Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 171125

Area Forecast Discussion
National Weather Service Phoenix AZ
425 AM MST Tue Oct 17 2017

...Updated Aviation...

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 end of the week.
High pressure returns quickly by the end of the weekend, sending
temperatures back to above normal readings.


Upper level ridge axis has cleared the area per evening streamline
analysis with the ML axis over western AZ/southern NV. Scattered to
broken high level cirrus is streaming into the region from the west;
a more visible signal of the flattening ridge heights and early
indication of the transition more zonally oriented flow is underway.
500mb heights will gradually lower a decameter or two through the
day with the 590dm line progged to sink southward as the first in a
series of shortwave troughs moves across the northern Rockies.
Daytime temperatures will continue their above normal trend again
with mid to upper 90s forecast for all the lower desert elevations.
Passing high cloud cover, albeit thin, in combination with the
lowering heights should shave a degree or two off of high`s compared
to those from Monday.

By mid-week, higher thicknesses and ridge heights will be removed
to our south and east as troughing moves onshore into the West.
One shortwave of interest in the increasingly zonal flow will have
a tropical origin and is currently west of the Baja Peninsula
around 120W. The feature will transition through SoCal and into
northern AZ Wednesday and Thursday bringing increased cloudiness and
some moisture into region. PWAT forecasts continue to bump into the
0.75-1.00" range (75% of the moving average per SPC Sounding climo
for Tucson) across the CWA and southern AZ. Majority of the moisture
remains in the mid layers and is not all together cohesive/deep as
forecast soundings indicate a few periods where dry layers develop
at different levels on and off Wednesday and Thursday. Sub-cloud
layer remains very dry and conditions do not look at all promising
to overcome the high temp/dewpoint spreads at the surface. Impacts
remain limited with the forecast, increasing mid/high level clouds,
plenty of virga and a few sprinkles possible overnight Wednesday
into Thursday.

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.


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

No major aviation concerns as only thicker high clouds above 20K ft
spread into the Southwest. Wind speeds will be very light, likely
not eclipsing 12kt at any point. Wind shifts in the Phoenix metro
will occur with near the same timing as Monday. Through SE
California, north and west winds will be favored.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday:
Weak disturbance moving through generally westerly flow aloft may
produce a few light showers on Thursday, but for the most part high
temperatures will stay well above normal and minimum RH values will
be on the low side between 10 and 15 percent. Locally breezy
southwest winds are expected during the afternoon hours. After that,
Friday into the weekend, a large area of low pressure will push
across the great basin bringing a cooling trend with the coolest day
likely to be Saturday. Desert highs by Saturday will fall into the
upper 80s to low 90s and minimum RH values will stay mostly in the
teens. It will be breezy again Friday into Saturday, especially in
far southeast California with winds favoring the west Friday, then
turning more to the north on Saturday. Strong high pressure aloft
will build back into the area from the west Sunday into Monday
bringing another warming trend with the warmest lower deserts
climbing back into the mid 90s by Monday. Drier air will accompany
the warming; by Monday minimum RH values will dive back into the
single digits over the deserts. Less wind is expected both Sunday
and Monday.

Extended period forecasts are coming together such that a much warmer
solution is looking more likely with strong ridging returning to
the region. This could result in another stretch of above normal
temperatures,with a shot at triple-digit readings not out of the
question by the end of the forecast period.



Tue Oct 179888102 in 2009

YUMA     ForecastNormalRecord
Tue Oct 17 100 90 104 in 1959


Spotters should follow standard reporting procedures.





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