WX352 Weather Forecasting

1
WX-352 Weather Forecasting
2
PG-13 Weather Joke

• This had most of the state of Michigan laughing
  for 2 days and a very embarrassed female news
  anchor who will, in the future, likely think
  before she speaks.What happens when you predict
  snow but don't get any. A true story...A
  female news anchor that, the day after it was
  supposed to have snowed and didn't, turned to the
  weatherman and asked "So Bob, where's that 8
  inches you promised me last night?"
• Not only did HE have to leave the set, but half
  the crew did too they were laughing so hard!

3
TV Weatherman in Serious Need of Medication
4
(No Transcript)
5
Four Forecasting Steps

• 1) Assess the present state, called the analysis,
  by plotting 6 hourly surface and 12 hourly
  sounding data
• 2) Predict a future state by running a computer
  model of weather changes
• 3) Interpret the model results, called a
  prognostic chart, given forecasting
  experienceapply ROTs
• 4) Translate forecast into something that is
  useful

6
Persistence Forecast Method
The persistence method works well when weather
patterns change very little and features on the
weather maps move very slowly. It also works
well in places like southern California, where
summertime weather conditions vary little from
day to day.
7
Trend Forecast Method

• Determining motion of systems, and projecting
  that motion into the future
• Frontal and air mass movement

8
Analogue Forecast Method
Figure 14.7
9
Forecast Using Climatology

• Using long-term average weather to predict future
  conditions
• Most useful for long-range forecasting
• Includes probability forecast methods

10
Climatology--Winds
11
Forecasting Using Probability
Figure 14.6
- Climate records can be used to generate
probability forecasts for a given event. --
Florida has less than a 5 chance of snow on
December 25th, while northern Minnesota has had
snow on that date for each of the past 30 years.
12
Numerical Weather Prediction (NWP)

• At 00Z and 12Z each day, data is collected and
  NWP models are run
• NWP is the 2nd largest user of super computers in
  the world

13
NWP--Limitations

• While NWP has greatly improved the accuracy of
  weather forecasts, it still haves significant
  limitations
• Usually the best method for forecasting weather
  12 hours to 5 days in the future

14
When Forecast Models Go Bad
15
Ensemble Modeling

• Numerical weather models are run multiple times
  with small changes in the initial conditions
• Gives forecaster a good estimate of the possible
  range of weather conditions at a future times

Spaghetti Chart helps forecaster to know
where/when to rely on the NWP model, and
where/when to ignore the NWP model
16
Weekly and Monthly Forecasts

• 30-90 day outlook of weather conditions
• Doesnt give a specific forecast
• Warmer or cooler (wetter/dryer) than normal
• Uses ensemble modeling and analogue methods
• Slightly better than climatology alone
• These 90 day outlooks were downloaded after
  Christmas, 2005. How accurate do you think they
  were?

Figure 14.8A
Figure 14.8B
17
Forecast Periods

• Nowcasting (less than 6 hours)
• Mainly surface observations, weather satellite
  and radar, and persistence or trend forecasts
• Short-range (lt3 days)
• Trend and analogue methods, weather charts and
  numerical weather forecast products
• Medium range (3-8 days)
• Analogue forecasts, numerical weather forecast
  products, climatology
• Long-range (gt8 days)
• Climatology NWP Ensembles

18
Accuracy and Skill

• How good are weather forecasts??
• 0-24 hours very good
• 2-5 days fairly good
• Beyond 7 days slightly more accurate than
  persistence
• Accuracywhat is considered right?
• If the TAF called for rain, and it rains in FWB
  (but not at HRT), the forecast is wrong
• Skillhow complicated/hard is the forecast
• Southern California in summereasy or difficult?
• Exactly where and when a tornado will touch
  downeasy or difficult?
• Where they are likely to form--not difficult 3
  days in advance
• Precise area struck--fairly easy a few minutes in
  advance
• Forecast skill is determined by comparing our
  forecast against either persistence or
  climatology
• If were no better than persistence or
  climatologywhy bother?

19
NWS Probability of Precipitation

• Probability of Precipitation (PoP) describes the
  chance of an event occurring at any point in the
  forecast area, over a certain period of time
  (usually 12 hours).
• PoP is defined as PoP Pa x Pc, where
• Pa the probability that precipitation will occur
  somewhere in the forecast area during the
  forecast period and           
• Pc the percent of the area that will receive
  measurable precipitation, if it does occur

20
Forecast Product Accuracy
21
Forecast Practice

• Scenarios

22
Forecasting with Surface Charts
This simplified 6 AM Tuesday surface weather map
is useful for short time interval predictions of
fronts and associated weather.
23
Surface Chart Predictions

• - 3-hour pressure tendencies plotted on isallobar
  maps help predict the movement of highs and lows
• Lows tend to move toward the region of greatest
  pressure fall, while highs move toward the region
  of greatest rise.

24
Upper Level Charts
Figure 14.11
Upper level winds guide the path of surface
pressure systems.
25
Observed Movement of Fronts
Surface weather observations from 6 PM Tuesday
and 6 AM Wednesday show how the fronts, pressure
systems, and precipitation have moved as
predicted.
26
Forecast

• Using both trend and persistence forecast
  techniques, what is your forecast for 24 hours
  from now in
• Denver? Dallas? Chicago?
  Memphis?
• Washington? Augusta?

Wed 6am
Tues 6am
27
Observed Weather for 6 AM Wednesday
28
Another Scenario

• Forecast for central CA coast for tomorrow
• Slowly falling barometer and increasing high
  clouds
• Weak low pressure in Pacific.....
• Will it rain, or not??

29
Surface Weather for 4 PM Sunday
Look at 500mb chart for clues
30
500 mb Chart for 4 PM Sunday
564 line across the area (local ROT) Omega High
presentbut cold advection and shortwave may
change the stagnant upper pattern upper-level
divergence??
31
12, 24, 36 hour Progs
Forecaster has access to several forecast models
-Which one is best? -- Choice is based on
initialization and experience.
32
Surface Weather Map for 4 AM Sunday
33
500 mb Chart for 4 AM Monday
34
IR Satellite for 645 AM Monday
35
Surface Weather Map for 4 AM Monday
36
Visible Satellite for 9 AM Tuesday
37
Forecast Applications
38
Weather Impacts on Aviation Mishaps

• 88 of all weather-related aircraft mishaps
  occurred under IMC (Instrument Meteorological
  Conditions)
• 83 of fatal crashes involved FAR Part 91
  (General Aviation) aircraft

39
Weather Impacts on National Airspace System
40
Wx Impacts on Flight Operations
41
lt VFR Conditionsceiling (broken or overcast
cloud bases) are more than 1,000 feet above the
ground, and the visibility is three miles or
more, the weather is VFR.
42
Types of Flight Weather Briefings

• Briefings and Advisories All pilots must obtain
  a formal weather briefing during preflight. Based
  upon the following pilot's information a briefing
  is given
• Type of flight (VFR or IFR), aircraft type,
  departure and destination airport, flight route,
  flight altitudes, estimated time of departure
  (ETD), estimated time of arrival (ETA)
• There are three types of briefings that are
  offered to pilots by "pilot weather briefers" who
  have been certificated by the National Weather
  Service, and work at local Flight Service
  Stations and Weather Service Offices.
• Pilots need to know which weather briefing to
  request
• The Standard Briefing
• The Abbreviated Briefing
• The Outlook Briefing

43
Types of Flight Weather Briefings

• 1. Standard Briefing is the most complete weather
  briefing and for the pilot who does not have an
  opportunity to gather any other weather data.
  Includes
• Adverse weather conditions expected
• VFR (visual flight rules) flight not recommended
• Synopsis
• Current conditions
• En route forecast
• Destination forecast
• Winds aloft
• Notices to airmen (NOTAM)
• ATC (air traffic control) delays

• 2. Abbreviated Briefing is for pilots who need to
  update previous weather data they have gathered,
  or who only need a few bits of weather data to
  complete their weather picture.
• 3. Outlook Briefing is requested by pilots whose
  departure time is 6 or more hours away. The
  briefer then gives forecast information
  appropriate to the proposed flight route. This
  will assist the pilot in making an initial
  decision about the feasibility of such a flight.