175
FXUS65 KPSR 180913
AFDPSR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PHOENIX AZ
213 AM MST SUN OCT 18 2026
KEY MESSAGES
- CHANCES FOR LIGHT RAIN WILL HEIGHTEN OVER ARIZONA THIS WEEKEND
WITH THE BEST CHANCES FOR ACCUMULATIONS FOCUSED OVER HIGHER TERRAIN
LOCATIONS.
- TEMPERATURES WILL GRADUALLY WARM CLOSER TO THE SEASONAL NORMAL
THROUGH THE MIDDLE OF NEXT WEEK.
- A PACIFIC LOW PRESSURE SYSTEM MOVING INTO THE SOUTHWEST DURING THE
MIDDLE AND END OF NEXT WEEK WILL RESULT IN MUCH BETTER RAINFALL
CHANCES WITH THE POSSIBILITY OF A FEW EMBEDDED THUNDERSTORMS, AS
WELL AS COOLER TEMPERATURES
SHORT TERM /TODAY THROUGH MONDAY/
THE REX BLOCK PATTERN THAT WAS POSITIONED OVER WEST CONUS HAS
ALREADY BEGUN TO BREAK DOWN AS THE HIGH PRESSURE IS NOW OVER THE
NORTHERN PLAIN STATES. THE HIGH PRESSURE WILL CONTINUE TO QUICKLY
MOVE INTO NE CONUS BY LATE THIS EVENING. MEANWHILE THE CUT OFF LOW
TO OUR SW HAS ALREADY BEGUN TO LOOSEN INTO AN OPEN WAVE. AS THE
LOW CONTINUES TO OPEN AND BEGIN TO BE ABSORBED INTO THE MAIN
WESTERLY FLOW ALOFT DECENT MOISTURE ADVECTION THROUGH THE MID
LEVELS WILL OCCUR, LEADING TO PWATS IN EXCESS OF 1" ACROSS MUCH OF
THE CWA. THIS IS NEAR 200-225% OF NORMAL, OR ABOVE THE 90TH
PERCENTILE OF NAEFS MEAN PRECIPITABLE WATER, VERY UNUSUAL FOR THIS
TIME OF YEAR. ADDITIONALLY, AS THE LOW OPENS IT WILL PLACE SOUTH
CENTRAL AZ IN A FAVORABLE POSITION FOR ASSOCIATED POSITIVE
VORTICITY, AIDING IN THE ASCENT OF THE AFOREMENTIONED MOISTURE. AS
A RESULT SLIGHT CHANCES FOR ISOLATED SHOWERS WILL CONTINUE TO
LINGER, WITH POPS BETWEEN 15-30% STRETCHING BETWEEN SOUTHERN
MARICOPA COUNTY THROUGH PINAL COUNTY AND INTO THE HIGHER TERRAIN
EAST OF THE PHOENIX METRO THROUGH THE MORNING HOURS INTO THE
AFTERNOON. BY THIS EVENING, THE UPPER LOW WILL FINALLY BEGIN
EJECTING INTO NORTHERN ARIZONA, BUT EVEN SO, MOIST UPSLOPE THROUGH
GILA COUNTY COULD STILL SUPPORT A COUPLE ISOLATED LIGHT SHOWERS
INTO MONDAY MORNING. BY MONDAY AFTERNOON MOST OF THE LOWER DESERTS
POPS ARE <10% WHILE 20-30% PERSIST OVER THE EASTERN HIGH TERRAIN,
HOWEVER IF ANYTHING DOES DEVELOP, TOTAL ACCUMULATIONS ARE
EXPECTED TO BE MINIMAL.
LARGE SPREAD IN NBM TEMPERATURES REMAIN FOR TODAY AND MONDAY WITH
THE UNCERTAINTY OF SHOWERS, AND DEPTH OF H5 HEIGHTS ASSOCIATED WITH
THE LOW. REGARDLESS, HIGH TEMPERATURES TODAY ARE FORECASTED TO BE IN
THE MID 80S ACROSS THE LOWER DESERTS. BY MONDAY, AHEAD OF THE NEXT
LOW PRESSURE SYSTEM, H5 HEIGHTS WILL INCREASE AS A RIDGE BUILDS OVER
EASTERN CONUS, IN TURN HEIGHTS ALOFT INCREASE SLIGHTLY OVER THE
DESERT SW RAISING TEMPERATURES A FEW DEGREES INTO THE MID TO UPPER 80S
THROUGH MID-WEEK.
LONG TERM /TUESDAY THROUGH SATURDAY/
BY TUESDAY, DEEP TROUGHING AND STRONG NEGATIVE HEIGHT ANOMALIES
WILL AMPLIFY AND INTENSIFY ALONG THE WEST COAST, HOWEVER CONFIDENCE
IN THE TIMING AND DEPTH OF THIS FEATURE CONTINUES TO BE LOW TO
MODERATE. THE DETERMINISTIC'S HAVE STARTED TO COME INTO BETTER
AGREEMENT COMPARED TO 24 HRS AGO BUT STILL SHOW KEY DIFFERENCES.
THE ECMWF FORECAST CONTINUES TO SHOW A SLIGHTLY SLOWER SYSTEM
HOWEVER MUCH SHALLOWER THAN BEFORE. MEANWHILE THE GFS DETERMINISTIC
REMAINS MOSTLY UNCHANGED, WITH THE EXCEPTION OF NOW LEANING TOWARDS
A MORE POSITIVELY TILTED TROUGH THAN COMPARED TO THE ECMWF, AND
THE PREVIOUS GFS RUNS, SPREADING OUT ANY VORTICITY ADVECTION AND
HINDERING BETTER CHANCES FOR STORM DEVELOPMENT. EVEN WITH THESE
DIFFERENCES COMPARED TO PREVIOUS RUNS, THE ECMWF CONTINUES TO
FORECAST A MORE MORE ROBUST RAINFALL SCENARIO IN COMPARISON TO THE
GFS. EITHER WAY, WEDNESDAY AND THURSDAY CONTINUE TO BE THE BEST
WINDOW FOR RAINFALL POTENTIAL, WHILE PATTERN RECOGNITION AND SOME
INSTABILITY PARAMETERS SUGGEST THE POTENTIAL FOR STRONG
THUNDERSTORMS AHEAD OF THE SHORTWAVE PASSAGE. LINGERING POPS
CONTINUE INTO FRIDAY MORNING BEFORE EXPECTING TO DRY OUT BY THE
WEEKEND.
ADDITIONALLY, WITH THE UNCERTAINTY OF THIS NEXT SYSTEM REMAINING,
A WIDE SPREAD IN NBM TEMPERATURES CONTINUE THROUGHOUT THE WEEK,
MOST NOTABLY ON THURSDAY WITH A SPREAD OF 11 DEGREES BETWEEN THE
25TH AND 75TH PERCENTILE. EVEN WITH THIS LARGE OF SPREAD BELOW
NORMAL HIGHS ARE LIKELY LATE THIS WEEK HEADING INTO THE WEEKEND.
AVIATION
UPDATED AT 0530Z.
SOUTH CENTRAL ARIZONA INCLUDING KPHX, KIWA, KSDL, AND KDVT:
NO MAJOR AVIATION CONCERNS ARE EXPECTED DURING THE TAF PERIOD.
WINDS WILL CONTINUE TO FAVOR AN E'RLY COMPONENT THROUGH SUNDAY
EVENING, WITH PERHAPS SOME ISOLATED GUSTS DURING THE MID TO LATE
MORNING HOURS. KSDL WOULD BE MOST LIKELY TO SEE THESE OCCASIONAL
BREEZY CONDITIONS DUE TO THEIR PROXIMITY TO TERRAIN FEATURES. VCSH
WILL CONTINUE TO MOVE OVER THE REGION TONIGHT, BUT OUTSIDE OF
INSTANCES OF ERRATIC WINDS, OPERATIONAL IMPACTS FROM THIS ACTIVITY
ARE NOT ANTICIPATED. CURRENT TAFS HAVE SHRA EXITING THE METRO BY
14-15Z AT THE LATEST, BUT TIMING MAY NEED TO BE ADJUSTED IN
SUBSEQUENT AMENDMENTS IF DIFFERENT FORECAST TRENDS ARE REALIZED.
ABUNDANT CLOUD COVER WILL BE PRESENT THROUGH SUNDAY MORNING BUT
SHOULD BEGIN TO SCATTER OUT LATER IN THE FORECAST WINDOW.
SOUTHEAST CALIFORNIA/SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH:
NO MAJOR AVIATION CONCERNS ARE EXPECTED DURING THE NEXT 24 HOURS.
OUTSIDE OF EXTENDED PERIODS OF LIGHT AND VRB CONDITIONS, WINDS
WILL CONTAIN A W'RLY COMPONENT AT KIPL, WHILE KBLH EXPERIENCES
OSCILLATIONS BETWEEN NW AND NE. CLOUD COVER WILL BEGIN TO SCATTER
OUT OVERNIGHT, WITH MOSTLY CLEAR SKIES RETURNING BY THE BACK-HALF
OF THE FORECAST WINDOW.
FIRE WEATHER
WIND SPEEDS THROUGH THE BEGINNING OF NEXT WEEK WILL BE MUCH WEAKER
THAN THE PAST SEVERAL DAYS WITH MINIMAL GUSTINESS. LIGHT SHOWERS
WILL AFFECT MUCH OF THE ARIZONA DISTRICTS THIS WEEKEND, AND
PARTICULARLY OVER EASTERN DISTRICTS, BUT THE CHANCE FOR WETTING RAIN
IS LOW (~10%). MINIMUM HUMIDITY LEVELS WILL IMPROVE CLOSER TO A 30-
50% RANGE DURING THE FIRST HALF OF NEXT WEEK, THEN LIKELY RISE
FURTHER DURING THE LATTER HALF OF THE WEEK. OVERNIGHT RECOVERY
SHOULD BE GOOD TO EXCELLENT. A PACIFIC WEATHER DISTURBANCE WILL
BRING INCREASED CHANCES OF WETTING RAINFALL, BREEZY WINDS, AND
COOLER TEMPERATURES DURING THE LATTER HALF OF NEXT WEEK.
PSR WATCHES/WARNINGS/ADVISORIES
AZ...NONE.
CA...NONE.
SHORT TERM...RYAN
LONG TERM...RYAN
AVIATION...RW
FIRE WEATHER...18/RYAN
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