Tomorrow's Weather

I'm scheduled to fly tomorrow from 2-4pm - what's the weather?

The Weather Network, which is probably one of the most popular TV stations in Canada, says that tomorrow afternoon there will be thunderstorms, with a 70% probability of precipitation. That's good enough for deciding whether to go to the beach or to a museum, but not good enough for flight planning.

Nav Canada's website is more specific. Unfortunately, aviation weather forecasts are coded as tightly as possible, a throwback to the days of teletypes that worked on 300 baud networks, which is about 30 characters per second. As a comparison, my home's broadband Ethernet connection is about 7,000,000 bits per second. Even dial-up is usually 56,000 bits per second. There is movement afoot to produce aviation weather forecasts in plain text, but change is slow. Heck, pilots still use slide rules.

The current weather:

METAR CYOW 100000Z 14003KT 15SM SCT060 BKN100 18/16 A2983 RMK SC4AC3 DIST SH ESE SLP103=

METAR - Meteorological report (of actual observed conditions)
CYOW - At Ottawa International Airport, Ottawa, Ontario
100000Z - as of 00:00 Zulu on the 10th day of the month (which is August). EDT is 4 hours behind Zulu, so this is the weather report at 8pm EDT
14003KT - Winds are from 140 degrees true, at 3 knots (anytime a direction is written down, it's in degrees true)
15SM - Visibility is 15 statute miles
SCT060 - Scattered clouds at 6000 feet Above Ground Level (AGL). With the SC4 we know that these clouds are strato-cumulus, and they cover 4/8 of the sky
BKN100 - Broken clouds at 10,000 feet AGL. With AC3 we know these are alto-cumulus clouds, and they cover another 3/8 of the sky
18/16 - Temperature is 18 degrees Celsius, dew point is 16 degrees
A2983 - Altimeter setting is 29.83
DIST SH ESE - Even though there is no rain at the observation station, there are distant rain showers observed to the east-south-east
SLP103 - Sea level air pressure is 1010.3 (29.83 inches of mercury is the same as 1010.3 millibars)

Tomorrow's forecast:

TAF CYOW 092338Z 100024 VRB03KT P6SM SCT060 BKN100
TEMPO 0002 P6SM -SHRA BKN050 BKN090
FM0200Z VRB03KT P6SM BKN080
BECMG 0608 09008KT
FM1100Z 10010KT P6SM BKN070
TEMPO 1113 P6SM -SHRA BKN030
FM1300Z 10010KT P6SM SCT020 BKN050
TEMPO 1324 5SM -SHRA BR OVC020
PROB30 1822 2SM TSRA BR OVC020CB
RMK NXT FCST BY 03Z=

TAF - Terminal Area Forecast - the weather forecast for within 5 nautical miles
CYOW - of Ottawa International Airport, Ottawa, Ontario
092338Z - Issued at 23:38 Zulu (19:38 EDT) on the 9th (of August)
100024 - Valid from 00 to 24 (a 24 hour period) on the 10th (of August)
VRB03KT - Winds will be Variable at 3 knots
P6SM - Visibility will be +6 (plus 6) statute miles
SCT060 - Scattered clouds at 6000 feet AGL
BKN100 - Broken clouds at 10,000 feet AGL

TEMPO 0002 - Temporary change from the forecast, from 00 to 02 (8pm to 10pm EDT).
P6SM - Visibility will remain at +6 statute miles
-SHRA - With light rain showers
BKN050 - Broken clouds at 5,000 feet AGL
BKN090 - Broken clouds at 9,000 AGL

Then at 0200Z (10pm EDT) the temporary conditions end, and we revert back to the original TAF (first line) as the basis for the rest of the forecast.

FM0200Z VRB03KT P6SM BKN080
FM (From) is a permanent change. Everything in line 1 (the TAF) remains the same unless specifically modified. From 02:00Z (10pm EDT) onwards, winds remain variable at 3kts, visibility remains at +6SM, but there is now only one layer of cloud, broken at 8,000 feet.

BECMG 0608 09008KT
There will be a gradual change (becoming) over the two hour time period from 06Z to 08Z (2am to 4am EDT) of the winds, from 090 degrees true at 8 knots. 0608 is the time period over which the change occurs, but once it does happen then the change is permanent for the rest of the forecast period (unless subsequently specifically modified). All other weather stays the same.

FM1100Z 10010KT P6SM BKN070
From 1100Z onwards there is another permanent change, the winds will now be from 100 degrees true at 10 knots, the visibility is still +6 statute miles, but the cloud base has descended another 1,000 feet to be broken at 7,000 feet.

TEMPO 1113 P6SM -SHRA BKN030
For the period from 11:00Z to 13:00Z (7am to 9am EDT) there will be light rain showers, and one broken cloud base which is at 3,000 feet ASL

FM1300Z 10010KT P6SM SCT020 BKN050
From 13:00Z onwards there is another permanent change: The wind continues to be from 100 degrees true at 10knots, the visibility is still +6 statute miles, but the cloud bases have changed once again, with one scattered cloud base at 2,000 feet ASL, and another broken cloud base at 5,000 feet ASL

TEMPO 1324 5SM -SHRA BR OVC020
From 13:00Z for the rest of the day there is a temporary change (this far into the future in unsettled weather this likely means temporary periods) of 5 statute miles visibility, light rain showers, mist (BR derives from the french word for mist), and overcast clouds (a solid cloud base) at 2,000 feet ASL

PROB30 1822 2SM TSRA BR OVC020CB
And just to make things interesting, there is a 30% probability, between 18:00Z and 22:00Z (2pm to 6pm EDT) of 2 statute miles visibility, thunderstorms and rain (TSRA), mist (BR), and a solid cloud base at 2,000 feet (OVC020) with embedded cumulonimbus (CB) - which are commonly known as thunderheads.

My scheduled flight time is from 2-4pm - right when there is a 30% chance of thunderstorms. I may not go flying in the afternoon. And it would be a good idea to get the grass cut in the morning.