000 
fxus65 kpsr 241245 aab 
afdpsr 


Area forecast discussion...updated 
National Weather Service Phoenix Arizona 
545 am MST Tue Jan 24 2017 


Update...updated aviation and fire weather discussions. 


&& 


Synopsis... 
troughing will remain stretched over the interior west through the 
end of the week, loosing access to the Pacific moisture feed. Enough 
moisture will linger over the higher terrain for showers to linger 
into today. Skies will return to more clear conditions as a drier, 
colder airmass settles over the region midweek. Some of the coldest 
low temperatures of this winter season are forecast for Wednesday and 
Thursday mornings. Clear skies, a slow temperature warming trend and 
breezy conditions will develop by the coming weekend. 


&& 


Discussion... 
surface cold front that passed through the forecast area Monday is 
now working eastward just into New Mexico this overnight. Shower 
activity lingers loosely in a line from Williams down through 
northeastern Maricopa County and down south to Tucson this hour. 
Very Stark boundary on overnight infrared imagery and sfc obs W/ pressure 
rises also place the front now to our east, with mostly clear skies 
spanning westward from central Phoenix across the southwest Arizona 
deserts and out towards Imperial/jtnp near and along the leeward side 
of the peninsular ranges. Rain shower activity has mostly wrapped 
for the Lower Desert locales in the wake of the surface front and 
Lower Valley Rain/Mountain snow showers will persist over the higher 
terrain of southern Gila County and on and off through the day. 
Winter Weather Advisory remains out through this morning for 
elevations above 6000ft where the greatest snow accumulations are 
expected and while snow levels continue to fall, a few flurries and 
flakes are possible down as low as 4000ft or so as the main trough 
axis cuts a slow path across northern AZ, fully exiting The Four 
Corners by Wednesday. 


Cold airmass will settle into the area, with afternoon 850mb 
temperatures progged only to peak around 0c this and Wednesday 
afternoons. Values this cool are near daily climo mins per Storm Prediction Center 
sounding climatology (using ktwc as a proxy). Below normal high 
temperatures will result, with many Lower Desert locales holding in 
the 50s and low 60s for the western forecast zones. This incoming 
airmass is the coldest so far this winter season and will be enough 
to drop minimum temperatures on the lower deserts into the low to mid 
30s, possibly 28 degrees both Wednesday and Thursday mornings. 


By the late week and coming weekend, temperatures will slowly recover 
to forecast values closer to seasonal averages if not just a little 
below average. Another low pressure trough will make a trip through 
The Four Corners and Colorado River valley regions from the north 
Friday. Given the overland trajectory of the system and little by way 
of moisture advection into the area ahead of it, pop chances remain 
at or close to zero for the weekend. Sensible weather impacts from 
this dry trough passage will likely be a prolonged period of 
northerly to northeasterly breeziness, with gap/pass winds filtering 
through the mountains northeast of Phoenix as well persistent gusty 
conditions along and west of the Colorado River Valley. High pressure 
will begins to build off the California coast and could gradually expand into 
southeast California and Arizona by early next week. Temperatures trending closer to 
late January normals are probable with dry conditions continuing. 


&& 


Aviation... 
south-central Arizona including kphx, kiwa,and ksdl: 


Weather system has exited to the east, and showers have pretty much 
ended across the greater Phoenix area. Threat for additional precip 
today is minimal so no mention will be made in the tafs. There will 
be lingering lower level moisture in the form of cumulus/SC decks; expect 
genly few-scattered decks in the 3-5k foot range and the 7-10k foot range. 
Cigs are possible in any of these decks but more often than not 
expect the decks to be scattered at most. Still, feel we may see a 
redevelopment of ceilings around 4-5k feet at ksdl and kiwa this 
afternoon - thus the broken decks in those tafs. Will be more 
optimistic at kphx and leave out any cigs today but cannot rule them 
out at times today. Otherwise, winds will struggle to turn to the 
southeast, especially at kphx, today in this Post frontal regime. 
Speed should mostly be below 12kt through the early evening today. 


Southeast California/southwest Arizona including kipl and kblh: 


A general dry west/northwest flow aloft will spread some mid/high 
clouds across the western deserts today with broken decks possible at 
times. May see a few lower SC/cumulus mainly during the afternoon with 
bases genly above 7k feet. Winds to favor the west at kipl and kblh 
today mostly below 12kt. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 


Thursday through monday: cool dry northwest flow aloft will settle 
into the area Thursday for generally sunny skies; high temperatures 
will be well below seasonal normals. For Friday into the weekend, 
high pressure aloft builds along the West Coast allowing a dry 
system to dive south across the Desert Southwest. This will be a dry 
system allowing humidity levels to fall but keeping temperatures on 
the cool side. Strong gusty north winds will develop over the 
western Arizona and far southeast California deserts Friday into 
Saturday with strongest gusts along the lower Colorado River valley. 
Although humidities do fall, minimum relative humidity values typically remain 15- 
20 percent over the lower deserts through the weekend. High pressure 
will remain to the west Sunday into Monday keeping dry north flow 
aloft across the area and allowing high temperatures to finally 
climb back near seasonal normals with warmer deserts into the low 
70s by Monday. Winds will decrease Sunday into Monday but remain 
locally breezy out of the north over the western deserts. 


&& 


Spotter information statement... 
spotters should follow Standard reporting procedures. 


&& 


Psr watches/warnings/advisories... 
Arizona...none. 
California...none. 
&& 


$$ 


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