The Weather Wire
April 2007 Volume 15 Number 04
Avg High 59.2
Avg Low 32.9
Snow - 6.7"
Season Snow - 71.7"
Precipitation - 0.57"
Avg High 60.9
Avg Low 34.2
Avg Snow - 9.1"
Avg Precip - 1.93"
Spring time in Colorado usually means plenty of wet weather including heavy wet snows. Storms that develop during the months of March and April provide their own special challenges to weather forecasters.
As we progress through these two months the sun becomes progressively higher in the sky and average daily temperatures warm from the 30s to the 50s and 60s. Therefore one of the problems we have to deal with is the warmer temperatures in the ground and paved areas. This is a critical consideration for those who have the job of removing the snow. It is especially difficult for the weather forecaster as not only does he have to sift through the various models to forecast the right amount of snow for the storm, but for this time of year he has to take into account soil and pavement temperatures. He may be right in forecasting 4-6 inches of snow and 4-6 inches did fall, but be wrong in that none accumulated on paved areas and only 2 inches stuck to grassy areas.
Another factor that becomes a major player this time of year is elevation. During the mid winter months just about all areas are cold enough for snow and the higher elevations can get heavier snows depending on wind direction. Upslope flow favors the foothills to the west and the Palmer Divide to the south. For the spring months it becomes a matter of where the rain/snow line is. For example, the snows that hit eastern Colorado the first week in April was very elevation dependant. The rain/snow line was about 5800 feet, as a result the higher elevations in Douglas County received 4-6 inches of heavy wet snow, while the Denver metro area all below 5800 feet saw only rain and showers and no snow at all.
Another challenge for the weather forecaster this time of year is convective snows. With the warmer temperatures we see in the months of March and April comes the increasing likelihood of convective snows. If you think of the afternoon summer thunderstorm caused by heating you won’t be far off with convective snows. During the months of March and April with the warmer surface temperatures, as we get storms in from the west they bring colder air with them at the upper levels. Push this over a warmer layer of air at the surface and we create a very unstable air mass that is capable of producing snows with embedded areas of convective snows. These areas of convection can produce snowfall rates of 2-3 inches per hour over a short period of time. Much like forecasting summer thunderstorms we would be right in saying there will be thunderstorms today, but some areas will see the heavier rains and some lighter amounts and other none at all. We face the same difficulty with convective snows. Forecasting a storm to produce 2-4 inches of snow might be right for most areas, but for those spots hit by convective snows might see 3-6” or even more from the embedded heavier showers. Know exactly where those heavier embedded showers will hit is almost impossible to forecast ahead of time and the best that can be done is to track those areas when they appear on radar. This now become more of a nowcast type of forecast, much like we see in the summer months.
So spring here in the Rockies has more than its share of challenges for the weather forecaster.
Little change, or perhaps improvement, in overall drought conditions across Colorado occurred in March.
The map below shows forecasted temperature deviances for April 2007. As can be seen, above normal to above normal temperatures are expected for Colorado for April 2007.
The map below shows forecasted precipitation deviances for April 2007. Normal precipitation is expected for Colorado for April 2007.
As can be seen in the below map, drought improvement is forecast for portions of Northeast Colorado through May.
For 28 days March 2007 flirted with becoming one of the top 5 warmest Marches in Denver weather history. But Mother Nature had other ideas by allowing 2 below normal days and 1 normal day at the end of the month to drop March 2007 as the 8th warmest March. The month finished with an average temperature of 46.1 degrees which is 6.5 degrees above the normal of 39.6 degrees. The warmest March ever occurred in 1910 with an 50.4 average, 104 degrees above normal. March 2007 had a temperature spread from 75 degrees on the 18th down to a low temperature of 14 degrees recorded on the 30th. Only one temperature record was tied during the month, 74 degrees on the 13th, last set in 2003.
Precipitation for March 2007 finished below normal. The month had a total of 0.57” which was 0.71 inch below normal. Most of that precipitation was confined to just two days, the 24th and the 29th. Snow was also below normal as the month finished with 6.7 inches, which is almost 5 inches below the normal of 11.7 inches. Most of this snow fell on the 29th when 4.1 inches was recorded at the old Stapleton Airport.
Although not recorded at DIA the first thunderstorms of the season for northeast Colorado were Observed on March 23rd in Morgan and Boulder counties. We might note that thunderstorms were also seen in Douglas, Jefferson and western sections of Denver County. 1 ¾ inch hail was reported 3 miles north of Fort Morgan.
Is a transitional month for most of the country and this is especially true for Denver. Almost any type of weather can occur and usually does. Tornadoes, thunderstorms, hail, snow and dust storm are all possible during the month. In addition, there are plenty of pleasant sunny days with temperatures ranging from the 60s to low 80s. On April 30th in 1992 the mercury climbed to the highest April temperature ever, 90 degrees.
In April, Denver is assaulted from all sides by varying types of air masses which accounts for the wide swings in the types of weather. Arctic air still occasionally invades the area and often teams up with an eastward moving Pacific storm producing snow in Denver. At other times, warm moist air from the Gulf of Mexico moves into the area and thunderstorms occur, sometimes occurring just before the snow. In addition, warm dry air from the Desert Southwest can bring warm temperatures and low humidities to eastern Colorado.
The temperature range for April is from 90 degrees down to 2 degrees below zero. April is the third snowiest month of the year with an average of 8.9 inches and is the windiest month of the year with an average speed of 10 mph. For the month you can expect 2 thunderstorms to occur.
April 2001 was highlighted by a strong winter storm that produced blizzard like conditions that moved across the metro area on the 10th and 11th. The storm dumped 10.0 inches on former Stapleton. The storm produced plenty of heavy wet snow with southeast Aurora reporting 18 inches and 5 foot drifts, Parker reported 16 inches with 3 foot drifts, Westminster 9 inches and Littleton 8 inches. Even though a great deal of snow fell, by the 12th nearly all of the white stuff had melted away.
Sunrise/Sunset (Denver area)
October 2006 to April 2007