Dew Point Bombs: Large Model Errors Provide an Obvious Target of Opportunity for Fire Weather Forecasters

As computer model accuracy continues to improve, it is difficult to find areas where meteorologists can make significant improvements to model guidance when producing forecasts. Meteorologists at the National Weather Service in State College, PA have identified one such weather pattern in which significant improvements are possible. When dry air sits above a well-mixed boundary layer, surface relative humidity (RH) values can fall well below forecast values with large and noticeable errors thanks to observed temperatures rising above expectation and dew point temperatures falling well below expectation. These RH errors have been reported by Pennsylvania fire weather partners, who regularly rely on National Weather Service forecasts. Feedback from partners indicated that they divide minimum RH forecasts by a factor of 2 in order to make better operational decisions.

Such feedback provided motivation to take a closer look at these significant errors and pursue forecast improvement. This project seeks to do the following:

(1) Develop a Central Pennsylvania climatology of so-called “dew point bomb” days when the dew point drops at least 10°F and then rises at least 5°F. Such a rule ensures the dew point drop is because of mixing and not simply dry air advection. Results indicate dew point bomb days occur most often during the typical eastern U.S. fire weather season (spring & fall) when RH errors significantly impact fire weather operations.

(2) Identify weather patterns and environmental parameters that are favorable for the development of deep-mixing and significant RH errors. This pattern identification has helped meteorologists communicate the potential for under-performing RH values and resulted in improved fire weather forecasts, partner relationships and trust.

NWS State College meteorologists have condensed the local climatology for “dew point bomb” days, along with synoptic and mesoscale ingredients that favor rapid and pronounced surface dew point drops, into a handy reference guide for staff meteorologists to use in operations. These local research efforts have also revealed that many parts of the eastern United States are susceptible to these events, provided the right atmospheric ingredients are in place. Future work will expand upon the initial pattern and environmental pattern recognition and involve development of tools to help forecasters more-accurately forecast temperature and relative humidity values based on model guidance and model soundings.