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Thursday, November 7, 2013

An Incredible Forecast from an Incredible Storm

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It was one year ago that Hurricane Sandy impacted the East Coast of the United States. Its incredible size allowed it to cause more damage than the average hurricane. Unlike many storms, whose effects are felt in a relatively more compact area, Hurricane Sandy unleashed her winds and precipitation over multiple states at the same time.

In fact, while New Jersey and New York were bracing for their direct hit, Hurricane Sandy was kicking up strong winds and rain all the way to Cleveland, Ohio. These winds and clouds caused some relatively minor (compared to NY/NJ) damage around the area and gave me a stretch of several days off from flying due to Terminal Area Forecasts (TAFs) such as this:

It's cold, rainy, and the winds are gusting to almost 50 knots. Let's not fly today.
I've had a few posts recently where I go into how to use a TAF and how not to. But what if you're still not sure what all that "secret code" means? Well, why don't we review how to read a TAF by decoding one you won't forget?

This screenshot is the combined METAR and TAF output from's lookup service. Where I fly, I usually pull up the forecasts for the two closest airports that have them to get a bigger, more accurate idea of what the weather is going to do, which is why this contains both KCLE (Cleveland-Hopkins) and KMFD (Mansfield-Lahm).

First the METAR:
KCLE 300251Z 34036G48KT 6SM +RA BR OVC014 06/04 A2951
I'll truncate the stuff beginning with RMK because while it's information that's "nice to know", it's not "need to know". I want to focus on the meat of the METAR without bogging you down in details.

KCLE: (Cleveland-Hopkins International Airport)
300251Z: Look at it as 30/0251Z - The 30th day of the month (October) at 2:51 a.m. Greenwich Mean Time (Zulu)

34036G48KT: Look at it as 340/36/G48 with a KT to remind you that it's knots. In other words, winds are from 340 (northwest; this is a true direction not magnetic, since this is a text report) at 36 knots with gusts (G) to 48 knots. That's 41-55 MPH. The light sport aircraft I was teaching in at the time would take off without moving in a wind like this.

6SM: The SM conveniently reminds you that this is something in statute miles. About the only thing ever given in aviation that isn't in nautical miles is visibility. Therefore, visibility 6 miles.

+RA: heavy (+) RAin

BR: mist. The reason it's not "MI" is because everybody hates Michigan so much that they'd rather use an abbreviation for the French word "brume", which means "mist".

OVC014: skies are OVerCast beginning at 1400 feet above ground level.

06/04: temperature 6° Celsius (43° Fahrenheit), dew point 4° C (39° F). The two numbers being so close usually means there will be some low clouds. That's kind of anti-climactic, since we just saw that the sky was totally covered in clouds (overcast) at only 1400 AGL.

A2951: Altimeter setting (i.e., the barometric pressure) 29.51" Hg. That's really low, especially considering Hurricane Sandy's center is hundreds of miles away.

Combining all these little facts into something useful, we can see that on the night of October 29-30, 2012, in Cleveland the weather was very chilly under solid clouds, and it was so windy that cats and small dogs were being blown away, then falling back to the ground as heavy rain. It's got to get better sometime, right?

That's where the TAF comes in. And then says, "Umm, no, actually."

KCLE 300253Z 3003/3106 35030G50KT P6SM -RA OVC015
   TEMPO 3003/3005 3SM RA SCT008 OVC015
   FM300500 34030G50KT 3SM RA BR OVC006
   FM301400 34025G40KT 4SM -RA BR OVC006
   FM302300 30018G30KT 5SM -RA BR OVC006

TAFs and METARs use similar terminology and format, so if you understand one, you understand about 80% of the other.

KCLE: (Cleveland-Hopkins International Airport)

300253Z: The date and time of a METAR are when the observation was taken. The date and time of a TAF are in the same format (in this case, the 30th day of the month at 2:53 a.m. Greenwich Mean Time) but are when the forecast was released.

3003/3106: Valid from the 30th at 3:00 a.m. Z to the 31st at 6:00 a.m. Z

35030G50KT: When this forecast was issued, the winds were from 350 (north) at 30 knots gusting to 50 knots.

P6SM: visibility is Plus (i.e., greater than) 6 statute miles

-RA: light (-) RAin

OVC015: skies are OVerCast beginning at 1500 feet above the ground

TEMPOrarily, from the 30th at 3:00 a.m. Z for the next two hours, it's going to get even worse. The visibility is going to decrease to only 3 statute miles, some scattered clouds are going to drop down around 800 feet above the ground and then it's going to be solid clouds starting at 1500 feet above the ground.

FRom the 30th at 5:00 a.m. Z, the winds are going to keep on blowing, the rain is going to become moderate instead of light, and it's going to stay cloudy, with the clouds coming down to 600 feet above the ground.

FRom the 30th at 2:00 p.m. Z, the wind is going to stay strong although slightly weaker, the visibility is still going to be cruddy but slightly better, and the rain is going to slightly but not completely let up.

FRom the 30th at 11:00 p.m. Z, ditto the preceding paragraph.

Condensing that forecast into something useful like we did with the METAR, we can see that the whole day is going to blow. Literally. To help you see just how much it blows, here are the surface analysis and weather depiction charts for that time:

Surface analysis for October 30, 2012 at 0300z. Hurricane Sandy stands out as the big L with the closely-spaced isobars surrounding it.

Weather depiction chart for October 30, 2012 at 0400z.

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