First east wind event of the season means high fire danger!

Most Septembers in the Pacific Northwest feature a transition from hot/dry weather to cool/wet/cloudier conditions. Every few years it happens in late August, but usually September is the transition month. Fall in our areas also features a return to occasional dry easterly wind periods when dry Canadian air moves south and west into the Pacific Northwest. It does that because cool & dense air begins developing over the continental areas = high pressure. Higher pressure to the east means wind blows from land toward the sea...easterly wind. That setup is generally not a problem fire-wise October through May because some rain falls from time to time (or regularly) and forests are moist/wet.

IF one (or more) of these dry east wind events shows up BEFORE our first fall rain? We’re talking very high fire danger because fuels are totally dry from 2 months of heat and no rain. For this reason, mid-August to mid-September is the most dangerous time of the year for fires west of the Cascades. Almost all of our major fires have occurred during this time. We have one of these dangerous periods on the way for two days...Friday and Saturday. Luckily this event will be nothing like the wind storm that struck on Labor Day 2020. That one fueled 5 megafires (>100,000 acres) on the west slopes of the Cascades and the worst firestorms seen in our area the past 80+ years.

First a quick summary...

1) Friday and Saturday will feature very high fire danger from the Cascades west to the coastline

2) Fire danger goes back to “normal” Saturday evening as east wind disappears

3) Easterly wind gusts will be far less intense than what occurred at Labor Day 2020. 25-35 mph in lowlands and 35-50 mph on mountain ridges.

4) Hot temperatures accompany the wind, around 90 for a high Friday and well into the 90s Saturday

5) The coastline will reach well into the 80s Friday, with a few spots even reaching 90 for one day!

6) 40,000+ PGE and Pacific Power customers may have power cut at some point Friday through Sunday through planned shutoffs. I am one of those customers.

Technical info...

A weak upper-level system is sliding by tonight, bringing cooler air through southern Canada and putting a “dent” in the huge heat dome over the SW USA

But then a new hot area of high pressure (in the upper atmosphere) pops up over the Pacific Northwest Friday and Saturday

At the surface, easterly wind arrives early Friday morning in the hills and spreads down into the urban areas and I-5 corridor during the daytime. Notice the highest fire risk areas are in the foothills, mountains, and Gorge...where the wind will really be blowing hard at times. The numbers here represent a “Fire Weather Index”. I don’t think I’ve seen it above a 4 west of the Cascades all summer.

Now let’s go hardcore...time for a WRF-GFS model “cross-section”. I apologize for the 1st grade style annotations. It was a quick snipping tool job...

Time goes from right to left and you’re looking at the lowest 10,000′ of the atmosphere over Portland. The “850″ horizontal line is around the 5,000′ elevation. Each little “wind barb” has one flag for each 10kts. A thick flag = 50kts. What do I see? Wind overhead will be northerly through about midnight Thursday night. Then easterly wind develops overhead the rest of the night. By Friday midday the wind is surfacing in the metro area. I’ve highlighted the strongest period of wind...Friday evening through sunrise Saturday. 50-60 mph wind over Portland around 2-3,000′. Labor Day 2020 speeds were forecast (and ended up being) quite a bit stronger. Note that the wind goes calm in the lowest elevations Friday night before returning Saturday. It’ll keep blowing through the night on hills and in mountains/foothills. Some of you in the hills will be having a 70 degree windy night! By the way, the red lines represent temperature in Celsius. Up to about +24 at 850(mb) Saturday afternoon. That’s about the hottest it can get on the 10th of September. The record for September 10th in Portland is 98. In fact we’ve never been at/above 97 after that date. So most likely we’ll be in the 94-97 degree range late Saturday afternoon.

Fire smoke may show up during this hot & windy weather too. Canadian fire smoke could drift south over us Friday. Here’s the high-level smoke forecast for Friday morning

Since the low level wind will also be coming in from the northeast, we might actually see some smoke in the air Friday/Saturday too. The low-level smoke forecast.

If we can get through Friday and Saturday, a cooler weather pattern, more typical for mid-September, is on the way. Check out the Euro ensemble temperatures for the next 15 days...an obvious downturn after Saturday