what's in a word?

One of the links on the National Weather Service (NWS) Forecast Office page for Boston is a link to an hourly weather forecast graph. Here's the SNOW portion of the graph for the upcoming blizzard:

From 5 PM today through 11 PM tomorrow the graph shows OCNL for snowfall.

OCNL? What does that mean?

Gridpoint Forecast Help on the NWS Caribou, Maine page showed me the English translation of the abbreviations:

Abbreviation	Meaning
OCNL	occasional
LIKLY	likely
CHC	chance
SCHC	slight chance
	no chance

Hmm... the top entry in the graph seems like it should be the represent the largest chance of snow. Occasional? Just below the table on the forecast help page I found an explanation:

* Please note that Ocnl (occasional) represents definite snowfall; however, the snowfall may not be continuous but may stop and start at times.

When I looked up the word occasional in a dictionary I found the definition "occurring, appearing, or done infrequently and irregularly" That seems like the opposite of occasional in the context of this gridpoint forecast, doesn't it?

The wording on the blizzard warning seems (to me) to say the opposite:

A BLIZZARD WARNING IS ISSUED WHEN SUSTAINED WINDS OR FREQUENT GUSTS OVER 35 MPH ARE EXPECTED WITH CONSIDERABLE FALLING AND/OR BLOWING AND DRIFTING SNOW.

I would love to know the origin of the use of the word occasional in this context.