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Forecast Discussion for Phoenix, AZ

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FXUS65 KPSR 181118

Area Forecast Discussion
National Weather Service Phoenix AZ
415 AM MST Wed Oct 18 2017

...Updated Aviation and Fire Weather...

Warm conditions will continue today as a weak disturbance
approaches from the west. This will bring a slight chance for
very light showers over south central Arizona Thursday morning,
but will quickly move out of our area by late Thursday afternoon.
A cooler airmass will settle in over the weekend before high
pressure returns, sending temperatures well above normal by early
next week.


High pressure remains over the desert southwest this morning
under a nearly zonal flow aloft. This westerly flow is bringing
in some high level moisture and clouds to our area while
conditions at the surface remain relatively calm. Elsewhere, a
Pacific trough is just beginning to edge its way into the Pacific
Northwest and will begin to influence our weather pattern come
later this week.

Temperatures will once again warm above average today as a weak
shortwave approaches from our west. While 850 mb temperatures
should hold steady through this afternoon, additional cloud cover
from the approaching wave could help cool temperatures a degree or
two from yesterday mainly across south central Arizona. By late
tonight and into tomorrow morning, the shortwave will create a
broad area of lift over Arizona. This combined with a bit of
elevated moisture over our area (PWs ~ 1.0 inches) will bring the
possibly of some very light showers to the lower desert of south
central AZ and areas to the east. However, below 10kft the
atmosphere will be very dry so many of these showers will likely
evaporate before reaching the ground. 500 mb temperatures will
also be dropping overnight with the coldest air settling over AZ
by early Thursday morning. This will act to destabilize the
atmosphere so wouldn`t be too surprised to hear a rumble or two of
thunder during Thursday morning if enough moisture is available.

This disturbance will quickly move off to our east by late
Thursday afternoon reducing the threat of showers to near 0.
Behind this, drier air will begin filtering in and the
aforementioned Pacific trough will be quickly approaching the
desert southwest. Unfortunately, the atmosphere with be rather dry
(PWs ~ 0.5 inches) when this trough and its associated dynamics
move overhead come late Friday. Therefore, no precipitation is
expected. The biggest change will be the cooler temperatures come
Friday and Saturday with some breezy conditions, mainly in SE
California. Beyond this, global circulation models are in good
agreement of rebuilding high pressure across the SW US bringing a
warming trend to our area starting Sunday. Guidance suggest
temperatures could warm 8 to 12 degrees above normal come early
next week with highs pushing the upper 90s here in Phoenix!


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

Minimal aviation impacts will exist through Thursday morning as only
mid/high clouds gradually thicken. Virga will be common over central
Arizona Thursday morning with more notable showers possible well
north and east of terminal sites. Daily wind shifts should occur
slightly earlier than Tuesday, while speeds once again stay below

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday:
Temperatures hovering near average over the weekend will rebound
back close to 10F above normal for the first half of next week. High
pressure and dry air will become locked over the districts ensuring
no chance of rain. Afternoon minimum humidity levels in a 10-20%
range over the weekend will fall into the single digits next week
while overnight recovery deteriorates from good down to poor to fair
levels. Gusty winds may be common over the weekend especially down
the lower Colorado River valley, but should remain well below any
critical thresholds.


Spotters should follow standard reporting procedures.





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NWS Phoenix, AZ (PSR) Office Forecast Discussions.
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