More Misuse of Water-Vapor Images

By: Lee Grenci, 12:31 AM GMT on September 29, 2015

If I see one more forecaster present a water vapor image and point to the ribbon of bright colors and bright white extending northward from the eastern Gulf of Mexico / Florida and describe it as a "plume of tropical moisture," I'm going to scream. First, those are high cloud tops (composed of mostly ice crystals) and NOT water vapor. Yes, high cloud tops contaminate water vapor images! Second, even if it were water vapor (and it's NOT), the temperatures of these fe...

Brief Commentary

Updated: 1:12 AM GMT on September 29, 2015

My Take on Erika

By: Lee Grenci, 2:56 PM GMT on August 30, 2015

I watched with interest the evolution of forecasts for Tropical Storm Erika as it approached the Caribbean Islands this past week. Fairly or unfairly, the first impression I got was that southern jog in the storm as it approached Hispaniola was a huge surprise (see track below). The associated detachment of the low-level center of circulation from the deep moist convection also seemed to catch some forecasters off guard.

Yet both of these dy...

Tropical Forecasting

Updated: 10:52 PM GMT on August 30, 2015

May 31, 1985: A Day I'll Never Forget

By: Lee Grenci, 4:12 PM GMT on May 28, 2015

This Sunday (May 31) will mark the 30th year since the terrible outbreak of tornadoes in Pennsylvania (there were also tornadoes in Ohio, Ontario, and New York on May 31, 1985). The local office of the National Weather Service just published an account of that horrific day (89 killed, over 1000 injured). If you scroll down the Web page, you'll see that I was quoted (among other quotes). It was 30 years ago, but I remember the exact moment (standing in front of the r...


Updated: 4:36 PM GMT on May 28, 2015

Super Typhoon Maysak: The Upper-Air Connection

By: Lee Grenci, 6:26 PM GMT on March 31, 2015

Just a quick blog to follow up on Dr. Masters' terrific blog on Super Typhoon Maysak.

Whenever a tropical cyclone undergoes changes in intensity (and structure), I routinely check for nearby weather systems that might have interacted with the cyclone. In the case of strong hurricanes or typhoons like Maysak, I first look to the upper troposphere to assess outflow jets that increase upper-level divergence (whose presence typically leads to intensification...

Tropical Meteorology Tropical Forecasting

Updated: 7:21 PM GMT on March 31, 2015

About Rain and Melting Snow

By: Lee Grenci, 1:43 PM GMT on March 25, 2015

Just a quick note on the topic of rain and melting snow in early spring.

When it rains on a snowpack in early spring, it would be easy to attribute the rapid melting of snow directly to warm raindrops falling on the snow. It can be shown, however, by invoking the conservation of enthalpy, that warm rain falling on snow cover increases the runoff by only 10% (raindrops directly melting snow). Thus, it would take sustained downpours to directly melt a rel...

Weather Forecasting Thermodynamics

Updated: 9:26 PM GMT on March 27, 2015

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About 24hourprof

Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.