Area Forecast Discussion
Issued by NWS Phoenix, AZ-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS65 KPSR 271041
Area Forecast Discussion
National Weather Service Phoenix AZ
341 AM MST TUE SEP 27 2016
-- Changed Discussion --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
-- End Changed Discussion --
-- Changed Discussion --Today and tonight...
Upper low centered roughly over the northern Sea of Cortez. One of
the vort lobes associated with the low is approaching the Mexico
border. 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.
-- End Changed Discussion --
South-Central Arizona Including KPHX, KIWA, and KSDL:
Easterly winds to persist for the Phoenix area terminals through the
evening. Another round of elevated and periodically gusty winds are
possible tomorrow. Initial band of clouds/virga/wind continue to
clear to the west-northwest with a narrow pocket of clearing that
should transition through the area over the next few hours resulting
in FEW to SCT skies briefly. Additional mid-level cloud decks will
then transition into the area towards the early morning hours with
additional potential for virga/very light rainfall. Earlier lofting
of dust from regionally strong easterly winds may impact slantwise
vsbys during the morning hours Tuesday.
Southeast California/Southwest Arizona including KIPL and KBLH:
Easterly winds will persist into the evening and make a gradual turn
to more northerly headings during the early Tuesday morning hours.
BKN to OVC mid and high level decks will also persist, along with
some lofted dust moving in from the east which may impact
slantwise/long-range vsbys in the morning. Some virga/sprinkle may
present overnight but be brief enough in nature that even VC mention
in the TAFs would be a bit much.
Aviation Discussion not updated for amended TAFs.
Thursday through Monday...A low pressure system across the Desert
Southwest will bring an increase in moisture that will persist
through Thursday. The chance for showers and thunderstorms will be
highest across northeastern Arizona and across the higher terrain
north and east of Phoenix. However, some rain is possible in the
lower deserts, particularly Thursday. Drier conditions are
anticipated Friday and through early next week as southwesterly flow
develops ahead of an area of low pressure in the Pacific Northwest.
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
.SPOTTER INFORMATION STATEMENT...
Skywarn radio Net activation will not be needed.
Visit us on Facebook...Twitter...and at weather.gov/phoenix