Upper level trough to keep Pacific Northwest cool and moist this weekA slow-moving upper level trough is forecast to pass through western North America this week. It is not expected to park over the Pacific Northwest for the entire week but the trajectory of this system should keep the region cool compared to the May average with periodic rain showers in some areas.
As usual, most of the shower activity can be expected in the mountains and west of the Cascades. That said, the Columbia Basin could have a few isolated showers and thunderstorms this week. The position of the upper level low makes it so that shower activity north of Kamloops will be less widespread.
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On Monday morning, the upper level trough was centred near a line from Crescent City, California, to Wells, British Columbia. By Tuesday the trough acts migrates to near Boise, Idaho, with it being near Billings, Montana, on Wednesday.
Northwesterly flow aloft is expected over the Pacific Northwest for several days as the trough moves eastward. Northwesterly flow brings in cool and moist air from the Gulf of Alaska, becoming the primary driver behind regional weather conditions.<div class="wp-block-jetpack-subscriptions__form-elements"><p id="subscribe-email"><labelid="subscribe-field-label"for="subscribe-field"class="screen-reader-text">Type your email…<inputrequired="required"type="email"name="email"class="no-border-radius "style="font-size: 16px;padding: 15px 23px 15px 23px;border-radius: 0px;border-width: 1px;" placeholder="Type your email…" value="" id="subscribe-field" title="Please fill in this field."></inputrequired="required"type="email"name="email"class="no-border-radius></labelid="subscribe-field-label"for="subscribe-field"class="screen-reader-text"></p><p id="subscribe-submit"><input type="hidden" name="action" value="subscribe"><input type="hidden" name="blog_id" value="231036007"><input type="hidden" name="source" value="https://ingallswx.com/?p=5585"><input type="hidden" name="sub-type" value="subscribe-block"><input type="hidden" name="app_source" value=""><input type="hidden" name="redirect_fragment" value="subscribe-blog"><input type="hidden" name="lang" value="en"><input type="hidden" id="_wpnonce" name="_wpnonce" value="637e8dc63b"><input type="hidden" name="_wp_http_referer" value="/?p=5585"><input type="hidden" name="post_id" value="5585"><button type="submit" class="wp-block-button__link no-border-radius" style="font-size: 16px;padding: 15px 23px 15px 23px;margin: 0; margin-left: 10px;border-radius: 0px;border-width: 1px; background-color: #113AF5; color: #FFFFFF;" name="jetpack_subscriptions_widget">Subscribe</button></p></div>
Cool temperatures in the mid- and upper-levels of the troposphere help to generate shower development over land. Low stratus will probably be in play most mornings west of the Cascades (especially in Washington, the Lower Mainland, and Vancouver Island) but sunshine later in the day and elsewhere will heat the ground allowing for rising air and cloud development.
Isolated thunderstorm activity appears likely under this regime. The best chance of lightning will be in mountain areas where the terrain helps to lift air upward for more dynamic cloud development.
For Thursday and Friday the upper-level pattern shifts to westerly (instead of northwesterly). A weak cold front may come in to bring a few hours of steady rain followed by postfrontal showers. We are too far out for specific timing details on this feature.
Rainfall totals don’t look all that impressive though stronger showers could produce brief heavy rain. The question becomes who will see these stronger showers and aside from knowing they are more likely in and near the mountains it’s hard to say until they actually begin to develop each day.
Through the end of the day Friday, rainfall expectations range from near 0 for Columbia Basin communities like Moses Lake and the Tri-Cities to about 30 mm (1.20 in) in the higher elevations of Vancouver Island. A total of 30 mm in a week is not particularly impressive for the rainforests of Vancouver Island.
Vancouver, Seattle, and Portland could come in somewhere between 5 and 15 mm (0.20 to 0.60 inches) for the week, which also is not particularly impressive.
Look for high temperatures a few degrees below average in the Pacific Northwest through the end of the week and possibly into the beginning of next week. Medium-range weather modeling is hinting at warmer air coming in for the second half of next week but this is far enough out to be taken with a grain of salt.
Still, May is the time of year when slightly below average temperatures can be quite pleasant (especially for the Columbia Basin and Willamette Valley). Highs at Pasco will probably be around 71ºF (21ºC). Portland and Eugene look closer to 66ºF (19ºC).
*The featured image is a satellite image showing the low pressure system off centred off the Oregon Coast on Sunday afternoon. (zoom.earth)*
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