Funnel Cloud in Arizona, September 16, 2017

Area Forecast Discussion
National Weather Service Tucson AZ
205 PM MST Sat Sep 16 2017

.SYNOPSIS...A slight chance of showers and thunderstorms mainly
east to south of Tucson through the upcoming week. Daytime
temperatures will remain near seasonal normals through midweek with
gradual cooling by Friday into the weekend.

&&

.DISCUSSION...Residual boundary layer moisture this afternoon,
especially from Tucson eastward where PWAT values are about 0.75
inches based on derived satellite values with a fairly widespread
cumulus field. However, even with the strong daytime heating, there
is a bit of an inversion just above the boundary layer which has
prevented build-ups resulting in showers and/or thunderstorms. We
can't completely rule out a brief shower this afternoon east of
Tucson but it's quite unlikely so we have removed PoPs below the
mentionable 10 percent level.

A broad but fairly flat trough over the western CONUS for much of
the upcoming week will keep westerly flow aloft across the area.
We've been watching Hurricane Norma situated to the south of the
Baja peninsula which is expected to weaken later this upcoming week
off the western coast of the southern Baja peninsula. It will be
close enough to influence our moisture, especially south and east of
Tucson but the broader westerly flow aloft this far north will keep
the bulk of the moisture to our south across Sonora. As moisture
seeps northward from time to time, there will be a slight chance of
a shower or thunderstorm especially for Cochise County with areas
north and west of Tucson expected to remain dry this upcoming week.
As the troughiness is fairly flat through midweek, heights will be a
bit higher than previously expected, so have bumped up temps upward
a few degrees Tuesday and Wednesday. Medium range guidance is
indicating a more amplified trough by Friday and Saturday moving
into the Desert Southwest which would enhance the afternoon breezes
and subsequently bring in more fall like temperatures. As the trough
moves in, there are some differences on just how much moisture will
lurk just to our south by Friday and Saturday that can be advected
north/northeast into areas mainly from Tucson eastward. Played a
middle of the road approach at this time with slight chance PoPs
east and south of Tucson.

&&

.AVIATION...Valid thru 18/00Z.
FEW-BKN clouds 5K-8K AGL from KTUS/KOLS east. SLY/SWLY SFC wind
diminishes after 17/02Z, becoming variable less than 10 kts through
the night. Winds restrengthen Sunday afternoon SLY/SWLY 10-15 kts
gusting to 25 kts with isolated -TSRA/-SHRA east of KTUS. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...A slight chance of mainly afternoon and early
evening showers/thunderstorms east to south of Tucson through the
forecast period. Afternoon winds increase on Tuesday, continuing
each afternoon through Friday as an upper trough deepens over the
western CONUS. Strongest winds occur Thursday with 20-25 mph
southwesterly flow. Temperatures also dip below seasonal normals
Thursday and Friday.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

GL

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