142
FXUS65 KTWC 181931
AFDTWC
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
1231 PM MST FRI SEP 18 2026
SYNOPSIS
THERE WILL BE A SLIGHT CHANCE OF SHOWERS AND
THUNDERSTORMS ALONG THE ARIZONA/NEW MEXICO STATE LINE AND IN THE
WHITE MOUNTAINS THIS AFTERNOON. OTHERWISE, DRIER AIR WILL MOVE
ACROSS SOUTHEAST ARIZONA OVER THE WEEKEND INTO EARLY NEXT WEEK.
MOISTURE THEN SPREADS BACK ACROSS PORTIONS OF SOUTHEAST ARIZONA
NEXT WEEK FOR A CHANCE OF SHOWERS AND THUNDERSTORMS, MAINLY EAST
OF TUCSON. HIGH TEMPERATURES WILL BE NEAR NORMAL THROUGH NEXT
WEEK.
DISCUSSION
THE UPPER PATTERN EARLY THIS AFTERNOON IS DEFINED
BY A WEAK SHORTWAVE OVER THE FAR SOUTHERN AZ/NM BORDER EXTENDING
INTO NE SONORA MEXICO, A TROUGH OF LOW PRESSURE OVER THE
PACIFIC NORTHWEST/NORTHERN GREAT BASIN AND A STRONG SUBTROPICAL
RIDGE SPREAD OUT OVER THE ENTIRE SOUTHEAST U.S.. THE LATEST GOES
PRECIPITABLE WATER IMAGERY INDICATED A MUCH DRIER AIRMASS ACROSS
SOUTHEAST ARIZONA THAN WHAT HAS BEEN RESIDING OVER US FOR THE
FIRST HALF OF THE MONTH (PWATS BETWEEN 0.80 AND 1.00 INCH WITH
POCKETS OF 0.65 TO 0.80 IN FAR WRN PIMA AND SW COCHISE). STILL
ENOUGH MOISTURE HANGING OUT ALONG THE AZ/NM BORDER TODAY, AND SO
WITH THE WEAK DISTURBANCE AROUND, THERE WILL REMAIN A SLIGHT
CHANCE OF SHOWERS/THUNDERSTORMS INTO THE EARLY EVENING HOURS.
THE DRY AIR WILL CONTINUE TO ADVECT INTO OUR NECK OF THE WOODS
SATURDAY, WITH PWATS BETWEEN 0.50 AND 0.65 INCHES ANTICIPATED
ACROSS THE ENTIRE CWA. THIS WILL OBVIOUSLY SHUT US DOWN COMPLETELY
FOR STORMS THIS WEEKEND.
TYPICALLY, THIS ENTHUSIASTIC DRYING USUALLY THROWS AN EXCLAMATION
POINT ON A WANING MONSOON SEASON, BUT THIS YEAR IT APPEARS THE
TRANSITION INTO OUR TROPICAL-SEASON/OCTOBER MAY BE MURKY. WITH
THE NORTHERN STREAM BECOMING MORE ACTIVE AND AMPLIFIED, THE
TROUGH ACROSS THE PACIFIC NORTHWEST/GREAT BASIN AND FOLLOW-UP
KICKER LOW CURRENTLY NEAR 140W BOTH TRACK EAST THIS WEEKEND,
SHOVING THE SUBTROPICAL RIDGE SW INTO NORTHERN MEXICO. THE
RESULTING SOUTHERLY FLOW ACROSS SONORA/CHIHUAHUA MX WILL DRAW
MOISTURE NORTHWARD AGAIN INTO THE EASTERN PORTIONS OF SOUTHEAST
ARIZONA, MAINLY EAST OF TUCSON. DEPENDING ON THE LOCATION OF THE
SUBTROPICAL HIGH AND THE KICKER LOW THAT SETS-UP ACROSS THE
GREAT BASIN, WE SHOULD HAVE ENOUGH LIFT/MOISTURE AROUND TUESDAY
FOR SCATTERED SHOWERS AND THUNDERSTORMS EAST OF TUCSON WITH
MOISTURE AVAILABLE FOR RECYCLING THE SECOND HALF OF NEXT WEEK.
THE EXTENDED IS LESS CLEAR. THE NHC HAS IDENTIFIED INVEST 99E
AS A BROAD AREA OF LOW PRESSURE 800 MILES SSW OF SW MEXICO THAT
HAS AN 80 PERCENT CHANCE OF FORMING INTO A TROPICAL SYSTEM
THROUGH 7 DAYS. THERE ARE DIFFERENCES WITHIN THE ENSEMBLE MODELS
ON HOW THEY WANT TO TRACK THIS FEATURE. THE 18/12Z ECMWF AND AI
ENSEMBLES ARE THE MOST AGGRESSIVE PULLING THIS FEATURE NORTH
ALONG THE COAST OF MEXICO TO EAST OF THE TIP OF THE BAJA WHEREAS
THE 18/12Z GEFS ENSEMBLES PUSHES IT WELL TO THE WEST OF THE
BAJA TIP. THIS IS MAINLY DUE TO HOW THE MODELS SHUNT THAT
SUBTROPICAL RIDGE INTO NRN MEXICO. THE ECMWF AND AI MODELS
WEAKEN THE FEATURE SIGNIFICANTLY WHEREAS THE GEFS KEEPS IT AS A
DOMINANT FEATURE PREVENTING IT FROM MOVING FARTHER NORTH. MY
HUNCH TELLS ME THAT THE GEFS ARE MORE CORRECT WITH THEIR
SOLUTION BECAUSE OF HOW STRONG THE FEATURE IS CURRENTLY
PRESENTING ACROSS THE SERN STATES. I THINK OF IT LIKE TRYING TO
SHOVE A BEACH-BALL UNDER WATER. SURE, IT WILL COMPRESS, BUT IT
STILL WANTS TO BE BIG AND SPREADS OUT. SO, MAYBE WE SEE SOME
SUBTROPICAL INFLUENCE THE SECOND HALF OF NEXT WEEK INTO NEXT
WEEKEND, BUT IT IS POSSIBLE THAT THE RIDGE CUTS OFF THE MOISTURE
SOURCE AND WE REMAIN DRY. PLENTY OF UNCERTAINTY, SO I THINK THE
BEST COURSE OF ACTION FOR WEDNESDAY THROUGH FRIDAY IS TO
MAINTAIN A CHANCE OF SHOWERS AND THUNDERSTORMS DUE TO RE-CYCLING
MOISTURE.
AVIATION
VALID THRU 20/00Z. MAINLY CLEAR SKIES/SKC CONDITIONS
WEST OF KTUS THRU THE FORECAST PERIOD. FROM KTUS EASTWARD, FEW-
SCT CLOUDS AT 8K-11K FT AGL AND SCT (LOCALLY BKN CLOUDS) AOA
20K FT AGL WITH A SLIGHT CHANCE OF -TSRA/-SHRA ALONG THE ARIZONA/
NEW MEXICO STATE LINE AND IN THE WHITE MOUNTAINS THROUGH 19/03Z.
SFC WIND LESS THAN 12 KTS THRU THE FORECAST PERIOD, FAVORING A
SWLY/WLY DIRECTION THIS AFTERNOON/EVENING AND AGAIN SATURDAY.
WIND VARIABLE IN DIRECTION AT OTHER TIMES. AVIATION DISCUSSION
NOT UPDATED FOR TAF AMENDMENTS.
FIRE WEATHER
THERE WILL BE A SLIGHT CHANCE OF SHOWERS AND
THUNDERSTORMS ALONG THE ARIZONA/NEW MEXICO STATE LINE AND IN THE
WHITE MOUNTAINS THIS AFTERNOON. OTHERWISE, MAINLY DRY
CONDITIONS INTO MONDAY. MOISTURE THEN SPREADS BACK ACROSS
PORTIONS OF SOUTHEAST ARIZONA NEXT WEEK FOR A CHANCE OF SHOWERS
AND THUNDERSTORMS, MAINLY EAST OF TUCSON. HIGH TEMPERATURES WILL
BE NEAR NORMAL THROUGH NEXT WEEK. MIN RH VALUES WILL BE IN THE
12-17 PERCENT RANGE WEST OF TUCSON THROUGH THE WEEKEND AND 17-27
PERCENT IN VALLEY LOCATIONS EAST OF TUCSON (25-40 PERCENT IN
THE MOUNTAINS). RH VALUES THEN INCREASE 7-15+ PERCENT NEXT WEEK
AS MOISTURE MOVES BACK INTO SOUTHEAST ARIZONA. 20-FOOT WIND WILL
BE LESS THAN 15 MPH INTO AT LEAST THE MIDDLE OF NEXT WEEK. GUSTY
AND ERRATIC WINDS WILL ACCOMPANY ANY THUNDERSTORMS.
TWC WATCHES/WARNINGS/ADVISORIES
NONE.
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