PORTLAND, Ore. (KOIN) — Welcome to the first weekend in May. We’re going to kick it off with showers, and assuming all the right ingredients, see thunderstorms across the state. This is quite a 360 from last month. Did you know that April ended up tying for the 5th warmest April on record and the fourth driest since 1940 at Portland International Airport.
The timing of the cold front today is key. No model is perfect but the latest projections indicate the cold front will arrive at the coast by late morning then continue on an eastward track. Right now it appears the window for thunderstorms over the Willamette Valley will be around noon to 2pm. Eastern Oregon may see thunderstorms sometime late in the afternoon. There won’t be much of a difference from the morning temperature in the low 50’s to the daytime high, upper 50’s.
So what can we expect with these thunderstorms and why is that map of Oregon in two shades of green? The light green color indicates that any thunderstorms that develop will not fit the criteria for severe.
The National Weather Service defines a severe thunderstorm as any storm that produces one or more of the following elements:
- A tornado.
- Damaging winds or speeds of 58 mph (50 knots) or greater.
- Hail 1 inch in diameter or larger.
Light green represents the non severe thunderstorm category. The dark green shows a moderate risk for severe storms. Further defined as an area of severe storms of either limited organization and longevity, or very low coverage and marginal intensity. Damaging winds and hail are possible but this storms would not be widespread.
We get cold fronts all the time. Why is this one more likely to produce thunderstorms? We look at large scale clues first and then dive down. If you ever hear meteorologists mention a negatively tilted trough, he or she is not referring to a pig trough that’s been turned upside down. A trough, or elongated area of low pressure, that has a negative tilt observed on a weather chart indicates there will be a lot of instability and wind shear associated with it. Naturally, there’s so much more to consider when forecasting severe weather, but for now just know that a negative tilt can spell trouble.
To learn more about thunderstorm development, view our latest weather lesson at koin.com/weather-kids