What in the world are 'MOS POPS'?

Mmmm...MOS POPS?

Blog originally posted Feb. 11, 2010:

I had someone ask me the other day: What in the world are "MOS POPS"?

A frosty organic treat to enjoy on a hot summer day? A new symphony set to debut in the rain forest? No, it's much more boring than that... It's a weather acronym.

The person found it by reading the National Weather Service Forecast Discussion which they update every 6 hours or so. That discussion was originally intended to be between other National Weather Service forecast offices so each one knew what the other was doing. But with the rise of the internet, it has blossomed into a more public discussion since anyone can easily read it now.

(And since it's more in the public eye, the restrictions for those writing it have changed as well. Not too long ago, all words in the discussion were restricted to 3-4 letters max to keep transmissions short. Now, Weather Service forecasters are free to write it conversationally. )

But the discussion is still thick with meteorological jargon that may have you scratching your head, and one of those you'll find frequently mentioned is about "MOS POPS."

To use it in a sentence from the discussion Wednesday morning:

"Models have sped up the next system, which is now due Wednesday afternoon. MOS POPS are quite high for the period 18Z-00Z Wednesday."

In layman terms, MOS POPS are what forecast models are saying about the likelihood of rain. It stands for "Model Output Statistics / Probability of Precipitation(s)"

A MOS is taking what a forecast model has generated for a region, and plucking the data for a specific location and turning it into a table format. For example, this link: shows a MOS for Seattle. You can learn how to decode it here for the extended GFSX MOS, or decode the short term GFS MOS's here.

Probability of Precipitation is basically the rain chance the model gives for that period. So "P24" means a 24 hour "POP" and a reading of 84 means an 84% chance of measurable rain during that period.

So if "MOS POPS are quite high for the period" that means models indicate a high chance of rain. ("18Z-00Z" is the time on "Zulu" time, or the old GMT time. That's 8 hours ahead of PST and 7 hours ahead of PDT.)

Also of better help, the NWS has started issuing a Graphical Forecast Discussion where they augment the written text with charts and other graphics to help explain what is going on. That product will also help with some of the mysterious acronyms that still make it into the discussion from time to time.