Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 280415

Area Forecast Discussion
National Weather Service Tucson AZ
915 PM MST TUE SEP 27 2016

.SYNOPSIS...An area of low pressure aloft will provide a chance of
showers and thunderstorms through Thursday. A slight chance of
showers and thunderstorms Friday will exist east of Tucson, then dry
conditions will prevail this weekend into early next week.


.DISCUSSION...Water vapor imagery and 28/00Z upper-air plots show a
low pressure system aloft over southwest Arizona and extreme
southeast California. Well to our southeast a high pressure system
is located over northeastern Mexico, near southern Texas. Between
these two systems, a strong south to southwest flow resides over
Arizona, with the 28/00Z KTWC sounding showing southerly winds in
the 750-425MB layer, with speeds in the lower levels of this range
around 15 to 25 knots, but in the higher levels they range from 35
to 40 knots and much higher above 400MB. Moisture remains relatively
high with a PW on the sounding of 1.1 inches. CIRA LPW Total imagery
shows values ranging from around 1.4 to 2+ inches over southern
Sonora and central and northern portions of the Gulf of California.
The southerly flow will continue to pump this moisture into Arizona
as the low slowly moves to the northwest during the next 24 hours or
so, as it moves into southern California and southern Nevada through
late Wednesday.

IR satellite imagery shows scattered cloudiness across parts of the
Desert Southwest with radar mosaic still detecting lingering
isolated to scattered showers and thunderstorms continuing to move
rapidly north. That said, will keep a mention of showers and
thunderstorms in the forecast for the remainder of the overnight
period as showers and storms can still be seen south of the border
and moving north.

Temperatures seem to be on track with regard to expected overnight
low temps, but will continue to monitor and will make adjustments
if needed.

See previous discussion below for details beyond tonight.


.AVIATION...Valid through 29/06Z.
Isolated to scattered -TSRA/-SHRA through Wednesday morning.
Isolated to scattered -TSRA/-SHRA to prevail Wednesday afternoon
KTUS vicinity east to the NM border. Otherwise, ceilings generally
9K-12K FT AGL with scattered clouds of 6K-8K FT AGL, and surface
wind easterly to southerly at 5-15 kts with gusts to 25 kts.
Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...A chance of thunderstorms exists area-wide through
Thursday evening, then a drying trend will begin late Thursday and
continue into this weekend. A slight chance of showers and
thunderstorms Friday will continue east of Tucson followed by dry
conditions Saturday through next Tuesday. Expect occasionally gusty
east winds through Thursday, then normal diurnal wind trends to
prevail Friday into early next week.


.PREV DISCUSSION...An upper low pressure system centered over far
northwest Sonora Mexico is progged to fill while moving northwest
into southern California Wednesday morning. The hi-res models
suggest that isolated showers/tstms will continue late tonight. A
north-to-south axis of deeper moisture will maintain isolated to
scattered showers/tstms mainly from Tucson eastward Wednesday and
Wednesday night.

Meanwhile, Tropical Storm Roslyn located at 18.8N/116.6W (or about
515 miles WSW of the southern tip of Baja California) valid 2 PM MST
this afternoon is expected to weaken to a remnant low in about 24
hours. However, the 27/12Z GFS/ECMWF/CMC depict some moisture from
this system to be entrained northeastward across the area Thur.
Thus, precip chances are expanded further west Thur versus Wed to
include the entire forecast area.

Isolated to scattered showers/tstms should continue Thur evening,
then a drying trend will ensue from west-to-east Friday as
southwesterly flow aloft increases. A slight chance of showers/tstms
continues across eastern sections Friday afternoon/evening.
Thereafter, the GFS/ECMWF/CMC were similar with depicting a
deepening upper low over the Pacific Northwest this weekend, then
moving this system eastward across the northern Rockies/northern
Plains Mon-Tue. Stronger westerly flow aloft will shunt the deeper
moisture well east of the area. Thus, dry conditions to prevail Sat-

High temps Wed are forecast to be about 5-10 degs warmer versus
temps achieved this afternoon. A few degs of additional warming will
occur Thur followed by no significant change in daytime temps Fri-
Sat. A very minor moderation in daytime temps is on tap early next
week, with high temps trending a few degs below normal.

A warming trend will occur Tue-Thur followed by no significant
change in daytime temps Fri-Mon. The upshot is that high temps
Thursday into early next week will trend closer to seasonal normals
for late Sep/early Oct.






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