EGOWS 2008

Systematic forecasting of weather “type” in the GFE

John BallyCAWCR

GFEHow do we describe the weather?

Shower

s

Sleet

Drizzle Patchy

HeavyWidespread

Scattered

RainIsolated

Snow

Light

Thunderstorm

s

Weather Grid

Potential Type of Precipitating Weather

Deep and Shallow Instability

Precipitating Weather Forecast Process Map

Snow Level

Upper Level Moisture

Cloud Cover

Probability of Precipitation

Expected Intensityof Precipitation

GFEWill the precipitation be convective ?

GFEThe model upper RH fields have skill.

Is there enough moisture for rain?

GFEDiagnose weather type from instability,

cloud cover and upper RH

GFEStart with the Probability of Precipitation

from a multi model ensemble.....

GFEMatch the statistics of ensemble PoP to

climatology.......Limit PoP to 70%

GFEUse PoP to delineate areas of weather

and assign coverage

GFEMake sure that the PoP and the most

expected precipitation amount match.....

GFEWhat effect will the wind field have on

Precipitation ?

GFEIncrease forecast precipitation where

convergence will trigger showers.....

GFEAnd set the weather intensity from the

expected precipitation rate.....

GFEThe previous day, with a cold front

crossing the west Australian coast .....

08 Jun 18Z 09 Jun 00Z 09 Jun 12Z09 Jun 06Z>>

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GFEForecasters expect more instability than

the model shows......

08 Jun 18Z 09 Jun 00Z 09 Jun 12Z09 Jun 06Z>>

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Weather Grid

Potential Type of Precipitating Weather

Define Feature

Feature Based Precipitating Weather Process

Associate Weather Type with Feature

Track Feature

Probability of Precipitation

Expected Intensityof Precipitation

GFEDraw in the axis of the cold front...

GFEAnd again 12 and 24 hours later....

GFEMake contours from these lines... Value

is the time of wind change .......

GFEUse select tool to dynamically pick out

the area from 3 hrs ahead of the change to 6 hours behind it .....

GFEThe cold frontal zone moving over

western Australia .....

08 Jun 18Z 09 Jun 00Z 09 Jun 12Z09 Jun 06Z>>

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GFEUse this “edit area” to assign “showers

and storms” potential weather type near the front .....

08 Jun 18Z 09 Jun 00Z 09 Jun 12Z09 Jun 06Z>>

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GFEAgain, calculate the forecast weather

from potential weather type, PoP and expected precipitation intensity ...

08 Jun 18Z 09 Jun 00Z 09 Jun 12Z09 Jun 06Z>>

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GFEAlso increase wind speed near the

change line... often under-forecast by model......

08 Jun 18Z 09 Jun 00Z 09 Jun 12Z09 Jun 06Z>>

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GFE Now we produce some words....

Weather Types are described by........

Coverages : How much of specified area is impacted ?

Intensities : how strong ?

Attributes : optional features of the weather type

Coverage, intensity and attributes combined .... e.g. “Widespread heavy showers with hail”

GFE Sample and summarise the grids ....

Take a time-series of grids and perform mathematical operations, including: Averaging, Min/Max, deciles etc Collection of weather keys

These operations produce samples, which represent the best numerical description over the space & time in question.

e.g. Wind : ((5, 10), (300, 340)) MaxT : (22, 28) Weather : ((SctSH+ 32%), (PaRa- 20%), (AreasTS 10%))

GFE Describe the situation over time.....

Examines the grids at the minimum time resolution, e.g. 3 hourly resolution

Identify times in the sequence where the samples change significantly

Choose significant transition points and produce one sub-phrase for each time chunk.

e.g. “Light wind becoming northerly 15-20 knots during the morning then tending southwesterly in the evening.

GFE Combine weather within a type.....

Look at similarity of weather between subphrases

Is it worth describing a transition from the chance of light rain to the chance of moderate rain?

Generally combine across one coverage OR intensity

Combined more aggressively in complex situations

GFE Describe the stats of each period...

Take the statistics for each chunk (time period)

Produce a scalar, vector, or weather description

Wind ((5, 20), (300, 340)) “north to northwest wind up to 20 knots”

Wx (isolSh-) “isolated light showers” or “chance of a light shower”

GFE Talking about changes.....

Transition style for trends while over-time, e.g. “becoming light around midday”.

Over-time style for isolated events, e.g. “Rain during the evening”.

Use transition words......eg Wx (isolSh-, WideRa) “isolated light showers

increasing to widespread rain” Wx (isolSh-, WideSh) “isolated light showers

becoming more widespread”

GFE and produce the worded forecast ...

GFE with some help from our testing infrastructure ...