Decoding social media 'weather geek speak'

Weather blog to the rescue!

Here are some of the common terms you might run in to, and what they mean.

"Just saw the 12Z GFS and it's going much colder than the NAM and ECMWF..."

The 12Z, or 0Z, or any number and a 'Z' is the time when the forecast model was started. The 'Z' stands for 'Zulu' and that's a military/government designation for what you might better recognize as GMT or Greenwich Mean Time -- basically the standard for using time internationally. (GMT is actually the old name. The French bought the naming rights and it's now "Universal Time Coordinated" (UTC) But that's for another blog.)

Z time is 8 hours ahead of PST, and 7 hours ahead of PDT. The forecast models come out four times a day -- at 0Z, 06Z, 12Z and 18Z. You can do the math for the Seattle times, but it's easier to remember this way:

06Z - midnight model run
12Z - early-morning model run
18Z - early afternoon model run
00Z - late evening model run

The GFS is one of the American forecast model names. There is also the NAM. The GFS is also moving to the WRF because NOAA apparently can't stand keeping a model name the same for too long (just ask defunct "NGM" and "MRF")

ECMWF is the European forecast model (also known as "Euro"). There is also the UKMET (British) and Canadian but those don't get as much play.

"The 516 line crosses Seattle on Saturday, and 850 temps are -13!"

The "516" line, or any kind of number in the 500s or a "522dm" is a measure of atmospheric thickness. Colder air is denser than warm air, so a column of cold air is less thick than a column of warm air. Forecasting models will measure a thickness between two known pressure levels and the lower the thickness, the colder the expected temperature.

For Seattle, we start to think snow once that thickness is around 522 decameters or lower (not sure why they picked decameters). Typical winter numbers are around 530-540 (anything under 540 is considered snow at Snoqualmie Pass) To contrast, in summer, our heat waves get to be around 575-580dm, maybe even 585. Anything over 570 is pretty warm.

The forecasting charts show thickness by a dotted line and will denote every 6 decameters. So when you see the 522 line, or even the 516 line, drift on top of our south of Seattle, that's cold enough to start thinking snow on the surface.

(Note, around the rest of the U.S., the traditional "rain/snow" thickness is about 540, so that's why numbers warmer than that are red and lower are blue on national forecast charts. For 75% of the country, the red/blue denotes rain/snow, but here it has to be colder due to moderating power of Pacific Ocean.)

"850 temps" or "850mb temps" is the measurement at the altitude where the atmospheric pressure is down to 850 mb -- roughly around 4,500-5,000 feet. The temps are in Celsius and typically you start playing the snow game when they get down to about -10 C -- that roughly translates to freezing at the surface.

520 is 0 mph and Sea-Blvu is 95 min. Go 2 I90

That probably means it has snowed on the 520 bridge, traffic is at a standstill, and you should move over to I-90 :)

One other random weather acronym:

MOS: Model Output Statistics. More on that here

If you see any other weird ones, post them in the comments below and I'll try to answer.

Happy Twittering!