MUWIC Newsletter - May 2003

(Issued: May 2, 2003)

Millersville University Weather Information Center (MUWIC)

Eric J. Horst, Director

************************************************************************

LEAD STORY: "Extreme Conditions Spring Forth"

Spring is a season of extremes and that was exemplified by our local weather conditions this April. The month began with a near record high of 80F on April 2nd, only to be followed by two inches of snow on April 7th and a record low maximum temperature of 34F on April 8th. This cycle repeated itself during the mid-month period with another near-record high of 85F on April 16th and a second record low max temperature of 44F on April 18th. I've heard numerous people remark about how unusual these dramatic changes have been-but are they really that usual for April?

Local climatology reveals that April is commonly a chameleon-like month in which highly variable conditions dominate. This bipolar personality results from a combination of increasing sun angle and growing warmth to our south, while snow cover and arctic air masses linger not far to the north. Therefore, Pennsylvania is in the right spot to experience vast swings in temperature and precipitation type as cold fronts from the north and warm fronts from the south take turns moving through the region. This is, in fact, how April weather normally behaves in our area, and you only have to look back a year to see similar occurrences. April 2002 yielded both a one-inch snowfall (6th) and a 90-degree record high (16th).

You may now rightfully conclude that when it comes to April weather it's normal to have widely varying abnormal conditions!

************************************************************************

EVENT OF THE MONTH

A nearly 50-degree turnaround in temperatures occurred on April 16th and 17th. Temperatures plummeted from a high of 85F at 5:00pm on the 16th to a low of 36F at midnight on the 17th. That's a 49-degree temperature swing in only 31 hours.

************************************************************************

STAT OF THE MONTH

April concluded with a slightly below normal average monthly temperature (-1.5 departure from normal). This is the seventh consecutive month with a negative departure from normal. Not since the fall and winter seasons of 1993/1994 have we experience a similar streak of below-normal conditions.

************************************************************************

APRIL CLIMATOLOGY (MU Weather Station)

Average High Temp: 61.2 F

Average Low Temp: 39.5 F

Average Monthly Temp: 50.4 F

(Departure from Normal: -1.5 F)

Snowfall: 2.0 inches

(Departure from Normal: +1.5)

Precipitation: 2.12 inches

(Departure from Normal: -1.35)

Annual Precipitation: 11.49 inches

(Departure from Normal: -0.84)

************************************************************************

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.

************************************************************************

MAY OUTLOOK

The last six months have featured a definitive lack of long-lasting above normal conditions-in fact, persistent normal conditions have even been a rarity since the middle of last fall! It is my sense, however, that we are on the cusp of a significant change in the global jet stream pattern, and I expect we'll break the seven-month streak of average monthly temperatures departing in the "negative" (per Stat of the Month above).

Two semi-permanent features since the beginning of this year have been unusually strong jet stream troughs over east-central Canada and along the U.S. west coast. I believe these two systems will virtually disappear over the next seven days, and in turn a more zonal (west-to-east) jet stream pattern will prevail across much of North America. If I'm right, we'll see an end to the repeating pattern of strong backdoor cold fronts dropping into PA from the north. Instead, we'll experience more persistent near-normal conditions with only the occasional weak cold front moving through from the west.

In the end, I suspect May will average out near to slight above normal in terms of temperature. Precipitation is always a tough call in the Spring since one heavy thunderstorm (or missed storm) can double (of half) the monthly total. Without a strong signal, I am tempted to defer to climatology and forecast rainfall near the long-term monthly average of 3.62 inches. However, I am inclined to lean toward a slight dry bias due to the evolving zonal pattern.