Now that it is mid-November, we all know Thanksgiving and Christmas are right around the corner. It also means winter and snow across most of Oregon and Washington are not that far away, either.
In the past, the National Weather Service would forecast a range of snow accumulation for a location, say 4 to 6 inches, provide a particular time frame, and that would be it.
Currently, the NWS is not only forecasting the range but is providing the full range of possibilities from the least amount to the maximum amount for a given storm. Using the 4- to 6-inch range I mentioned above, that would be considered the “most likely” accumulation for a location. What this means is the forecaster is most confident that these amounts will occur in that particular area. However, does the forecaster think there is a better chance of 4 inches or 6 inches?
Also, though 4 to 6 inches is the most likely range of accumulations, there is still a chance the snowfall could be greater than 6 inches. Is this chance a high probability or low probability of occurring? At the same time, there is a chance the snow could be less than 4 inches. Again, is that a high percentage or a low percentage chance?
Over the last year or so, the NWS in Pendleton began forecasting probabilistic snow amounts. National Weather Service meteorologists predict the “most likely” accumulations, based on their expertise and interpretation of the available meteorological guidance. A full set of snowfall forecast information is then developed that combines NWS forecaster skill and 50-plus U.S. and international weather models to give a range of possibilities, not just the current expectation.
Thus a more complete picture is formed with all this information that lets us know the likelihood of exceeding any given snowfall amount. Specifically, we use all this forecast data to compute a reasonable lower-end and higher-end snowfall amount based on many computer model simulations of possible snowfall totals.
The lower end amount represents a 90% chance that more snow will fall and only a 1 in 10 (or 10%) chance that less snow will fall. Conversely, our reasonable upper end snowfall amount represents a more unlikely scenario, with only a 1 in 10, or 10%, chance that more snow will fall, and a 9 in 10, or 90%, chance that less will fall. The upper end amount is considered a “reasonable worst case scenario” and has been proven to be useful for planning purposes for many of our partner agencies.
The most likely accumulations, “low end”, and “high end” are then plotted on maps. Additionally, other maps are made available for the probabilities of exceeding 1 inch, 2 inches, 4 inches, 6 inches, 8 inches, 12 inches and 18 inches or more of snow across the area.
Lastly, data is provided numerically for several cities and towns and by county. For example in Umatilla County, data is provided for Hermiston, Meacham, Milton-Freewater, Pendleton, Tollgate and Ukiah.
So, going back to my example of 4 to 6 inches, again, that is the most likely amount. Hypothetically, the chance of 8 inches or more could be only 10%, or it could be 40% or more depending on the scenario. These numbers can give you an idea if the accumulations look to be on the upper end or lower end of the forecast range, or even if they will be exceeded or come up short.
Also, the “high end” or “reasonable worst case” values can sometimes end up being fairly close to the most likely forecast. When this happens, it generally means there is good confidence in the forecast with little variation. Greater disparities between the high end, expected, and worst case amounts point to lesser confidence and a more complex winter storm system. Often, you will see quite a bit of variability (sometimes with accumulation differences in feet) over the Cascades and mountains of Northeastern Oregon and Southeastern Washington.
These probabilistic forecasts are generally updated twice a day around 5 a.m. and 5 p.m., but they can be updated at any time if the forecaster makes a significant change to the snow accumulations. The maps and data can be found on the National Weather Service Pendleton’s website at weather.gov/PDT/winter.
Hopefully, you will find this information useful the next time the flakes fly across Northeastern Oregon or Southeastern Washington.