On March 5, 2024, the National Weather Service office in Portland is issuing an update to the geography of their public forecast zones. These are the preset boundaries they use to issue a number of their weather alerts, including those for snow, freezing rain, wind, and fog. This update is for Portland’s county warning area which is Northwest Oregon and Southwest Washington, including Portland, Eugene, Longview, and Astoria.
Before the creation of the National Digital Forecast Database, most forecasts users saw from the National Weather Service were zone forecasts for these preset areas instead of the modern “point forecasts” presented on their website today. Zone forecasts are still issued and available both online (though they are not the default) and on NOAA Weather Radio.
The current forecast zones are pretty large. Everyone from Forest Grove to Troutdale and Wilsonville to Scappoose are all in the same “Greater Portland Metro Area, OR” zone (006 on the above map). This is a large area with numerous microclimates and significant elevation changes. It is common, for example, for a wind advisory to be issued for the entire region just for an east wind event in the east metro – in effect alerting people in Hillsboro about strong winds that are only expected east of I-205. Similarly, the entire zone will also get winter weather advisories for snow that is limited to the upper slopes of the West Hills.
The Greater Portland Metro Area zone is not the largest zone in this county warning area, but it is the most populated in Northwest Oregon and Southwest Washington. Most of the other old zones also contain significant microclimate diversity. On top of this, you may notice that the zone boundaries either follow fairly smooth lines or county lines. Neither of these boundary types reflect the elevation diversity of the region.
In contrast, the new zones that have been created by the Portland office are smaller, capture microclimates better, and instead of having smooth lines follow elevation contours quite a bit better. The old Greater Portland Metro Area zone is being changed into six separate zones:
- 109: Tualatin Valley
- 110: West Hills and Chehalem Mountains
- 111: Inner Portland Metro
- 112: East Portland Metro
- 113: Outer Southeast Portland Metro
- 115: East Central Willamette Valley
These changes reflect advances in technology that allow meteorologists at the National Weather Service to issue more detailed forecasts such as better computers in the forecast office and improved modeling. Once these changes are made on March 5, people in Hillsboro won’t get alerts for wind in Gresham and the evergreen wintertime confusion created throughout the city for a little snow in the West Hills should be reduced some.
The zones the Portland office uses for fire weather alerts are also getting an update with several zones being split up and reconfigured after consultation with users of those forecasts in such as the National Forest Service and Portland General Electric.
Other National Weather Service offices throughout the country have made similar changes in the last decade or so. Seattle’s office split up the forecast zones around the Puget Sound, Pendleton split up zones on the east slopes of the Cascades, and the office in Charleston, South Carolina split a few of their zones – all to better reflect microclimate variation that changes over short distances and to minimize the number of irrelevant alerts people receive.
The changes in forecast zones won’t change how some weather alerts are issued, such as tornado and severe thunderstorm warnings and a few kinds of flood alerts. Rather than being issued for a whole forecast zone, these alerts are created using custom drawn polygons.
If March 5 ends up having adverse weather conditions, the Portland office will postpone the update to a later time to avoid causing confusion. For more details on these changes, follow these links for the Public Forecast Zone changes and the Fire Weather Zone changes.
