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  • STAFF PHOTO BY BOB HAMMERSTROM


    A lack of rain meant water levels were low on area rivers including the Souhegan in Milford, Tuesday, April 3, 2012.
Saturday, May 5, 2012

April dry, but temperatures were above normal

Doug Webster

April’s weather was almost a letdown after the March we enjoyed.

March was more like April than it typically is. Despite the return to reality last month, we still saw more nice weather than April normally dishes out.

Rainfall continued to be a problem until nearly 3 inches of badly needed moisture from April 22-23 brought some improvement. While not a drought-buster, this rain was essential in temporarily knocking down the extreme fire danger and adding some much needed moisture to the ground for spring growth.

Despite the welcome rain later in April, we still saw a monthly total below normal, and the calendar year deficit continues to build. Since Jan. 1, we’ve seen only 9.69 inches of precipitation, which is nearly 6 inches below normal. The first four months of 2012 represent the 11th driest start to any year since precipitation records began in 1884.

While the rainfall deficit sounds bad, we have to consider the time of year and that during the winter, the amount of water needed for nature’s water budget is much less than during the summer half of the year. We are now into the time of year when it’s much more important to receive adequate rainfall to maintain a good groundwater supply for the summer and fall.

Prior to the late April rainstorm, southern New Hampshire was on the brink of a moderate drought, according to the Climate Prediction Center’s Palmer Drought Severity Index. The PDSI indicates prolonged and abnormal moisture deficiency or excess on the time frame of months or years.

As of April 28, and after the rain we’ve seen, the PDSI recovered to just a little drier than normal for New Hampshire’s southern division.

If we see rain amounts not too different from normal during the next few months, we shouldn’t have any major groundwater problems this summer.

The Crop Moisture Index differs from the PDSI. The CMI represents the short-term or current status of purely agricultural drought or moisture surplus and can change rapidly from week to week. The CMI takes into account only the top foot or so of the topsoil and is of great value to the farming community.

The CMI has gone from slightly dry prior to the rain to what the Climate Prediction Center refers to as wet for the southern New Hampshire division as of April 28.

It’s important to note that the PDSI and CMI can be opposite one another. It’s possible to have a severe drought ongoing and a rainstorm can come along and raise CMI conditions to levels that are too wet. A reverse situation is also possible.

For more information on drought, the Climate Prediction Center has plenty of information at www.cpc.ncep.noaa.gov/products/Drought.

Nashua’s rainfall total for April was 3.78 inches, producing a deficit of 0.58 inches. April tied for the 78th driest on record since 1885.

No snow was recorded during April, and that isn’t unusual. Measurable snow is recorded about every other April, and we only have to go back to last year to find a total of 4.5 inches.

Temperatures during April were quite variable, as would be expected for the time of year. The first two weeks, as well as the final week, were on the cool side of normal, while the third week saw readings that averaged well above normal. A record daily high of 90 was recorded April 17.

The wide range of temperature is quite normal for spring. Readings in the 20s one morning can be followed by 80s just an afternoon or two later in April. One might scrape frost from a windshield in the morning, only to open the windows in the house the same afternoon after readings jump 40 or 45 degrees.

While two-thirds of April saw cooler-than-normal temperatures, the month still averaged above normal because of the very warm period from April 15-22. The average temperature for the month as recorded at Pennichuck Water Works in north Nashua was 48.6 degrees. The monthly average temperature was 2.9 degrees above normal and ranked as the ninth warmest April since 1885.

The reason for the return of temperatures a little more like we would expect was tied to something that was absent from North America all winter. For the first time since the middle of last fall, we saw a roadblock develop across Greenland and eastern Canada, allowing the jet stream to be deflected south across the northeastern U.S.

The blocking high to our northeast was also responsible for allowing a storm to develop along the East Coast later in the month, delivering the much needed rain. The storm center tracked to our west, keeping us on the mild side. If this storm had taken a path about 250 miles farther east, we might have seen a late-season snowstorm like what was seen through western New York and western Pennsylvania.

The return to a cooler temperature regime for April should be no surprise. After the very warm Marches of 1945 and 1946, the April and May weather tended to be cooler than normal. Weather and climate are always in a state of change, and the pendulum always swings between warm and cold and dry and wet.

Large-scale climate changes are likely in the months before fall and winter as the La Nina of the last three years dies across the Pacific Ocean. There are strong indications that La Nina’s opposite cycle, El Nino, will develop in the next few months. El Ninos are noted for cold and wet conditions across the southern U.S. during winter.

The weather in May can be friendlier to the outdoor enthusiast, but still present cool, wet weather at times. When the wind blows from the west, we can see temperatures soar to summer levels, but the east wind brings chill and clouds.

For the gardener, hardy plantings are good anytime, but other vegetables that can be damaged by frost shouldn’t be planted until Memorial Day. Frosts aren’t unusual through mid-May and can occur infrequently until late month.

Weather & Climate appears the first or second Saturday of each month, depending on when final weather data are available. Doug Webster, of Hudson, is senior meteorologist at Telvent DTN in Woburn, Mass.