I ran across a great blog entry Tuesday from Senior Weather Channel meteorologist Jonathan Erdman, Sr. detailing the textbook conditions that were in play this week for yet more severe tornado outbreaks across the Midwest.
He outlined five different ingredients you need for tornado outbreaks, and the current conditions in the Midwest are extreme in all five categories.
For me, it was a great reminder of just how difficult it is to get severe weather here in the Northwest, as we rarely get any of these ingredients -- much less all five. And thankfully so.
First ingredient needed: Low level moisture. Looking at dew points is a good rough gauge. That's the temperature at which the air would become completely saturated. Dew points over 60 are considered quite moist, with dew points over 65-70 considered extremely humid. As of Tuesday afternoon, dew points were in the low 70s in Oklahoma.
Around the Northwest, dew points rarely get over 55 and it is exceedingly rare to get over 60. (Last September set a monthly record with a dew point of 67). That's because the cool Pacific Ocean keeps our dew points in the comfortable 40s to low 50s. The only way to get really hot air around here is to get an offshore wind, but that wind comes from Eastern Washington -- where it's dry already -- and gets drier as air sinks down the Cascades. Dew points on our hot summer days can be in the 20s and 30s!
About the only way we can manage any kind of real dew point is to have low pressure to our south in the summer tap into some monsoon moisture in the Desert Southwest and spin it up here in near-due southerly flow -- not east/southeast as that'll dry out from the mountains, and not west/southwest as it'll cool over the Pacific. That's the only way we tend to get the summer thunderstorms. (Or, we can get storms that form in Eastern Washington or along the Cascades that blow west into the I-5 corridor.)
So low-level moisture gets an "un-check."
Ingredient number two: Instability. This occurs when you have much colder air moving in aloft, which allows air parcels to rise higher into the atmosphere and create larger storms. Erdman talks about a "CAPE" index -- "Convective Available Potential Energy" -- a measure of how much lifting energy is available to create massive storms. A CAPE of over 2,500 is considered strong, over 3,500 is extreme and some of these areas are predicted to have values over 4,000 this week. Incredible!
Around here, it's so low it's barely tracked. Some unstable air was expected to be around this weekend, but values are well under 1,000. In fact, on Tuesday it registered "1.8" over Western Washington. There is also something called "Lifted Index" which measures temperature differences. Positive numbers are fairly stable, the higher the positive number, the more stable the atmosphere. Negative numbers indicate increased instability. Around the Northwest, we get to 0 or minus 1, that's pretty unstable for us. In Oklahoma, it was -5.6 -- extreme! In Seattle today, it was +8 -- extremely...stable.
Other ingredients needed: low level wind shear -- we can get that on occasion, but not to the levels of the Midwest. We need a surface boundary too -- that's simply something like a cold or warm front. That will help provide the trigger to lift the warm air into the primed atmosphere. Obviously fronts are common everywhere but thankfully we don't have the other factors in play to where the fronts matter so much.
And finally: A convective cap. This is an essential "lid" when you've got a layer of warm-to-hot air a few thousand feet up. That keeps the warm air from rising too high and instead builds pressure in the lower levels of the atmosphere. But as the day progresses, the heat from the sun could eventually create so much pressure that it overwhelms the cap -- or even better yet, another trigger, such as an approaching front or other exterior can break the cap and allow for explosive cloud development - much like how steam blows through a bubbling pot of tea.
We usually don't get anything like that around here -- you need to have the prior ingredients in place -- namely the warm humid air and instability -- to where the cap matters.
How do we get tornadoes?
Washington and Oregon each average about one tornado per year somewhere in the state. Usually for us it occurs when we have an area of low pressure that has tapped into some cold air from the Gulf of Alaska. The cold air brings the instability, and the low pressure trough can add some lift - two of the ingredients needed. The warm, moist air is lacking -- which is why any tornadoes we get are generally quite weak -- but sometimes we can fudge some conditions just close enough to get a very weak tornado.
But living in the Northwest, we almost exclusively bat 0-for-5 when it comes to tornadic conditions. And I think we'll take it every time.