890  
FXUS66 KPQR 211009  
AFDPQR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE PORTLAND OR  
308 AM PDT THU SEP 21 2023  
   
SYNOPSIS
 
DRY WITH WARMER TEMPERATURES FOR THURSDAY AND FRIDAY,  
WARMEST ON FRIDAY AS A SHORTWAVE RIDGE MOVES OVERHEAD. AN ACTIVE JET  
STREAM WILL BRING A MUCH WETTER AND COOLER WEATHER PATTERN SATURDAY  
INTO NEXT WEEK AS A SERIES OF MOISTURE-RICH FRONTAL SYSTEMS PUSH INTO  
THE PACIFIC NORTHWEST.  
 

 
   
DISCUSSION...THURSDAY MORNING THROUGH NEXT WEDNESDAY
 
 
AN UPPER LEVEL LOW WAS CENTERED OVER EASTERN OR EARLY THURSDAY  
MORNING, BRINGING CLOUD COVER AND AREAS OF RAIN TO MUCH OF  
CENTRAL/EASTERN OR. WESTERN WA/OR REMAINED MOSTLY CLEAR WITH LIGHT  
WINDS, AS THE UPPER LOW IS TOO FAR TO THE EAST TO IMPACT LOCATIONS  
WEST OF THE CASCADE CREST. THIS SETUP HAS RESULTED IN CHILLY  
TEMPERATURES ACROSS THE AREA, WITH OBSERVED TEMPS ALREADY IN THE MID  
40S TO LOWER 50S FOR MOST LOWLAND LOCATIONS AS OF 3 AM THURSDAY.  
MODEL GUIDANCE CONTINUES TO SUGGEST THE UPPER LOW IN EASTERN OR WILL  
RETROGRADE WESTWARD A BIT THURSDAY AFTERNOON, WHICH MAKES SENSE AS  
THE STEERING FLOW WILL BE WEAKENING AT THAT TIME. THE SLIGHT WESTWARD  
MOVEMENT OF THIS LOW WILL BRING A RENEWED CHANCE OF SHOWERS TO THE  
OREGON CASCADES (20-40% CHANCE), AND A 10-15% CHANCE IN THE SOUTH WA  
CASCADES. STILL EXPECTING LOCATIONS WEST OF THE CASCADES TO REMAIN  
DRY WITH PLENTY OF SUNSHINE AND PLEASANT AFTERNOON TEMPERATURES IN  
THE MID 70S, EXCEPT 60S AT THE COAST AND 50S IN THE HIGH CASCADES.  
 
MODELS SHOW A SHORTWAVE RIDGE BUILDING INTO WESTERN WA/OR THURSDAY  
NIGHT INTO FRIDAY, WHICH WILL HELP PUSH THE AFOREMENTIONED UPPER LOW  
EASTWARD INTO SOUTHERN IDAHO AND NORTHERN NEVADA. THIS SETUP WILL  
BRING MOSTLY CLEAR SKIES AND RELATIVELY LIGHT WINDS TO THE AREA,  
ALONG WITH WARMER TEMPERATURES FOR FRIDAY. THE DETERMINISTIC NBM HAS  
CONSISTENTLY SUGGESTED HIGHS WILL TOP OUT IN THE MID TO UPPER  
70S ACROSS THE LOWLANDS, EXCEPT LOWER 70S AT THE COAST. OVERALL  
FRIDAY LOOKS LIKE A GREAT DAY TO RECREATE OUTDOORS, DO YARD WORK, AND  
ENJOY THE SUNSHINE (ESPECIALLY SINCE FRIDAY LOOKS TO BE THE LAST  
SUNNY DAY FOR THE FORESEEABLE FUTURE). EVEN WITH THIS WARMUP, IT IS  
WORTH MENTIONING THAT ANYONE WITH OUTDOOR PLANS IN THE HIGH CASCADES  
SHOULD BE PREPARED FOR CHILLY TO NEAR FREEZING TEMPERATURES, MAINLY  
DURING THE OVERNIGHT HOURS THURSDAY NIGHT/FRIDAY MORNING.  
 
BEYOND FRIDAY, MODELS AND THEIR ENSEMBLES CONTINUE TO SUGGEST THERE  
WILL BE A NOTABLE PATTERN SHIFT TO FALL-LIKE WEATHER WITH SEVERAL  
DAYS OF COOL, CLOUDY, RAINY, AND BREEZY CONDITIONS. THIS PATTERN  
SHIFT IS DUE TO THE LOCATION OF THE PACIFIC JET STREAM, WHICH LOOKS  
TO AIM DIRECTLY AT WESTERN WA/OR AND SEND A SERIES OF  
MOISTURE-RICH FRONTAL SYSTEMS INLAND. THE EXACT TIMING OF THE FIRST  
FRONTAL SYSTEM IS STILL UNCLEAR AS SOME ENSEMBLE GUIDANCE KEEPS  
INLAND LOCATIONS MAINLY DRY ON SATURDAY WITH RAIN HOLDING OFF UNTIL  
SUNDAY, EXCEPT AT THE COAST WHERE CONFIDENCE IS HIGH RAIN WILL ARRIVE  
BY LATE SATURDAY (>70-80% CHANCE). EVEN IF RAIN DOES HOLD OFF INLAND,  
IT WILL EVENTUALLY ARRIVE BY MONDAY AT THE LATEST. IN FACT, NBM 12-HR  
POPS ARE OVER 90% MONDAY THROUGH TUESDAY ACROSS THE  
ENTIRE FORECAST AREA, WHICH ALSO APPEARS TO BE THE WETTEST TIME  
PERIOD WITH THE HEAVIEST RAINFALL RATES AND BEST FORCING FOR ASCENT  
DUE TO FAVORABLE JET STREAM DYNAMICS AND QG FORCING.  
 
WHILE CONFIDENCE IS HIGH THAT WIDESPREAD MODERATE TO LOCALLY HEAVY  
RAINFALL WILL OCCUR EARLY NEXT WEEK, CONFIDENCE IS QUITE A BIT LOWER  
REGARDING EXACT RAIN AMOUNTS. NBM V4.1 PROBABILITIES FOR EXCEEDING  
0.5 INCHES OF RAIN FOR THE 48-HR PERIOD ENDING AT 5AM  
WEDNESDAY ARE GENERALLY ABOVE 90% FROM THE COAST TO THE CASCADE  
CREST. PROBABILITIES ARE ALSO QUITE HIGH FOR RAIN AMOUNTS OVER 1 INCH  
(70-95% CHANCE, HIGHEST AT THE COAST AND LOWEST ALONG THE CASCADE  
CREST). PROBABILITIES FOR AMOUNTS OVER 2 INCHES DECREASE  
SIGNIFICANTLY TO 10-30%, EXCEPT 40-65% ALONG THE COAST AND COASTAL  
MOUNTAINS AND 60-80% ALONG WESTWARD-FACING SLOPES IN THE CASCADES.  
REGARDLESS OF THE EXACT OUTCOME, IT IS A SAFE BET TO SAY A WIDESPREAD  
SOAKING RAIN IS ON THE WAY. THIS WILL PUT A SERIOUS DAMPER ON FIRE  
SEASON. WOULD LIKE TO REITERATE THAT THERE IS STILL A NON-ZERO  
CHANCE OF DEBRIS FLOW CONCERNS OVER RECENTLY BURNED AREAS, BUT THE  
ACTUAL LIKELIHOOD IS STILL VERY UNCLEAR AS IT WILL COME DOWN TO  
HOURLY RAIN RATES AND THE POTENTIAL FOR EMBEDDED CONVECTION. IT IS  
STILL TOO SOON TO GET A GOOD HANDLE ON HOURLY RAIN RATES, BUT WILL  
MONITOR FOR THE POTENTIAL CLOSELY OVER THE COMING DAYS. ALSO WANT TO  
NOTE THAT THESE WILL BE RELATIVELY WARM WEATHER SYSTEMS WITH SNOW  
LEVELS ABOVE 7000-8000 FEET, SO NO SNOW IMPACTS ARE EXPECTED ALONG  
THE CASCADE PASSES. -TK  
 

 
   
AVIATION
 
HIGH PRESSURE WILL BUILD OVER THE REGION TODAY,  
RESULTING IN CLEAR SKIES. HOWEVER, WITH CLEAR SKIES, LIGHT WINDS  
AND A COOL AIR MASS, THERE IS AROUND A 20% PROBABILITY FOR PATCHY  
IFR/LIFR FOG DEVELOPMENT ACROSS THE REGION THROUGH 16Z THURSDAY.  
ANY LOWER FLIGHT LEVELS THAT DEVELOP WILL LIKELY SCOUR OUT BETWEEN  
16Z-18Z THURSDAY. AFTERWARDS, EXPECT VFR CONDITIONS THROUGH THE  
REMAINDER OF THE TAF PERIOD.  
 
KPDX AND APPROACHES...VFR UNDER CLEAR SKIES FOR THE MAJORITY OF  
THE TAF PERIOD. 20% PROBABILITY FOR PATCHY IFR/LIFR FOG DEVELOPMENT  
THROUGH 16Z THURSDAY. ANY LOWER FLIGHT LEVELS THAT DEVELOP WILL  
LIKELY SCOUR OUT BETWEEN 16Z-18Z THURSDAY, WITH VFR CONDITIONS  
EXPECTED THROUGH THE REMAINDER OF THE TAF PERIOD. /42  
 

 
   
MARINE
 
HIGH PRESSURE WILL HOLD OVER THE REGION THROUGH FRIDAY,  
WITH NOT MUCH WIND (15 KT OR LESS) AND SEAS HOLDING 5 TO 7 FT.  
A SERIES OF FRONTS WILL START TO ARRIVE SATURDAY, BRINGING SOUTHERLY  
WINDS ACROSS ALL WATERS. THESE FRONTS WILL MARCH ACROSS THE  
REGION THROUGH SUNDAY AND INTO THE START OF THE WEEK. SUNDAY MAY  
BRING FIRST ROUND OF MODEST SOUTHERLY GALES, ALONG WITH SEAS  
BUILDING INTO THE LOW TO MID TEENS. THIS ACTIVE PATTERN WILL  
CONTINUE WELL INTO NEXT WEEK, WITH YET ANOTHER STRONG FRONT  
ARRIVING WEDNESDAY. /42  
 

 
   
PQR WATCHES/WARNINGS/ADVISORIES
 
 
OR...NONE.  
WA...NONE.  
PZ...NONE.  
 

 
 

 
 
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