Norbert. Odile. Dolores. Javier. Four named tropical systems in the eastern Pacific since 2014. All with vastly different impacts on Tucson. What makes forecasting tropical influences on the Arizona Monsoon so difficult? Here’s three reasons.
1) The data void of Mexico
Upper air balloon data is vital to forecasting. Not only are we able to get a snapshot of what’s happening above our heads, the data also goes into the computer models. In the United States, there are dozens of sites that release weather balloons twice a day. The above map shows you those locations.
How many sites are in Mexico? 10. Some don’t release weather balloons for weeks. This is a serious problem when trying to determine conditions within a tropical system off Baja California. It makes the models go into virtual “best guess” mode. Issues that arise from this include precip timing, storm track and amount of moisture in the atmosphere.
2) Too much of a good thing
To borrow a phrase from University of Arizona Meteorologist Mike Leuthold, it can be “too wet to rain”. How is that possible? When the atmosphere is saturated with tropical moisture, it doesn’t take long for rising air to cool, condense and become a cloud. Sounds like a good thing, but quick cloud development can also choke off instability needed for thunderstorms. Thus, turning a 1-3″ rain prediction into spotty showers and 50 shades of a gray forecast bust.
3) Chilling off Baja California
Tropical systems feed off warm ocean water. It’s the fuel that drives them from a tropical depression into a hurricane. Conversely, cold water is kryptonite. And there’s a lot of kryptonite off the central and northern Baja coast, as the image above depicts water temps in these areas a chilly 68-77°F (20-25°C). Forecast tracks become dicey for tropical system moving north along the Baja coast. Many times I’ve seen a cat 1 hurricane dissipate into a small remnant circulation near the Baja Spur, keeping moisture bottled up south of the border.