Transcript Mid-latitude Cyclones and Weather Forecasting

Mid-latitude Cyclones and
Weather Forecasting
AT351
Lab 9
March 26, 2008
Mid-latitude (Extratropical) Cyclone
 A cyclone (area of low pressure) in the middle
(35°-70°) latitudes
 Important for global heat transport
 Help to redistribute energy between the tropics
(equator) and the poles
 Often associated with significant weather events
 Described by the Polar Front Theory
 Form on boundaries between warm and cold air
 Polar front is the boundary between polar cell and
Ferrel cell
 Cold Polar air meeting warm tropical air
Features of a Mid-latitude Cyclone
 Deep low pressure area with attached cold and warm
fronts
 Often an occlusion forms, the triple point lending to the
formation of severe weather
 Precipitation associated with the cold and warm fronts
organizes in typical “comma cloud” structure
Stages in Wave Cyclone
Development
Polar Front -->
Polar Front Theory
 Initially, there is a stationary front that acts as the
boundary separating cold, continental polar air from
warm, maritime tropical air
 Winds blow parallel to this front on either side
 Flow around the highs
Central Pressure
Cyclogenesis
 A wave forms on the front due to a shortwave
disturbance
 Frontal Wave
 Incipient Cyclone
 The front develops a "kink" where the wave is
developing
 Precipitation will begin to develop along the front
 Overrunning and lifting
Strengthening
 The cyclonic circulation around the low becomes more
defined
 The central pressure intensifies
 The cold front and warm front have more organized
motion
 Cyclone usually pushed east or northeast by the winds
aloft
Mature Cyclone
 The cold front catches up with the warm front and an
occlusion forms
 Where does the energy to do all of this come from?
 The cyclone is at its strongest at this point
 Severe weather often develops near the “triple point”
 Note: Precipitation in 3 locations
Dissipation
 The occlusion grows with time
 Eventually, the occlusion is so great that the supply of
warm, moist air into the low is cut off
 Cold air on both sides
 When this happens, the system starts to dissipate
Family of Cyclones
Strengthening
 A low pressure area’s strength is defined
by how low the pressure is
 Hence, all a low needs to strengthen is for
its pressure to drop in the center
 How might that happen?
Strengthening Low
 We know that for the pressure to drop, air
must leave the column
 It isn’t leaving at the surface, because the
wind is spiraling inward
 Therefore, it must leave at the upper levels
 Rising motion will cause this
What is this showing?
What do the dashed lines represent?
Long Waves vs Short Waves
 Uneven heating causes troughs and ridges to form
around the globe
 An almost constant ring of trough/ridge patterns exist,
called longwaves
 The Polar Vortex
 Waves usually move west-east, but can sometimes
appear to move westward
 Called retrograde motion
 Within these longwaves, disturbances exist called
shortwaves
 Shortwaves deepen in longwave troughs
 Shortwaves weaken in longwave ridges
 Shortwaves move faster (generally)
Long Waves
Short Waves
Interaction with Upper Levels
 Previous model for cyclone development only includes
surface characteristics – but what happens higher up
can determine what happens below
 Remember this picture?
 Turns out that divergence aloft can help to remove mass
from a column, hence lowering the surface pressure
even more
Interaction with Upper Levels
 Downstream of an upper level trough, the air
tends to diverge
 If a surface low is located slightly downstream of
an upper level trough, the divergence will be
located above the low and help to intensify it
Mid-latitude Cyclones :
The Upper Level
 A 500 mb trough moves into place directly above a
surface stationary front
 If a shortwave trough moves into the main flow, the
flow pattern is disturbed
 As the 500 mb trough deepens, the associated upper
level divergence strengthens, helping to intensify the
surface low
 Stronger winds aloft force the upper level trough to move
eastward faster, and eventually it becomes located
above the surface low
 When the surface and upper level low are “stacked”,
convergence at both levels starts to “fill” the low pressure
area, weakening the cyclone
Another Example – Jet Streaks
 A jet streak is an area within a jet stream that
has the highest wind speeds
 Typically jet streaks are thought of as made up
of four quadrants – separated by left and right,
entrance and exit
 The right entrance region and the left exit region
both contain divergence (aloft) and so they
promote the development of surface low
pressure systems
 Due to shifts in the Coriolis Force brought about by
acceleration of the wind
Weather Forecasting - Qualitative
 Most information for a qualitative forecast can be seen
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on a simple weather map
If you see a low pressure area that has been moving
eastward towards Colorado, what type of weather might
you expect?
If a cold front is moving southward through Wyoming,
what would you expect the temperature to do?
If it’s going to be cloudy tomorrow, will it be warmer or
colder than it was today?
Often the best forecast is persistence: if it’s warm and
sunny today, and it was warm and sunny yesterday, the
odds are pretty good that it will be warm and sunny
tomorrow (unless you know something else)
Weather Forecasting - Quantitative
 In order to predict specific quantities in a
forecast (temperature, humidity, rainfall) we rely
on computer models
 Numerical weather prediction (NWP) uses a
system of equations that describes the behavior
of the atmosphere
 NWP model uses the current state of the
atmosphere as its initial condition and steps
through a small time step, recalculating every
number for each step until the forecast time is
reached
Initialization
 A computer model has a set of equations in it
that dictate what will happen based on what we
already know
 Takes the actual initial conditions and applies
the equations to create a picture of the weather
5 minutes from now
 Calls that prediction the “new data” and runs
another 5 minutes
 Repeats until you get the desired forecast (12
hours, 24 hours, etc)
MOS
 The output from a model can be shown in the
form of Model Output Statistics (MOS)
 MOS is a summary of the predicted condition of
the atmosphere at each forecast time
 MOS is often called “model guidance”, because
forecasters will use the generated numbers as a
guide to make their forecast
 MOS isn’t perfect though, and forecasting takes
some intuition
What models produce output for
use in MOS?
See UCAR website.
 The Eta Model: runs forward 84 hours
 The GFS (Global Forecasting System) model runs
forward for 10 days.
 Combines the old “Aviation” and “Medium-Range Forecast”
(MRF) models
 The NGM (Nested Grid Model): an older model that is
largely replaced by the Eta although it has a high quality
climatological dataset for use in MOS.
The premise behind MOS
 The (preceding) models produce output describing the
weather over No. America and vicinity at grid points or in
“wave space”.
 Conditions for this time of year at a specific city may
have occurred similarly in the historic past (i.e., old
climate data are used).
 Forecasts of current conditions can be made for a city
using the current model output and based on the historic
weather outcomes.
Downfalls of MOS
 There isn’t just one model that is used for NWP
 Multiple models are used that have differences
in resolution and in the equations used and
assumptions made
 The models never agree on everything
 A good forecaster will look at multiple model
predictions and have a feel for which model
performs the best under certain circumstances
So why aren’t forecasts always right?
 Observations aren’t good enough!
 A model is only as good as its initial conditions
 Even having an observation for every square meter of the planet would leave out
smaller details…and we don’t have even close to that many observations
 Computers aren’t fast enough!
 In order to truly create a perfect forecast, one would have to use the exact
equations on a really, really small spatial scale
 In order to create a model that will create a 12 hour forecast in less than 12
hours, we must approximate certain parts of equations and run the model on a
grid with spacing of multiple kilometers
 ~1.2 million grid points x thousands of calculations > 1 Billion calculations per time step
 More than 1 Quadrillion calculations per time step at 1 meter resolution (1012)
 More than 4 Quadrillion calculations to simulate 12 hours
 Forces us to make approximations to the equations that govern the atmosphere
 Little bit of error added at each time step
 Chaos reigns supreme!
 Ever heard of the butterfly effect? It’s more than just a movie
Chaos Theory
 Also known as the “Butterfly Effect”, coined by Dr.
Edward Lorenz in the 60’s (he’s still teaching at MIT)
 Basically says that even the most insignificant change to
initial conditions will magnify into drastic changes
 The smallest disturbance will eventually grow into a large difference –
this limits the range of forecasts to just a few days
 Even if the initial conditions and computing power were
perfect, chaos theory would limit us to a reasonable
range of about 2 weeks
 Computers cannot possibly predict the movement of a
butterfly, or how hard you step on the gas pedal
So How far have we come?
 100 years ago we just looked to the west.
 The first attempt at numerical weather prediction
by Lewis Fry Richardson was done by hand.
 On a WWI battlefield as part of an ambulance unit in
Northern France
 Just wanted to predict the pressure change over the
next six hours
 Calculation took him six weeks!
 And he was off by over 140 mb…
Richardson’s Forecast Factory
How far have we come?
 In 1937, the US started using weather balloons.
 It wasn’t until WWII that the existence of a jet stream
was confirmed.
 In 1948, ENIAC was put together by John von Neumann
in a 30 by 50 room.
 In April 1950, the first 24-hr forecast was attempted.
 Took more than 24 hours due to breakdowns
 8 years later, forecasts began to show signs of skill.
 As it stands today, we can have a great deal of faith in a
weather forecast out to about 3 days.
 Just 30 years ago, we could only do 2 days
 Beyond that, the accuracy drops dramatically
 If you see a 15 day forecast…don’t believe it
Improvements
 As computers improve and we can run better
models, we may be able to extend the range to
4 or 5 days in our lifetimes
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Better numerical techniques
Fewer approximations
Better measurements
Better understanding of interactions with other
systems
 However, because of chaos, and that sensitive
dependence on initial conditions it is unlikely that
we will ever go much farther than that
Station Name – Artesia, NM
Date and Time the Model was run
Decoding MOS – Max and Min
Temperature
KMIA
AVN MOS GUIDANCE
DT /SEPT 4
/SEPT 5
HR
12 15 18 21 00 03 06
X/N
89
TMP 79 85 86 84 81 80 79
DPT 77 76 75 75 75 76 76
CLD BK BK BK OV OV OV OV
WDR 00 19 17 17 14 13 16
WSP 00 01 04 03 01 01 02
P06
20
44
25
P12
45
Q06
0
2
0
Q12
1
T06
33/ 0 64/ 0 21/ 0
T12
66/ 0
CIG
7 7 7 6 6 7 7
VIS
7 7 7 7 7 7 7
OBV
N N N N N N N
9/04/2003
09 12
77
79 80
76 77
OV OV
17 20
05 03
20
30
0
1
19/ 0
7
7
N
15 18
84
76
OV
23
09
42/
42/
7 6
7 7
N N
85
75
OV
22
09
36
2
0
0
4
7
N
0600 UTC
/SEPT 6
21 00 03 06
89
83 80 79 77
74 74 75 75
OV OV OV OV
24 24 23 24
08 04 04 04
55
46
66
4
4
4
80/ 0 37/ 0
93/ 0
5 4 4 4
7 7 7 7
N N N N
09 12
76
77 79
75 76
OV OV
21 22
06 06
44
68
2
5
37/ 0
5
7
N
15 18
85
75
OV
25
11
39/
58/
6 5
7 7
N N
88
74
OV
25
11
35
4
0
0
5
7
N
/
00 06
91
82 78
74 74
OV BK
26 24
06 05
30 15
54
0 0
4
22/ 0
66/ 0
7 7
7 7
N N
Decoding MOS – 3 hourly
temperature forecasts
KMIA
AVN MOS GUIDANCE
DT /SEPT 4
/SEPT 5
HR
12 15 18 21 00 03 06
X/N
89
TMP 79 85 86 84 81 80 79
DPT 77 76 75 75 75 76 76
CLD BK BK BK OV OV OV OV
WDR 00 19 17 17 14 13 16
WSP 00 01 04 03 01 01 02
P06
20
44
25
P12
45
Q06
0
2
0
Q12
1
T06
33/ 0 64/ 0 21/ 0
T12
66/ 0
CIG
7 7 7 6 6 7 7
VIS
7 7 7 7 7 7 7
OBV
N N N N N N N
9/04/2003
09 12
77
79 80
76 77
OV OV
17 20
05 03
20
30
0
1
19/ 0
7
7
N
15 18
84
76
OV
23
09
42/
42/
7 6
7 7
N N
85
75
OV
22
09
36
2
0
0
4
7
N
0600 UTC
/SEPT 6
21 00 03 06
89
83 80 79 77
74 74 75 75
OV OV OV OV
24 24 23 24
08 04 04 04
55
46
66
4
4
4
80/ 0 37/ 0
93/ 0
5 4 4 4
7 7 7 7
N N N N
09 12
76
77 79
75 76
OV OV
21 22
06 06
44
68
2
5
37/ 0
5
7
N
15 18
85
75
OV
25
11
39/
58/
6 5
7 7
N N
88
74
OV
25
11
35
4
0
0
5
7
N
/
00 06
91
82 78
74 74
OV BK
26 24
06 05
30 15
54
0 0
4
22/ 0
66/ 0
7 7
7 7
N N
Decoding MOS – Cloud Cover
KMIA
AVN MOS GUIDANCE
DT /SEPT 4
/SEPT 5
HR
12 15 18 21 00 03 06
X/N
89
TMP 79 85 86 84 81 80 79
DPT 77 76 75 75 75 76 76
CLD BK BK BK OV OV OV OV
WDR 00 19 17 17 14 13 16
WSP 00 01 04 03 01 01 02
P06
20
44
25
P12
45
Q06
0
2
0
Q12
1
T06
33/ 0 64/ 0 21/ 0
T12
66/ 0
CIG
7 7 7 6 6 7 7
VIS
7 7 7 7 7 7 7
OBV
N N N N N N N
9/04/2003
09 12
77
79 80
76 77
OV OV
17 20
05 03
20
30
0
1
19/ 0
7
7
N
15 18
84
76
OV
23
09
42/
42/
7 6
7 7
N N
85
75
OV
22
09
36
2
0
0
4
7
N
0600 UTC
/SEPT 6
21 00 03 06
89
83 80 79 77
74 74 75 75
OV OV OV OV
24 24 23 24
08 04 04 04
55
46
66
4
4
4
80/ 0 37/ 0
93/ 0
5 4 4 4
7 7 7 7
N N N N
09 12
76
77 79
75 76
OV OV
21 22
06 06
44
68
2
5
37/ 0
5
7
N
15 18
85
75
OV
25
11
39/
58/
6 5
7 7
N N
88
74
OV
25
11
35
4
0
0
5
7
N
/
00 06
91
82 78
74 74
OV BK
26 24
06 05
30 15
54
0 0
4
22/ 0
66/ 0
7 7
7 7
N N
Decoding MOS - Cloud Cover
 CL = Clear, no clouds
 SC = Scattered, between clear and the
sky ½ filled with clouds
 BK = Broken, between sky ½ filled with
clouds and totally cloudy
 OV = Overcast, sky filled with clouds
Decoding MOS – Winds
KMIA
AVN MOS GUIDANCE
DT /SEPT 4
/SEPT 5
HR
12 15 18 21 00 03 06
X/N
89
TMP 79 85 86 84 81 80 79
DPT 77 76 75 75 75 76 76
CLD BK BK BK OV OV OV OV
WDR 00 19 17 17 14 13 16
WSP 00 01 04 03 01 01 02
P06
20
44
25
P12
45
Q06
0
2
0
Q12
1
T06
33/ 0 64/ 0 21/ 0
T12
66/ 0
CIG
7 7 7 6 6 7 7
VIS
7 7 7 7 7 7 7
OBV
N N N N N N N
9/04/2003
09 12
77
79 80
76 77
OV OV
17 20
05 03
20
30
0
1
19/ 0
7
7
N
15 18
84
76
OV
23
09
42/
42/
7 6
7 7
N N
85
75
OV
22
09
36
2
0
0
4
7
N
0600 UTC
/SEPT 6
21 00 03 06
89
83 80 79 77
74 74 75 75
OV OV OV OV
24 24 23 24
08 04 04 04
55
46
66
4
4
4
80/ 0 37/ 0
93/ 0
5 4 4 4
7 7 7 7
N N N N
09 12
76
77 79
75 76
OV OV
21 22
06 06
44
68
2
5
37/ 0
5
7
N
15 18
85
75
OV
25
11
39/
58/
6 5
7 7
N N
88
74
OV
25
11
35
4
0
0
5
7
N
/
00 06
91
82 78
74 74
OV BK
26 24
06 05
30 15
54
0 0
4
22/ 0
66/ 0
7 7
7 7
N N
Meteorology Wind Direction
180 = From
the South
090 = From
the East
270 = From
the West
000 = From
the North
Decoding MOS – Chance of
measurable precipitation
KMIA
AVN MOS GUIDANCE
DT /SEPT 4
/SEPT 5
HR
12 15 18 21 00 03 06
X/N
89
TMP 79 85 86 84 81 80 79
DPT 77 76 75 75 75 76 76
CLD BK BK BK OV OV OV OV
WDR 00 19 17 17 14 13 16
WSP 00 01 04 03 01 01 02
P06
20
44
25
P12
45
Q06
0
2
0
Q12
1
T06
33/ 0 64/ 0 21/ 0
T12
66/ 0
CIG
7 7 7 6 6 7 7
VIS
7 7 7 7 7 7 7
OBV
N N N N N N N
9/04/2003
09 12
77
79 80
76 77
OV OV
17 20
05 03
20
30
0
1
19/ 0
7
7
N
15 18
84
76
OV
23
09
42/
42/
7 6
7 7
N N
85
75
OV
22
09
36
2
0
0
4
7
N
0600 UTC
/SEPT 6
21 00 03 06
89
83 80 79 77
74 74 75 75
OV OV OV OV
24 24 23 24
08 04 04 04
55
46
66
4
4
4
80/ 0 37/ 0
93/ 0
5 4 4 4
7 7 7 7
N N N N
09 12
76
77 79
75 76
OV OV
21 22
06 06
44
68
2
5
37/ 0
5
7
N
15 18
85
75
OV
25
11
39/
58/
6 5
7 7
N N
88
74
OV
25
11
35
4
0
0
5
7
N
/
00 06
91
82 78
74 74
OV BK
26 24
06 05
30 15
54
0 0
4
22/ 0
66/ 0
7 7
7 7
N N
Decoding MOS – Quantitative
Precipitation Forecast
KMIA
AVN MOS GUIDANCE
DT /SEPT 4
/SEPT 5
HR
12 15 18 21 00 03 06
X/N
89
TMP 79 85 86 84 81 80 79
DPT 77 76 75 75 75 76 76
CLD BK BK BK OV OV OV OV
WDR 00 19 17 17 14 13 16
WSP 00 01 04 03 01 01 02
P06
20
44
25
P12
45
Q06
0
2
0
Q12
1
T06
33/ 0 64/ 0 21/ 0
T12
66/ 0
CIG
7 7 7 6 6 7 7
VIS
7 7 7 7 7 7 7
OBV
N N N N N N N
T06, T12:
9/04/2003
09 12
77
79 80
76 77
OV OV
17 20
05 03
20
30
0
1
19/ 0
7
7
N
15 18
84
76
OV
23
09
42/
42/
7 6
7 7
N N
85
75
OV
22
09
36
2
0
0
4
7
N
0600 UTC
/SEPT 6
21 00 03 06
89
83 80 79 77
74 74 75 75
OV OV OV OV
24 24 23 24
08 04 04 04
55
46
66
4
4
4
80/ 0 37/ 0
93/ 0
5 4 4 4
7 7 7 7
N N N N
09 12
76
77 79
75 76
OV OV
21 22
06 06
44
68
2
5
37/ 0
5
7
N
15 18
85
75
OV
25
11
39/
58/
6 5
7 7
N N
88
74
OV
25
11
35
4
0
0
5
7
N
/
00 06
91
82 78
74 74
OV BK
26 24
06 05
30 15
54
0 0
4
22/ 0
66/ 0
7 7
7 7
N N
probability of thunderstorms / severe t-storms over 6, 12h
Decoding MOS – Precip.
Amount
 0 = no precipitation
 1 = 0.01 to 0.09 inches
 2 = 0.10 to 0.24 inches
 3 = 0.25 to 0.49 inches
 4 = 0.50 to 0.99 inches
 5 = 1.00 to 1.99 inches
 6 = 2.00 inches or greater
Decoding MOS – Local forecast
KMIA
AVN MOS GUIDANCE
DT /SEPT 4
/SEPT 5
HR
12 15 18 21 00 03 06
X/N
89
TMP 79 85 86 84 81 80 79
DPT 77 76 75 75 75 76 76
CLD BK BK BK OV OV OV OV
WDR 00 19 17 17 14 13 16
WSP 00 01 04 03 01 01 02
P06
20
44
25
P12
45
Q06
0
2
0
Q12
1
9/04/2003
09 12
77
79 80
76 77
OV OV
17 20
05 03
20
30
0
1
15 18
84
76
OV
23
09
85
75
OV
22
09
36
2
0600 UTC
/SEPT
21 00 03
89
83 80 79
74 74 75
OV OV OV
24 24 23
08 04 04
55
66
4
4
6
06 09 12
76
77 77 79
75 75 76
OV OV OV
24 21 22
04 06 06
46
44
68
4
2
5
High Temperature: 89F
Low Temperature: 77F
Precipitation Category: Cat-4
15 18
85
75
OV
25
11
88
74
OV
25
11
35
4
/
00
91
82
74
OV
26
06
30
54
0
4
06
78
74
BK
24
05
15
0
Decoding MOS – Zone Forecast
KMIA
AVN MOS GUIDANCE
DT /SEPT 4
/SEPT 5
HR
12 15 18 21 00 03 06
X/N
89
TMP 79 85 86 84 81 80 79
DPT 77 76 75 75 75 76 76
CLD BK BK BK OV OV OV OV
WDR 00 19 17 17 14 13 16
WSP 00 01 04 03 01 01 02
P06
20
44
25
P12
45
Q06
0
2
0
Q12
1
T06
33/ 0 64/ 0 21/ 0
T12
66/ 0
9/04/2003
09 12
77
79 80
76 77
OV OV
17 20
05 03
20
30
0
1
19/ 0
15 18
84
76
OV
23
09
85
75
OV
22
09
36
2
42/ 0
42/ 0
0600 UTC
/SEPT 6
21 00 03 06
89
83 80 79 77
74 74 75 75
OV OV OV OV
24 24 23 24
08 04 04 04
55
46
66
4
4
4
80/ 0 37/ 0
93/ 0
09 12
76
77 79
75 76
OV OV
21 22
06 06
44
68
2
5
37/ 0
15 18
85
75
OV
25
11
88
74
OV
25
11
35
4
39/ 0
58/ 0
/
00 06
91
82 78
74 74
OV BK
26 24
06 05
30 15
54
0 0
4
22/ 0
66/ 0
Tonight: Mostly cloudy with scattered showers and thunderstorms. Lows in the mid 70s.
Light southeast winds. Chance of rain 30%.
Tomorrow: Mostly cloudy with scattered to numerous showers and thunderstorms. Highs
near 90. Light southwest winds. Chance of rain 70%.
Tomorrow Night: Mostly cloudy with scattered showers and thunderstorms. Lows in the
mid 70s. Light southwest winds. Chance of rain 70%.
MOS on the web
 http://www.weather.gov/mdl/synop/product
s.php
Introduction
 July 1996: USA changes to a more
international system for meteorological
data reporting: METAR/TAF. Replaced
surface airways code.
 METAR: A routine aviation weather report
 SPECI: non-routine wx reports
 TAF: Aerodrome (airport) forecasts
METAR Observations
Example METAR Report
METAR KABC 121755Z AUTO
-RA BR BKN015 0VC025
WSHFT 1715 VIS 3/4V1
CIG 017 RWY11 PRESFR
21012 58033 TSNO $
21016G24KT 180V240 1SM R11/P6000FT
06/04 A2990 RMK A02 PK WND 20032/25
1/2 VIS 3/4 RWY11 RAB07 CIG 013V017
SLP125 POOO3 6OOO9 T00640036 10066
KABC - ICAO STATION (location) IDENTIFIER Four character ICAO
location identifier. K = USA; PA = Alaska; PH = Hawaii
121755Z - DATE/TIME All dates and times in UTC using a 24-hour
clock; two-digit date and four-digit time; always appended with Z to
indicate UTC.
AUTO REPORT MODIFIER AUTO: Indicates a fully automated report
with no human intervention. It is removed when an observer logs on
to the system. COR: Indicates a corrected observation. No modifier
indicates human observer or automated system with human logged
on for oversight functions.
METAR Observations
Example METAR Report
METAR KABC 121755Z AUTO
-RA BR BKN015 0VC025
WSHFT 1715 VIS 3/4V1
CIG 017 RWY11 PRESFR
21012 58033 TSNO $
21016G24KT 180V240 1SM R11/P6000FT
06/04 A2990 RMK A02 PK WND 20032/25
1/2 VIS 3/4 RWY11 RAB07 CIG 013V017
SLP125 POOO3 6OOO9 T00640036 10066
21016G24KT - WIND DIRECTION AND SPEED Direction in degrees
from true north (first three digits); next two digits: speed in whole
knots; if needed, include character as: Gusts (character) followed by
maximum observed speed; always appended with KT to indicate
knots; 00000KT for calm
180V240 - if direction varies by 60o or more and speed greater than 6
knots, a Variable wind direction group is reported, otherwise omitted.
If wind direction is variable and speed 6 knots or less, replace wind
direction with VRB followed by wind speed in knots.
1SM VISIBILITY Prevailing visibility in statute miles and fractions with
space between whole miles and fractions; always appended with
SM to indicate statute miles (5280 feet); values <1/4SM reported as
M1/4SM.
Runway Visual Range (RVR)
 R11/P6000ft: runway 11has 6000 ft
visibility. [Others: R11R or R11L or R11C]
 P: RVR is higher than the sensor reported
 M: RVR less than sensor (not illustrated
here)
 V: RVR Variable. Example:
R06L/2000V4000ft for runway 6 left
METAR Observations
Example METAR Report
METAR KABC 121755Z AUTO
–RA BR BKN015 0VC025
WSHFT 1715 VIS 3/4V1
CIG 017 RWY11 PRESFR
21012 58033 TSNO $
21016G24KT 180V240 1SM R11/P6000FT
06/04 A2990 RMK A02 PK WND 20032/25
1/2 VIS 3/4 RWY11 RAB07 CIG 013V017
SLP125 POOO3 6OOO9 T00640036 10066
-RA BR – Current WEATHER PHENOMENA
- Intensity or Proximity “-” Light, “no sign” Moderate, “+” Heavy, VC
Vicinity
- Precipitation: DZ Drizzle, IC Ice Crystals, UP Unknown in automated
observations, RA Rain, PL Ice pellets, SN Snow, GR Hail, SG Snow
grains GS Small hail/snow pellets; SH showers; TS thunderstorm
- Obscuration: BR Mist (< or = 5/8SM), SA Sand, FU Smoke, HZ
Haze, VA Volcanic Ash, PY Spray, DU Widespread Dust
- Other: SQ Squall, FC Funnel Cloud, SS Sandstorm, +FC Tornado/
Waterspout, DS Duststorm, PO Well developed dust/sand whirls.
METAR Observations
Example METAR Report
METAR KABC 121755Z AUTO
–RA BR BKN015 0VC025
WSHFT 1715 VIS 3/4V1
CIG 017 RWY11 PRESFR
21012 58033 TSNO $
21016G24KT 180V240 1SM R11/P6000FT
06/04 A2990 RMK A02 PK WND 20032/25
1/2 VIS 3/4 RWY11 RAB07 CIG 013V017
SLP125 POOO3 6OOO9 T00640036 10066
BKN015 OVC025 - SKY CONDITION. Cloud amount and height: CLR
(In automated METAR reports only, no clouds detected below 12000
feet.); (Human Observers: clear is SKC) Sky CLeaR 0/8; FEW 1/82/8; SCattered 3/8-4/8; BroKeN 5/8-7/8; OVerCast 8/8; 3-digit height
of base in hundreds of feet; followed by Towering CUmulus or
CumulonimBus if present. More than 1 layer may be reported.
06/04 - TEMPERATURE/DEW POINT. Each is reported in whole
degrees Celsius using two digits; values are separated by a slash;
sub-zero values are prefixed with an M (minus).
METAR Observations
Example METAR Report
METAR KABC 121755Z AUTO
–RA BR BKN015 0VC025
WSHFT 1715 VIS 3/4V1
CIG 017 RWY11 PRESFR
21012 58033 TSNO $
21016G24KT 180V240 1SM R11/P6000FT
06/04 A2990 RMK A02 PK WND 20032/25
1/2 VIS 3/4 RWY11 RAB07 CIG 013V017
SLP125 POOO3 6OOO9 T00640036 10066
A2990 ALTIMETER. Altimeter setting (in U.S. reports) is always
prefixed with an A indicating inches of mercury; reported using four
digits: tens, units, tenths, and hundredths.
RMK REMARKS IDENTIFIER. Remarks includes clarifying or
augmenting data concerning elements in the body of the METAR,
additive coded data and maintenance data.
AO2 TYPE OF AUTOMATED STATION. AO1; automated station
without a precipitation discriminator. AO2; automated station with
precipitation discriminator.
METAR Observations
Example METAR Report
METAR KABC 121755Z AUTO
–RA BR BKN015 0VC025
WSHFT 1715 VIS 3/4V1
CIG 017 RWY11 PRESFR
21012 58033 TSNO $
21016G24KT 180V240 1SM R11/P6000FT
06/04 A2990 RMK A02 PK WND 20032/25
1/2 VIS 3/4 RWY11 RAB07 CIG 013V017
SLP125 POOO3 6OOO9 T00640036 10066
PK WND 20032/25 - PEAK WIND. PK WND dddff(F)/(hh)mm; direction
in degrees, speed in whole knots, time in minutes after the hour.
Only minutes after the hour is included if the hour can be inferred
from the report.
WSHFT 1715 - WIND SHIFT. WSHFT followed by hours and minutes
of occurrence. The term FROPA may be entered after the time if it is
reasonably certain that the wind shift was a result of a frontal
passage.
METAR Observations
Example METAR Report
METAR KABC 121755Z AUTO
–RA BR BKN015 0VC025
WSHFT 1715 VIS 3/4V1
CIG 017 RWY11 PRESFR
21012 58033 TSNO $
21016G24KT 180V240 1SM R11/P6000FT
06/04 A2990 RMK A02 PK WND 20032/25
1/2 VIS 3/4 RWY11 RAB07 CIG 013V017
SLP125 POOO3 6OOO9 T00640036 10066
P0003 - HOURLY PRECIPITATION AMOUNTPrrrr; in tens, units, tenths
and hundredths of an inch since last regular hourly METAR. A trace
is reported as P0000.
60009 - 3- AND 6-HOUR PRECIPITATION AMOUNT. 6RRRR;
precipitation amount, including water equivalent, to nearest 0.01
inches for past 6 hours reported in 00, 06, 12, and 18 UTC
observations and for past 3 hours in 03, 09, 15, and 21 UTC
observations. A trace is 60000.
7**** Not on this report - 24-HOUR PRECIPITATION AMOUNT. 7****;
precipitation amount to nearest 0.01 inches for past 24 hours
reported in 12 UTC observation; e.g., 70015 indicates 0.15 inches of
precipitation for past 24 hours.
METAR Observations
Example METAR Report
METAR KABC 121755Z AUTO
–RA BR BKN015 0VC025
WSHFT 1715 VIS 3/4V1
CIG 017 RWY11 PRESFR
21012 58033 TSNO $
21016G24KT 180V240 1SM R11/P6000FT
06/04 A2990 RMK A02 PK WND 20032/25
1/2 VIS 3/4 RWY11 RAB07 CIG 013V017
SLP125 POOO3 6OOO9 T00640036 10066
T00640036 - HOURLY TEMPERATURE AND DEW POINT. TsnTaTaTa
snT'aT'aT'a; reported to nearest tenth of oC; sn: 1 if temperature or
dew point below 0oC and 0 if temperature/dew point 0oC or higher.
10066 - 6-HOUR MAXIMUM TEMPERATURE. 1snTxTxTx; maximum
temperature for past 6 hours reported to nearest tenth of degree
Celsius; reported on 00, 06, 12, 18 UTC reports; sn = 1 if
temperature below 0oC and 0 if temperature 0oC or higher
21012 - 6-HOUR MINIMUM TEMPERATURE. 2snTnTnTn; minimum
temperature for past 6 hours reported to nearest tenth of degree
Celsius; reported on 00, 06, 12, 18 UTC reports; sn = 1 if
temperature below 0oC and 0 if temperature 0oC or higher.
For you slackers out there…
 Metar decoders:
 http://lwf.ncdc.noaa.gov/oa/climate/conver
sion/swometardecoder.html
 www.met.tamu.edu/class/METAR/metar.html
Ultra-cool METAR Displays
 www-frd.fsl.noaa.gov/mab/metar
 www.rap.ucar.edu/weather/surface
 Click on Java applets large version