Fire Weather

[kylep]

The point where High becomes Very High is different in WA - it won't go over until the index hits 32. We have always used the Grassland ratings here in WA - the bottom of the Fire Weather Forecast explains it. The Forest index needs fuel loadings and drought factor inputs, which can vary greatly in a very small area..

Also remember - its a 'forecast', not a 'nowcast'

That last bit is very important, and the fire behaviour that goes along with it is Predicted Fire Behaviour not what is going to happen. Even on very bad days, the actual weather and behaviour can be much lower than the forecast, and vice versa. The more you learn about this and observe it, the better you become at preparing yourself and assessing the situation. I know I had limited exposure to it over there, but have dramatically increased it now. I'd recommend to all firefighters that they learn as much as they can about this.

[newbie]

The rating was based on the new fire danger index as taken on by FESA etc, sure fuel loadings and other factors were averages so some areas would have not been as bad and other areas might have been worse.
I guess the point that I was skirting around was that all the agencies that fight predominatly forest fires in Australia, ie DEC, DSE etc all go off the forest fire danger index for forest areas, so surely if you have a fire danger sign in a predominantly forest area like at the top of Greenmount hill you would base it off the forest fire danger index, all it would take is for the ranger to call DEC instead of going off the rating from the BOM.

[colin]

[quote="newbie"]all it would take is for the ranger to call DEC instead of going off the rating from the BOM.[/quote]

Or DEC could post it on thier web site then we could all see/use it.....

[newbie]

Thats a good idea! I'm sure there would be a lot of BFB's that would like to have that info readily available.

[WPXZBP]

Down here each Brigade has at least one Fire Weather Officer. At the morning scheduled on-air net they pass on the weather details and the situation is analysed. The BoM produce their forecast. What they use is not known to me. Our Weather Officers here use the grassland scale as a lot of the area is farmland.

[Fastlane]

First Total Fire Ban issued since the amendments to the Bush Fires Act came into effect??

https://internet.fesa.wa.gov.au/mediare ... ItemId=114

[newbie]

Yep I think it is the first one!
The new rating system is an interesting one for areas like the wheatbelt, due to the wind and low humidity etc that they get basicly puts the fire danger ratings into the top rating categories on most days. The fear is that having to issue fire weather warnings all the time for those areas will dilute the effectivness of warnings when the danger rating is unusually high... There was an idea of changing the rating system in areas that are constantly in the higher end of the scale so that it better reflects localised average and above average conditions and doesn't excessively issue warnings.
The old fire rating system had a similar thing done to it at the point where high ticked over to very high. It was altered to kick in at a slightly higher rate so that it didn't put undue restrictions on hazard reduction burning for brigades.

[Fastlane]

The old fire rating system had a similar thing done to it at the point where high ticked over to very high. It was altered to kick in at a slightly higher rate so that it didn't put undue restrictions on hazard reduction burning for brigades.

This is still the case. See http://www.bom.gov.au/catalogue/warning ... n_FW.shtml - In WA we don't reach a Very High FDR until the index is 32, whereas this happens at an index of 25 in all other staes.

[pacestick]

Its not looking good up here at the moment weve got thunder around cant see any lightning yet fortunantly.

[Tyranus]

well east gidgie and West Gidgie are turning attending a pine tree hit by lightning, I also saw lightning earlier.

[pacestick]

well east gidgie and West Gidgie are turning attending a pine tree hit by lightning, I also saw lightning earlier.

yeh Ive just got home from a lightning strike fire in clackline.
rather scary when theres strikes happening around you

[Aus_Samurai]

The formula that BoM use to calculate the forecast fire danger index is listed below:

Fire Danger Index Calculation McArthur Mk 4 Grassland Fire Danger Meter (as used by BoM 2009-2010)
GFDI = EXP(-1.523+1.027*LN(Q)-(0.009432*(100-C)^1.536)+0.02764*T+(0.6422*V^0.5)-0.2205*(RH)^0.5

where
Q = fuel quantity (t/ha) (NB. BoM use a standard of 4.5 t/ha)
C = degree of curing (%) (NB. BoM assume 100% curing)
T = air temperature (°C)
V = average wind speed (km/h) (NB. not gusts and measured 1 - 2m above ground)
RH = relative humidity (%)

Modification to McArthur as used by BoM
if wind speed >=50 km/h then
FDI = FDI*50/wind speed

Fire Danger Rating (WA 2009-2010)
Low Less than 3
Moderate 3-11
High 12 – 31
Very High 32 – 49
Severe - 50 - 74
Extreme 75 - 100
Catastrophic Greater than 100

You should be able to copy this into an excel spreadsheet and calculate your own GFDI's based on local conditions.

The key thing to remember is that BoM use a fixed 4.5 t/ha for fuel load which may be less than the actual in most areas.

The reason that BoM use the GFDI is because it uses more readily available data, the FFDI uses a dryness factor which requires very specific local calculations involving the KBDI for the area. Also in most instances the forecast GFDI will be fairly similar to the FFDI, usually no more than one rating lower.

McArthur Mk 5 Forest Fire Danger Index formula (Noble, et al)
FFDI = 2*EXP(-0.45+0.987*LN(DF)+0.0338T-0.0345RH+0.0234V

DF = ((0.191*(KBDI+104)*(DSLR+1)^1.5)/(3.52*(DSLR+1)^1.5+LR-1))

Where KBDI = Keech-Byram Drought Index
DSLR = Days Since Last Rain
LR = Last Rain (mm)

For example, the BoM forecast GFDI for Bickley tomorrow is 13 (High), the calculated FFDI for the same area is 24 which is still High. If you tweek the figures, you can get the FFDI to jump one rating above the GFDI, but not any more.

Hope this helps rather than confuses the issue, fire weather is a complex subject that requires a lot of understanding, but in many instances rough calculations on the ground will suffice.

[pacestick]

Thankfully today isnt as bad as predicted.

I just hope we dont get the afternoon thunderstorm.

whats the weather like around the place?

[WPXZBP]

www.bom.gov.au/weather/wa

[Helitak_670]

You will find that the forecast FDI under estimates the current fire conditions we are seeing. I was certainly surprised with the activity of late of some of the fires that I have seen in the past week. All I can say to all the fire fighters here be careful. Just remember that the books etc that we have under estimate the power of fire and what it can do. It is extreamly dry and difficult to control any fire. We still have 3+ months of fire fighting ahead of us.