Low IFR in Roanoke

Roanoke Regional Airport/Woodrum Field (KROA) was featured in a recent issue of IFR Refresher because of it's LDA approaches and rugged surrounds. That LDA may come in handy, but perhaps not handy enough this morning. Currently KROA has low IFR (with a model index of 71) with winds from 150 at 4 knots, visibility 1/2 miles in light rain and fog. Overcast layer at 100. Winds favor runway 15, but the LDA is for Runway 6 and the only ILS approach is for Runwnay 33. Current pressure and density altitudes are 1175 and 4220 feet, respectively.

METAR:
KROA 041354Z 15004KT 1/2SM -RA FG OVC001 13/13 A2992

TAF:
2009/05/04 14:16
KROA 041416Z 0414/0512 15005KT 1/2SM FG OVC001
FM041500 12005KT 2SM -SHRA OVC005
FM041600 24006KT P6SM VCSH SCT012 OVC025
FM042100 27010KT P6SM BKN035
FM050100 31007KT P6SM BKN025
FM050800 VRB04KT 6SM BR OVC007

Champaign-Urbana

This morning, University of Illinois/Willard Airport is reporting low IFR conditions with a model index of 72:

KCMI 301245Z 14014KT 1 1/4SM -RA BR BKN003 BKN015 OVC095 17/16 A2991

The winds are from 140 at 14 knots, visibility 1.25 miles in light rain and mist. Broken layer at 300, broken 1500, overcast 9500. Temperature 62.6*F, dewpoint 60.8*F, relative humidity is 94 percent. The barometric pressure is 29.91

Expect Runway 18 - pressure altitude is 794, and density altitude is 4531

METAR Decoding

Here is the current base METAR for San Antonio:
KSAT 281253Z 08006KT 1/8SM R12R/2600V3000FT -RA BR VV001 19/18 A3013

The wxJabber model index is 78 and conditions are LIFR. The METAR is decoded: San Antonio reporting winds from 080 at 6 knots, visibility 0.125 miles, and vertical visibility at 100 feet in light rain and mist. The RVR for Runway 12R is variable between 2600 and 3000 feet. The temperature 66.2*F, dewpoint 64.4*F, relative humidity is 94 percent. The barometric pressure is 30.13.

Historical data reveals that conditions at San Antonio have been LIFR since at least 3:00 A.M., and the TAF indicates soggy conditions for the rest of the day:

http://flightaware.com/live/airport/KSAT/wx

Snowy IFR

Fresh snow! The first big snows of the year have hit the north central and northeast states this week, and have brought with them the usual obscuration that accompanies any precipitation. According to Thomas Sullivan (Northeast Weather Blog), a very January-like pattern has set up for the Northeast this week and there will be an enhancement of snow showers followed by a reinforcing shot of cold air and a reorganization of the lake effect activity.

This morning, 4F dispatchers issued a snowy IFR flight from Wilkes-Barre in Pennsylvania to Nantucket Memorial Airport in Massachusetts. For departure, you can lift off VFR because Wilkes-Barre/Scranton is reporting winds 350 at 10 knots, visibility 10 miles. Scattered clouds at 3000, broken 3700. Temperature 27F, dewpoint 12F - the wind chill is 16F.


KACK 191453Z 36022G31KT 2SM -SN BR SCT014 BKN022 OVC045 00/M02 A2991

As you can see above the METAR indicates that sometime enroute you'll want to pick up that IFR clearance. Nantucket is reporting winds 360 at 22 knots, gusting to 31 knots, visibility 2 miles in light snow and mist. Scattered clouds at 1400, broken 2200, overcast 4500. The temperature dewpoint spread is a little tighter too - with temperature 32F, dewpoint 28F, and relative humidity at 86 percent. Snow is likely to continue to fall until this afternoon as the skies clear, but look for gusty winds to continue through early evening.

Decoding Lesson #1

Austin-Bergstrom International Airport (KAUS) is located in the capitol of Texas. Austin is typically a relatively dry place compared to most of the state to the south, yet conditions can be stormy, or as is the case today, foggy! Here's the current surface observation:

KAUS 221253Z 00000KT 1/4SM R17L/1200V1600FT FG OVC001 17/17 A3000

At 1253Z (7:53 AM CDT) winds were calm (00000KT) and visibility was a quarter of a mile (1/4SM). At this hour Austin is also reporting Runway Visual Range (RVR). The code follows the visibility token because RVR is calculated from visibility, ambient light level, and runway light intensity, and is the maximum distance at which the runway, or the runway lights can be seen.

In this case, the RVR is for Runway 17L, the only runway at Austin that has CAT II/III approaches. The 1200V1600FT part of the RVR token in this METAR indicates that the runway is visibile from between 1200 to 1600 feet (V=variable).

If you were wondering what was causing the poor visibility in the Lone Star State's capitol this morning, just refer to the next token: "FG" - it's the symbol used for fog. Also, the ceiling is reported overcast at 100 feet above ground level.

Finally, the temperature-dew point spread definitely indicates fog. Fog is probable when the temperature-dew point spread is 10°C (50°F) or less and decreasing. Fog usually forms when the dew point and the temperature are within a few degrees of each other, and will start to lift when the temperature-dew point spread begins to increase.

Finally, the surface pressure is reported at 30.00 inches of Mercury (A3000).

I'm listening to Austin Departure and they are reporting that the RVR is currently 6000 feet, so it sounds like conditions are improving - likely that temperature-dew point spread is helping out.

Lots of Abbreviations But Only One Format

In 1996, with the beginning of airline deregulation, the U.S. started using a new format to disseminate current weather and forecasts. Prior to that time the U.S. (and Canada and Mexico) used Surface Aviation Observations (SAO) for current conditions and Terminal Forecasts (TF), while the rest of the world used METARS and TAFS. METAR is an abbrevaition of the French phrase "message d’observation météorologique régulière pour l’aviation", literally "aviation weather observation". A typical METAR includes the date and time the report was sent (in UTC), the winds, could cover and height above ground, temperature and dew point, and barometric pressure. For example, the METAR for this hour for Pontiac, Michigan is:

KPTK 202349Z 29008KT 3SM -RA BR OVC008 11/09 A3008

The four-letter identifer is an ICAO-format desginator for Oakland-Pontiac Airport. 202349Z is interpreted as the 20th day of the current month at 2349 Zulu (Zulu is slang for UTC 'Universal Time Coordinated' or GMT 'Greenwich Mean Time'). The wind direction is presented followdd by the wind speed; here's it's reported as coming from 290 degrees at 8 knots. The next token is the visibility - here's it's 3 statute miles. RA means rain, but the minus in front mean light rain (and +RA means heavy rain). BR is a token for "mist". The cloud cover is overcast (OVC) at 800 feet above ground level (AGL). The temerature is 11 degrees Celsius, and the dewpoint is 9 degrees Celsius - not much of a spread and perhaps indicative of potential icing conditions. The final token "A3008" is interprested as "Altimieter 30.08 inches of Mercury - the surface pressure reading.

As you can see, reading METARS requires a good understanding of the conventions used to report surface conditions using the METAR format. This format, used all over the world now with only minor changes, as spelled out by exceptions for only a handful of airports, makes it easier for pilots to read the current weather at almost any station in the world.