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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 171829

Area Forecast Discussion
National Weather Service Phoenix AZ
1129 AM MST Wed Jul 17 2019

.UPDATE...Updated Aviation Discussion.


An increase in moisture today will lead to slightly cooler
conditions, but temperatures will remain above normal at least
into the weekend. Higher moisture levels will only allow for
storms across the eastern Arizona high terrain with the lower
deserts remaining dry through at least Saturday. The monsoon looks
to become much more active across the Desert Southwest by early
next week, likely leading to day to day thunderstorm chances
through much of the region.


The subtropical high center is now situated over extreme 
southeast Arizona and is allowing for a modest low level moisture
surge out of northern Mexico. Surface dew points have risen well
into the 60s across southwest Arizona and near 60 degrees through
much of the rest of southern Arizona. Despite the moisture surge,
flow around the high will largely keep much of Arizona under a
fairly dry mid and upper level southwesterly flow through Friday.
Moisture across southwest and south-central Arizona is fairly 
shallow and will not be enough to support any convection today, 
but moisture is shown to be deeper over southeast Arizona and 
likely enough to support some late afternoon and evening storms. 
Hi-res models generally show any thunderstorm activity today will 
remain well to our southeast with only a slight chance of an 
outflow reaching the lower deserts across Pinal County. 

Temperatures today will start off quite warm, but the increase in
boundary layer moisture will keep highs mainly below 110 degrees
across the lower deserts. Even though highs will fall well short
of what we have seen the past couple days, the higher humidity
should make it feel almost as uncomfortable. The subtropical high
center will drift a bit to the northeast Thursday into Friday and
heights aloft will decrease enough to bring further cooling to 
the area. This should lower daytime highs another degree or so, 
but temperatures should remain slightly above normals. No 
additional moisture surges are seen Thursday into Friday, so each 
day will allow for boundary layer moisture to decrease as drier 
air aloft mixes down. Afternoon surface dew points are forecast to
drop from the low to mid 50s today to the middle 40s Thursday and
lower 40s on Friday. This decrease in moisture will also lower 
storm chances across the eastern Arizona high terrain, but at 
least slight chances will remain across far eastern Arizona. 

Models remain in good agreement that a significant pattern change
will be coming late in the weekend into next week. The 
subtropical high is forecast to strengthen and shift northward 
Sunday into Monday with the high center settling over the Four 
Corners region. The first in what should be several inverted 
troughs is shown reaching far southern Arizona on Saturday and 
this will begin the significant moisture increase over the region.
Saturday still looks to be mainly dry over our area, but storm 
coverage should be increasing across the high terrain of southeast
and east-central Arizona. Though moisture levels will likely 
continue to increase on Sunday and likely spread across the 
remainder of southern Arizona, very weak southeasterly steering 
flow should keep most of the thunderstorm activity over the high 
terrain. The lower deserts will likely have to rely on storm 
outflow interactions for storm development on Sunday and this 
should only result in a few isolated storms near the Phoenix area.

There is growing confidence next week may be quite active for 
storm activity as the high center over the Four Corners region 
strengthens and moist east-southeasterly flow across Arizona and 
southern California increases. The GFS and European mostly agree 
the bulk of the Desert Southwest will be under a favorable monsoon
flow for much of next week. Another positive sign is the mid and 
upper level flow is shown to become fairly strong, likely leading 
to more organized storm activity by the middle of next week. Day 
to day temperatures next week should be near seasonal normals, but
will likely depend on the timing and extent of monsoon 
thunderstorms and cloud cover.


.AVIATION...Updated at 1825 UTC

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Expect light to moderate westerly winds to become elevated by mid
afternoon with sustained speeds between 10-15 kts and gusts 
between 20-25 kts. Storms initiating across southeast Arizona this
afternoon will have the potential to produce outflows and blowing
dust that could propagate towards terminals. KIWA, being 
furtherest southeast, looks to have the best odds. Otherwise, 
skies will remain mostly clear today before upper-level clouds 
increase later this afternoon and evening. 

Southeast California/Southwest Arizona including KIPL and KBLH:

At KBLH, winds favoring the south will be very breezy to windy 
with gusts of 24-29 kt. Gustiness will diminish after sunset with
winds becoming light and variable by late tonight. At KIPL 
moderate southeasterly winds will prevail for most of the 
afternoon. By late afternoon winds will switch to the west and 
become very breezy with gusts to 25 kt. By late tonight around 
09Z, light to moderate southeast winds will return. 

Southwest flow aloft will continue to allow for smoke from a 
nearby fire in northern Baja California to infiltrate over 
southeast California and reduced visibility concerns may become 
an issue. Outside of any haziness, expect limited cloud cover 

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday: 
Temperatures will remain seasonal, although still hot. RH values
will drop close to 15-20% during the day but with decent overnight
recovery. Moisture will favor a return of thunderstorms, first
(and mostly) along the higher terrain of central, southern and
eastern Arizona...but eventually with storm chances increasing for
the desert. Right now, Monday and Tuesday afternoons look to have
the best potential for possibly the most active day of storms yet
this summer. Otherwise, winds will follow typical diurnal
tendencies with afternoon breezes.


Spotters should follow standard reporting procedures.