VISIT: Meteorological Interpretation Blog

(Courtesy NOAA/NWS - July 15, 2008) 

Strong heating over the elevated (Mexican Plateau) desert southwest CONUS causes an area of low pressure to form known as a thermal low.  Since the air pressure is relatively higher over the nearly adjacent ocean areas (Gulf of California and the Tropical Pacific) to the south and west, air flow (from high pressure to low pressure)  begins to bring much more humid air toward the thermal low.  Instability with this lower source (level) of moisture will help in developing thunderstorms in which rain can actually reach the ground (instead of just virga storms) which will additionally add to the boundary layer moisture and help in increasing thunderstorm chances for a prolonged period…at least until the cycle reverses in late summer/early fall (when land temperatures decrease some and the oceanic waters reach their maximum).  Mid and upper level flow around high pressure aloft will also bring mid and upper level moisture into the region from the Gulf of Mexico. 

Of course there can be much variability with the North American Monsoon as to where and how intense the moisture and thunderstorms tend to be.  Where both the thermal and upper level lows/highs set up is of major importance and can mean the difference between all or nearly nothing.  For example, while a good strong upper level ridge over the great plains area will help drive moisture into the southwest CONUS, a weaker ridge or one located further west over New Mexico or northern Mexico will keep the moisture located to the east and over the great plains.  There are also a large number of other variables which can adversly affect the monsoon (see this short paper).  Currently the thermal low is located over N to NWrn Mexico and the upper level ridge is centered over the Rio Grand Valley region of Texas/Mexico. 

According to the the National Weather Service (NWS) out of Tucson, who track the North American Monsoon and its progress, the monsoon “officially” began here in the United States around the 2nd of July, 2008 (when average dewpoints in the Tucson region remain at least 54 deg F. or higher) - see the Monsoon Tracker page.  Past 24 hour rainfall (as of July 15, 2008) is depicted at the top of the page and is very typical for an early season monsoon pattern.  

For a more detailed and fascinating look at the North American Monsoon, see the NWS Tucson’s Monsoon section. 

See also these papers:

Adams, D.K., and A.C. Comrie, 1997: The North American Monsoon. Bull. Amer. Meteor. Soc., 78, 2197-2213.

Douglas, M.W., R.A. Maddox, K Howard, and S. Reyes, 1993: The Mexican monsoon. J. Climate, 6, 1665-1667.

Carleton, et.al., 1990: Mechanisms of Interannual Variability of the Southwest United States Summer Rainfall Maximum. J. Climate, 3, 999-1015.

Higgins, R.W., Mo, K.C. and Yao, Y., 1998: Interannual Variability of the U.S. Summer Regime with Emphasis on the Southwestern Monsoon. J. Climate, 11, 2583-2606.

Higgins, R.W. and Shi, W., 2000: Dominant Factors Responsible for Interannual Variability of the Summer Monsoon in the Southwestern United States. J. Climate, 13, 759-776.

Higgins, R.W. and Shi, W., 2001: Intercomparison of the Principal Modes of Interannual and Intraseasonal Variability of the North American Monsoon System. J. Climate, 14, 403-417.

Castro, C.L., McKee, T. B. and Pielke, R.A., 2001: The Relationship of the North American Monsoon to Tropical and North Pacific Sea Surface Temperatures as Revealed by Observational Analyses. J. Climate, 14, 4449-4473.

Vera, C. et. al., 2006: Toward a Unified View of the American Monsoon Systems. J. Climate, 19, 4977-5000.

Castro, C.L., Pielke, R.A. and Adegoke, J.O., 2007: Investigation of the Summer Climate of the Contiguous United States and Mexico Using the Regional Atmospheric Modeling System (RAMS). Part I: Model Climatology (1950-2002). J. Climate, 20, 3844-3865.

Castro, C.L., Pielke, R.A. and Adegoke, J.O., 2007: Investigation of the Summer Climate of the Contiguous United States and Mexico Using the Regional Atmospheric Modeling System (RAMS). Part II: Model Climate Variability. J. Climate, 20, 3866-3887.

(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  �

Here is a paper/discussion presented at this year’s 13th Conference on Aviation, Range, and Aerospace Meteorology by Haig Iskenderian (from MIT) titled, “Cloud-to-Ground Lightning as a Proxy for Nowcasts of VIL and Echo Tops.”  Although geared toward the aviation community, this inforamtion can be valuable to any WFO with forecasting/warning duties in their CWA in which radar coverage is depleated, such as: 1) sparse at the edges with no good adjacent radar coverage. 2) Has terrain/city/etc blockage. 3) A coastal office with no radar coverage offshore, etc. 4) Radar outages.

Relationship proxy determinations have been sought between cloud-to-ground lightning data and the radar fields of VIL and echo tops for use in the event of degraded or lost radar data. A probability matching methodology was applied to lightning and radar data to develop the proxy relationships.

It’s worth taking note of if for no other reason than to aid in those tricky warning situations when just a little bit more info is needed.  “To issue, or to not issue…that is the question.” 

Below are some links to, or copies of, some perhaps lesser known meteorological training materials.  Many of these have been born out of Aviation Weather programs either here in the USA through the military Air Force Weather Agency (AFWA) or the FAA, or from up north and our Canadian Neighbors.  Although some of the information is based on region specific examples, all of it (the principles, etc.) can be applied to most of our own geographic areas. 

First up is a link to NAV Canada, a privately run non-profit organization that operates Canada’s Civil Air Navigation Service.  This section contains an in depth training manuals section that is broken into six geographic regions that cover the whole of Canada.  Truly indispensable stuff here for all but tropical forecasters: NAV Canada Maunuals

Next up is the Air Force Weather Agency’s “Meteorological Techniques” which is an in depth compilation of many various weather forecasting parameters and techniques.  It is another truly indispensable item to be used for review, support, rules of thumb (tricks of the trade).   Also by AFWA, great training, practice, and supplemental review is the manual of the Mesoscale Forecast Process. 

This paper by John Mecikalski and Kristopher Bedka titled, “Forecasting Convective Initiation by Monitoring the Eveolution of Moving Cumulus in Daytime GOES Imagery” is a little long in the tooth (title-wise), but is definitely worth a read. 

More in the way of research papers / training materails will be posted here from time to time.  If you know of some lesser known, but valuable training for those of us in the weather business, please send us the information so that we may pass in on to others.�