Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 171231

Area Forecast Discussion
National Weather Service Phoenix AZ
531 AM MST Tue Sep 17 2019

.UPDATE...Updated Aviation Discussion.


A low pressure system to the north will slowly exit to the
northeast today with lingering chances of showers across far
eastern Arizona. Drying conditions today and Wednesday will bring
slightly warmer temperatures and a return to sunny skies across
the lower deserts. Generally dry and mostly clear conditions will
persist into the upcoming weekend with temperatures within a
couple degrees of normal.


After an active evening, only minimal shower activity is currently
seen north of Phoenix as the trough axis stretching through
central Arizona slowly drifts northeast. The Pacific low center
currently over northern Nevada is forecast to quickly weaken today
and eject northeastward leaving broad cyclonic westerly flow
across the Desert Southwest. There is still a good amount of
boundary layer moisture in place across the eastern half of
Arizona, but increasingly dry subsident westerly flow behind the
exiting trough will prohibit showers and storms from all areas
except for the eastern Arizona high terrain. Highs today across
the lower deserts will again be near normal readings, but sky
conditions today will generally be clear.

Another Pacific trough will quickly follow behind the current one
exiting the region, again diving as far south as the Great Basin
on Thursday. However, this time around moisture will be much more
limited, likely confining rain chances to the higher terrain north
and east of Phoenix. Eventually the trough will exit to the
northeast on Friday leaving behind a slightly cooler airmass with
highs temperatures dropping into the middle to upper 90s.

The synoptic pattern into the weekend will keep broad troughing
over the Western U.S., but with weaker flow temporarily across the
Desert Southwest. Little change in moisture availability is
seen for the weekend as dry and mostly clear conditions prevail
over our region. Temperatures will likely warm slightly Friday into
Sunday with highs topping out around 100 degrees over the lower
deserts by Sunday.

Although the monsoon season is not technically over, the forecast
synoptic flow pattern with trough after trough coming into the
Western U.S. shows very little chance of any meaningful monsoon
moisture returning over the bulk of Desert Southwest. We are
currently watching a potential significant upper level low shown
diving into the Southwestern U.S. for early next week, possibly
stalling out for a few days. There is fairly little model spread
through Sunday, but model spread increases thereafter with
considerable differences among ensemble members on how to handle
this potential cut-off Pacific low. For now, we have gone slightly
above median guidance for PoPs since the median was generally
less than 10% for eastern Arizona next Monday into Tuesday. Have
also lowered high temperatures a couple degrees to account for the
potential deep low centering itself over the Desert Southwest.
The track of this low, especially how far south and west of
Arizona it tracks, will likely determine rain potential for early
next week. We also cannot rule out the slight potential for more
significant moisture being drawn northward out of Mexico into
Arizona as shown by the 00Z GFS operational run. This possibility
would likely be contingent upon a remnant tropical disturbance
being drawn northward which the 00Z GFS also depicts. It is a week
away and many details will probably change, but it is definitely
something to watch.


.AVIATION...Updated at 1220Z

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Expect a return to a basically diurnal wind regime with mainly
light easterly winds this morning turning to the west during
midday. Wind speeds this afternoon should generally remain around
7-11 knots with isolated gusts near 15-18kt. Expect winds to
become light and variable in the early evening, possibly as a
result of weak outflows from distant isolated thunderstorms to the
south and the east of the area. Skies will be mostly clear during
the period with occasional few-sct low, mid and high clouds from

Southeast California/Southwest Arizona including KIPL and KBLH:

Expect mostly light and variable winds at KIPL through this
morning before picking up during the early afternoon out of the
west with some gusts over 25 kt into this evening. Winds will then
taper off and become northwesterly in the evening. Also expect
very gusty southwesterly and then westerly winds to develop by
early afternoon at KBLH with gusts approaching 25 kt. There is
also a slight chance of slightly reduced visibilities in patchy
areas of blowing dust during the afternoon across the region.
Expect clear to mostly clear skies with periodic few mid to high
clouds through the period.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday:
A low pressure system will move through the Great Basin on
Thursday bringing a slight chance for wetting rains across the
higher terrain east of Phoenix and widespread breezy conditions.
Drier conditions and lighter winds are then seen Friday through
Sunday with near normal temperatures. Humidity levels will mostly
range from 10 to 20% with gradually lower readings into Saturday.
Another low pressure system seems likely by next Monday,
potentially bringing decent rain chances, higher humidities, and
breezy to windy conditions.


Spotters should follow standard reporting procedures.




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