If you think rainfall at your location is representative of the region....think again.
This June there were huge precipitation contrasts.
Consider the accumulated rainfall from June 1 to yesterday (June 26 is shown below).
Wow. The windward (western) side of the Olympics received 5-7 inches so far in June. In contrast, only 1-1.5 inches fell over Puget Sound and under a half-inch dampened Sequim, northwest of the Olympics.
Over TEN INCHES at some locations on the western slopes of the Cascades, declining to under a half inch along the eastern slopes of the Cascade and into the Columbia Basin.
A closer look at the contrasts from Seattle to the Cascades is shown below. From 0.79 inches over western Seattle to 11.30 inches in the terrain.
The western side of Washington and particularly the western slopes have been wetter than normal while the eastern slopes of the Cascades have been relatively dry. Why?
This month the winds approaching the region have been stronger than normal. Stronger-than-normal winds from the west. Such a wind field enhances upslope flow and thus precipitation on the western sides of mountain.
In contrast, enhanced winds from the west produce stronger descent and rain shadowing on the eastern side of barriers---producing a strong rainshadow.
Below are the heights at 850 hPa, which you can think of as pressure around 5000 ft. Lower values over the Gulf of Alaska and higher values to the south. This pressure pattern produces west-southwesterly winds approaching our coast (white arrow).
How was the situation this month different from normal? That is shown below (the anomaly of 850 hPa height--think pressure at 5000 ft) . This month we have had a stronger than normal low over the Gulf of Alaska, which helped enhance the southwesterly flow and brings more moisture into the region.
Thanks, Cliff, for yet another insightful and informative blog.
ReplyDeleteI'm trying to reconcile the concept of hydrometeor drift with your comment:
"In contrast, enhanced winds from the west produce stronger descent and rain shadowing on the eastern side of barriers---producing a strong rainshadow."
The paper below speaks to the topic, but in the context of atmospheric rivers. In an extreme example (Mid November 2021 AR) I've heard secondhand that hydrometeor drift caused precipitation to essentially skip an entire mountain range (Selkirks) to affect a more easterly mountain range (Purcells) in BC. I know you didn't speak to AR events specifically in this blog but could you please help enlighten us on resolving the differences between orographic uplift and hydrometeor drift... AR vs. 'garden variety' precip events?
Thanks again!
https://journals.ametsoc.org/view/journals/wefo/34/5/waf-d-18-0176_1.xml
Correction? Cliff - In your last sentence, did you mean to say that we had "a stronger than normal low over the Gulf of Alaska" (as shown in the anomaly graph)?
ReplyDeleteThis has been the coolest first half of the year since 2011 at BLI - about 1.5F below normal on average
ReplyDeleteThe mean April-June temp was ~0.5F colder with 0.05" more precipitation than that period during 2022;
DeleteI have lived in the convergence zone for most of my 48 years. I currently live just south of Mill Creek and it seems this is in the middle of the convergence zone. Often it is hard to determine if the rain predictions actually apply to our area. I have had days when we have clear skies but there is rain all around us. More commonly, it is clear in Bellevue or maybe even Bothell but raining at our house. Go a bit further north and it is clear again. So, having a variation in rain levels based on the predictions is almost a fact of life for us.
ReplyDeleteI often use the UW weather radar to determine if it looks like we will get rain or not. That seems to be the most reliable method for planning our daily walks.