Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 211237

Area Forecast Discussion
National Weather Service Phoenix AZ
537 AM MST Fri Jul 21 2017

Updated Fire Weather section.


Thunderstorm activity...most numerous over the higher
terrain...will continue through the weekend with chances expanding
into southeast California Monday. Drier southwesterly flow aloft
develops next week. This will lead to drier air and warmer
temperatures along with fewer storms region-wide.


Shower activity over metro Phoenix wrapped up a while ago and
appeared to be associated with a moisture convergence zone (as
depicted in SPC mesoanalysis) as well as reduced CIN as seen in
temperature profile trends in the PSR sounding between 00Z and
03Z. Meanwhile, light showers continue over portions of southern
Gila County. They have been slowly weakening and this trend will
continue. Elsewhere, only isolated shower activity over northern
and southeast Arizona. Looking at the flow pattern over North
America and adjacent northeast Pacific, dry southwesterly flow
aloft is in place across the NW CONUS and a strong anticyclone
continues to be centered over the central CONUS. Another area of
anticyclonic flow in the mid troposphere is an area from off the
southern California coast extending into the Great basin (with
multiple anticyclonic circulations). In between, is something of a
deformation area over Arizona with some weak northerly flow over
much of our forecast area. A cutoff upper low is located west of
Baja. Further south, the tropical East Pacific is quite active (no
direct impact from those systems in the foreseeable future).

Today looks to be roughly the same environment as yesterday with
weak steering flow aloft and good moisture availability with
1000-700 mb mean mixing ratio values for this afternoon of 10-11
g/kg. However, CAMs are not overly enthusiastic about storm
activity today. This is likely due to some warming aloft and
slightly drier air coming in (as depicted in the some of the forecast
soundings from NAM, GFS). That`s not to say that things will be
quiet as the higher terrain will be active once again and that
means adjacent lower terrain areas will continue to have a chance
of storms during the favored late afternoon and evening hours.
Given the weak steering flow, there is potential for localized
heavy rain to occur (most likely over the higher terrain).

Saturday through Monday...
Northeasterly flow aloft develops Saturday and slowly trends to
east and even southeasterly by Monday with anticyclonic flow
becoming centered over the Great Basin and then into Colorado.
This is a favorable steering flow configuration for lower terrain
areas of central and western Arizona and thus storm chances
continue. In fact, ensemble and deterministic model guidance
indicates that PoPs expand into California early next week with
associated moist advection. There may also be also be an inverted
trough the moves through to aid things. Hard to put a lot of
confidence in model forecasts of features like that this far out.

Tuesday through Thursday...
The Westerlies dig southward along the coast next week leading to
a turn of drier southwesterly flow aloft. This will lead to a
downtrend in storm activity - even over the higher terrain. With
drier air, highs will increase but not to excessive levels.


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

Convective activity will be confined to the higher terrain during
this TAF period, however, an outflow cannot be ruled out during
the late afternoon/ early evening hours (similar to what we saw
today). However, chances for thunderstorms over the terminals will
be very limited today due to a slightly more stable environment.
For planning purposes however, included a northeasterly wind shift
from an outflow in this TAF package at 04Z. However, if an
outflow from the east doesn`t make its way to Phoenix terminals,
it may become difficult for the winds to shift to the east solely
due to diurnal tendencies and thus winds would stay out of the
west through the TAF period.

Southeast California/Southwest Arizona including KIPL and KBLH:

Rather dry airmass will keep FEW to SCT mid- and high- level
cloud layers in place through the taf period. Winds will follow
typical diurnal trends at KIPL. Winds to remain mainly southerly
at KBLH with a few afternoon gusts up to 15-20 kts.

Aviation discussion not updated for amended TAFs.


Sunday through Thursday:
Thunderstorm chances expand into southeast California early next
week with east and southeast flow aloft. A less humid airmass is
expected to develop next week with storm chances becoming only
slight and limited to the higher terrain of south-central AZ.
Afternoon and evening breezes will be a bit stronger as well.


Spotters should follow standard reporting procedures.




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