Area forecast discussion 
National Weather Service Phoenix Arizona 
435 am MST Tuesday Apr 21 2015 


..updated aviation and fire weather... 


Synopsis... 
the region will get one more day of above normal temperatures before 
a broad area of low pressure brings changes starting on 
Wednesday...lasting through the end of the work week. We can expect 
considerably cooler temperatures for Thursday and Friday along with 
chances for showers and possibly a thunderstorm both days. High 
pressure and warmer temperatures will eventually settle back into 
the region for the coming weekend. 


&& 


Discussion... 
southerly flow aloft has settled over the region ahead of an 
approaching upper level low that is currently centered just off the 
Southern California coast. Current satellite imagery reveals little 
in the way of any middle and upper level moisture near and around the 
low circulation as only ocean stratus is present. Water vapor 
imagery indicates very dry air around the southern periphery of the 
broad trough...extending into southern Arizona. There was enough 
boundary layer moisture to spark off a few showers and thunderstorms 
in a favored vertical accent area just east of the low circulation 
in an area over the northern Baja California mountains late yesterday 
afternoon. As this first low circulation drifts eastward today 
moving over Southern California by this afternoon...model guidance 
shows the possibility of more showers and thunderstorms...mainly 
focused over the inland mountains of Southern California. Slight 
chance probability of precipitation for our jtnp area are in the forecast for this 
afternoon. Elsewhere...dry conditions will remain with some slight 
cooling as the low enters the region. Highs this afternoon should 
peak in the middle to upper 80s across our western deserts and upper 
80s in south-central Arizona. 


The evolution of the broad low pressure trough remains somewhat 
uncertain as model differences remain with subtle changes from run 
to run. Compared to the previous couple days model runs...the latest 
12z and 00z model runs of the GFS and European both scale back the 
strength of the second shortwave trough forecast to move through the 
region on Thursday. This slightly weaker wave solution likely won't 
affect how much low level moisture makes its way into Arizona as 
this has not changed...but weaker broad scale upper level forcing 
and slightly weaker upslope low level southerly flow keeps US from 
raising probability of precipitation any further. Thursday afternoon into Thursday night 
still looks to be the best time frame for any showers and maybe a few 
thunderstorms with the highest probability of precipitation over the higher terrain north 
and northeast of Phoenix. There is still quite a bit of doubt 
whether the lower deserts will see much in the way of measurable 
rainfall. 


Temperatures will take a big hit for Thursday with highs most likely 
in the upper 70s to around 80 across the deserts...but with the 
possibility of showers during the day some locations may struggle to 
reach above the low to middle 70s. The main shortwave trough should 
quickly exit the area late Thursday night...but guidance continues 
to show at least a slight chance of some lingering showers over 
south-central Arizona into Friday morning and possibly into the 
afternoon. 


The overall synoptic pattern remains progressive going into the 
weekend with another weak shortwave trough moving through sometime 
Friday night or Saturday. For now...this feature seems to be too 
weak to add in any probability of precipitation for Saturday...but we will have to keep any 
eye on how the models resolve this feature in coming model runs. 
Temperatures will gradually recover this weekend with highs back to 
around normal by Sunday. Even warmer air is shown by both the GFS 
and European for early next week as a rather strong Pacific Ridge 
slides over the western United States. This may result in high 
temperatures well into the 90s by the early to middle of next week. 


&& 


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


Only few-scattered middle/high clouds through Wednesday morning yielding 
little to no aviation concerns. Light winds this morning will become 
occasionally gusty during the afternoon and early evening. A 
westerly component possibly touching 20 knots will be common for central 
Arizona...while stronger S to SW gusts 20-30kt will be possible at 
kblh and kipl. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Thursday through Monday... 
cooler and more humid conditions will prevail Thursday and Friday as 
a weather disturbance crosses the district. Isolated to scattered 
showers and thunderstorms will also be possible...however coverage 
of wetting rains will be extremely limited and more favored towards 
the higher elevations of central/eastern/northern Arizona. Afternoon 
humidity levels will only fall into a 15 to 30 percent range with 
good overnight recovery. More typical late April weather will return 
over the weekend with much warmer temperatures and substantially 
drier conditions...however winds will remain rather light limiting 
fire weather concerns. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 
California...none. 


&& 


$$ 


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


Discussion...Kuhlman 
aviation...MO 
fire weather...MO 






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