Area forecast discussion 
National Weather Service Phoenix Arizona 
245 am MST Sat Jan 31 2015 


Synopsis... 
rain shower chances will continue for much of the day 
today...especially across south central Arizona...before diminishing 
tonight. Temperatures will remain on the cool side over the central 
deserts...warming to near 70 across southeast California. A return 
to dry weather and a slow warming trend will begin Sunday and 
continue through the end of the next work week. 


&& 


Discussion... 
a deep and wet upper level low pressure system remained situated 
across the Desert Southwest as well as northern Baja California/northwestern Mexico early 
this morning. Main upper low center appeared to be spinning over far 
southern Nevada...as seen in the latest vapor imagery...while at the same 
time a series of vorticity lobes/disturbances rotated around the main low 
mainly skirting the Southern California coast and moving into northern Baja California and far 
southern Arizona. Radar at 2 am showed scattered to numerous showers moving 
northeastward across central and southeastern Arizona...with much less echo 
present over the western deserts and over southeastern California. Airmass is very 
moist across the Lower Desert with precipitable water values running around 1 
inch. However...it is not particularly unstable...as 500 mb temperatures are on 
the warm side running around minus 15-16c per the latest plot data. 
A few lightning strikes have been seen overnight...but mainly to the 
south of Arizona. 


As the day progresses...forecasts continue to call for the main low to 
start shifting off to the southeast...and most of the vorticity lobes or 
disturbances will mostly skirt by to our south as the low moves 
along. The main upper jet has already moved east of our area and 
winds aloft will become quite light for the rest of the day. As 
dynamics continue to taper off...but moisture remains very high...we 
will be primed for continued instability showers across the 
deserts...most numerous across south central Arizona. Probability of precipitation will 
remain in the chance to likely category east of the lower Colorado River 
valley for most of the day...with the highest values over southern Gila 
County. Do not expect much in terms of quantitative precipitation forecast today...most new rainfall 
totals should stay less than one quarter of an inch. We will still 
call for a slight chance of mainly afternoon thunderstorms over the 
central deserts...and should a storm develop...rainfall totals could 
quickly approach one half inch over small areas. Temperatures today will 
stay well below seasonal normals given the cloudy moist 
atmosphere...and we should see highs in the greater Phoenix area 
hover in the low to middle 60s for the most part. 


Overnight tonight...the low will continue to push off to the 
southeast...and as such a drier and more subsident northeast flow 
aloft will spread into the area. We can expect a lingering slight 
chance for mainly evening showers over the central deserts...with 
low end chances across southern Gila County...and by Sunday morning there 
will be just some lingering low/middle cloudiness across south central 
Arizona including the greater Phoenix area. Patchy fog is likely east of 
the lower Colorado River valley Sunday morning...and will be mentioned in 
the forecast package...but lingering clouds will limit the amount of 
radiational cooling and reduce the density and coverage of fog that 
does develop. Otherwise expect partly to mostly sunny skies during 
the day Sunday along with somewhat warmer high temperatures. Look for 
central desert highs climbing into the middle to upper 60s on 
Sunday...reaching to near seasonal normals. 


Monday through Friday... 
the ridge slowly builds into the west Monday through Friday for a 
gradual warming trend...and we can expect desert highs to reach back 
into the low to middle 70s by next Wednesday...with the warmer western 
deserts rising to near 80 by next Thursday...with little change on 
Friday. 


&& 


Aviation... 
south-central Arizona including kphx...kiwa and ksdl... 
short term: moderate to good confidence that widespread rainfall and 
IFR ceilings have ended for the evening. However...scattered showers are still 
developing SW of Phoenix...and may survive into terminals later 
tonight. Visibilities have improved...though mountain obscuration 
will still be a problem. Some areas of MVFR ceilings around the Phoenix 
metropolitan...and may begin to fill in as rain re-enters the area later 
tonight after 06z. 


Sat morning-afternoon: lower confidence that residual moisture will 
allow MVFR br/haze to form Sat morning...and may be more associated 
with incoming new round of rainfall. Somewhat better confidence that 
the next round of showers will impact terminals in the 12z-18z time 
frame...with associated MVFR visibility/ceilings likely. Arrival time may be 
sooner based on recent trends. More scattered activity Sat afternoon...and 
even an isolated ts may be possible...though chances are too remote to 
place in taf package. 


Southeast California and southwest Arizona including kipl and kblh... 
showers should stay south and west of California terminals through 
at least 12z Sat...though residual moisture could form local MVFR 
br/haze (especially at the more favored kipl site). Scattered rain showers and 
potentially brief periods of MVFR ceilings/visibility may be possible later 
Saturday morning as the upper low pivots through the area...though 
confidence is only low to moderate. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Sunday through Thursday... 
drier and significantly warmer weather is expected...as high 
pressure begins building back into the region. High temperatures in 
the lower deserts will start out in the upper 60s on Sunday and 
gradually climb into the upper 70s by Thursday. Winds are expected 
to be light and variable for most of the forecast period. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


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


Discussion...cumulonimbus 
aviation...MO 
fire weather...McLane 


















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