Synoptic Summary
The month began with a cold frontal passing through the state, which stalled just south of Pennsylvania. A cyclone developed in the lee of the Rockies moved ENE along this front and passed over Lake Erie and into Southeastern Canada on the 2nd. This system brought a bout of widespread rain throughout the state from the passage of both a warm and cold front. Afterward, a dome of high pressure settled in for several days. Keeping precipitation minimal across most of the state from the 3rd to the 7th.
A cold front crossed the state from NW to SE on the 7th, which hung up across the Ohio River valley and quickly developed a wave of low pressure once again moved through the lakes and brought episodes of rain across PA. The series of frontal waves continued, as yet another potent system, which resembled a classic inland coastal storm, swept across the state on the 14th. This storm brought welcome drought relief, with its widespread heavy rainfall. Reports of one inch or more of rain were common.
An area of high pressure following the storm cleared the weather for the 16th and 17th, but gave way to a considerably powerful cold front on the 18th. This front and its associated low-pressure system dropped another inch of rain across much of the state. Afterward, the mother of all mid-May High-pressure systems made a visit to the eastern United States. This high averaged 1030 millbars and passed from Lake Michigan through the Ohio Valley and finally into Georgia where it eventually weakened. This system brought record-breaking chill to the state. Several hundred record lows were broken across the Northeast. Low temperature readings as low as the upper 20's were widespread in central and Northwestern PA. There were no notable precipitation events, with the exception of very unusual lake effect snow showers throughout the period of May 19th through the 25th. This also applied for the entire nation east of the Mississippi River while this high dominated.
The month ended with a stationary front wavering across the state. This resulted in cloudiness and showers to the region. A slightly more substantial front passed on the 31st, which brought more significant rainfall. In short, the month of May 2002 can be best remember for the positive change in the general weather pattern, because the change brought major drought relief to much of the state, as well as the record breaking minimum temperatures on the week of the 19-25th.
Means and Extremes
Disregarding the cold outbreak, temperatures remained near normal for the remainder of the month. There were surges of warmth ahead of frontal waves, and a minor warm period at the end of the month. However, the outbreak sent most of the state into much below normal temperatures for a typical May.
Statewide precipitation reports ranged near an inch and a half at Harrisburg to approaching ten inches at LaPorte. The statewide monthly average precipitation was approximately 5.5".
As mentioned before, the entire state was below average in the temperature
department. However, the abnormally chilly areas of the state were the
north and west. The Southeast area of the state was the warmest, and in
general, the driest.
--- Dan Swank
| Value | Locations(s) | Day of Month | |
| Maximum High Temp | 90 | Franklin Institute, Philladelphia | 31 |
| Minimum Low Temp | 19 | Kane | 4 |
| Max Monthly Precipitation | 9.97 | Laporte | n/a |
| Min Monthly Precipitation | 1.21 | Harrisburg | n/a |
| Maximum Snowfall | 4.10 | Genesse | n/a |