Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 211136

Area Forecast Discussion
National Weather Service Phoenix AZ
436 AM MST Thu Sep 21 2017

.UPDATE...To Aviation and Fire Weather Discussions...


An area of low pressure and associated cold front will sweep through
the region over the next two days. The end result will be
substantially cooler temperatures along with occasionally breezy
conditions. Precipitation chances will be neglible and limited to
only some sprinkles over the higher terrain locations. Temperatures
will only slowly warm back to near average next week.


A positively tilted, yet seasonally deep negative height anomaly
through the NW quarter of the Conus will nearly stall over the Great
Basin and Southwest through early next week as downstream high
amplitude ridging temporarily blocks the progression of the westerly
flow pattern. H5 heights below 570dm will hover over the northern
half of Arizona through the weekend easily providing the coolest
weather of this burgeoning autumn season. The blocked flow regime
will be stubborn to relinquish its hold on the country even into the
middle of next week with somewhat lower heights likely lingering
over the forecast area towards the end of the month. Despite this
blocking pattern and larger anomaly framework, ensemble output
displays remarkable agreement through early next week yielding
better than expected forecast confidence.

Though the first vestiges of height falls and cold advection aloft
are already impinging on SE California early this morning, the more
pronounced impacts will be experienced late this afternoon and
evening with a cold front and much lower theta-e airmass crossing
the San Diego mountains. Fropa timing looks quite favorable for
tapping and transferring momentum in the H8-H7 layer (40 kt above
the shallow windward marine layer) and enhancement through the
formation of stronger mountain rotors across the western half of
Imperial County. The wind advisory was expanded slightly east
towards terrain features into central Imperial County though impacts
will be most evident along the I-8 corridor from El Centro westward.
Otherwise further east into central Arizona, there is model evidence
of enhanced sfc pressure packing along a pre-frontal trough such
that some localized blowing dust coincident with 30 mph gusts along
I-8 from Gila Bend to Casa Grande is also possible.

The aforementioned cold front will forge through the state Friday,
finally clearing into the New Mexico on Saturday. Moisture
availability along this boundary will be tenuous, at best with only
a few model iterations suggesting a narrow moist tongue present
within a frontal based orographic upslope regime (albeit before more
supportive cooler temperatures aloft advect eastward). Based on
forecast soundings, little more than a brief shower/sprinkle looks
possible along this front. Winds will remain occasionally gusty
Friday afternoon though begin to weaken after fropa and sunset
Friday evening.

More notable through the weekend will be the unusually cool
temperatures. Ensemble means depict heights and temperatures aloft
within the trough core some 3 normalized standard deviations below
normal, or below the 2nd percentile for the end of September. Closer
to central Arizona, anomalies will be closer to the lowest 10th
percentile, that is, historically equating to highs only in the
middle/upper 80s. Even more amazing will be overnight lows in the
50s and lower 60s and these readings some 10F-15F below average will
persist into early next week.

The overall troughing structure will be maintained over the SW Conus
into the middle of next week, however heights will slowly rise as
weakening and filling of the trough begins. Downstream eastern Conus
ridging will also weaken allowing for a pattern shift/adjustment by
the end of next week. At this point, operational and ensemble output
diverge with respect to how much vorticity/circulation remains
centered over Arizona or gets displaced by high pressure building
from the east Pacific. Regardless, temperatures will undoubtedly
moderate much closer to the seasonal normal beyond the middle of the


South-Central Arizona including KPHX, KIWA, and KSDL:
Typical east-southeasterly AM winds will turn southerly midday
while beginning to pick up in speed. A continued swing to west-
southwest headings will occur between midday and early afternoon
with gusts 20-25kts possible. Westerly oriented winds will stay
longer in the terminals overnight than usual, switching early
Friday AM to easterly headings for a brief period. Conditions will
remain dry with limited thin high clouds possible early Friday AOA

Southeast California/Southwest Arizona including KIPL and KBLH:
Strong cool season frontal passage will generate windy conditions
for the terminals today and into the overnight. Wind gusts
25-35kts will possible, more likely at KIPL than KBLH, from west-
southwest headings by the afternoon. Skies will remain mostly
clear and winds will subside into the overnight Thursday.

Aviation Discussion not updated for amended TAFs.


Saturday through Wednesday:
Post-frontal weekend temperatures will fall below seasonal normals
for the districts with periodic breeziness possible over area
ridgetops. Conditions will largely remain dry and under mostly
clear skies as troughing holds over the region. Some moisture from
the east may begin to be wrapped into trough circulation mid next
week, raising area dewpoints and introducing some very slight
chances for precipitation over southeast AZ and the White Mtns.
Overall, humidities will fall mainly into the 15-30 percent range
with good overnight recoveries.


Spotters should follow standard reporting procedures.


CA...Wind Advisory from 11 AM this morning to 1 AM PDT Friday for



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