ATM 111 Forecast Discussion

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ATM 111Forecast Discussion

• R. Grotjahn
• W 2005

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Forecast Discussion

• III. Making a forecast
• A. Overview of general forecast presentation
• 1. Primary charts
• a. text discussions (e.g. prog. discussions,
  QPF, DIFAX prog. chart).
• b. begin with current surface map -- indicate
  features to be tracked
• c. time sequences of
• 500 mb Z with vorticity -- motion, intensity,
  PVA/NVA
• sea level pressure -- (frontal movements, low
  centers, highs, special wx conds.)
• precip -- types amounts
• d. model comparisions (including ensembles)
• e. MOS (difax) -- max/min, PoP for selected cities

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Forecast Discussion

• III. Making a forecast
• A. Overview of general forecast presentation
• 1. Primary charts
• a. text discussions (e.g. prog. discussions,
  QPF, DIFAX prog. chart).
• b. begin with current surface map -- indicate
  features to be tracked
• c. time sequences of
• 500 mb Z with vorticity -- motion, intensity,
  PVA/NVA
• sea level pressure -- (frontal movements, low
  centers, highs, special wx conds.)
• precip -- types amounts
• d. model comparisions (including ensembles)
• e. MOS (difax) -- max/min, PoP for selected
  cities
• 2. Secondary charts (as needed to justify
  explanations information presented above)
• a. hemispheric maps (including N. Pac.) for
  longer range discussion
• b. forecast meteograms for selected cities

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Forecast Discussion

• a. text discussions These give an overview.
  These help point out features to notice in the
  charts to be shown. Generally, it is not useful
  to examine the specific city forecast by the NWS
  or MOS just yet. These text discussions are to
  help you formulate your presentation. Generally
  you do not show these, though they should be
  acknowledged if you paraphrase their comments.
• i. find general model information, including
  recent performance notes in the NWS text
  discussions. Also prognostic, hemispheric and QPF
  discussions.
• ii. A short summary can be found (along with
  helpful schematic sfc weather charts) on the
  DIFAX as the 36 48 hr prog (or similar title)

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Forecast Discussion

• current surface map.
• usually discussed already by the person handling
  the review of current weather and again with the
  model performance. So, there is no need to go
  into much detail.
• main purpose now is to indicate what specific
  features will be tracked in this forecast
  discussion. Typically the main highs and lows,
  the positions of the main fronts or troughs, main
  areas of precipitation/weather

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Forecast Discussion

• forecast map time sequences.
• i. The best organization is to show sequences of
  maps. Typically, analysis (0), 12, 24, 36, and
  48 hr fcsts are shown. Show a complete sequence
  for one variable at a time. While this may seem
  repetitive, in fact it is more understandable to
  the audience since it avoids stating too many
  details at a time. Also, this places the emphasis
  on how things may evolve rather than what is
  happening at a particular time.
• ii. Choose a model as your archetype. Show these
  forecast sequences for that model. It is logical
  to show output from a model that is currently
  doing well. (see map review.) Later, where other
  models agree or disagree can be mentioned. The
  goal here is to present a reasonable estimate of
  the coming weather. Some maps have more than one
  of the three quantities below on the same chart,
  which can be convenient and efficient. A typical
  choice here is the ETA, sometimes NOGAPS,
  sometimes GFS. (A given model may not be
  available on a given day.)

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500 hPa Z and vorticity (absolute or relative)

• 1. deduce motion and changes of intensity of the
  vorticity. Identify where each significant
  vorticity max is at successive times. Notes
  Short wave troughs, which may be the seed for
  future developing storms, or may intensify or
  modify existing systems can often be spotted more
  easily using vorticity. However, do not neglect
  to identify the trough that goes with the
  vorticity max. (A trough is a line of maximum
  curvatuve in the contours.)
• 2. identify areas of PVA and NVA. Purpose is to
  deduce a prime forcing factor in vertical motion.
  Of course, WAA or CAA may reinforce or cancel the
  effect. In most situations, upward motion may be
  linked to cloudiness and precipitation, while
  downward motion may be linked to windy conditions.

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sea level pressure and temperature (or thickness)

• the goal is to deduce movements of fronts, low
  centers, highs, and various special weather
  conds.)
• 1. Finding high and low center locations and
  intensities is easy, though a bit tedious. One
  wants to deduce the general direction and speed
  of movement and to notice intensification or
  decay that may be occuring. Note topography
  effects sfc P movement (Carlson sect. 9.2, p.
  205)
• 2. fronts are generally not plotted, but must be
  deduced. A thickness field is very useful in that
  regard. Some rules for finding frontal locations
• 1. warm air side of gradient in thickness
• 2. wind shift
• 3. SLP trough
• 4. SLP trough directly under thickness ridge is
  probable occlusion
• 5. use past location fronts can be followed over
  time (see II.B.6.a.vi.)

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Precipitation (past 12 hrs)

• note the areas where precipitation is forecast,
• 1. note whether it is liquid or frozen, and
• 2. note the amount, particularly if the amount
  forecast is large.
• 3. try to find a cause for the precipitation. Is
  it
• 1. associated with a front?
• 2. associated with PVA or WAA or both?
• 3. caused by upslope conditions? (Check low level
  wind directions relative to topography)
• 4. a local effect (e.g. lake-effect snows?)
• 5. convective? (e.g. consult a forecast map of
  CAPE)

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Significant Weather

• Try to find these on the forecast maps
• B. Forecasting various significant weather
  situations 4.7
• 1. chinooks, downslope winds 4.7
• 2. fog 4.8
• 3. Topographic enhancement of precipitation 4.9
• 4. freezing rain 4.11
• 5. snow line 4.12
• 6. lake effect snows 4.13
• 7. forecasting convection 4.14
• 8. Some specific aviation forecast needs 4.16
• C.Significant weather situations for the
  Sacramento Valley 4.18
• i. CAA NW wind 4.19
• ii. WAA NW wind 4.20
• iii. strong prefrontal southerly winds 4.21
• iv. strong post-frontal southerly winds 4.22
• v. heavy rain 4.23
• vi. CAA hard freeze (non-radiative) 4.24
• vii. persistent fog 4.25
• viii. summer heat wave 4.26

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model comparisons (including ensembles).

• The goal is to see what the models agree on and
  what the models disagree upon. If the models
  agree, then one has more confidence in the model
  guidance. There are many model solutions one
  might examine. It is realistic only to check a
  couple of these models. In a presentation,
  obviously one need not show model solutions that
  agree. When models disagree, one need not show
  the model results if the difference can be
  summarized in a few words.
• i. Global models AVN NOGAPS MRF (including
  ensemble runs) ECMWF, etc.
• ii. Regional models ETA meso-ETA NGM MM5
  (e.g. U. Wash. version)
• Basic procedure check consistency of the main
  features. Generally, the things checked fall into
  three categories
• 1. location intensity of 500 mb cut-off lows at
  24, 36, 48 hr
• 2. location central (sea-level) pressures of
  lows highs at 24, 36, 48 hr
• 3. the general distribution of precipitation and
  regions of larger amounts. Due to model
  differences, it is not important to compare
  specific amounts, just the general locations.

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model comparisons Eta

• 48hr

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model comparisons MM5

• 48hr

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model comparisons NGM

• 48hr

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model comparisons Avn

• 72hr

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model comparisons CMC

• 72hr

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model comparisons UKMet

• 72hr

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forecast ensembles

• ensemble mean with normalized spread
• spagetti diagram

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other significant weather

• The goal check other sources of weather
• Here
• LI
• RH

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other significant weather

• The goal check other sources of weather
• Here
• Jets
• Z300

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Point Forecasts

• Cities with interesting events
• Forecast contest cities

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Longer-term outlook

• Look for large-scale changes
• Here
• SLP
• Z500

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Longer-term outlook

• Look for large-scale changes
• Here
• SLP
• h
• P

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End of Forecast Discussion
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