and remained mostly over the barren foothills as it too tracked to the north-northwest and into southern Wyoming - however, with no additional (apparent) damage.

Interesting atmospheric severe weather set-up for not only the front range of northern Colorado, but for the entire high and central plains region with many more strong tornadoes showing themselves and wreaking havoc in Kansas and Nebraska.  Even the west coast of the USA was not untouched by tornadoes on this day  - they too being influenced by the massive-deep-digging late season upper level trough.

For more concerning the morning tornadoes of northern Colorado please go to this satellite oriented report at: http://rammb.cira.colostate.edu/case_studies/20080522/    

Or, The NWS BOU/DEN report at: http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=bou&storyid=8556&source=0

Or, for yet another look at the storms and set-up, please go to the CIMMS blog: http://cimss.ssec.wisc.edu/goes/blog/archives/660  �