596  
FXUS66 KPDT 142334  
AFDPDT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE PENDLETON OR  
434 PM PDT SAT SEP 14 2024  
   
UPDATED FOR AVIATION
 
 
SHORT TERM...THIS AFTERNOON THROUGH MONDAY NIGHT...ISOLATED  
SHOWERS ALONG THE EAST SLOPES OF THE WASHINGTON CASCADES ARE  
EVIDENT IN RADAR AND SATELLITE IMAGERY. 12Z HREF SOUNDINGS SUGGEST  
SURFACE-BASED CAPE OF 150-300 J/KG CAPPED AROUND 15 KFT AGL,  
LIKELY TOO SHALLOW FOR LIGHTNING. RADAR RETURNS PAINT A MORE  
OPTIMISTIC PICTURE FOR LIGHTNING CHANCES WITH RETURNS AS HIGH AS  
20-25 KFT, WHERE TEMPERATURES ARE ROUGHLY -25 TO -30C, PERHAPS  
JUST SUFFICIENT FOR AN ISOLATED STRIKE.  
 
LOOKING AHEAD, A COUPLE VORTICITY LOBES OVER THE PACIFIC WILL  
CARVE OUT A DEEPENING UPPER-LEVEL TROUGH JUST OFFSHORE THROUGH THE  
REMAINDER OF THE DAY AND TONIGHT. ENSEMBLE GUIDANCE IS IN  
EXCELLENT AGREEMENT (>95% CHANCE) THAT THE TROUGH WILL DEVELOP  
INTO A CLOSED LOW THROUGH SUNDAY WITH THE MAIN CIRCULATION CENTER  
OVER SOUTHWEST OR AND NORTHWEST CA. THIS WILL PLACE OUR CWA UNDER  
SOUTHWESTERLY TO SOUTHERLY FLOW IN THE MID- TO UPPER LEVELS, WHILE  
LOW-LEVEL FLOW WILL BECOME NORTHERLY. WITH SUFFICIENT MOISTURE IN  
PLACE, RAIN SHOWERS ARE ANTICIPATED TO DEVELOP OVERNIGHT THROUGH  
SUNDAY ACROSS THE EASTERN MOUNTAINS AIDED BY SYNOPTIC-SCALE  
LIFT FROM AN UPPER-LEVEL JET. SUNDAY AFTERNOON, LOW (10-15%)  
CHANCES OF EMBEDDED THUNDERSTORMS ARE FORECAST FOR SOUTHEASTERN  
PORTIONS OF OUR CWA, PRIMARILY SOUTHEASTERN DESCHUTES, CROOK, AND  
GRANT COUNTIES.  
 
MONDAY THROUGH MONDAY NIGHT, THE UPPER-LEVEL LOW WILL EJECT  
NORTHEASTWARD ACROSS THE NORTHWESTERN GREAT BASIN. WHILE THE  
UPPER JET WILL WEAKEN OVERHEAD, NWP GUIDANCE IS SUGGESTING  
DIFFLUENT FLOW ALOFT COUPLED WITH A MODESTLY UNSTABLE ATMOSPHERE  
OVER THE BLUE MOUNTAINS. AFTERNOON CONVECTION IS MORE LIKELY OVER  
THE IDAHO ROCKIES, BUT ACTIVITY MAY TRACK INTO OUR CWA.  
PROBABILITIES OF ADDITIONAL UPSLOPE SHOWERS RANGE FROM CHANCE  
(25%) TO LIKELY (60%) ACROSS THE NORTHERN AND EASTERN BLUE  
MOUNTAINS OVERNIGHT.  
 
WHILE CONFIDENCE IS VERY HIGH IN THE OVERALL SYNOPTIC PATTERN,  
ENSEMBLE CLUSTERS SUGGEST SOME UNCERTAINTY REMAINS REGARDING THE  
PRECISE TRACK OF THE LOW AS IT COMES ONSHORE AND TRACKS ACROSS  
THE REGION. THE MOST IMPACTFUL FORECAST VARIABLE THAT THE TRACK  
WILL MODULATE IS QPF. WHILE NBM CHANCES OF WETTING RAIN (0.10" OR  
GREATER) ARE MEDIUM-HIGH (40-70%) FOR THE 48-HR PERIOD TONIGHT  
THROUGH MONDAY NIGHT ACROSS MOST OF THE BLUE MOUNTAINS, INDIVIDUAL  
GLOBAL ENSEMBLE QPF CLUSTERS PLACE A SHARP SOUTHWEST-NORTHEAST  
GRADIENT ACROSS EASTERN OREGON; THE MOST LIKELY SOLUTION (~50% OF  
MEMBERS) PLACES THE HEAVIEST QPF (CLUSTER MEAN OF 0.10-0.50") OVER  
SOUTH- CENTRAL AND SOUTHEAST OREGON INTO WESTERN IDAHO, THE  
REMAINING CLUSTERS ARE SPLIT (25%/25%) IN PLACING A SUBSTANTIAL  
(0.5-1.5") OR LIGHTER (UP TO 0.50") AMOUNT OF QPF OVER THE BLUE  
MOUNTAINS, HEAVIEST OVER NORTHEAST OREGON.  
 
WINDS WILL SWITCH TO WESTERLY AND BECOME BREEZY TO WINDY MONDAY  
EVENING THROUGH MONDAY NIGHT. PLUNKETT/86  
 
LONG TERM...TUESDAY THROUGH SATURDAY...  
 
KEY MESSAGES:  
 
1. BREEZY WINDS MIDWEEK.  
 
2. SCATTERED SHOWERS TUESDAY AND WEDNESDAY.  
 
3. BELOW NORMAL TEMPERATURES EXTEND THROUGH THE PERIOD.  
 
THE EXTENDED PERIOD IS CHARACTERIZED BY AN INCOMING UPPER LEVEL  
TROUGH TUESDAY AND WEDNESDAY, FOLLOWED BY AN UPPER LEVEL RIDGE  
BUILDING ACROSS THE AREA THROUGH THE REMAINDER OF THE WEEK. THE  
DIGGING UPPER LEVEL SYSTEM AND ASSOCIATED COLD FRONT WILL BRING  
SCATTERED SHOWERS TO THE AREA THROUGH WEDNESDAY BEFORE DRYER AND  
WARMER CONDITIONS RETURN. THESE SYNOPTIC FEATURES WILL ALSO ALLOW  
PRESSURE GRADIENTS TO TIGHTEN ACROSS THE CASCADES, LEADING TO BREEZY  
CONDITIONS THROUGH THE GORGE AND KITTITAS VALLEY. THE MOST IMPACTFUL  
CONCERNS WILL PRIMARILY BE FOCUSED ON THESE ENHANCED WINDS THROUGH  
THE GORGE, FOOTHILLS, AND THE KITTITAS VALLEY TUESDAY. SUSTAINED  
WINDS OF 25-35 MPH WILL BE POSSIBLE WITH GUSTS OF 30-40 MPH OUT OF  
THE WEST-NORTHWEST OVER THESE AREAS. ELSEWHERE, 10-15 MPH SUSTAINED  
WINDS AND GUSTS OF UP TO 20 MPH ARE EXPECTED. CONFIDENCE IS MODERATE  
TO HIGH (60-80%) IN THESE WIND VALUES AS THE NBM ADVERTISES A 70-90%  
CHANCE OF SUSTAINED WINDS OF 30 MPH AND GUSTS OF 35 MPH ACROSS THE  
GORGE AND KITTITAS VALLEY OVER TUESDAY AFTERNOON. PEAK TIMING FOR  
THESE INCREASED WINDS WILL BE LATE MORNING THROUGH THE EARLY  
AFTERNOON ON TUESDAY BEFORE WINDS LOOK TO SLACKEN THROUGH THE  
EVENING INTO WEDNESDAY MORNING.  
 
SHOWERS ASSOCIATED WITH THE INCOMING SYSTEM ON TUESDAY WILL  
PREDOMINATELY OCCUR OVER MOUNTAINOUS TERRAIN ACROSS THE CASCADES AND  
BLUES UNTIL THE AFTERNOON, BEFORE EXTENDING INTO CENTRAL OREGON,  
BLUE MOUNTAIN FOOTHILLS, AND PORTIONS OF THE BASIN BY LATE  
AFTERNOON. THIS IS IN RESPONSE TO THE TROUGH AXIS DROPPING TO OUR  
SOUTH AS THE ASSOCIATED COLD FRONT PASSES THROUGH THE REGION  
OVERNIGHT INTO WEDNESDAY MORNING. WETTING RAIN (>.10") CHANCES WILL  
BE LOW TO MODERATE (10-35%) FOR THE AREA, WITH THE EXCEPTION OF  
WALLOWA COUNTY AND THE EAST SLOPES OF THE CASCADES (60-80%). RAIN  
CHANCES (20-45%) WILL LINGER INTO WEDNESDAY AFTERNOON ACROSS THE  
CASCADES AND THE EASTERN MOUNTAINS BEFORE UPPER LEVEL RIDGING BEGINS  
TO PUSH INTO THE AREA FROM THE WEST. THE CHANCE FOR A WETTING RAIN  
OVER MOUNTAINOUS TERRAIN IN THE CASCADES AND EASTERN MOUNTAINS ON  
WEDNESDAY IS RATHER LACKING, AS ONLY 10-30% IS SHOWCASED BY THE NBM.  
 
ENSEMBLES AND DETERMINISTIC MODELS ARE IN GOOD AGREEMENT WITH THE  
OVERALL PATTERN, BUT SLIGHT DIFFERENCES IN SYSTEM SPEED IS OBSERVED  
VIA THE 500MB EOF PATTERNS. THIS WOULD CORRELATE TO A SLIGHTLY  
EARLIER ONSET OF PRECIPITATION AND ELEVATED WINDS TUESDAY  
MORNING/AFTERNOON. HOWEVER, THIS IS ONLY DEPICTED BY 49% OF ENSEMBLE  
MEMBERS, SO IF IT DOES SHIFT EARLIER IT IS EXPECTED TO BE SLIGHT.  
44% OF MEMBERS ALSO HINT AT A STRONGER INCOMING SYSTEM, VERSUS 34%  
SUGGESTING A WEAKER SYSTEM, AND 22% STAYING SIMILAR TO THE CURRENT  
FORECAST. A STRONGER SYSTEM WOULD LEAD TO A BETTER POTENTIAL FOR  
SHOWERS, COOLER TEMPERATURES, AND HIGHER WINDS, BUT CONFIDENCE IN A  
STRONGER SYSTEM IS CURRENTLY LOW TO MODERATE (20-40%) DUE TO THE  
VARIABLE SPREAD OF ENSEMBLE MEMBERS. HOWEVER, ENSEMBLES ARE IN GOOD  
AGREEMENT WITH THE INCOMING UPPER LEVEL RIDGE MOVING INTO THE REGION  
LATE WEDNESDAY INTO THURSDAY AND STRENGTHENING INTO THE WEEKEND,  
LEADING TO A 80-100% CHANCE OF HIGH TEMPERATURES STAYING 5 TO 10  
BELOW NORMAL THROUGH THIS PERIOD. 75  
 
AVIATION...  
00Z TAFS...VFR CONDITIONS EXPECTED FOR THE TAF PERIOD. GUSTY  
WINDS TO AROUND 25 KTS AT DLS AND 20 KTS AT PDT, RDM AND BDN WILL  
LINGER INTO EARLY EVENING BEFORE TO LESS THAN 10 KTS. BY EVENING  
ALL SITES WILL BE 10 KTS OR LESS EVERYWHERE FOR THE REMAINDER OF  
THE TAF PERIOD.  

 
   
PRELIMINARY POINT TEMPS/POPS
 
 
PDT 50 65 49 72 / 10 30 30 20  
ALW 53 67 50 75 / 10 40 40 20  
PSC 56 72 55 80 / 10 10 20 10  
YKM 46 74 47 82 / 10 10 10 0  
HRI 54 71 53 78 / 10 20 20 10  
ELN 47 74 48 81 / 10 10 10 0  
RDM 44 64 42 69 / 10 20 30 20  
LGD 49 64 48 69 / 30 60 60 40  
GCD 47 63 48 67 / 30 60 60 40  
DLS 52 75 53 80 / 0 10 10 0  
 

 
   
PDT WATCHES/WARNINGS/ADVISORIES
 
 
OR...NONE.  
WA...NONE.  

 
 

 
 
SHORT TERM...86  
LONG TERM....75  
AVIATION...77  
 
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