MUWIC Newsletter - May 2004

(Issued: May 3, 2004)

Millersville University Weather Information Center (MUWIC)

Eric J. Horst, Director

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LEAD STORY: “Drought Conditions Unlikely This Year”

Less than two years ago the region was in the midst of the worst drought on record with no end in sight. Eighteen months and seventy-five inches—yes, over six feet!—of rain later, the region has garnered a Palmer Drought Index (PDI) classification of “unusually moist”. The PDI estimates long-term drought—over several months to a year—based on a supply-and-demand model of soil moisture.

Fitting the long-term trend, April brought us above normal rainfall with over 4.5 inches recorded at the MU Weather Information Center. Given a relatively dry January and March, however, the year-to-date rainfall remains slightly below normal. Still, the “moist” Palmer Index classification should give farmers and gardeners some comfort as we proceed toward the Summer season.

Looking ahead, there are no strong signals regarding long-term weather trends. The progressive and relatively showery pattern of recent weeks is likely to continue into early May. Beyond that, the best bet may be near normal temperatures and rainfall into the Summer months. Of course, almost every season features brief dry spells, but in the long run we can hope for a year devoid of the “D word”!

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EVENT OF THE MONTH

April 19^th’s high temperature of 89 degrees fell just two degrees shy of the record (91) set in 1976.

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STAT OF THE MONTH

April showers were even more common than usual this month as 15 out of 30 days featured measurable precipitation. In fact, seven consecutive days—21^st through 27^th—yielded some showers on our way to the wettest April in twelve years!

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APRIL CLIMATOLOGY (MU Weather Station)

Average High Temp: 62.3 F

Average Low Temp: 43.3 F

Average Monthly Temp: 52.8 F

(Departure from Normal: +0.9 F)

Snowfall: Trace

(Departure from Normal: -0.3)

Precipitation: 4.53 inches

(Departure from Normal: +1.06)

Annual Precipitation: 11.07 inches

(Departure from Normal: -1.26)

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MAY NORMALS, RECORDS & FACTS

Normal daily temperatures will shoot up 9 degrees this month. The normal high/low ranges from 70/44 on May 1st to 79/54 by month’s end. The record high for May is 98 degrees (on 5/23/25) and the record low is 27 F (on 5/1/78). Normal monthly rainfall is 3.62 inches. The rainiest May on record occurred in 1989 (10.07 inches) while the driest May was in 1939 when just a half-inch fell.

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MAY OUTLOOK

This monthly outlook is especially difficult, because I see opportunities for both unseasonable chill and unseasonable warmth. A large jet stream ridge, now building across the southern plains, will likely progress to the east coast by late this week. In fact, there are some indications that it could develop into a classic Bermuda High by early next week—if so, the period May 6 – 12 could be summer-like in the East. Then again, the upper-level flow pattern across Canada hints that a backdoor cold front will affect the Northeast sometime between May 7^th and 9^th. Consequently, it possible that, say, May 9^th could feature a high of 82F (the former scenario) or 62F, if the backdoor cold front sets in.

The bottom line: the first half of May will likely feature some wild swings in temperature as we experience a couple dramatic warm-ups (into the 80s) as well as a few chilly periods as cool air drops in from Eastern Canada. In aggregate, the first two weeks will likely average out near normal, even though very few days will actually be near normal!

What I don’t see are any signs of major storminess anytime soon. We are entering the time of year when significant low pressure systems are less common, and our precipitation instead comes from garden variety showers and thunderstorms. Given the current pattern, I’m inclined to forecast near normal precipitation for the month ahead. Climatologically, the threat of severe storms increases as we near Memorial Day weekend (the peak “severe storm season” in the Lancaster area is from Memorial Day to the Fourth of July). In summation, I see no basis for a bold forecast that departs too far from the long-term climatological averages for May.