April 2009 Newsletter

The next SKYWARN meeting is 21-Apr @ 7:00 PM, the third Tuesday of the month.

About 29 people attended the 17-Mar meeting including quite a few new people from last Tuesdays class. Topics discussed included:

  • Staffing the booth at the Home Show on March 27-29.
  • Deuel NSL is organizing WX radio events with HY-Vee and Cabelas.
  • Marv NFJP is photographing sweet spots.
  • Using the 147.105 repeater as the primary method for Amateur Radio communications.
  • City Watch alerts.

No activations for Steele County. We reflect on the needs of the Fargo-Morrehead area.

SKYWARN Training
About 31 individuals attended the SKYWARN Spotter training class on 10-Mar. It may have been a stretch to think about tornadoes when the wind and snow was blowing hard that evening. One of the graphs shown as part of the training indicated Minnesota tornadoes have appeared in March. Chris G. and Dave KCUVY did a great job presenting the new material. Dave commented the material is so new they were getting updates earlier that afternoon. The biggest change in the material is the focus on properly identifying the updrafts and downdrafts in a supercell.

If you were not able to make the 10-Mar class, other classes are being held in the region. A partial list is included on the Calendar page. Links to the class schedules by the NWS and Metro SKYWARN are included there. The NWS publishes the Basic Spotters's Filed Guide online.

Hailstorm Changes From Multiple SOurces
Hail is pretty common in many states, especially in the central United States. The offices in the Central Region of the National Weather Service have decided to change their criterion for issuing a severe thunderstorm warning for hailstorms from 0.75 inch (penny diameter) to 1 inch (quarter dollar diameter). The rationale is that hail smaller than one inch diameter does not typically cause much property damage. This, of course, will reduce the number of hailstorm warnings that are issued. A large number of warnings can make people less responsive when the danger is far greater. In other words, they want you to pay attention when a warning is issued. The change is effective 01-Apr for the Chanhassen National Weather Service office. All offices in the entire central region of the National Weather Service will change the hail criteria by June 1.

Severe Weather Awareness Week
Severe Weather Awareness Week is April 20 - 24, 2009. Mark your calendar for 23-Apr. This is the day for the state-wide tornado Drill. More information on how to protect you and your family is available at MN HSEM.

Vernal Equinox From Multipel Sources
The overhead (vertical) sun passes over the equator (0 degrees latitude) on its seasonal migration into the northern hemisphere on 20-Mar. Length of daylight is roughly 12 hours everywhere. The variations are due to time zone geography, angle of the sun relative to the curvature of the Earth, and other features. Daylight hours will continue to lengthen all the way to the summer solstice on June 21st.

La Nia Advisory From Weather Underground
NOAA's Climate Prediction Center has issued a La Nia Advisory. This is triggered when the 1-month mean temperature anomaly in the equatorial Eastern Pacific cools below -0.5C and is expected to persist for three consecutive months. Ocean temperatures in the region have warmed since late January, but many El Nio forecast models predict a continuation of La Nia conditions through May of 2009. Potential impacts in the US include above-average precipitation in the Ohio and Tennessee Valleys and below-average precipitation across the South, particularly in the southwestern and southeastern states. Other potential impacts include below-average temperatures in the Pacific Northwest and above-average temperatures across much of the southern United States.

Flood Gauges
The flooding in the Fargo-Moorehad area has certainly been in the news with river and flood levels. Steele County has several river gauges, and SKYWARN participated in routine monitoring of the gauges in 2006. Gauges may or may not indicate water levels realtive to sea level. The gage may simply reference as arbitrary standard set ages ago by those who first monitored the flows on a river at a particular location. It may refer to a river height above the bottom of the channel or above a bridge footing or some other reference point. All of the gauges in Steele County ultimately have a correlation to sea level. Any reference to the level of a river must be taken into context with the local geography and the reference point. Local residents that may be affected by the river know what numbers correspond to minor, moderate, or major flood stages.

Barometric Pressure From USA Today
When it comes to air pressure, everything is relative. There is no absolute threshold value that indicates high or low pressure. A general rule of thumb is a high pressure system is 1020 millibars and a low pressure system isless than 1000 millibars. Barometric (or atmospheric) pressure can be thought of as the weight of the air on top of us. It may be difficult to recognize that air has weight. It is the weigt of air molecules stacked from the top of our heads to the edge of the atmosphere that result in an absolute air pressure of approximately 14.7 pounds per square inch. When the pressure is going down, essentially the weight of the air column above us is decreasing. This means that the air above us is moving upward and spreading out. An increasing air pressure means that the air is coming from the surrounding area and sinking. Air at the bottom of the column cannot go into the ground, so it spreads outward from the high pressure center.

So, why does a high pressure system turn clockwise and a low pressure system counter-clockwise? The spinning earth produces a force called Coriolis. If the earth were not spinning, air would move in a straight line from high pressure to low pressure. The Coriolis effect deflects moving air (wind) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Air moving inward toward low pressure is deflected to the right, resulting in counter-clockwise flow. Air moving outward from high pressure is deflected to the right, resulting in clockwise flow.

So far, so good? Why then, does high pressure indicative of good weather? As the air moves upward in a low pressure system, the cooling water vapor within the air condenses, forming clouds and storms. With a high pressure system, the air gets heavier, making it harder for water vapor to condense. The result is increasing pressure readings with clear skies and fair weather.

Natural Disaster Deaths From Weather underground
Major natural disasters grab the headlines because ofthe magnitude of the single event. The cumulative effect of the smaller events is often ignored. Heat waves, cold winter weather, severe thunderstorm winds, and flooding all kill more people in the U.S. This is illustrated in the NOAA graphic to the right. The big difference in the 10-year and 30-year average for hurricanes is due to the 1800 deaths associated with Katrina in 2005. The message is that heat- and cold-related extreme weather are probably the deadliest weather hazards in the U.S.

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Steele County SKYWARN
Owatonna, MN