Tuesday, March 24, 2015

Cloud "Mystery" Solved

If you are interested in weather phenomena and like a good detective story, then this is for you.

About two weeks ago people noticed strange cloud formations in the skies above San Antonio, TX. A deck of altostratus clouds were moving across the area, and people noticed wide, winding openings in the cloud deck and in some cases large circular openings. Photos began being posted to the NWS San Antonio Facebook page with inquiries about what caused these unusual formations.

Cloud formation seen above San Antonio on March 13, 2015

The phenomenon is known as "hole punch" clouds, or fallstreak holes. They are not that uncommon, especially near major airports and along air traffic routes, but the conditions have to be just right for them to occur. What happens, in simple terms, is that aircraft cause precipitation to occur as they fly through these clouds of supercooled droplets. The precipitation falls out of the cloud, but evaporates in the typically dry air underneath. When the cloud layer is thin and there is no supply of moisture to replace that which precipitated out, a hole remains.  The wispy streaks you often see beneath the hole are ice crystals precipitating out of the cloud. The difference between a "canal" in the clouds or a circular hole depends on whether the aircraft was cruising through the cloud layer or descending or ascending through the layer.

This is a photo of a "hole punch" cloud I took over western Indiana
on October 17, 2014

What is really neat about the San Antonio occurrence is that the forecasters at the National Weather Service, through a little "detective" work, were able to determine which individual airplane caused these hole punch clouds using sounding, satellite, and air traffic data! You can read the details about "The Mystery of the 'Cloud Hole Punch'" at the NWS San Antonio web site.

There is also more information about hole punch clouds at from NASA at this link. Some striking photographs and more explanation of hole punch clouds can be found at EarthSky.

There are more spectacular photos and explanation on the Cloud Appreciation Society web site. Keep your eyes to the sky - you never know what you may see.

Wednesday, March 11, 2015

The Yin and Yang of the Winter of 2014-2015

Yin and yang. Warm and cold. Buried and barren. Winners and losers.

There are a number of ways to describe this past winter, but my pick for a description would be something along the the lines of "sudden and weird".

The winter of 2013-2014  was a severe winter in many parts of the country, especially in the Northern Plains, Great Lakes, Midwest, and parts of the Northeast. Winter weather settled in during December and it was pretty consistently cold and snowy in the aforementioned areas until mid-March. Winter was relentless.

This year, however, Old Man Winter toyed with us. A typical first 10 days of November gave way to record cold and snow . On November 17 more than half of the lower 48 states had snow on the ground and was the highest level for the season until January 4.  Then came December, which was warmer than average across the entire country. The northern and central Rockies, some Great Lake snow belts, and western Maine were the only areas to see average to above average snowfall.

January's temperature departure pattern had hints of the warm west, cold east pattern that ultimately defined the winter. Snow was above normal in the northern Rockies, the southern Rockies, higher elevations of Arizona, New Mexico, and west Texas through the first three weeks of the month. Other than the dramatically cold and snowy weather for a couple of weeks in November, winter failed to materialize for much of the country.

Mean 500 millibar pattern
for January 24-February 28, 2015
Everything changed the last week of January. A building upper level ridge in the western U.S. was complemented by a deepening trough over the central and eastern U.S., a pattern that would remain more or less the same through February. For much of the country east of the Mississippi River winter began that week, and it came in at full speed with no let up. The start of that week saw much warmer than normal weather extended into the northern Plains under a building upper ridge, while to the east the stage was being set for the first of what would be several major snowstorms of the season to hit the Northeast.

For the six-week period ending March 7 snowfall was much above normal in a wide band from the southern Rockies eastward to the Carolinas, north to the southern Great Lakes and northern New England. Late February and early March storms brought one to two feet of snow to Kentucky, and damaging ice storms from Tennessee east to the Carolinas and south into the northern Gulf States.

February was an incredibly cold month for the country east of the Rockies, with temperatures averaging 10 to 18 degrees below normal across the northeastern third of the country. There were 23 states that recorded a top-ten coldest February. Much warmer and drier than normal weather continued over the western U.S. with record warmth in six states.

For the period beginning December 1 snow cover across the U.S. reached it's season low on December 12 at 17.5 percent. (It had dropped to 16 percent on November 24 after the mid-November peak of 50.4 percent). It rose to 53 percent on January 4, then fluctuated up and down before dropping to 21 percent in early February. The storms the last three weeks of February led to a peak snow cover of 63.4 percent on March 1.

Chart showing percent of U.S. (lower 48 states) covered by snow each day since November 1.

Today, after several days of mild weather and some rain, snow cover has plunged to 16.1 percent. Much of that snow cover is in the Northeast which is still 98 percent snow covered with an average depth of 21 inches. It will be a few weeks before the last vestiges of the Sudden Winter of 2014-2015  disappear from the landscape there.

Snow depth and extent on March 1, 2015 (left) and March 11, 2015 (right).
Source: NOAA National Operational Hydrologic Remote Sensing Center