MUWIC Newsletter - July 2004

(Issued: July 1, 2004)

Millersville University Weather Information Center (MUWIC)

Eric J. Horst, Director

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LEAD STORY: “June 2004: Wet & Wild!”

After the warmest May on record, June turned stormy with frequent rains that produced isolated flash flooding. Nearly six inches of rain fell, in aggregate, making for the wettest June since 1986 and the twelfth wettest June on record.

The heaviest rains occurred during the week of June 14 – 18 as the remnants of a tropical disturbance moved across the region. On the 14^th, a single severe thunderstorm produced a one- to two-inch downpour in a line from around Manheim to Lampeter to Quarryville. Then on the 17^th, widespread storms dropped a couple more inches of rain countywide, with isolated pockets of three to four inches. Flashing flooding was reported in several locations on the evening of the 17^th.

Given the frequent precipitation so far this year, the Climate Prediction Center’s Long-Term Drought Indicator maintains a 90^th percentile rating for the Lancaster area. This unusually moist rating represents a complete reversal from the record dryness the area experienced just two years ago.

June Rainfall

Millersville University - 5.82”

Lancaster Airport - 5.25”

Holtwood - 6.07”

Lancaster Water Works - 6.76”

New Holland - 6.52”

Ephrata - 5.81”

Harrisburg - 4.31”

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EVENT OF THE MONTH

On June 17^th, severe thunderstorms crossed the county producing flash flooding, gusty winds, a couple of funnel clouds, and a tornado touchdown (F1) in the county park. A tropical air mass provided deep moisture for several storms that moved through the area between 2:00 p.m. and 8:00 p.m. Rainfall reports of one to three inches were common across the county and, possibly, as much as five inches in southeast parts of the county (according to radar estimates).

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JUNE CLIMATOLOGY (MU Weather Center)

Average High Temp: 79.5 F

Average Low Temp: 60.7 F

Average Monthly Temp: 70.1 F

(Departure from Normal: -0.2 F)

Precipitation: 5.82 inches

(Departure from Normal: +1.94)

Annual Precipitation: 19.98 inches

(Departure from Normal: +0.19)

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JULY NORMALS, RECORDS & FACTS

Climatologically the warmest month of the year, daily average temperatures peak on July

18th and 19th with a normal high/low of 88/63. The record high for July is 104 degrees set on three different occasions (July 3, 1966, July 10, 1936, and July 21, 1930) and the

record low is 42 set on Independence Day in 1918. Normal precipitation for the month is

4.51 inches. The rainiest July on record occurred in 1941 when 9.48 inches fell. The

driest July was in 1955 when only 0.60 inches fell.

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JULY OUTLOOK

The character of the weather in recent weeks has been one of wet and occasionally cool conditions with no extreme heat. In fact, June produced only one 90-degree day compared to the long-term average of five days. I believe July will be a very different month, with building heat over the first 10 days and a good chance of a typical summer heatwave (3 consecutive days in the 90s) sometime during the middle or latter portion of the month.

The cause of this change: The persistent June jet stream pattern of a deep trough over Eastern Canada is showing signs of relaxing, so the mean jet stream position should shift northward to a more climatologically average location near the Canadian border. This will allow heat now building in the Plains to expand eastward during the first week of July. A bonafide Bermuda high may develop during the second week of July pushing temperatures to 90 or above. Still, a weakness in the upper-level flow may remain over the Great Lakes; if so, there’d still be a tendency to bring weak cold fronts into our region from time to time. No, these fronts are not likely to bring refreshingly cool air as in June, but they should help take the edge off the heat for a day or two.

Given the return of the Bermuda high, increasing humidity, and the prospect of occasional fronts to wander into the region, I’d expect a normal amount of shower and thunderstorm activity in July. Therefore, monthly precipitation may be within an inch of normal (4.51”), but not as extremely wet we experienced in June. The normal number of thunderstorm days for July is seven.