Rain and Heavy Rain

Rainfall and heavy rainfall go together.  That is, when there is a pattern of above-average and more frequent rainfall, it is also more likely that there will be a downpour.  Warm weather and heavy rainfall also go together.  When the atmosphere is warmer, there is more capacity to hold moisture in the air, which increases the potential for downpours.  As the average temperature in our region has risen since the middle of the 1900s, so has the frequency of heavy rains.  Also, as the average annual precipitation began to rise in the 1990s, the downpours have been increasing.  The flooding problem across our region the past 20 years can be tied to both of these trends.  The future of flooding in our region is likely going to be tied to both of these trends.  Additional warming appears likely but a further increase in annual precipitation is probably less likely.   Meteorologist John Wheeler

Rising Seas?

Although the interior of the Antarctic continent is getting colder, one of the most rapidly warming locations on Earth the past few decades is along the Antarctic Peninsula.  An ice shelf known as Larsen C, the fourth largest ice shelf in the world at more than 20,000 square miles, adjoins the Antarctic Peninsula and has been discovered to be melting at a rapid rate due to the warmer air temperatures and the warmer seas underneath.  A research team from the British Antarctic Survey has reported a growing crack in the shelf which could cause it to break up some time in the next 100 years.  This would not lead directly to a rise in sea levels because this ice is already floating in the ocean.  However, if the region continues to warm, the loss of the ice shelf could allow glaciers presently land locked to melt into the ocean faster.  Sea-level rise is one of the greatest concerns associated with Global Warming.   John Wheeler

California Drought

Tree ring analysis suggests the present drought in the Southwestern U.S. may be the fourth most severe in approximately the past 1000 years.  The nation’s two largest reservoirs, Lake Powell and Lake Mead, both on the Colorado River, are at their lowest levels since construction.  These two impoundments provide much of the water needs for desert metropolises such as Las Vegas and Phoenix.  This melted snow also provides considerable water for the Los Angeles Basin and the California agriculture industry, which is where a large portion of America’s grocery store produce is grown.  The lakes are at approximately 40 per cent of capacity this spring, and will drop more over the summer.  Hopefully, next winter’s El Nino will help generate an above-average snow pack in the Rockies next winter or the situation will continue to worsen.     Meteorologist John Wheeler

Hurricane Forecast

There are three major, large-scale factors which can have large scale effects on the Atlantic hurricane season.  One is the general sea-surface temperature.  If the tropical part of the Atlantic Basin is warmer than average, there is more thermodynamic energy available for hurricanes.  Another is the amount of dust from the Sahara.  Actually, it isn’t the dust, but the deep layer of dry air which accompanies these dust clouds that rob tropical storms of moisture.  The other key is the overall strength of upper level winds.  When the winds aloft are strong, tropical storms tend to shear apart before they can grow large and powerful.  The presence of a building El Nino such as there is at present often produces stronger winds above the Atlantic, usually signaling a quieter Atlantic hurricane season.   Meteorologist John Wheeler

Very, Very Cold May

The cold weather earlier this week prompted many inquiries as to the relative unusualness of having snow and temperatures so cold in the middle of May.  Fargo did set a daily record cold maximum temperature Monday at 40 degrees.  Old record was 44 set in 1890.  May snowfall is also rare, but not without precedent.  I would describe such weather in our region as rare, but not unusual; the sort of thing that happens every few years or so.  The most unusually wintry May weather on record in the Fargo Moorhead area was in 1907.  There was measurable snow in Fargo on four separate days; the 2nd, 13th, 14th, and 19th; for a total of 4.2 inches.  The first ten mornings in May were all in the 20s (except for May 2 when it was 17).  There were three more hard freeze mornings on the 14th, 20th, and 27th.      Meteorologist John Wheeler

Drought Beginnings and Endings

 

It was just one week ago that it started raining.  Rain over the seven days since has been plentiful enough to eliminate all concern about it being dry.  It is interesting to note that our weather condition can switch from dry to wet in such a short time.  On the other hand, it cannot switch from wet to dry so quickly.  Of course, it can stop raining tomorrow, but the lingering effects of recent rain will keep the soil soggy and the grass growing for some time.  The winter drought began last September 5.  That was the day after the last heavy rain of 2014.  We were in a drought through the fall because the drier weather caused us no problems.  The drought really began to manifest itself this spring with agriculture’s need for germinating rainfall along with a rash of grass fires.  Now suddenly, all that is over.  I wonder when the next drought will begin.  Tomorrow, perhaps?

Meteorologist John Wheeler

Warm Days and Rain

 

The average daily high temperature is not meant to be a reflection of what the temperature is supposed to be.  Rather, it is the average of all the daily highs over a period of record.  So with that in mind, the average daily high has now reached 70 degrees for the first time since September.  There have already been 17 days with a high temperature of at least 70 degrees this year.  The first was back on March 15.  It happened 11 times during a warm, dry April.  The warmest has been 87 degrees on May 2.  We had a lot of warmer than average afternoons from mid-March through early May due to the early snowmelt and exposed dry soils.  Now that the ground is wet, it will become a little harder to get so many above average afternoon temperatures.  Unless, of course, it stops raining again and the ground becomes dry.   Meteorologist John Wheeler

 

Tornadoes and Climate Change

Is our warming climate effect the threat of tornadoes and severe thunderstorms?  From the political sides of this debate, this question can be quickly and easily answered either “Yes!” or “No!” depending on the politics.  In the real world, the answer is a lot trickier.  The trend across the United States in recent decades is a decrease in the overall number of tornado days along with an overall decrease in the quantity of tornadoes.  But coupled with this decrease in bad storms is an increase in those few terrible tornado outbreak days, those days when dozens or even hundreds of tornadoes causing staggering property damage and loss of life.  So although severe storms are not exactly increasing, their variability is.  Some of this statistical clustering is skewed by the year, 2011, when several huge tornado outbreaks swept the Midwest and South.  Ongoing research offers no consensus yet of how tornadoes will be affected by climate change in the future.

 

Meteorologist John Wheeler

Shower vs. Rain

I had an inquiry earlier this week about the difference between a rain shower and just plain rain.  The classic meteorological definition is that a shower is brief and/or intermittent whereas rain is steady.  But there is no specific time limit for when a shower crosses the threshold and becomes just rain.  For most of us in the business of forecasting, a shower differs from rain by being convective in nature.  This means that showers are caused by smaller scale updrafts of air and so are briefer or more intermittent than a general area of rain which is caused by a general, gradual rising motion over a large area.  But here, also, there is no cutoff at which rising air is of too large of a scale to be considered a shower-making updraft.  In general, if it is brief, short, or longer but highly variable in intensity, it is a shower.     Meteorologist John Wheeler

From Dry to Wet

It took one storm exactly two days to take eastern North Dakota and western Minnesota from too dry to plenty wet.  This is often how it works in spring because of the fact that average precipitation through fall, winter, and spring is just not that much.  The six months of November through April produce just a fourth of our average annual precipitation.  So if those six months are very dry and we get just half of the average, we can make it up with one good rain storm like the one Sunday and Monday.  Warm season precipitation is the important stuff.  If we were to go from May through October with half of average rainfall, the moisture deficit would be more than eight inches which would cause much more of a problem and would be much harder to make up.  So the weather picked a good time to go dry this fall, winter, and early spring.       Meteorologist John Wheeler