458  
FNUS76 KPDT 212300  
FWSPDT  
 
SPOT FORECAST FOR HWY 20 IFR UNITS 93/81...USFS  
NATIONAL WEATHER SERVICE PENDLETON OR  
400 PM PDT WED MAY 21 2025  
 
FORECAST IS BASED ON IGNITION TIME OF 0600 PDT ON MAY 22.  
***THUNDERSTORMS IMPLY GUSTY AND ERRATIC WINDS***  
***WINDS ARE 20 FOOT 10-MINUTE AVERAGES***  
***CWR-CHANCE OF WETTING RAIN 0.10 OR GREATER***  
***VENTILATION INDEX IS DERIVED FROM THE  
TRANSPORT WIND MULTIPLIED BY THE MIXING  
HEIGHT IN 100'S OF FEET. INDEX CATEGORIES  
ARE A GENERALIZED INDICATOR ONLY.***  
 
IF YOU HAVE QUESTIONS OR CONCERNS WITH THIS FORECAST, PLEASE  
CONTACT OUR OFFICE AT (541) 276-8134.  
   
DISCUSSION
 
A WEAK WEATHER SYSTEM WILL PASS OVERHEAD TONIGHT  
THROUGH THURSDAY, FACILITATING A CHANCE OF RAIN SHOWERS. CHANCE  
OF WETTING RAIN IS VERY LOW (6%) AND CHANCE OF LIGHTNING IS ALSO  
VERY LOW (6%). ANOTHER WEAK SYSTEM IS FORECAST THURSDAY NIGHT  
THROUGH FRIDAY, BUT LITTLE TO NO PRECIPITATION IS ANTICIPATED  
FOR THE BURN SITE WITH THE BEST CHANCE OF SHOWERS ALONG THE CASCADE  
CREST AND EAST OVER THE BLUE MOUNTAINS. WINDS WILL BE PREDOMINANTLY  
DIURNALLY DRIVEN, STRONGEST IN THE AFTERNOON AND EVENING BOTH DAYS.  
   
THURSDAY
 
 
SKY/WEATHER.........PARTLY TO MOSTLY CLOUDY (40-70 PERCENT). CHANCE  
OF RAIN SHOWERS IN THE MORNING.  
CWR.................6 PERCENT IN THE MORNING.  
CHANCE OF LIGHTNING.6 PERCENT.  
MAX TEMPERATURE.....58-61.  
MIN HUMIDITY........45-50 PERCENT.  
WIND (20 FT)........LIGHT WINDS EARLY MORNING, BECOMING NORTHWEST  
6-10 MPH WITH GUSTS OF 15-20 MPH LATE MORNING  
THROUGH AFTERNOON.  
RIDGETOP WIND.......LIGHT WINDS EARLY MORNING, BECOMING NORTHWEST  
6-10 MPH WITH GUSTS OF 15-20 MPH LATE MORNING  
THROUGH AFTERNOON.  
MIXING HEIGHT.......INCREASING TO 3500-4000 FT AGL BY AFTERNOON.  
TRANSPORT WINDS.....WEST 10-15 MPH.  
VENTILATION INDEX...POOR (170) INCREASING TO FAIR (480) IN THE  
AFTERNOON.  
 
TIME (PDT) 6 AM 8 AM 10 AM NOON 2 PM 4 PM  
CHC OF LTNG (%).0 0 0 10 10 10  
TEMP............38 44 53 56 58 60  
RH..............91 81 59 51 47 45  
20 FT WIND......W 3 NW 4 NW 6 NW 8 NW 8 NW 8  
20 FT WIND GUST.4 6 12 15 15 15  
   
THURSDAY NIGHT
 
 
SKY/WEATHER.........PARTLY CLOUDY (30-40 PERCENT).  
CWR.................0 PERCENT.  
CHANCE OF LIGHTNING.1 PERCENT.  
MIN TEMPERATURE.....33-36.  
MAX HUMIDITY........93-98 PERCENT.  
WIND (20 FT)........NORTHWEST WINDS 5-9 MPH WITH GUSTS OF 10-15 MPH  
IN THE EVENING, BECOMING DRAINAGE WINDS 1-4 MPH  
OVERNIGHT.  
RIDGETOP WIND.......NORTHWEST WINDS 5-9 MPH WITH GUSTS OF 10-15 MPH  
IN THE EVENING, BECOMING DRAINAGE WINDS 1-4 MPH  
OVERNIGHT.  
MIXING HEIGHT.......DECREASING TO NEAR SURFACE OVERNIGHT.  
TRANSPORT WINDS.....WEST 10-15 MPH IN THE EVENING, DECREASING TO 4-8  
MPH OVERNIGHT.  
VENTILATION INDEX...FAIR (270) DECREASING TO POOR (60) OVERNIGHT.  
   
FRIDAY
 
 
SKY/WEATHER.........MOSTLY SUNNY (15-25 PERCENT).  
CWR.................0 PERCENT.  
CHANCE OF LIGHTNING.1 PERCENT.  
MAX TEMPERATURE.....68-71.  
MIN HUMIDITY........30-35 PERCENT.  
WIND (20 FT)........NORTHWEST WINDS 2-5 MPH IN THE MORNING, INCREASING  
TO 6-10 MPH WITH GUSTS OF 10-15 MPH BY AFTERNOON.  
RIDGETOP WIND.......NORTHWEST WINDS 2-5 MPH IN THE MORNING, INCREASING  
TO 6-10 MPH WITH GUSTS OF 10-15 MPH BY AFTERNOON.  
MIXING HEIGHT.......INCREASING TO 6000-6500 FT AGL BY AFTERNOON.  
TRANSPORT WINDS.....WEST 4-8 MPH IN THE MORNING, INCREASING TO 8-12  
MPH BY AFTERNOON.  
VENTILATION INDEX...POOR (70) INCREASING TO FAIR (420) IN THE  
AFTERNOON.  
 

 
FORECASTER...PLUNKETT  
REQUESTED BY...TJ WHITEHALL  
TYPE OF REQUEST...PRESCRIBED  
.TAG 2514475.0/PDT  
.DELDT 05/21/25  
.FORMATTERVERSION 2.0.0  
.EMAIL TJ.WHITEHALL@USDA.GOV  
 
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