Snow forecasting Techniques

1. Snow forecasting Techniques

2. Session Objectives • Understanding of the impact of snow on aviation operations • Understanding of the strengths and weaknesses of a variety of snow forecasting techniques • Be able to apply the techniques to a real case study.

3. Impact of snow when aircraft in flight • Poor visibility and low cloud base • Snow ‘packing’ restricting: • airflow into engines • preventing retraction of landing gear • Blocking or Pitot tubes • ‘Wet’ snow (T>0°C) will result in airframe icing.

4. Impact of snow on aircraft at airfield • Poor visibility and low cloud base • Snowfall accumulation on airframe: • Aerodynamics • all up weight of aircraft • windshield obscured • Runway contamination: • degrading braking action. • obscuring runway and runway lights • 1mm of rain = 1cm of snow.

5. Boston Blizzard January 2005

6. Probability of snow 90% 70% 50% 30% 10% Thickness (gpm) 5180 5238 5258 5292 5334 1000 - 500 hPa Thickness Advantages: • Easy to use Disadvantages: • Not necessarily representative of the lowest levels of atmosphere

7. 1000-500hPa thickness chart SNOW PROBABILITY (AMSL): 528.0 DM ≈ 40% 522.0 DM ≈ 80% 516.0 DM ≈ 95% 534.0 DM <10% WHAT IS THE SNOW PROBABILITY AMSL AT POINTS: A? B? C? B A C 30-40% >95% <10%

8. Height of wet-bulb freezing level Probability of snow Mainly Readily turns Mainly Snow snow to snow rain very rare Height of wet-bulb <300 M <600 M 600 M 900 M 0 °C level AGL Advantages: • Easy to use • Takes account of evaporative cooling (though not precipitation intensity) Disadvantages: • Too course in borderline situations Watch for cold surface air undercutting warm air!

9. Wet-bulb freezing level ≈900m AGL Snow unlikely HEIGHT OF WET-BULB FREEZING LEVEL 850 3 5 2 0 C 7 900 950 9 1000 10 0

10. Wet-bulb freezing level ≈600m AGL Rain readily turning to snow HEIGHT OF WET-BULB FREEZING LEVEL 850 3 5 2 0 C 7 900 950 9 1000 10 0

11. Probability of snow 90% 70% 50% 30% 10% Height of 0 °C isotherm AGL (hPa) 12 25 35 45 61 Height of zero degree isotherm Advantages: • Easy to use Disadvantages: • Too coarse in borderline situations • Takes no account of precipitation intensity or evaporative cooling if low level air is dry.

12. 0°C isotherm level ≈110hPa AGL <10% probability of snow HEIGHT OF ZERO DEGREE ISOTHERM 850 3 5 2 0 C 7 900 950 9 1000 10 0

13. 0°C isotherm level ≈45hPa AGL 30% probability of snow HEIGHT OF ZERO DEGREE ISOTHERM 850 3 5 2 0 C 7 900 950 9 1000 10 0

14. Probability of snow 90% 70% 50% 30% 10% Surface temp (°C) 0.3 +1.2 +1.6 +2.3 +3.9 Surface temperature Advantages: • Easy to use Disadvantages: • Takes no account of warm air aloft • Takes no account of precipitation intensity

15. Surface temperature 90% 70% 50% 30% 10% 0.3 +1.2 +1.6 +2.3 +3.9 WHAT IS THE SNOW PROBABILITY AMSL AT POINTS: A? B? C? aa 70% C 40% 20% A B

16. 90% 70% 50% 30% 10% C 1281 1290 1293 1298 1303 Boyden’s Snow Forecasting Technique Where: C is the corrected value of the 1000–850 hPa thickness (gpm) A is the actual 1000–850 hPa thickness (gpm) H1000 is the height of the 1000 hPa surface AMSL HGR is the height of the station AMSL Probability of snow

17. Boyden’s Snow Forecasting Table

18. 90% 70% 50% 30% 10% C 1281 1290 1293 1298 1303 Boyden’s Snow Forecasting Technique Example 1000-850: 128.7DM MSLP: 992hPa HGR: 100M 1hPa ≈ 10m C = 1287 + (-80-100)/30 = 1287 – 180/3 = 1287 – 6 = 1281 = 90%

19. Boyden’s Snow Forecasting Table

20. Boyden’s Snow Forecasting Technique Advantages: • Samples crucial low levels of atmosphere • Gives precise values Disadvantages: • Inaccurate if there is a cold or warm undercut near surface • Takes no account of precipitation intensity

21. DRY Low level air temperature initially above freezing Snow falls into the lower levels of this atmosphere Snow falling into a layer with an above freezing temperature melts and may evaporate if layer is unsaturated Large amounts of latent heat required 3 5 2 Melting starts as 0°C isotherm is reached 850 7 9 1000 RAIN TURNING TO SNOW AT SURFACE 0 C 10 0

22. Cooling occurs as snow melts just below this level Temperature profile changes Profile starts to follow the 0°C isotherm down toward the surface Dew point increases slightly 3 5 2 850 7 9 1000 10 0 RAIN TURNING TO SNOW AT SURFACE 0 C

23. Rough guide 1hr continuous melting snow - 600 feet of isothermal 4hrs continuous melting snow - 1200 feet of isothermal Rain increasingly turning to snow at surface RAIN TURNING TO SNOW AT SURFACE 3 5 2 0 C 850 7 9 1000 10 0

24. If melting snow is of heavy intensity then isotherm can reach surface LESSON: In borderline snow situations, if precipitation is forecast to be heavy and prolonged, forecast snow. RAIN TURNING TO SNOW AT SURFACE 3 5 2 0 C 850 7 9 1000 10 0

25. Summary • Each snow forecasting technique has strengths and weaknesses • Crucial forecasting points: • Temperature and humidity of the lowest 1500M of the atmosphere • Intensity and duration of precipitation • Height of airfield • Small changes in 1 to 3 above can lead to big forecast errors • Each technique is a probability forecasting assuming that precipitation is occurring • If it is dry then probability of snow = 0 no matter how cold it is!

26. Percentage probability of snow

27. Any questions?

28. UK, 25th November 2005 Snow case study Newquay Airport

29. Scenario • Please write down the following: • Newquay Airport: EGDG • 51°N 05°W • Height 150M • You will be given snow forecasting information for 0300Z, 0900Z and 1500Z • Calculate the snow probability using the techniques taught this morning • Use the tephigrams to forecast the intensity of of any precipitation eg TEMPO +SHRA.

30. Time is now 250300Z 1000-850: 129.9DM 1000-500: 525.6 MSLP: 1006hPa T: +4.0°C TEMPO -SHRA Probability of snow 90% 70% 50% 30% 10% 1000-500hPa Thickness (gpm) 5180 5238 5258 5292 5334 Probability of snow Mainly snow Rain turns to snow Mainly rain Snow rare Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M Probability of snow 90% 70% 50% 30% 10% Height of 0 °C isotherm AGL (hPa) 12 25 35 45 61 Probability of snow 90% 70% 50% 30% 10% Surface temp (°C) 0.3 +1.2 +1.6 +2.3 +3.9 Prob of snow 90% 70% 50% 30% 10% Boyden C 1281 1290 1293 1298 1303 50% 300m 60hPa <10% 1296

31. Time is now 250900Z 1000-850: 130.1DM 1000-500: 519.4 MSLP: 1000hPa T: +4.0°C TEMPO SHRASN Probability of snow 90% 70% 50% 30% 10% 1000-500hPa Thickness (gpm) 5180 5238 5258 5292 5334 Probability of snow Mainly snow Rain turns to snow Mainly rain Snow rare Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M Probability of snow 90% 70% 50% 30% 10% Height of 0 °C isotherm AGL (hPa) 12 25 35 45 61 Probability of snow 90% 70% 50% 30% 10% Surface temp (°C) 0.3 +1.2 +1.6 +2.3 +3.9 Prob of snow 90% 70% 50% 30% 10% Boyden C 1281 1290 1293 1298 1303 80% <300m 35hPa <10% 1296

32. Time is now 251500Z 1000-850: 128.3DM 1000-500: 515.9 MSLP: 995hPa T: +0.0°C TEMPO +SHSN Probability of snow 90% 70% 50% 30% 10% 1000-500hPa Thickness (gpm) 5180 5238 5258 5292 5334 Probability of snow Mainly snow Rain turns to snow Mainly rain Snow rare Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M Probability of snow 90% 70% 50% 30% 10% Height of 0 °C isotherm AGL (hPa) 12 25 35 45 61 Probability of snow 90% 70% 50% 30% 10% Surface temp (°C) 0.3 +1.2 +1.6 +2.3 +3.9 Prob of snow 90% 70% 50% 30% 10% Boyden C 1281 1290 1293 1298 1303 >90% <300m 12hPa 90% 1276

33. Now lets’ see what really happened!