Area forecast discussion 
National Weather Service Phoenix Arizona 
805 PM MST sun Apr 20 2014 


Synopsis... 
high pressure will build into the area early this week leading to 
sunny days...clear nights...and sharply warmer afternoon high 
temperatures. Breezy to windy conditions will develop over much of 
the area Tuesday afternoon into Wednesday as a dry weather system 
passes to the north. Temperatures will fall only slight in response 
to this weather system...closer to seasonal normals for the 
remainder of the week. 


&& 


Discussion... 
clearing skies were noted over the forecast area at 03z. A high 
pressure system is still forecast to build over the region 
Monday...followed by a very strong Pacific storm into the western 
states Tuesday. The Tuesday system will be potent for places over 
northern/central Nevada/Utah Tuesday afternoon...however over our 
forecast area...southeast California to south central Arizona...dry and windy 
weather with considerable high clouds are expected. Also in our 
forecast area Tuesday...there is potential for wind gusts in the 25 
to 35 miles per hour range area-wide which have implications for blowing 
dust...and enhanced fire weather danger. Current forecasts look OK 
for now. No updates planned. Previous discussion below still applies. 




Previous discussion...145 PM MST... 
despite a rather subsident...dry northwest flow across Arizona on the 
backside of a departing upper low...sufficient boundary layer 
moisture...sunshine...instability and weak upper cyclonic flow have 
combined to produce considerable cumulus across the Mogollon Rim and 
White Mountains this afternoon. In fact...infrared imagery even showed a 
bit of fair weather cumulus forming over higher elevation locations 
across the Lower Desert. We may see an isolated high based 
thunderstorm drift off the eastern rim and into the eastern portions of zone 
24 later this afternoon and early evening...mainly affecting areas 
to the east of Globe and Punkin Center. Otherwise expect skies 
becoming generally clear early this evening after sunset and the 
associated loss of solar insolation. 


Upper level high pressure ridge will build into the Desert Southwest 
on Monday...leading to some more warming with high temperatures over the 
warmer deserts rising into the middle 90s to near 100 degrees...under 
generally sunny skies. 


Guidance remains quite consistent in calling for a rather vigorous 
and progressive open wave upper trough to move across the area Tuesday 
through the day on Wednesday...bringing breezy to windy conditions 
along with a cooling trend...but no precipitation. This is a very typical 
weather pattern for late April...having the trough pass to the north 
with dry conditions seen across the lower Arizona desert. The most 
significant wind will occur Tuesday afternoon and evening...mainly 
across the western deserts and higher terrain areas of Joshua Tree 
National Park. At this time it appears the winds will remain a bit below 
Wind Advisory levels...but combined with drier air moving in we will 
likely have fire weather concerns...and a Fire Weather Watch may 
need to be issued within the next 24 hours. Otherwise...expect 
cooler conditions on Wednesday with high temperatures dropping from the middle 
to upper 90s down into the middle to upper 80s over the lower deserts. 
Less wind is expected Wednesday however it will still be on the 
breezy side during the afternoon hours. 


The low level frontal passage should clear the forecast area 
Wednesday morning...with modest height falls and surface-700 mb cold air advection. This 
will only be sufficient to cool the region back towards the 
climatological normal Wednesday afternoon...with shortwave ridging 
and 500 mb heights rebounding at or above 576dm by Thursday and Friday. Ensemble 
spread continues to grow for the weekend time frame with each 
forecast iteration. The model trend of negative height anomalies 
covering the northeast Pacific basin remains consistent...however 
the subsequent progression of shortwaves into the western/central Continental U.S. Is 
highly uncertain. 


Operational members have retained good run to run continuity...with 
the GFS bodily deepening a significant single wave into the southwestern 
Continental U.S. Over the weekend...while the European model (ecmwf) ejects several weaker lead 
waves through the downstream quasi-zonal flow while maintaining 
general east Pacific troughing. The operational Gem is somewhat of 
a compromise of these two extremes...but tends to eventually favor 
some measure of deeper troughing near the forecast area. Ensemble 
members exhibit a vast array of possible outcomes...resulting in 
very low forecast confidence next weekend. At this time...unless the 
system digs even deeper to the west of California...could not argue 
for higher precipitation chances with the current shortwave 
propagation trajectories (thats not to say it couldn/T happen...just 
very difficult this time of year). Rather...the primary forecast 
challenge will be determining if/when pronounced cooling affects the 
region. 


&& 


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


Through 19z Monday...clear skies. Light and variable wind under 8 knots. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Wednesday through Sunday... 
building high pressure will move in starting Wednesday...but 
widespread breezy conditions will still persist much of Wednesday as 
the gradient remains somewhat tight behind the exiting low pressure 
system. After light winds on Thursday...breezy conditions return for 
Friday and Saturday as another Pacific low pressure system nears the 
region. Dry conditions along with near to slightly above normal 
temperatures are expected through the coming weekend. Minimum 
humidities will be in the single digits across the lower deserts 
through Friday...before seeing an uptick for the weekend with values 
mainly in the teens. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


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


Discussion...vasquez/cb/MO 
aviation...vasquez 
fire weather...ck 



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