FXUS65 KPSR 180230

Area Forecast Discussion
National Weather Service Phoenix AZ
730 PM MST Sun Dec 17 2017

Moderately cool weather will linger through Monday as skies clear
and a weather disturbance moves to the east. Quiet and warm weather
will hold Tuesday and Wednesday before another system ushers in
windy and cool, but still dry conditions Thursday. The cool weather
will remain in place Friday and only exhibit modest warming through
the weekend.


Satellite, radar, and objective analysis depicts a vertically
stacked circulation center southwest of Tucson with shower activity
largely confined to the immediate cold core, as well as downstream
in the warm advection conveyor belt. At this point with the system
generally in a filling/decaying stage, any radar echoes rotating
back towards the forecast area have been mostly evaporating as
drier/downslope air has wrapped into the northern periphery of the
circulation. Best performing high resolution models still suggest
some isold light showers through Gila County tonight, but little
elsewhere. As such, have cut and refined POPs through the evening
and into Monday for much of central AZ.

A sharp delineation across the CWA with respect to cloud cover and
dewpoints may yield an odd distribution of overnight lows where
clear, dry SE CA could cool rapidly tonight while clouds and
residual boundary layer moisture through central AZ constrains
better nocturnal cooling. So in addition to some near term
adjustments in dewpoints considering recent trends, have made some
larger and targeted changes to Monday morning lows. Overall, looks
like a fairly typical mid-December forecast for Monday and Tuesday.


/116 PM MST Sun Dec 17 2017/
Next system will approach Wednesday and swing through Thursday.
Trend continues that this will be a dry system but it'll bring
about an increase in winds and a drop in temperatures. Northerly
winds will be strongest Thursday and we may end up exceeding
advisory level winds. Added patchy blowing dust to the forecast
Thursday for our western CWA where winds are expected to be

The cold air will (H7 temps near -10C) will push afternoon
temperatures down into the 40s and 50s Thu/Fri with morning lows
in the 30s and 40s. We may see sub-freezing temperatures across
the lower elevation valleys of southern Gila County, including the
Globe area. We'll monitor for a potential Freeze Watch.

Broadly looking toward next weekend, the long wave pattern across
North America will feature a pronounced/anomalous ridge over
Alaska which will promote troughing across the western CONUS (and
another ridge over the East Coast). We would expect this pattern
to yield below normal temperatures and a potential for traversing
short waves/precipitation chances. Details will come into focus as
we get closer.


South-Central Arizona including KPHX, KIWA, and KSDL:
Minimal aviation impacts from weather through Monday evening as the
current weather system moves well east of terminal sites. Cigs 10K-
15K ft will scatter and lift by Monday morning with any lingering
light rain staying over mountain locations east of Phoenix. Light
east winds will be preferred overnight though some variability may
occur. A light west wind will finally develop by early/mid Monday

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation problems through Monday evening under clear skies.
Steady north winds near 10kt will weaken overnight with nearly calm
(or light and variable) conditions widespread Monday morning and

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday...
Winds will be light and variable with subtle trends toward
drainage slopes and diurnal patterns. Dry northwest flow will
linger for Tuesday and Wednesday before another colder and more
potent storm system sweeps in from the northwest. Breezy west-
northwest winds will develop overnight Wednesday and into
Thursday, impacting area ridgetops first then mixing into the
lower desert elevations during the daytime. Model forecasts have
backed off any precipitation chances with this system, so
conditions will stay mostly dry over the Districts. Daytime
humidities will be range higher than the last few weeks, 15 to 20
percent for the lower elevations and 30 percent or greater for the
higher terrain. After temperatures rebound in the early week to
slightly above normal readings, they will drop to near normal
readings (60s for the deserts and 40s/50s for the foothills/higher
terrain) by Thursday into the coming weekend.


Spotters should follow standard reporting procedures.





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