Unlike the prevalent dry soil and persistent hot temperatures that are often associated with major wildfires in the southwestern U.S., Rocky Mountains, and West Coast, significant wildfires in Pennsylvania are linked to anomalously strong northwest flow on the western side of an upper level trough located just off the Atlantic coast. Long dry periods occasionally precede active periods for wildfires.
The most favorable time of year in Pennsylvania for large wildfires (defined as those burning >100 acres) is during a 6 week period from late March to Early May.
The use of numerical model anomalies based on departure from 30-year climatology were used to examine parameters such as 850 hPa Temperature, Precipitable Water, 850 hPa and 250 hPa U and V Wind components, Surface Pressure, and 500 hPa height, to establish a signal for potentially active Fire Weather periods.
Preliminary results suggest several large fires were associated with above normal dryness and an anomalous upper level ridge (500 hPa height > 2 standard deviations above normal) prior to the fire. In a few severe cases, characterized by fires burning for several days or affecting an area of over 3000 acres, strong anomalous southerly or westerly flow (850 hPa winds > 2 standard deviations above normal) ahead of an approaching front was associated with the fire. Positive 850 hPa temperature and 500 hPa height anomalies combined with below normal precipitable water anomalies also were noted to precede large wildfires.
This paper will examine climatological anomalies and illustrate a quantitative relationship between the severity of the fires (acres burned and cost of human and mechanical resources, such as helicopters used for water drops) and the degree of the anomaly in key weather parameters. Using the National Centers for Environmental Predictions-National Centers for Atmospheric Research re-analysis data set, the antecedent conditions were examined for 153 wildfires of >100 acres that occurred across Pennsylvania during the period 1983-2001.