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Forecast Discussion for Phoenix, AZ

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FXUS65 KPSR 101124

Area Forecast Discussion
National Weather Service Phoenix AZ
424 AM MST Mon Dec 10 2018

.UPDATE...Updated Aviation Discussion.


Thick clouds will cover the region early this week ahead of a couple
fast moving weather systems approaching the area. Despite all the
clouds and weather disturbances, precipitation chances will be
neglible with temperatures remaining very seasonable. Clearing
conditions will exist later in the week though temperatures should
cool a few degrees.


An elongated trough was stretched through the east Pacific basin
early this morning with the majority of shortwave energy lifting
into the NW Conus and western Canada. However, a smaller PV anomaly
was detaching southward tapping an enhanced southern stream jet and
supporting a long fetch of thick upper level moisture directed into
the forecast area. This vorticity center will only slowly propagate
eastward and the thicker high clouds will be a forecast fixture
through the next 36-48 hours. As is typical this time of year, these
high clouds will likely narrow the diurnal temperature range though
a seasonably warm atmospheric profile should still ensure afternoon
highs a few degrees above normal.

The southern stream wave will shear eastward through the forecast
area late Tuesday and Wednesday as a stronger upstream kicker
shortwave descends into the Great Basin. Model forecasts remain
insistent that better moisture will be relegated to the upper levels
with little to no lower level WV transport. Better ascent fields
will also be displaced well east of the area as deepening/wave
amplification occurs over the southern plains well after the jet
core has passed Arizona. Therefore, have removed any remnant mention
of precipitation associated with these weather systems. The
resulting synoptic scale pattern amplification still looks to force
stronger sfc pressure rises north of the region allowing cooler air
to spill into the CWA. However, this will only be a glancing blow
and H5 heights will rapidly recover by the end of the week allowing
temperatures to hover close to the seasonal normal.

Forecast ensemble spread is rather moderate heading through the
weekend though the preponderance of evidence suggest a more
progressive quasi-zonal flow pattern should persist over the SW
Conus. Previous operational ECMWF runs had shown a deeper trough
through the SW (this models propensity to forecast deeper systems
over the region subjectively seems to be a recurring theme this
winter); however given the good ensemble agreement of an intense
Gulf of Alaska storm system this weekend, a downstream deep negative
height anomaly over the SW seems very unlikely. Very modest POP were
kept over higher terrain areas as some subtropical moisture could be
pulled into the region with a weaker wave, though the most likely
outcome points towards a continuation of dry, seasonally typical mid
December weather.


South-Central Arizona including KPHX, KIWA, and KSDL:
No major changes with minimal aviation concerns anticipated
through the TAF period. High clouds will continue to thicken this
morning as surface dewpoints continue to gradually decrease.
Thereafter, broken to overcast cloud decks aoa 20 kft will
persist through Tuesday morning. High pressure building across the
southern Rockies will result in a prolonged period of easterly
5-10 kt flow during the afternoon with brief gusts up to 15 kts
possible. Winds may try to back to north- northeast late in the
afternoon and evening hours, but should remain weak.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather impacts are expected as light diurnal winds
persist under broken to overcast cirrus cloud decks through
Tuesday morning.

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday:
Temperatures will cool slightly during the latter half of the week
though remain not too far from the seasonal average with dry
conditions for all the districts. A period of breezy north winds
will be possible Thursday, and especially across ridge tops of
central Arizona. Minimum afternoon humidity levels will likely not
fall below 20-30% through the weekend with good to excellent
overnight recovery.


Spotters should follow standard reporting procedures.





Previous Forecast Discussions may be found at
NWS Phoenix, AZ (PSR) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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