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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