The Weather Wire
June 2008 Volume 16 Number 06
Avg High 70.7
Avg Low 41.0
Snow - 3.4"
Season Snow - 46.3"
Precipitation - 1.56"
Avg High 82.1
Avg Low 53.0
Avg Snow - Trace
Avg Precip - 1.56"
Severe Weather in Colorado
The spring months of May and June typically bring most of the severe weather to Colorado's Front Range. We have already witnessed with many hailstorms so far this year and an unforgettable tornado that tore through the town of Windsor. Why does most of the severe weather occur in the spring versus the summer?
There are a couple reasons why the spring is more active with severe weather than during the rest of the summer months. One reason is that cold air still remains above the surface as we transition out of winter into spring. The cold air aloft helps create severe weather in a couple ways. First, the cold air helps de-stabilize the atmosphere as warm air at the surface wants to rise to form thunderstorms. The ground warms rapidly with the increasing sunshine during the spring months with the longest day of the year being the first day of summer or the summer solstice which occurs in June. Second, the cold air allows hail to form in the updrafts at lower elevations in the cloud which makes the hail stones larger and more likely to reach the surface. Many summer storms contain hail as well, but the hail travels through a larger warm layer and often melts before reaching the ground. A second reason is wind shear or changes in wind speed, or direction, or both with height in the atmosphere. In the spring the jet stream can still be rather active bringing disturbances to Colorado area before it retreats northward during summer. The stronger winds aloft associated with the jet stream can be tapped into by strong thunderstorms. Typically, but not always the strongest winds aloft are from the southwest ahead of an approaching trough of low pressure. Winds at the surface often vary from the upper level winds by both speed and direction. This in turn causes the wind shear with height in the atmosphere. When a strong thunderstorm develops in this these conditions the updraft carrying moisture laden air from the surface upwards can rotate. Storms with a rotating updraft are called super cell thunderstorms, like the storm in May that resulted in a strong tornado that devastated the Windsor area. The rotation of the updraft can be concentrated by stretching the column of air like a figure skater spinning faster and faster as she brings her arms into her body.
There area many other factors that come into play when it comes to severe weather, but cold air aloft and wind shear are two of the major players when it comes to spring severe weather in the state of Colorado. As we move into the summer months the jet stream weakens and travels north impacting the state less often and reducing wind shear. As the overall atmosphere warms in the northern hemisphere during summer the air aloft is just not as cold near the surface allowing hail to fall as large rain drops instead of frozen precipitation. Hail still quite often occurs here in Colorado during the summer months due to our elevation above sea level, but is typically smaller in size and more isolated.
Little in the way of meaningful drought across Colorado as of early June though some dry areas do persist across Eastern Colorado.
The map below shows forecasted temperature deviances for June2008. As can be seen, it is expected that all of Colorado to have normal temperatures for the month of June 2008.
The map below shows forecasted precipitation deviances for June2008. Normal precipitation is anticipated across all of Colorado for June 2008.
Little change in conditions is anticipated over the next 3 months, with most of the state not expected to see drought conditions. Extreme southeastern Colorado is anticipated to experience drought conditions persisting with some improvement.
May of 2008 marks the 7th month in a row for below average precipitation. There was a monthly total of 1.56" which is 0.76" below the average of 2.32". For the year now the official measuring site is an incredible 4.22" below normal. With April and May being some of the wettest months of the year it is probably safe to say that this year will end up being well below average unless a very wet monsoon season occurs. Temperatures were near normal for highs and below normal for lows with a monthly mean of 1.4 degrees below normal. The highest temperatures observed was 89 degrees on the 21st and the lowest was 21 on the 3rd, none of which tied or broke any records. Snowfall was above average, but with 3.4" for the month the 2007-2008 winter season will end up being 15.4" below normal for the Stapleton site. There were 11 days with measurable precipitation reported which is right at the average. One record was broken for the month with a maximum wind gust of 61mph from the south, the old record was 60mph set back in 1991.
June is the transition month from spring to summer and is typically the severe weather month for Denver and surrounding areas. Nearly half of all tornadoes that are reported in eastern Colorado and along the Front Range during the spring and summer months occur during the month of June. One storm many of you may remember occurred on June 13, 1984. This strong thunderstorm brought large hail to the metro area and is one of the worst and costliest hail storms in Denver history. The temperatures really begin to rise during June with six days with highs of 90 degrees or above on average. The record high for the month is 104 recorded on the 26th in 1994. Cold weather can still occur in early June as the record monthly low of 30 degrees was recorded on the 2nd in 1951. There have only been 5 years with a trace of snow or greater, so it is safe to say that there will not be any more snow to fall this winter season. Precipitation records range from a trace to 4.96" with an average of 1.56'. Even if we matched the record rainfall for the month we would still be below average for the year.
Sunrise/Sunset (Jan - June Denver area)
May 2008 to September 2008