One of the most frequent questions that I get when posting about weather events over on Tri-Cities Weather is some form of “why does the alert stop at <insert county line here>?” The short answer is that it has to do with how the National Weather Service, the source of all official weather alerts in the United States, is organized and the system it uses to organize the things it does.
There are two main reasons why a weather alert in the Pacific Northwest would stop at a state or county line. First, each National Weather Service office has an area of responsibility, termed a County Warning Area (CWA), that it issues forecasts and alerts for. Second, each weather service office divides their County Warning Area into a set of forecast zones which provide the geographical boundaries for most weather alerts. Details on both below.
COUNTY WARNING AREAS (CWAs)
As the name implies, each CWA is a set of counties assigned to the individual office. The Tri-Cities lies in the Pendleton office’s CWA, as does Yakima and Bend, but Othello and Moses Lake (Adams and Grant Counties) are assigned to the Spokane office. Pendleton isn’t going to issue an alert for Othello, and Spokane isn’t going to issue an alert for Connell. Even the few weather alerts that aren’t based on forecast zones, like flood, tornado, and severe thunderstorm warnings, won’t cross CWA lines.
Washington and Oregon are covered by six National Weather Service offices located in Boise, Medford, Pendleton, Portland, Seattle, and Spokane. Neighboring offices often talk with one another during events such as snowstorms or severe weather outbreaks to be on the same page with each other, but when it comes down to it each office is only issuing products for their CWA.
FORECAST ZONES
Forecast zones are an artifact of older days when the National Weather Service was dealing with lower computer power. Despite now offering point forecasts on a 2.5 kilometer grid (meaning if you go 2.5 km either direction you get a different forecast), forecast zones are useful for the National Weather Service. Most forecast zones are at least partially based on county lines.
Most weather alerts are based on Public Forecast Zones (see map above). Winter storm warnings are an example of this. Every winter storm warning issued by the National Weather Service will be for one or more forecast zones. Each of these alerts will provide a list of forecast zones that it covers, then some of the main cities in those zones. Here’s an example from 2021:
Lower Columbia Basin of Washington-
Including the cities of Connell, Prosser, and Tri-Cities
950 PM PST Fri Feb 12 2021
...WINTER STORM WARNING REMAINS IN EFFECT UNTIL 4 PM PST
SATURDAY...
Sometimes the text of the product will give more details on the location, such as above a certain elevation or south of a certain point, like this example also from 2021:
East Slopes of the Washington Cascades-
453 AM PST Sat Jan 30 2021
...WINTER WEATHER ADVISORY IN EFFECT FROM 10 PM THIS EVENING TO
10 PM PST SUNDAY ABOVE 4000 FEET...
* WHAT...Snow expected above 4000 feet. Total snow accumulations
of 6 to 10 inches through Sunday night. More snow is likely
through the first of the week.
That said, usually even when the text of an alert says something like “above 4000 feet,” most mapping services will draw it to cover the entire forecast zone. Public forecast zones are also still used for general forecasting. If you know where to look you can find them online (such as this one for the Lower Columbia Basin of Washington covering the Tri-Cities), but in my opinion the biggest benefit is that this is an easy way for the National Weather Service to broadcast the forecast on their weather radio system.
There are other types of forecast zones that are different than Public Forecast Zones and they are worth mentioning. Fire Weather Zones (see map above) are used for fire weather alerts, such as red flag warnings, and Marine Forecast Zones are essentially Public Forecast Zones but for the oceans and the Great Lakes.
SEVERE THUNDERSTORM & TORNADO WATCHES
Unlike most weather alerts, severe thunderstorm and tornado watches (just the watches, not the warnings) are issued by the Storm Prediction Center in Norman, Oklahoma instead of by a local National Weather Service office. These are based on counties, rather than forecast zones, but always come with this tag at the bottom, reminding people that just because the alert is based on county lines the actual weather isn’t:
REMEMBER...A Severe Thunderstorm Watch means conditions are
favorable for severe thunderstorms in and close to the watch area.
Persons in these areas should be on the lookout for threatening
weather conditions and listen for later statements and possible
warnings. Severe thunderstorms can and occasionally do produce
tornadoes.
Text from past NWS alerts was sourced from the Iowa Environmental Mesonet archive. The featured image is a screenshot from RadarOmega during a winter weather event in 2021.
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