A Dry Storm is Hitting the Northwest
During the past few days have you noticed the dryness of the air? See a spark when you touched a doorknob or that your skin seemed unusually dry? Or a dry throat and the urge to drink more water? Garden soils seem desiccated?
You are not alone. Many on the western side of Washington State noted the phenomenon, which I will term a Dry Storm. And it is about to get revved up again!
To get some perspective, here is a plot of the relative humidity at Seattle Tacoma Airport over the past six months. Two days ago, the relative humidity dropped under 15%, the lowest since October 29th.
As many of you know, relative humidity describes the degree to which the air is saturated: the amount of water vapor in the air divided by the maximum amount of water vapor air can hold at the current air temperature. Since warm air can "hold" more water vapor than cold air, there are two factors that influence relative humidity: the amount of moisture in the air and temperature. Thus, with the same amount of water vapor in a sample of air, the relative humidity is lower during the warmer time of the day.
So what about the amount of water vapor in the air, say at Olympia. A good measure of water vapor content is the dew point temperature: the temperature to which air has be cooled to saturate it (100% RH). If air is full of water vapor, you don't have to cool it as much--thus, higher dew point. The plot below shows a plot of dew point at Olympia over the past six months. It was quite low, but not as low as October 29th. The very low RH on Monday was the combination of low water vapor content and a relatively warm day.
So why was the relative humidity so low on Monday and why is it going to happen AGAIN on Thursday? Good question.
It has to do with the weather pattern we have been in, with a large upper level ridge and inland high pressure pushing dry air from the Northwest interior westward into western Washington.
Let me show you a series of relative humidity forecast maps for our region...and I warn you, you might put on some lip balm before you view it.
This morning we have had a bit of a moist reprieve. The forecast surface relative humidity map for 5 AM shows higher values (blue colors) as moist air flooded western Washington. There are lots of low clouds this AM. No wonder I slept better last night.
But tonight, cooler air and high pressure will move into Montana and Idaho, creating offshore (easterly) flow over our region. The forecast map for 5 AM Thursday (below) shows this, with blue colors indicating cooler temperatures. Look closely (or click on image to expand) and you will view the surface winds being offshore-directed over Washington State. Note the air warming as it descended the western slopes of the Cascades--that is due to compression warming as air goes from low pressure aloft to higher pressure at the surface.
The relative humidity forecast forecast for 5 PM Thursday (below) is, well, withering west of the Rockies, with some values in western Oregon and Washington getting below 15%. We can call it Dry Storm2.
The surface air west of the Cascade crest will not only be dry but warm, with highs in the upper 60s over Puget Sound and 70s in the Willamette Valley (see temperature forecast below for 5 PM Thursday). Reds and pinks are the warmest. And yes...no rain. This persistent pattern has resulted in essentially no precipitation over us for the past two weeks and none is expect during the next 7 days.
You are not alone. Many on the western side of Washington State noted the phenomenon, which I will term a Dry Storm. And it is about to get revved up again!
To get some perspective, here is a plot of the relative humidity at Seattle Tacoma Airport over the past six months. Two days ago, the relative humidity dropped under 15%, the lowest since October 29th.
At Olympia, Monday brought the lowest relative humidity for the entire six months.
So what about the amount of water vapor in the air, say at Olympia. A good measure of water vapor content is the dew point temperature: the temperature to which air has be cooled to saturate it (100% RH). If air is full of water vapor, you don't have to cool it as much--thus, higher dew point. The plot below shows a plot of dew point at Olympia over the past six months. It was quite low, but not as low as October 29th. The very low RH on Monday was the combination of low water vapor content and a relatively warm day.
So why was the relative humidity so low on Monday and why is it going to happen AGAIN on Thursday? Good question.
It has to do with the weather pattern we have been in, with a large upper level ridge and inland high pressure pushing dry air from the Northwest interior westward into western Washington.
Let me show you a series of relative humidity forecast maps for our region...and I warn you, you might put on some lip balm before you view it.
This morning we have had a bit of a moist reprieve. The forecast surface relative humidity map for 5 AM shows higher values (blue colors) as moist air flooded western Washington. There are lots of low clouds this AM. No wonder I slept better last night.
But tonight, cooler air and high pressure will move into Montana and Idaho, creating offshore (easterly) flow over our region. The forecast map for 5 AM Thursday (below) shows this, with blue colors indicating cooler temperatures. Look closely (or click on image to expand) and you will view the surface winds being offshore-directed over Washington State. Note the air warming as it descended the western slopes of the Cascades--that is due to compression warming as air goes from low pressure aloft to higher pressure at the surface.
The relative humidity forecast forecast for 5 PM Thursday (below) is, well, withering west of the Rockies, with some values in western Oregon and Washington getting below 15%. We can call it Dry Storm2.
The surface air west of the Cascade crest will not only be dry but warm, with highs in the upper 60s over Puget Sound and 70s in the Willamette Valley (see temperature forecast below for 5 PM Thursday). Reds and pinks are the warmest. And yes...no rain. This persistent pattern has resulted in essentially no precipitation over us for the past two weeks and none is expect during the next 7 days.
Comments
Post a Comment