Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 271540
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
840 AM MST TUE SEP 27 2016

.UPDATE...

&&

.SYNOPSIS...
A low pressure system over northwest Mexico will slowly move north
through the Desert southwest the next few days. This will lead to variable
cloudiness, cooler temperatures, and a threat of showers and
thunderstorms. Best chances for precipitation look to be today and
tonight. Dry and stable weather will return Friday through next
Monday.

&&

.DISCUSSION...

Looking at current radar, areas of showers and scattered convection
are located along two different max vortices, one in northern Arizona
and another in southern Arizona. For the remainder of the morning, a
little nose of higher MUCAPE along with an area of theta-e
convection will slowly creep northward towards Phoenix, which will
allow shower and storm activity to generate in Pinal county and
primarily south of I-10 in La Paz and Maricopa county. This morning,
the better chances for storms will be in Pinal county where the main
axis and nose of higher MUCAPE values will be. As for the remainder
of the day, another batch of instability driven storms will develop
in south-central and southeastern Arizona, with convective activity
pushing northward (greatest chances for showers and storms from
Maricopa county eastward). With the forecast Phoenix sounding for
this afternoon and evening showing equilibrium levels no higher than
30kft, not anticipating convection to be too strong, however, with
some decent 0-6 km shear and instability forecast, a few stronger
storms may develop and produce small hail and strong winds.

&&

.Previous DISCUSSION...
Today and tonight... Upper low centered
roughly over the northern Sea of Cortez. One of the vort lobes
associated with the low is in southern Arizona. It is helping to
produce a small batch of thunderstorms. The NCAR ensemble picked up
on this development. It goes on to show thunderstorm activity moving
northward and nibbling at the Phoenix metro by mid morning but mostly
stays south of there. Latest HRRR however keeps it south of the
Mexico border but then redevelops activity over Pima County by midday
spreading it into northern Pinal and southwest Maricopa County in the
afternoon. There is some support for this scenario from other hi-res
models (from NCEP and UofA). The main reason for the convection is
cooling aloft (given that we`ve already had moist advection) as the
low drifts northward. Interestingly, despite the deep southeast and
southerly flow, precipitable water is advertised by multiple models
to decline over southern AZ the rest of the morning (especially
portions of southeast AZ). Despite having similar PWAT values, there
are significant differences in the 1000-700mb mean mixing ratios
between the GFS and NAM. The NAM is about 2 g/kg more moist and thus
it has more CAPE. The RAP agrees with the NAM. The SREF shows
increasing CAPE today but median values stay below 500 j/kg over
southern AZ. There will be some CIN to contend with though and thus
PoPs are relatively conservative. SSEO reflects the presence of the
CIN and keeps emphasis of convection south of metro Phoenix today.
There will be enough vertical wind shear for longer lived storms with
more coherent motion than we often see in the Monsoon. SREF shows
median values of 0-6km bulk shear remaining below 30 kts for our area
(but close). Best shear mainly over Cochise and Graham counties where
there will be less moisture. Anticipate some blowing dust issues with
storms moving from Pima County into Pinal and southwest Maricopa this
afternoon. Before the afternoon activity though, we may see some
weaker shower activity as there have been batches of weak echoes
overnight - most noticeably over northern Gila County. With the
approaching vort lobe expect we will see more as the morning
progresses.

Wednesday through Friday...
During the middle and latter part of the week, models show
amplification of the flow pattern over North America and the
northeast Pacific such that anticyclones over the interior West and
northwest Mexico phase up over the Plains while troughing deepens
over eastern North America and the northeast Pacific/western Canada.
In the process, the upper low currently to our south will move slowly
northward and weaken into a short wave and be followed by another
short wave. Lingering moisture and the passing waves keep precip
chances going through Thursday with only very slight chances for our
easternmost areas Friday. The preponderance of the 00Z (plus the 03Z
SREF) indicate that storm activity might be most potent on Thursday
but will mainly be limited to the eastern third of Arizona. Highs
stay near to below normal.

Saturday through Monday...
Dry southwest and westerly flow aloft is in place over the weekend
as the Pacific trough slowly moves inland. Looking at breezy to windy
conditions but nothing overly strong looking yet. Near normal high temps
Saturday then decreasing several degrees by Monday.

&&

.AVIATION...

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

Upper low spinning to our south will keep elevated amounts of
moisture present across the TAF sites over the next 24 hours, with
variable mid and upper cloud decks mostly aoa 10k feet. Hard to
time/track disturbances rotating around the low will bring mostly
light showers from time to time and due to uncertainly will leave
out precip from TAFs until late this afternoon. Model guidance
suggests somewhat better chance of a shower tonight so will leave a
broad brushed VCSH mention in from around 23Z thru the end of the
taf period. Thunder is possible this afternoon and evening but way
too low confidence factor to include in the TAFs at this time.

Gusty east wind is expected today with best potential for gusts over
20kt from 16z thru 22z. Chance of wind turning to the west later
this afternoon at KPHX is low.

Southeast California/Southwest Arizona including KIPL and KBLH:

Upper low spinning to the south will bring variable mid/high cloud
decks to the area next 24 hours with bases mostly aoa 12k feet.
Winds to favor the north through the east with speeds mostly aob
12kt. There will be an outside threat of mostly light showers off
and on but too low confidence to mention in the TAFs at this time,
also an outside threat for thunder this afternoon/evening but again
too low confidence to put vcts in the TAFs.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday...Moist southwest flow aloft will continue
Thursday leading to continued cooler than normal and unsettled
conditions east of the lower Colorado River Valley with the best
chance of showers or afternoon thunderstorms over higher terrain
north and east of Phoenix.

Drier conditions are anticipated Friday and through early next week
as stronger southwesterly flow develops ahead of an area of low
pressure in the Pacific Northwest. Near seasonal normal high
temperatures Friday and Saturday will cool a few degrees below
normal starting on Sunday. Humidity will remain elevated Friday with
most deserts above 20 percent, then a steady drying trend from the
southwest will take place over the weekend and into Monday with
minimum RH values dropping ranging mostly between 10 and 15 percent
by Monday. Winds will follow typical diurnal headings and remain on
the lighter side, with the exception of some afternoon breezes up to
15 to 20 mph in southwest Arizona and southeast California Saturday
and Sunday.

&&

.SPOTTER INFORMATION STATEMENT...
Skywarn radio Net activation will not be needed.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

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DISCUSSION...Hernandez
PREVIOUS DISCUSSION...AJ
AVIATION...CB
FIRE WEATHER...Hernandez/CB



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