Area forecast discussion 
National Weather Service Phoenix Arizona 
848 PM MST Friday Dec 19 2014 


Synopsis... 
high pressure building in from the west and a dry northwesterly flow 
is expected to keep skies mainly clear...with just some high 
cloudiness passing overhead from time to time...and allow a slow 
warming trend to progress through at least the middle of next week. 
Cooler temperatures are then possible by the end of next week as a 
rather strong and cold weather system begins to move inland from the 
Gulf of Alaska into the western states. 


&& 


Discussion... 
fairly quiet weather day across the region with passing high level cloud 
bands and observed temperatures just a degree above /or below/ 
seasonal normals for the phx area. Broad northwesterly flow 
continues to stream into the West Coast off the Pacific...with 
several north-S orientated bands of clouds noted on evening infrared/WV 
imagery. Latest band into our forecast area currently at the Colorado 
River valley and will continue to track across Arizona through the 
overnight hours. If this band were more widespread or slower in it's 
movement...it could play a role on in adding a few degrees of warmth 
to overnight temperatures. While overnight lows seem on the cool side 
/compared to more mild readings from earlier in the month/ they are 
on track for near normal readings...low to middle 40s for the lower 
deserts and low to middle 30s across the foothill and higher terrain 
locations. 


Only minor updates needed in the form of aligning sky forecasts with 
current trends and the addition of patchy fog mainly across portions 
of Pinal and Yuma counties in Arizona and Imperial County in California. 
Dewpoints generally remain unchanged from readings over the last 
couple of nights and since fog has made repeat performances over the 
last couple of mornings...felt adding mention would not Hurt. Rest 
of the dry weekend and early next week forecast remains on 
track...which is outlined in the previous discussion below. 


&& 


Previous discussion /issued at 130 PM MST/1230 PM PST/... 
tonight through Wednesday... 
dry conditions and mostly clear skies have returned to our County Warning Area today 
as the long-wave upper trough that brought US several days of 
unsettled weather has now moved off well to the east...into the 
Southern Plains. High pressure ridging that is now just beginning to 
become established over the eastern Pacific has now pushed 
light...and dry northwesterly flow back over the region. 
Northwesterly flow is expected to remain in place over the Desert 
Southwest through the middle of next week...with 500mb heights 
rising from their present level near 570dm up into the 576-582dm by 
Monday...then hold at these levels through at least Wednesday. This 
airmass warming is expected to allow Lower Desert highs to warm from 
today/S low to middle 60s up into the 70-75f range by Monday...then 
hold at those levels through at least Wednesday. The dry 
northwesterly flow aloft is also expected to keep skies mainly 
clear...except for some periods of high cloudiness tonight and 
Saturday as a weak trough embedded in this northwesterly flow pushes a 
bit of high-level Pacific moisture inland over the region. A 2nd 
shortwave is then expected to pass well to our north and east on 
Monday and Monday night...but the only impact to our County Warning Area from this 
system will be to increase winds a bit on Monday afternoon...and to 
push even drier air near the surface into our County Warning Area. 


Thursday through Saturday... 
although a change toward cooler weather still looks likely during 
this period...considerable model disagreement has now cropped back 
up on the magnitude of this expected cooldown...with the Euro and 
Gem going back to the idea of a much deeper...and colder trough moving 
into our region...while the operational GFS...and the parallel 13km 
GFS keeps the trough much further to our north...with just some slight 
cooling over our County Warning Area. At this point...given all of this model 
disagreement...with leave the extended forecast alone pending future 
model runs. 


&& 


Aviation... 
south-central Arizona including kphx...kiwa and ksdl...and 
southeast California and southwest Arizona including kipl and kblh... 


Passing middle to high level clouds expected at periods through the 
weekend...generating broken to overcast skies and remaining mostly above 
20kft. Could still see some areas of limited ground fog by the early 
morning...but confidence not high enough for development near any of 
the terminals to warrant inclusion in evening taf packages. Surface 
winds will remain light...gnly 8kts or less...and follow typical 
diurnal headings. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Sunday through Thursday... 
upper level ridging will dominate through the early part of next 
week with warming temperatures and slightly drier 
conditions. Highs will warm to above normal starting Sunday with 
Lower Desert highs in the low to middle 70s for the middle of next 
week. Humidities will remain fairly high through the period with 
minimum values staying above 25 percent. Winds will be light for the 
most part through Wednesday with the exception of occasional 
afternoon breeziness. A Pacific low pressure system may affect the 
region late next week bringing the potential for gusty 
winds...cooler temperatures...and slight chances for precipitation 
across higher terrain areas. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


Visit US on facebook...twitter...and at weather.Gov/Phoenix 


Discussion...Nolte 
previous discussion...percha 
aviation...Nolte 
fire weather...Kuhlman 
















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