WeatherWatcher: Weekly Weather - unseasonably cold air until Friday

• General weather pattern outlook
• Recognizing the size of the EF-5 Oklahoma Tornado
• About the EF scale used for tornadoes
  
• Last week's data

Wednesday - Sunday we will be in a showery partly sunny, cloudy type weather pattern. We have a low pressure system with unseasonably cold air around it parked off of the coast of Washington State right now that is expected to hang out with us until Friday. Due to the colder air in the atmosphere and the sun breaks we will have the potential for some unstable atmosphere conditions. Ground heating from sun breaks can generate convection leading to isolated thunderstorms in the afternoons and evenings not unlike last Monday. We may see that type of weather off and on through Friday. The weekend looks slightly drier but we may still have some showers. The threat of lightning should be gone by the weekend. Our high temperatures will range in the low-mid 50's rising barely into the 60's going into the weekend.

Tornado - I wanted to take a moment here to recognize the amount of power, and force that a thunderstorm released in Moore, Oklahoma. The city of Moore is roughly the same size as the city of Shoreline population wise, however is spread out over a larger physical area of land. The tornado was rated by the National Weather Service as an EF-5, the strongest a tornado can get with winds exceeding 200mph. The tornado lasted for about 40 minutes, traveled 17 miles, with a destruction path about 1 and a half miles wide. 17 miles is about the distance from Downtown Seattle to the Lynnwood-Mountlake Terrace border, with a path about a quarter of the width of Shoreline. Thankfully the biggest a tornado has ever been recorded here in Western Washington is an F-3 (about the same as an EF-3) in Vancouver, Washington, and they are very rare at that. Most occur in Southwest Washington where there is less interference from mountains and buildings and such. EF-1's generally won't cause the kind of widespread total destruction that happened in Moore, Oklahoma. Western Washington averages about one EF-0 tornado a year, not very often at all. We may complain about our weather, but I for one am thankful it is tame compared to what can and does happen every now and then across the middle of our Nation.

EF stands for Enhanced Fujita scale which was used by the National Weather Service starting in 2007. This scale is used to rank the strength of tornados in the United States and Canada based on the amount of damage they cause. Damage caused is what the scale is more based on than actual wind speed due to how localized the wind speeds are with tornadoes. The ratings are as follows in relative relationship to wind speeds:

EF-0: Winds of 65-85mph

EF-1: Winds of 86-110mph

EF-2: Winds of 111 - 135mph

EF-3: Winds of 136-165mph

EF-4: Winds of 166-200mph

EF-5: Winds of 200mph or greater.

Strongest winds ever recorded at the surface on earth was 302mph, in an EF-5 tornado which also occurred in Moore, Oklahoma, May 3, 1999. By comparison, hurricane force winds range from 75mph - 165mph.

Last week's weather data May 11th - 17th:

High temperature: 83.5ºF (Saturday)

Low temperature: 44.6ºF (Wednesday)

Rainiest day: 0.17 inches (All from one thunderstorm on Monday afternoon in less than 30 minutes)

Total rainfall: 0.25 inches

Warmest day: 64.1ºF (Saturday)

Coldest day: 53.1ºF (Wednesday)

Average temperature last week: 57.6ºF

Previous week was 60.9ºF due to our sunny weather.

Warmest and coldest days are based on average temperature of the entire day, starting at midnight. All other averages are based on the whole week, starting Saturday morning at midnight. All weather data unless otherwise noted is sourced from Carl's Shoreline Weather.

For weather related updates or storm warnings, check out my weather station web page or follow me on Twitter: @SWeatherWatcher