638
FNUS75 KBYZ 300935
FWSBYZ

Spot Forecast for North Derby Unit 3...USFS
National Weather Service Billings MT
335 AM MDT Wed Sep 30 2026

Forecast is based on ignition time of 0700 MDT on September 30.
If conditions become unrepresentative...contact the National Weather
Service.

Please contact our office if you have questions
or concerns with this forecast.

.DISCUSSION...

.TODAY...
We are still expecting mild and dry conditions over the burn site
through Thursday. Westerly winds will prevail today. Gusts 20-25
mph are expected to be common by midday, with occasional gusts
to near 30 mph possible (25% probability). Look for winds to turn
more to the northwest in the afternoon. Humidity readings are
expected to bottom out in the 30-35% range today. Winds shift to
the east this evening and remain easterly into Thursday. This will
likely result in less mixing Thursday afternoon with decreased
ventilation rates. RH readings will be higher Thursday as well,
remaining above 40%. BT


Sky/weather.........Mostly Sunny. High clouds at times.
CWR.................0 percent.
Chance of lightning.0 percent.
Max temperature.....63-67.
Min humidity........29-34 percent.
Wind (20 ft)........
 Slope/valley.......Light W/NW winds early, then West winds 8 to 13 mph. Gusts up
                    20-25 common with up to 31 mph in the afternoon.
 Ridgetop...........West around 15-20 mph. Becoming NW in the aftn.
Mixing height.......500 ft AGL early in the morning increasing to
                    5500 ft AGL in the afternoon.
Transport winds.....West 14 to 19 mph.

TIME (MDT)      7AM 8AM 9AM 10A 11A 12P 1PM 2PM 3PM 4PM 5PM
Sky (%).........16  12  3   0   2   3   4   4   6   8   5
Weather cov.....
Weather type....
Tstm cov........
Chc of ltng (%).0   0   0   0   0   0   0   0   0   0   0
Temp............52  51  54  58  58  60  62  63  63  64  63
RH..............47  47  45  41  42  38  36  33  33  34  35
20 FT wind dir..W   W   W   W   W   W   W   W   W   NW  NW
20 FT wind spd..6   7   8   10  12  13  13  12  11  11  10
20 FT wind gust.14  13  14  17  18  23  26  25  25  24  19

.TONIGHT...

Sky/weather.........Increasing clouds.
CWR.................0 percent.
Chance of lightning.0 percent.
Min temperature.....39-43.
Max humidity........68-79 percent.
Wind (20 ft)........
 Slope/valley.......Northwest winds 5 to 10 mph with gusts to
                    around 17 mph shifting to the east 5 to 8 mph
                    in the mid evening, then shifting to the
                    southeast up to 7 mph overnight.
 Ridgetop...........West around 10-15 mph.
Mixing height.......3500 ft AGL early in the evening decreasing to
                    500 ft AGL overnight.
Transport winds.....West around 13 mph becoming easterly around
                    12 mph in the evening, shifting to the
                    southeast 7 to 10 mph late.

TIME (MDT)      6PM 7PM 8PM 9PM 10P 11P MID 1AM 2AM 3AM 4AM 5AM
Sky (%).........11  14  18  39  56  56  36  37  35  33  19  28
Weather cov.....
Weather type....
Tstm cov........
Chc of ltng (%).0   0   0   0   0   0   0   0   0   0   0   0
Temp............62  59  55  53  52  51  49  48  46  45  44  42
RH..............37  43  50  56  60  62  65  67  69  70  72  77
20 FT wind dir..NW  N   NE  E   E   SE  SE  SE  SE  SE  SE  SE
20 FT wind spd..10  9   8   7   7   7   6   6   6   6   6   5
20 FT wind gust.17  16  14  12  11  10  10  10  10  10  10  8

.THURSDAY...

Sky/weather.........Mostly sunny.
CWR.................0 percent.
Chance of lightning.0 percent.
Max temperature.....54-58.
Min humidity........49-54 percent.
Wind (20 ft)........
 Slope/valley.......Southeast winds up to 5 mph early in the
                    morning becoming light, then becoming east up
                    to 8 mph in the afternoon.
 Ridgetop...........West around 10-15 mph.
Mixing height.......increasing to 3600 ft AGL in the afternoon.
Transport winds.....Mainly easterly 6 to 9 mph.

TIME (MDT)      6 AM      9 AM      NOON      3 PM
Sky (%).........42        37        38        43
Weather cov.....
Weather type....NONE      NONE      NONE      NONE
Tstm cov........
Chc of ltng (%).0         0         0         0
Temp............42        43        48        54
RH..............77        70        64        49
20 FT wind......SE 5G7    NE 3G5    NE 5G7    E 6G10

$$
Forecaster...Tesar
Requested by...Greg Oyler
Type of request...PRESCRIBED
.TAG 2622650.2/BYZ
.DELDT 09/30/26
.FormatterVersion 2.0.0
.EMAIL gregory.oyler@usda.gov