ATM 111

Forecast Discussion

R. Grotjahn

W 2007

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

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

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)

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

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 WRF, sometimes NOGAPS, sometimes GFS. (A given model may not be available on a given day.)

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.

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.)

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)

Significant Weather

• Watch for these situations (patterns) 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– ix. summer sea breeze (“delta breeze”) 4.27

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: GFS; NOGAPS; GFS ensemble runs; ECMWF, etc.– ii. Regional models: NAM (WRF); 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.

model comparisons – Eta• 48hr

model comparisons – MM5• 48hr

model comparisons – NGM• 48hr

model comparisons – Avn• 72hr

model comparisons – CMC• 72hr

model comparisons – UKMet• 72hr

forecast ensembles• ensemble mean with normalized spread• “spagetti” diagram

other significant weather - convection• The goal: check other sources of weather• Here:

– LI– RH

other significant weather – jet streaks• The goal: check other sources of weather• Here:

– Z300

– Jets

isotachs

Point Forecasts• Cities with interesting events• Forecast contest cities

Longer-term outlook• Look for changes in large-scale pattern(s)• Here:

– SLPcontours– Z500

colors

Longer-term outlook• Look for changes in large-scale pattern(s)• Here:

– SLP– h– P

End of Forecast Discussion