Scientific Forecaster Discussion
fxus65 kpsr 180545
Area forecast discussion
National Weather Service Phoenix Arizona
1045 PM MST Tue Sep 17 2019
Update...updated aviation discussion.
drying conditions today and Wednesday will bring slightly warmer
temperatures and a return to sunny skies across the lower deserts.
Generally dry and mostly clear conditions will persist into the
upcoming weekend with temperatures within a couple degrees of
normal. Storm chances increase Sunday through early next week.
the Pacific trough that brought Arizona showers and storms last
night has quickly skirted off to the northeast and is currently
situated over Wyoming/Montana. Dry west-southwesterly flow exists
in the wake of this trough and will keep storm chances low in our
forecast area today and even lower tomorrow. Even though there is
enough lingering moisture today across southwest and south-central
Arizona, enough subsidence aloft will really limit storm
development. There is still the possibility for an isolated storm
or two to develop, but mainly North/East/south of Phoenix. Highs
today will be a bit below normal in the upper 90s to low 100s and
tomorrow highs will be a few degrees warmer, mainly in the low
100s. Thanks to the dry west-southwest flow aloft, skies through
tomorrow will remain mostly clear.
Another Pacific trough will quickly follow behind the current one
exiting the region, again diving as far south as the Great Basin
on Thursday. However, this time around moisture will be much more
limited, likely confining rain chances to the higher terrain north
and east of Phoenix. Eventually the trough will exit to the
northeast on Friday leaving behind a slightly cooler airmass with
highs temperatures dropping into the middle to upper 90s.
The synoptic pattern into the weekend will keep broad troughing
over the western U.S., But with weaker flow temporarily across the
Desert Southwest. Little change in moisture availability is
seen for the weekend as dry and mostly clear conditions prevail
over our region. Temperatures will likely warm slightly Friday into
Sunday with highs topping out around 100 degrees over the lower
deserts by Sunday.
Although the monsoon season is not technically over, the forecast
synoptic flow pattern with trough after trough coming into the
western U.S. Shows very little chance of any meaningful monsoon
moisture returning over the bulk of Desert Southwest. We are
currently watching a potential significant upper level low shown
diving into the southwestern U.S. For early next week, possibly
stalling out for a few days. There is fairly little model spread
through Sunday, but model spread increases thereafter with
considerable differences among ensemble members on how to handle
this potential cut-off Pacific low. For now, we have gone slightly
above median guidance for pops since the median was generally
less than 10% for eastern Arizona next Monday into Tuesday. Have
also lowered high temperatures a couple degrees to account for the
potential deep low centering itself over the Desert Southwest.
The track of this low, especially how far south and west of
Arizona it tracks, will likely determine rain potential for early
next week. We also cannot rule out the slight potential for more
significant moisture being drawn northward out of Mexico into
Arizona as shown by the 00z GFS operational run. This possibility
would likely be contingent upon a remnant tropical disturbance
being drawn northward which the 00z GFS also depicts. It is a week
away and many details will probably change, but it is definitely
something to watch.
Aviation...updated at 0545z
South-central Arizona including kphx, kiwa, ksdl, and kdvt:
No aviation concerns through the taf period. Typical diurnal wind
trends are favored through Wednesday, with speeds around 8 kts or
less. A few westerly wind gusts to 15-18 kts are possible
Wednesday afternoon. Skies should remain mostly clear with a few
high terrain cumulus to the east Wednesday afternoon.
Southeast California/southwest Arizona including kipl and kblh:
Dry southwesterly flow aloft to keep skies mainly clear through
Wednesday. Winds to remain mainly westerly through the taf period
at kipl and westerly to southwesterly at kblh, with some
gustiness at both terminals Wednesday afternoon/early evening.
Aviation discussion not updated for amended tafs.
Friday through tuesday:
drier conditions and lighter winds will be seen on Friday through
Sunday with near normal temperatures. Humidity levels will mostly
range from 10 to 20% with gradually lower readings into Saturday.
Another low pressure system seems likely by next Monday,
potentially bringing decent rain chances, higher humidities, and
breezy to windy conditions through Tuesday.
Spotter information statement...
spotters should follow Standard reporting procedures.