Yup… gonna snow locally again this weekend. The weather models have been predicting this for about a week now; pretty much indicating a sure thing in the weather prediction business.

Here’s what the National Weather Service says about this weekend from this afternoon’s “area forecast discussion”:1 for the Bay Area.

bq.. NO DOUBT THE BIG STORY HERE WILL ONCE AGAIN BE VERY LOW SNOW LEVELS ESPECIALLY FOR MID MARCH. BY 18Z FRIDAY THE GFS HAS 700MB TEMPS OF -15 WITH 1000-500MB THICKNESS VALUES AROUND 525 DM. THIS TRANSLATES TO A SNOW LEVEL OF AT LEAST 2000 FEET…POSSIBLY LOWER. BY SATURDAY MORNING ANOTHER REINFORCING SHOT OF COLD AIR DROPS DOWN WITH 700 MB TEMPS TO -16 (VERY COLD) AND 1000-500MB THICKNESS TO AROUND 523 DM. IN CONVECTIVE SHOWERS AT NIGHT OR EARLY MORNING THIS COULD BRING SNOW LEVELS TO NEAR SEA LEVEL.

p. Again, Z is Greenwich Mean Time, MB is millibars, DM is decameters and temperatures are in degrees Celsius. The 1000-500MB thickness is a good proxy for how cold the air will be from the surface up to 500MB of pressure. Cold air is denser than warm air, so cold air will have a lower 1000-500MB heights. 525MB height is pretty low… especially for March.

What is driving the potential for snow this weekend? Well, it’s artic air being driven down from Alaska. Here’s an animated image of the 850MB temperatures (which correlate strongly to surface temperatures) from Wednesday to late on Friday.

[1(Bay Area AFD)]http://www.srh.noaa.gov/fwd/productviewnation.php?pil=MTRAFDMTR&version=0