Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 221534
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
832 AM MST Fri Sep 22 2017

.SYNOPSIS...On this first day of fall, with the equinox occurring at
102 pm this afternoon, expect another day of gusty winds. This is
due a low pressure system over the western United States. The gusty
winds will also continue on Saturday. Cooler temperatures arrive
today, with readings holding below average through at least the
middle of next week.

&&

.DISCUSSION...Strong southwest flow will continue across the
forecast area today ahead of a deep west coast trough and associated
low pressure system currently centered over northern Nevada. Falling
heights/thickness values will result in 3-6 degrees of cooling today
across southeast Arizona, with more substantial cooling expected
over the weekend as the trough axis swings across the state.

Otherwise, satellite imagery shows two distinct northeast-to-
southwest oriented cloud bands across Arizona. One extends over
central Arizona, from Prescott to Gila Bend into far western Pima
county. The other from Show Low to just east of Tucson to Nogales.
The 06Z GFS model run initialized these clouds bands very well, and
indicates the western most band will shift northeast today, while
the eastern band hangs in across eastern portions of the forecast
area until the trough axis passage during the day Saturday.

Overall, current forecast package in good shape. No updates this
morning. For more information on the latter periods of the forecast
please refer the PREV DISCUSSION section below.

&&

.AVIATION...Valid thru 23/12Z.
Isolated -TSRA/-SHRA this afternoon mainly across the White
Mountains northeast of KSAD. Otherwise, VFR conditions into Saturday
morning. Surface wind late this morning and this afternoon swly at
12-20 kts with gusts to near 30 kts. Surface wind variable in
direction mainly less than 12 kts at other times. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...There is a slight chance of thunderstorms mainly
across the White Mountains this afternoon and Saturday. Dry
conditions will prevail area-wide Sunday into Monday. A slight
chance of thunderstorms returns to the White Mountains Tuesday, then
a slight chance of thunderstorms exists east of Tucson Wednesday and
Thursday. Expect gusty southwest 20-foot winds late this morning
through this afternoon and then again Saturday afternoon. 20-foot
winds will then be terrain driven less than 15 mph Sunday into next
Thursday.

&&

.PREV DISCUSSION...Gusty winds will continue today and Saturday as
an upper level trof slowly moves through the intermountain west.
This feature will also bring cooler temperatures, especially this
weekend, along with a slight chance of precipitation to mainly the
White Mountains on Saturday. Could see a few sprinkles later today
but better chance of measurable on Saturday.

Upper level trofiness will remain over the area most of next week
with some reinforcing energy dropping into the state around mid-week
keeping temperatures below normal. Still some uncertainty in the
models/ensembles on where a potential closed low may develop, which
would bring an increase in moisture from the east. At this time kept
slight chance of showers/t-storms across the eastern areas for the
Tue-Fri time frame.

&&

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

&&

$$

DISCUSSION...Zell

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