Page 1© Crown copyright Snow forecasting Techniques

© Crown copyright Page 1

Snow forecasting Techniques


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.


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.


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.


Boston Blizzard January 2005


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


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?

A

B

C

30-40%

>95%

<10%


Probability of snow Mainly Readily turns Mainly Snowsnow to snow rain very rare

Height of wet-bulb <300 M <600 M 600 M 900 M 0 °C level AGL

Height of wet-bulb freezing level

Watch for cold surface air undercutting warm air!

Advantages: Easy to use Takes account of evaporative

cooling (though not precipitation intensity)

Disadvantages: Too course in borderline situations


HEIGHT OF WET-BULB FREEZING LEVEL

0 10

850

1000

2 3 5

79

Wet-bulb freezing level ≈900m AGL

Snow unlikely

0 C950

900


HEIGHT OF WET-BULB FREEZING LEVEL

0 10

850

1000

2 3 5

79

Wet-bulb freezing level ≈600m AGL

Rain readily turning to snow

0 C950

900


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


Disadvantages: Too coarse in borderline situations Takes no account of precipitation

intensity or evaporative cooling if low level air is dry.


HEIGHT OF ZERO DEGREE ISOTHERM

0 10

850

1000

2 3 5

79

0°C isotherm level ≈110hPa AGL

<10% probability of snow

0 C950

900


HEIGHT OF ZERO DEGREE ISOTHERM

0 10

850

1000

2 3 5

79

0 C950

900

0°C isotherm level ≈45hPa AGL

30% probability of snow


Probability of snow 90% 70% 50% 30% 10%Surface temp (°C) 0.3 +1.2 +1.6 +2.3 +3.9

Surface temperature


Disadvantages: Takes no account of warm air aloft Takes no account of precipitation

intensity


1 0

1 0

5

5

0

0

5 0

5 05 0

5 55 55 5

059999M02

1010

SCT050FEW035

039999

00

1006

BKN080FEW020

7000SHRA

00

M05

999

CAVOK

00

M04

997

CAVOK

M019999M01

1004

FEW012

02

M02

1005

CAVOK

004000M01

998

-SHGSBKN025CB

SCT010

9999 G33

009999M06

999G23

FEW040

019999M01

1016G29

BKN100SCT045

BKN016CB

059999

02

986

BKN015CB

002000

00

1009

SHSNSCT010

00

M05

1002

CAVOK

00

M05

1003

CAVOK

01

M05

1001

CAVOK

046000M03

1005G26-SHGS

FEW025

009999M03

1005

FEW022

2800 G49-SHRA

00

M06

1000

CAVOK

029999M02

994

OVC050SCT025

029999M01

1015

FEW015

009999M01

1011

-SHSNBKN050SCT025FEW012

039000

00

1012G26SHRAGR

BKN023CBSCT017CB

02

M02

1006

CAVOK

9999

9999

9999 A

009999M04

1003

FEW045

019999M03

1004

FEW030

00

M05

996G33

CAVOK

029999

00

1016

SCT060FEW018CB

049999M01

996G35

BKN033FEW024

9999

3200-SN

009999M05

1002

FEW030

029999

00

1008

VCSHSCT025TCU

FEW010

9999 A

9999 A

01

M05

1000

CAVOK

9999 G35

Surface temperature

aa

WHAT IS THE SNOW PROBABILITY AMSL AT POINTS:

A?

B?

C?

A

B

C

90% 70% 50% 30% 10%0.3 +1.2 +1.6 +2.3 +3.9

70%

40%

20%


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 AMSLProbability of snow

90% 70% 50% 30% 10%C 1281 1290 1293 1298 1303

30

)( 1000 GRHHAC


Boyden’s Snow Forecasting Table


Example


1000-850: 128.7DM MSLP: 992hPa HGR: 100M

1hPa ≈ 10m

C = 1287 + (-80-100)/30 = 1287 – 180/3 = 1287 – 6 = 1281

= 90% 90% 70% 50% 30% 10%C 1281 1290 1293 1298 1303


Boyden’s Snow Forecasting Table



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


RAIN TURNING TO SNOW AT SURFACE

0 10

850

1000

2 3 5

79

Melting starts as 0°C isotherm is reached

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

0 C


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 slightly0 10

850

1000

2 3 5

79

0 C



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

0 10

850

1000

2 3 5

79

0 C



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.

0 10

850

1000

2 3 5

79

0 C



Summary

Each snow forecasting technique has strengths and weaknesses

Crucial forecasting points:1) Temperature and humidity of the lowest 1500M of

the atmosphere2) Intensity and duration of precipitation3) 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!


Percentage probability of snow

TECHNIQUE 90% 70% 50% 30% 10%

Ht of 0°C isotherm hPa 12 25 35 45 61

Based on 900 hPa 108m 225m 315m 405m 550m

Surface temperature -0.3°C 1.2°C 1.6°C 2.3°C 3.9°C

Ht of 0°C wet-bulb temp <250m 370m 600m 750m 900m

500-1000 hPa thickness 5180m 5238m 5258m 5292m 5334m


Any questions?

Snow case studyUK, 25th November 2005

10

10

5

5

0

0

50

5050

555555

059999M02

1010

SCT050FEW035

039999

00

1006

BKN080FEW020

7000SHRA

00

M05

999

CAVOK

00

M04

997

CAVOK

M019999M01

1004

FEW012

02

M02

1005

CAVOK

004000M01

998

-SHGSBKN025CB

SCT010

9999 G33

009999M06

999G23

FEW040

019999M01

1016G29

BKN100SCT045

BKN016CB

059999

02

986

BKN015CB

002000

00

1009

SHSNSCT010

00

M05

1002

CAVOK

00

M05

1003

CAVOK

01

M05

1001

CAVOK

046000M03

1005G26-SHGS

FEW025

009999M03

1005

FEW022

2800 G49-SHRA

00

M06

1000

CAVOK

029999M02

994

OVC050SCT025

029999M01

1015

FEW015

009999M01

1011

-SHSNBKN050SCT025FEW012

039000

00

1012G26SHRAGR

BKN023CBSCT017CB

02

M02

1006

CAVOK

9999

9999

9999 A

009999M04

1003

FEW045

019999M03

1004

FEW030

00

M05

996G33

CAVOK

029999

00

1016

SCT060FEW018CB

049999M01

996G35

BKN033FEW024

9999

3200-SN

009999M05

1002

FEW030

029999

00

1008

VCSHSCT025TCU

FEW010

9999 A

9999 A

01

M05

1000

CAVOK

9999 G35

Newquay Airport


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.

1000-850: 129.9DM

1000-500: 525.6

MSLP: 1006hPa

T: +4.0°C

Time is now 250300Z

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 rareHeight of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M



Prob of snow 90% 70% 50% 30% 10%Boyden C 1281 1290 1293 1298 1303 30

)( 1000 GRHHAC

50%

60hPa

300m

<10%

1296

TEMPO -SHRA

Time is now 250900Z

30

)( 1000 GRHHAC





Prob of snow 90% 70% 50% 30% 10%Boyden C 1281 1290 1293 1298 1303

1000-850: 130.1DM

1000-500: 519.4

MSLP: 1000hPa

T: +4.0°C

80%

35hPa

<300m

<10%

1296

TEMPO SHRASN

Time is now 251500Z

30

)( 1000 GRHHAC





Prob of snow 90% 70% 50% 30% 10%Boyden C 1281 1290 1293 1298 1303

1000-850: 128.3DM

1000-500: 515.9

MSLP: 995hPa

T: +0.0°C

>90%

12hPa

<300m

90%

1276

TEMPO +SHSN

Now lets’ see what really happened!

Documents

Page 1© Crown copyright Snow forecasting Techniques