Plaincast decodes the National Weather Service's Area Forecast Discussion — the real forecast, written 3 to 4 times a day by the meteorologist on shift — into plain English. The discussion is where a forecaster explains their reasoning: which models disagree, what they are watching, and how confident they are. Below is the current edition for Portland, with its shorthand expanded. The full experience — an AI plain-English summary beside the annotated original — loads with JavaScript, and every one of the 68 forecast offices has its own edition.
Area Forecast Discussion · National Weather Service Portland (PQR) · issued Sunday, August 23 at 2:25 PM PDT.
Synopsis
Upper level ridging and low-level offshore flow develops Monday, continuing into Tuesday, bringing warmer and sunnier conditions with high temperatures in the mid 80s to lower 90s across the interior lowlands, warmest on Monday. Wildfire smoke will also begin to increase over the Columbia River Gorge, Upper Hood River Valley, and the greater Portland/Vancouver metro area Monday morning and then linger through Tuesday; however smoke is not expected to be dense for most locations. Onshore flow arrives by Wednesday and continues thereafter, which will clear any lingering smoke out of the area and bring relatively cooler temperatures through the end of the week and into the weekend.
Discussion
Tonight through Saturday Night. Water vapor satellite imagery reveals the upper level shortwave trough lifting across NE Washington into Canada this afternoon. Heights aloft will continue to increase through tonight allowing for clearing skies. As a ridge of high pressure builds aloft over the region, a surface thermal trough develops along the coast extending into western WA early Monday morning. This will allow for light offshore flow to develops, especially over the western slopes of the Cascades with east winds increasing through the Columbia River Gorge later Monday morning. Winds are not expected to be very strong, but gusts up to 25-30 mph will certainly be possible through the western gorge. Temperatures at 850 mb are also expected to rapidly rise to around 20C by Monday evening. Downsloping winds and afternoon mixing will therefore allow for surface temperatures to warm into the upper 80s to lower 90s across much of the inland lowlands, as well as through Cascade valleys, while temperatures at the coast will likely warm into the 70s before winds shift as sea-breezes occur early in the afternoon. This offshore flow will also cause wildfire smoke to begin to filter back down into the valleys, including into the Portland/Vancouver Metro area, Columbia River Gorge and Upper Hood River Valley. However, smoke is not expected to be dense for most locations.
By Monday evening, the surface thermal trough is expected to shift east into the Willamette Valley, and potentially to the Cascade crest, allowing for onshore flow to return across much of the area. Lingering low pressure in the valley will likely allow for very light offshore flow to return over the Cascade foothills Monday night into early Tuesday. This will likely maintain elevated overnight temperatures in the lower to mid-60s where the easterly winds persist. A moderate risk of heat related illnesses will therefore exist Monday into Tuesday. Wildfire smoke is also likely to linger into Tuesday as well for northern portions of the forecast area.
Early Tuesday, a shortwave trough will lift just offshore, skirting the coast, as a deeper upper level low in the Gulf of Alaska drops south. This should cause a southerly wind reversal to occur along the Oregon coast and into the Willamette Valley. Heights aloft still remain high with the axis of the upper level ridge over the Intermountain West, but 850 mb temperatures cool to around 16-17C. Still expecting Tuesday temperatures to warm into the mid to upper 80s (about 4-8 degrees above normal). By Tuesday evening, surface high pressure building along the coast will bring a strong marine push of westerly winds. Any lingering wildfire smoke should shift back east of the Cascades by Wednesday.
Models and their ensembles are showing fairly good agreement that the upper level trough over the NE Pacific gradually pushes east over the Pacific NW Wednesday through Thursday, with troughing likely persisting over the region through the end of the week and into the weekend. There is high confidence that temperatures trend cooler to near or slightly below average. Confidence is low regarding precipitation, if any will occur later this week. But, if any rain does fall, it would most likely be very light. Best chances would be drizzle along the coast with a weak frontal passage Thursday morning; and again on Saturday, but chance of rain remains around 15% or less. /DH
This is the plain-text edition, decoded without AI so it loads anywhere. Read the whole Portland discussion at /o/PQR/, see what changed since the last one, or open The National Desk for where the weather is today.