Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 172101
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
200 PM MST Tue Oct 17 2017

.SYNOPSIS...
High pressure will persist across the region through the middle of
the week allowing for continued very warm and dry conditions. High
temperatures for lower desert communities will generally reach
into the middle and upper 90s. Somewhat breezy and cooler weather
with more cloud cover will arrive during the latter portion of the
week, including a slight chance for very light showers Thursday over
south central Arizona. High pressure returns quickly by the end of
the weekend, sending temperatures back to above normal readings.

&&

.DISCUSSION...
Early this afternoon, strong high pressure aloft persisted across
the desert southwest; according to the 12z 500mb plot data, heights
were down just 10m and still running about 591dm over southern
Arizona. Temperatures over the central deserts were running nearly
the same as 24 hours ago and despite a modest increase in high
clouds, highs today should end up about the same with warmer deserts
reaching into the upper 90s. Again, the ridge was starting to shift
just a bit to the southwest and as it flattened it allowed some
subtropical high clouds to spill into the area with some of the
thickest clouds moving into far southeast Arizona.

Although there have been continued changes in the details over the
past many model runs valid for the middle of the week, the
operational guidance has been relatively consistent with the big
picture for far southeast California and southern Arizona. The big
picture is generally dry westerly flow with very little cooling.
Over the next couple of days the upper ridge will very slowly weaken
as a series of weak waves - embedded in the westerly flow aloft -
move inland across the state. These features will not be strong
enough to bring any consequential cooling; in fact the warmer
deserts will stay in the mid to upper 90s both Wednesday and
Thursday. There will be just enough QG forcing (very modest amounts
of mid level q convergence will move across southern AZ Wednesday
night through Thursday) to act on a bit of elevated mid level
moisture to kick off a few very light showers across south-central
Arizona. POPs have been changed very little and will stay mostly in
the low teens to single digits with best chances for light precip
starting after midnight Wed night and running thru Thursday
afternoon. Considering how dry the subcloud layer is forecast to be,
there is a much better chance that we will simply see some virga and
there should be little or no measurable rainfall with the passing
waves.

By Friday, another stronger Pacific trough will move into the
Pacific Northwest with heights trailing southward through the
Great Basin. Trough heights will weakly swing through the Southwest
overnight Friday into Saturday ushering in a drier airmass and
slightly cooler temperatures for the weekend. Best pocket for height
falls and jet influenced ascent have been drawn well to our north
with the latest deterministic and ensemble runs. Increasing winds
will be the first sensible impact across the region with breezy to
windy conditions for Friday, and to a lesser extent Saturday
afternoons. Gusts around 20mph will be possible over the lower AZ
desert locales while spots downwind of the Peninsular Ranges in
southeast CA could see gusts nearing 30mph at peak on Friday. While
timing is still a ways out, could not rule near advisory conditions
in the SW corner of Imperial County sometime Friday into Saturday as
winds through the gap/pass respond well to trough passages. Second
sensible impact with be a cooling trend for temperatures dipping
closer towards seasonal normals for the weekend.

Model agreement in the extended is pretty good for being 5-6 days
out with run to run continuity further boosting forecast confidence.
Upper level pattern reamplifies with a strong ridge of high pressure
developing over much of the U.S. West Coast. Initial 500mb height
forecasts are stronger than the ridging from Monday (that produced a
high of 99 and 97 for Phoenix and Yuma respectively) and NAEFS
percentiles already are in the 90-97.5 percentiles for temperatures
and heights across much of CA and eastward into AZ. Resultant
forecast has temperatures returning to the mid 90s for Sunday and
mid to upper 90s for Monday and Tuesday for the lower elevations.
Forecasts could run 10 degrees above late October normals and
threaten records in some locations.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL, and
Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation concerns at any of the terminals for at least 24 hours.
Gradients are now rather weak at the low levels, and winds in the
greater Phoenix area should follow typical diurnal tendencies
through Wednesday afternoon, with light west wind through the
evening becoming mostly east after midnight. Winds at KBLH and KIPL
will mostly be light variable although KIPL could favor light west
winds overnight tonight. Otherwise expect variable amounts of mainly
high clouds this afternoon through the day tomorrow with decks
mostly SCT to BKN above 15k feet.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday:

A broad area of low pressure will pass by mainly to the north of
Arizona Friday into Saturday, bringing dry northwest flow aloft
along with cooler high temperatures. High temperatures over the
warmer lower deserts will drop to around 90 by Saturday. Expect
breezy mainly west winds Friday afternoon becoming more northerly on
Saturday. Minimum RH values Friday will be a bit elevated and range
mostly from the mid teens through low 20s; drying will occur
Saturday as RH values fall into the 10 to 15 percent range. Sunday
into early next week we can expect strong high pressure to quickly
build into the area from the west bringing much warmer and drier
conditions; by Monday high temperatures over the warmer lower
deserts should reach back into the mid to upper 90s and minimum RH
values will fall well into the single digits. Winds each day will be
light across south central Arizona, with some local breezes from the
north across the western deserts each afternoon.

&&

.CLIMATE...

PHOENIXForecastNormalRecord
Tue Oct 179888102 in 2009

YUMA     ForecastNormalRecord
Tue Oct 17 100 90 104 in 1959

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

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

&&

$$

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DISCUSSION...Nolte/CB
AVIATION...CB
FIRE WEATHER...CB
CLIMATE...Nolte



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