Ceaseless Wind

With all the talk of this being an above normal season for tropical storms and hurricanes in the Atlantic, it seems the last few storms, “Bonnie” and “Colin”,  (editors note: Tropical Depressions 4 & 5 too!) have had some issues getting it together!  For that reason, I thought I would list two things that will kill or at least injure a tropical storm or hurricane.

Wind shear

It’s funny, but a hurricane, the largest and most powerful storm on earth, must start in fairly benign conditions. In fact, strong changes in wind speed and/or direction called wind shear will tear apart developing tropical systems.

While the middle latitudes often have shearing winds during the summer, the lower latitudes often do not and hence tropical storms can take hold in these areas.

Shear migrates with the weather systems they are a part of so a tropical storm may either try to form in some shear and then later move out of it or will get torn apart by increasing shear. The latter is more likely as storms travel more northward. Both “Bonnie” and “Colin” had some issues with shear this season.

Dry air

Since a tropical storm is really a well-organized thunderstorm machine, it follows that anything which robs a thunderstorm of its strength will also inhibit a developing tropical storm.

Dry air in the middle levels of the atmosphere will literally eat away at cloud towers which make up a storm.  Dry air will enter the side of a storm and evaporate the water droplets it is made of, thereby weakening the updraft of the storm and decreasing its size.

This dry air is usually supplied by nearby continental region.  Many developing storms coming off of Africa are fighting the dry air from that region.

Vivian, SD

Aurora, NE

Coffeeville, KS

Note:  It’s official, a new record!

A record-holding hailstone fell in Aurora, Nebraska., on June 22, 2003, from was has been referred to as “the mother of all supercells”.  It measured 7 inches in diameter with a circumference of 18.75 inches.  Now, seven years later, it appears as if that record may be broken.

Severe thunderstorms last Friday, July 23rd, produced numerous reports of large hail and even a tornado in eastern and northeastern South Dakota.  Among the reports, the town of Vivian has the hardest hit with extensive hail damage.  While surveying the ground for hail stones, a particularly large “spiked” stone was spotted.  Turns out this could be the largest ever found!

The National Weather Service office in Aberdeen, South Dakota weighed the stone at 1.9375 pounds which is a world record weight.  The circumference prior to melting was 18.5 inches with a diameter of 8 inches.  This is likely enough to establish this as the largest hailstone on record!

Of local interest, the hailstone which held the record prior to the two abovementioned fell just west of the Ozarks in Coffeeville, Kansas on September 3, 1970  It had a diameter of 5.7  inches and a circumference of 17.5 inches.

Gust Front Nearing Greene Co from the Northeast on Tuesday

We have had a rather high number of outflow boundaries move into the Ozarks this spring and summer.

Yesterday, we had at least two in the Ozarks. We had another on Monday and numerous others earlier in the year.

Outflows are like mini-cool fronts produced by the collective cool downdraft(s) of a storm(s).  Sometimes, one storm produces a downdraft which spreads out in all directions from the storm.  Most recently, lines of storms have contributed to a collective larger outflow boundary.  These boundaries are of interest because they can help to develop additional storms or make storms stronger or even tornadic!

How these boundaries move and whether they can produce additional showers and storms greatly depends on the strength of the outflow, the instability of the atmosphere and the overall wind field from the earth’s surface up to the jet stream level.

Strength is mostly determined by the the temperature contrast but there are other factors such as the depth of the cool air and also whether the boundary is moving with the steering currents or against them.  The chances for producing showers and storms increases if these boundaries can lift the air and this lift comes from the convergence of air or rising air produced by temperature contrast itself.

But all of this can come to a screeching halt if the air along the boundary is not unstable, weakly unstable or is capped by warm air which prevents currents from rising.  Essentially, the boundary can give the air a shove upwards but if it is not buoyant (warmer and less dense when compared to the surrounding air) it will resist rising.

These boundaries are hard to forecast since they are produced by thunderstorms which in turn are hard to nail down at any given hour.  The boundary which came from the northeast on Tuesday had just enough lift and instability to produce the “surprise” rain and showers in Springfield and the Greene County.

This is a phrase which I’ve actually heard more in connection with the volcanic activity which lines the Pacific Ocean basin but it has also been used to describe the ring of thunderstorms which is sometimes observed in the central United States during the summer months.

Thunderstorms should love summer in the U.S. because the air is hot and humid making it unstable.  But as the jet stream retreats northward during this season, it is replaced aloft by an area of high pressure.  This feature promotes sinking air which has the effect of “snuffing out” storms which are by there very nature creatures of strongly rising air. 

Other features which can help lift the air such as fronts or strong jet streams aloft are circulating around this high, turning in a clockwise direction.  This activity is mostly observed on the northern edge of the high.

A ring is a circle of course.  To complete the effect, storms are often active on the western edge of the high near the Rocky Mountains where a weak jet stream and orographic lifting are in play.  Also, weak easterly waves, sea breeze circulations and a deeper humidity supply found near the Gulf of Mexico round out the southern edge of the ring.

Do to the overwhelmimg popular demand for this, I will be broadcasting live on the web at the following times today.  A live chat room is featured, bring questions or comments!:

1:00 pm  A weather briefing I give to my students at MSU.  About ten minutes.

4-5 pm  A chat room will be open and I will cycle through some maps.  Limited live talking due to TV preparation but I’ll pop in from time to time!

6:30 pm Live from the Weather Lab!  About half and hour.

11:00 pm The regular broadcast of “Upstream” from my house.  Half hour or more.

The link to all of these broadcasts on Ustream.tv

I use the graphing product from Iowa State University in my posts quite a bit:

http://www.meteor.iastate.edu/~ckarsten/bufkit/image_loader.phtml?site=ksgf

This is for Springfield (SGF).  You can select another location if you like by going here:

http://www.meteor.iastate.edu/~ckarsten/bufkit/data/index.html

Select the city then, “visualize data”.

The lines represent data from two different models, the NAM (Eta) and the GFS, and shows the last two runs of both models.  Run times and z time blog.  Also represented is the National Weather Service forecast in green and observed data in black.