MUWIC Newsletter - May 2006

(Issued: May 3, 2006)

Millersville University Weather Information Center (MUWIC)

Eric J. Horst, Director

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MONTH IN REVIEW: “Drought Watch Issued”

For the first time in three years, the Commonwealth is under a Drought Watch. On April 11^th, the DEP issued a Drought Watch for all sixty-seven counties. A Drought Watch is the lowest of three levels of drought status, asking for a 5 percent reduction in water use by residents. March concluded as the driest on record with only two-thirds of an inch of rain falling in the Lancaster area. The first three weeks of April continued the dry pattern, before a heavy rain event on April 22^nd.

A plot of the 120-day moving average of local rainfall reveals recent improvement thanks to the late-April rainfall. Worse off are parts of Maryland and New Jersey that have received less than 50 percent of normal rainfall over the last 90 days. Comparatively Lancaster has received about 88 percent of normal year-to-date precipitation.

The bottom line: Conditions are far from dire, however a continuation of the current pattern would lead to a worsening drought by summer. Follow the links below for a regional view of the current mini-drought.

Drought Watch map: http://www.erh.noaa.gov/marfc/drought_2006.gif

90-day Precip map: http://www.erh.noaa.gov/marfc/Maps/PANY_counties_90_color.htm

Lancaster Water monitor: http://pa.water.usgs.gov/monitor/precip/120day/Lancaster120.gif

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EVENT OF THE MONTH

A slow-moving storm dropped 3 inches of rain across the region from April 21 – 25 with 2.41 inches falling on April 22^nd (a new daily record).

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APRIL CLIMATOLOGY (MU Weather Station)

Average High Temp: 66.5 F

Average Low Temp: 43.1 F

Average Monthly Temp: 54.8 F

(Departure from Normal: +2.9 F)

Snowfall: Trace

(Departure from Normal: -0.3)

Precipitation: 4.36 inches

(Departure from Normal: +0.89)

Annual Precipitation: 10.48 inches

(Departure from Normal: -1.85)

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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 is a very difficult monthly outlook since I’m issuing it during a time of significant jet stream blocking. The persistent feature in recent weeks has been downstream blocking and a tendency for northwest flow to bring us dry air from Canada. This pattern looks to continue short-term, while warmth will be building over the nation’s midsection. Consequently, temperatures in the Plains will almost certainly average above normal while parts of the Northeast may actually be a bit below normal.

Pennsylvania is located in the transition zone between a ridge over the Plains and a trough off the East coast. Resultant northwest flow will drive a few cold fronts through the region followed by cool, dry air masses. However, milder southwest flow can still develop ahead of these approaching cold fronts, and this holds promise for periodic one- to two-day warm ups, with highs occasionally reaching the 80s. The passage of cold fronts may trigger some scattered showers and thunderstorms, although the total monthly precipitation will likely average below normal (baring a good soaking rain).

As for the future of the drought watch, it’s my sense that it will take about an inch of rain per week to have the watch lifted as some point this month. Looking at the first week of May, I don’t see this amount of rain and therefore the watch will likely last into mid-month or beyond.