As summer rolls on here in the Pacific Northwest, those of you who peruse the National Weather Service forecast discussion, or follow a dedicated weather geek on Twitter or Facebook will probably hear some terms used over and over again that might make military jargon sound positively kindergarten.
So here is a handy guide to decode some of that weather-geek speak, with some actual examples from the National Weather Service:
"Temperatures aloft cooling with the 850 mb temperatures lowering from the lower plus 20C range down to around plus 16-18C Tuesday afternoon and plus 11-13 C Wednesday afternoon and to plus 8-10C Thursday afternoon."
850 milibars is the pressure level at roughly speaking about 5,000 feet high and the temperature is always given in Celsius. It's one of the good gauges we use to get a sense of how hot the air mass is as the temperature at 5,000 feet is better to take out the variability of day vs. night like a surface temperature has.
Of course, temperatures at 5,000 feet are cooler than at the surface (unless you have a monster inversion going, but not an issue much in the summer) but you can get a gauge of how much cooler using the average "lapse rate" of roughly 2 degrees C per 1,000 feet, so it's about 10 degrees (C) cooler at 850mb than the surface, but that is rough estimate and can fluctuate depending on how moist the air is and any surface events such as a low level marine push or easterly downslope winds.
But in this case, assuming little surface events (which was pretty much the case during our heat event at the end of June/early July), a +22C 850 temp would translate to about 32C surface temperature, or 90 degrees. A 17C becomes about 81F, and a 12C becomes 72F. (As it turned out, the models were pretty good. That Monday's high in Seattle was 89, then it was 83 on Tuesday. It did manage to get to 79 on Wednesday but dropped to 71 on Thursday).
Again, marine pushes will help it be a bit cooler and easterly winds can make it hotter. In the winter, when you talk snow, it's not quite the same because if the atmosphere is moist (and snowy) the air doesn't cool as rapidly with altitude -- usually about -8C is the magic line to start thinking snow in Seattle.
"Onshore surface gradients kicked late this morning and have increased through the afternoon and early evening hours. At 04Z...KUIL-KBLI was plus 4.4 mb with KOTH-KSEA and KHQM-KSEA near plus 2 mb. The cross Cascade gradient has increased to over plus 5."
Wow, that's a mouthful, taken from the NWS discussion on the afternoon of July 1 as the heat wave was beginning its end. Surface gradients are an essential part of a Western Washington summer forecast as they determine if and when a cooling marine push is coming and how strong it'll be.
A "pressure gradient" is a measure in difference of air pressure between two points. Since air flows from higher pressure to lower pressure, checking the pressure numbers around the Sound can give you an idea of which way the low level winds are going to flow. The greater the difference in pressure, the stronger the wind potential.
Some of the most useful gradients are the difference between Forks (KUIL -- the "UIL" comes from Quillayute) and Bellingham (KBLI) for determining the wind down the Strait of Juan de Fuca, the gradient between Hoquiam (KHQM) and Seattle (KSEA) for a push of air off the coast into Seattle, and the gradient between Seattle and Yakima to see if the west winds will extend through the Cascades into Eastern Washington. (KOTH is North Bend, Oregon, also a good idea of a marine push.)
A positive gradient means the pressure is higher in the first location than the second, and a negative gradient means the pressure is lower in the first spot than the second. Around here, gradients are set up to where positive numbers mean a west or south wind while a negative number means an east or north wind, depending on the alignment of the cities. So in the summer, a positive gradient indicates cooler marine flow while negative gradients mean warmer, offshore flow.
A +4.4 mb gradient from Forks to Bellingham is a strong west wind down the Strait -- indiciative of a strong sea breeze and/or building marine push. Rule of thumb is you can take that number and multiply it by 10 to get a rough estimate of expected wind speed (in knots) in the Central Strait of Juan de Fuca -- in this case, ~40 knots! That's a lot of cooling and why the North Sound had some drastic cooling that afternoon (It was 89 in Seattle, but just mid 70s around Everett.)
A +2 Hoquiam to Seattle gradient is also indicative of a push, but a weak to moderate one. A light marine push is probable when the HQM-SEA number is between +1.0 and +2.0. A moderate push would be +2.0 to +3.5, and strong push would be greater than +3.5. If it's between -1.0 and +1.0, that's a weak flow day, which would be sunny and warm. A strong negative number (less than -1.0) would mean a strong offshore flow day, and probably very warm temperatures in summer.
But the cross Cascade gradient of +5 also indicates a push will make it into the Puget Sound region and perhaps even cool off Central Washington. A day when you've got a cross Cascade gradient that is +7-9 or higher, it means a solid marine push is likely for Seattle and also some cooling but breezy west winds for Central Washington. A negative number across the mountains and that means we have the hot, east winds coming into Seattle from Eastern Washington and it's likely a near-record breaking or record-breaking hot day.
You can see the current gradients at this chart.
(Also, "4Z" is "Zulu" time which is also the old Grenwich Mean Time. It's 7 hours ahead of Pacific Daylight Time and 8 hours ahead of Pacific Standard Time. So 04Z is 9 p.m. PDT)
"Thicknesses are set to warm to 570, then increase to about 576 before cooling back to the 560s"
Thickness is a measure of temperature using air density and how tall the column of air is between 1000 mb (near surface) and 500 mb (about 18,000 feet). The warmer the air, the less dense it is, so a column of air will be taller in a warmer atmosphere than a colder one. We give the measurement in decameters so a "thickness of 570" means the air is 5700 meters tall between that 500 and 1000 mb levels.
These readings are usually indicated by dashed lines on the surface charts:
570 is the signal for pretty warm weather around Seattle, typically indicating highs in the mid 80s or so. The higher the number, the greater the heat potential -- we were over 580 in the last heat wave, with anything over 585 extremely toasty. Seattle usually holds in the 560-565 range in the summer. Anything below 550 is Juneuary. (We use this in the winter too -- 540 is about the "Snow and Snoqulamie Pass" line while getting around 516-518 indicates a potential for lowland snow in Seattle. 540 and below is noted in blue on the forecast charts because for much of the nation, 540 is the "rain/snow" line)
You'll also see discussions refer to "500 heights" - that is similar to thickness -- it's the height in decameters of where the 500 mb pressure level is -- again, the higher the number the warmer the air. But it's usually a higher value than a thickness because surface pressures will be higher than 1000 mb. Here, anything greater than 580 is also pretty hot -- Seattle was close to 590 in the recent heat event.
Hopefully that helps a little bit next time you're wading through a forecast discussion!