Let me illustrate what happened by showing you the temperatures at Bellingham during the past few days (see below). On 14th and 15th temperatures were in the teens, as uber cold air from the interior of British Columbia, pushed through the Fraser River Valley and then over Bellingham and northwest Washington. Temperatures slowly warmed on the 16th. But suddenly over one hour, temperatures surged 20F to 45F. Instant defrost. But then temperatures dropped just as suddenly two hours later, back to the low 20s. Startling.
On the 17th temperatures dropped back down to 15F and everything froze solid again. But that would not last long. First, temperature rose rapidly to around 30F and held around 30F for much of January 18. But later that day, temperatures surged 15F in one hour to 45F, rose to near 50F, and then dropped back to 35F over one hour. How weird is that?
So what is going on? How can temperature rise and fall tens of degrees in an hour?
To get a hint, let's plot the wind direction at Bellingham for the same period.
Ah ha! For most of the period, winds at Bellingham were from the northeast, associated with cold, arctic air flowing out of the Fraser River Valley. But look carefully. There was a switch to southeasterly winds early on the 16th when temperatures started to warm, and a sudden spike to easterly winds later on the 16th when the temperatures surged! And later on the 18th there was a sudden shift to southerly winds associated with the sudden warm-up on that day,
A better approach is to look at local weather maps before and after. At 1 AM on the 16th, cool northeasterly air was over the Bellingham.
But by 3 AM, strong southeasterly flow had surged in bringing warm air (see below). But the cool air was not far away. A meteorological battle was going on between cold and warm airstreams!
A similar situation occurred two days later.
So why this big change on the 16th? As shown below (surface map at 10 PM that day), a strong low center approached on the 16th. On one hand this low produced a large pressure difference between high pressure in British Columbia and the low center, which would tend to produce cold northeasterly flow in the Fraser Valley. But the low center also resulted in strong southerly/southeasterly flow over the lowlands, and for a few hours that warmer, southerly flow was dominant--thus the surge in temperatures. After the low moved through, the cold northeasterlies returned to Bellingham.
Other areas of Washington State can also get large temperature swings. For example, shallow cold air in eastern Washington can be rapidly displaced by strong downslope flow from local mountains. And Portland can have rapid temperature changes depending on the air flow in the Columbia River Gorge.
You can imagine the future impacts of these huge temperature swings.
Could sensor location have anything to do with it? Warm source one direction and open exposure the other for instance.
ReplyDeleteNot really. And of course it was not just one reading from one station. It's simply the direction of the wind. South wind is warm, 50F plus while the wind from the North is well below freezing. Simply science. No conspiracy involved.
DeleteBham is where I currently live, and boy was it interesting to be in those swings of temp/weather when the wind direction changed.
ReplyDeleteFreaky! Learning more about the atmosphere and terrain is like discovering a new world. Very interesting explanation, Cliff.
ReplyDeleteBig strong convergence zone from south seattle to lynnwood, heavy heavy rain. All in the red. I'm not in it, but I wish I was! Time: 3:15
ReplyDeleteI guess folks here in the NW are relatively isolated with the moderating influence of the Pacific with temp extremes that are not even unusual in the rest of the US. It would have been nice of Cliff to provide context. On the East Coast, 12 hour temp swings of 50F are not uncommon. Experience this myself in Norfolk VA. One day it was 72, next morning it was 19F and I had no gloves for the bike ride to the base for classes.
ReplyDeleteWhy should Cliff provide context... after all this is a NW weather blog. What makes these temp swings highly irregular even by East Coast standards is that they swing drastically several or more times in a day- Which I believe is irregular and highly unlikely even by Norfolk, VA standards... this back and forth temp swing multiple times a day simply does not happen in most environments in the continental or East Coast U.S. It cannot be explained simply by North wind is cold and South wind is warm as previously stated. It has more to do with cold air pooled to the East behind the Cascades and the moderating influence of the Pacific while the winds drive the two forces upon us.
DeleteHey, Cliff, are you covering the tornado warning in effect in Aberdeen, WA?
ReplyDeletehttps://twitter.com/search?q=ABERdeen%2C%20wa%20tornado&src=typed_query
Many convergence zones slamming north seattle and west, (from what I saw on the radar)
ReplyDeleteThey must have at least an inch since midnight!
Short lived, but extremely heavy downpour in east bellvue, 9:50 pm. I was in bed and heard the rain on the roof, it sounded like a roaring fire!
ReplyDeleteI'm glad you posted about this- feeling it firsthand was pretty wild! It was also interesting how much colder Bellingham was than the rest of the forecast area during much of the winter storm. It's usually a couple degrees chillier, but for several days we never got out of the teens and were 10-20 degrees below points south! And from what we noticed from reporting and from firsthand accounts from friends in Seattle, our snow was much drier and more powdery than the heavy, wet stuff most people in western WA experienced. Gotta love the Fraser River Outflows!
ReplyDelete