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FXUS65 KPSR 090509

Area Forecast Discussion
National Weather Service Phoenix AZ
1009 PM MST Sun Dec 8 2019

Updated Forecast Discussion.


Abundant moisture will continue to bring cloudy skies with 
isolated showers across the lower deserts this afternoon.
Additional isolated to scattered showers are possible again Monday
morning and afternoon. Afterwards, drier conditions with near to 
slightly above normal temperatures are then expected for the rest 
of the week.


Patchy fog continues to develop over portions of the Phoenix metro
tonight, particularly the West Valley and northern portions from
Deer Valley to Scottsdale. At the same time, showers and even a
few thunderstorms are developing along an axis from Ajo to Casa
Grande to San Carlos in agreement with several hi-res models.
This convection appears to be developing in the right entrance
region of an upper-level jet streak ahead of a positively-tilted
upper-level trough currently moving through southern California.
Aircraft soundings into Phoenix do show modest instability at
mid-levels with in-cloud lightning observed with this activity
moving into western Pima and central Pinal County, so cloud to 
ground lightning cannot be ruled out. Thus, we have added in a
slight chance of thunderstorms just south of the Phoenix metro
with this activity, along with increasing rainfall amounts
slightly. Most hi-res models focus this activity along its current
axis south of the Phoenix metro, but there is still the chance of
some isolated to scattered convection slipping into the Phoenix
metro overnight most likely after 2-3 AM. 

This aforementioned convection will play a strong role in how
widespread fog formation becomes for La Paz, Maricopa, and
northern Pinal Counties overnight considering winds should be
relatively calm and dew points should remain steady or increase 
slightly overnight. If showers develop, that should promote enough
mixing to prevent widespread fog and even some patchy dense fog 
from developing, but we would still see some reduced visibilities
with the shower activity and mist. If showers remain to the south,
there would be a threat of widespread fog including at least 
patchy dense fog developing overnight, particularly for areas that
have relatively clear skies with only high clouds aloft. We may 
still see a period of this before any shower activity begins 
regardless, but the period would be brief and thus less impactful.
At the current moment, we still expect enough shower coverage 
late in the overnight hours to prevent dense fog from developing. 
However, this will have to be monitored heavily overnight. 

Finally, we have added in the mention of isolated thunderstorms
tomorrow afternoon for the Phoenix metro and areas east as the
cold core low aloft passes overhead. This may end up materializing
east of the Valley as storms develop along the higher terrain of
northeast Maricopa and Gila Counties, but the elevated instability
already present combined with forcing from the weak surface cold
front should at least produce a few cells capable of producing
lightning strikes. Rain chances should quickly end by late
afternoon as the front clears the region and skies begin to clear.

For this afternoon and evening, several models indicate showers 
redeveloping along the higher terrain north and east of Phoenix 
and south of the Valley over Pinal County. The Phoenix area may 
see some isolated activity but the areal extent of rainfall will 
be less compared to yesterday. During the overnight and into 
Monday afternoon, convectively enhanced showers are possible as 
instability increases when the cold core of the low moves 
overhead. These showers may produce locally heavy downpours and 
some locations may see a few more tenths of additional rainfall. 
While thunderstorms are not anticipated, there may be enough 
instability to support an isolated cell or two capable of 
producing small hail on Monday afternoon. Farther west, tightening
of the pressure gradient will create breezy conditions across 
southeast California and along the lower Colorado River. By Monday
night, the low will be southeast of the forecast area and our 
conditions will begin to dry. 

A dry disturbance will pass to our north keeping atmospheric 
heights suppressed through midweek. As such, daytime temperatures 
will remain near seasonal normal values before a ridging takes 
hold later in the week allowing the 70s to return.


.AVIATION...Updated at 0006Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Light shower activity will generally remain east of the terminals
through the evening hours as SCT-BKN ceilings lift above 6 kft
with an overcast deck around 10 kft. Isolated to scattered 
showers should return overnight after 09Z with BKN-OVC ceilings 
lowering to 4-6 kft. MVFR ceilings and visibilities will be 
possible with any heavier showers or patchy fog, particularly at 
KIWA. Ceilings will gradually lift tomorrow morning as we 
experience another lull in shower activity after 13-14Z until 
another round of isolated convection occurs with a weak front 
early Monday afternoon. Very isolated thunderstorms will also be 
possible due to elevated instability associated with the cold core
low aloft, but confidence is too low to include those chances in 
the TAFs at this time. Winds will generally be light and variable 
before becoming westerly at 5-10 knots behind the front early 
Monday afternoon before becoming easterly Monday evening. 

Southeast California/Southwest Arizona including KIPL and KBLH: 

VFR conditions should generally prevail through the TAF period as
midlevel cloud decks lift and clear out behind a weak cold front
that will push through the region overnight into Monday morning.
Light and variable winds across the region will gradually become
westerly this evening, with downsloping westerly 10-15 knot winds
expected at KIPL tonight. Isolated showers are expected to develop
overnight along the front as it pushes into the Colorado River
Valley and southwest Arizona, with the best chances between 
09-12Z at KBLH. Skies should gradually clear behind the front on 
Monday morning, with northwest winds gusting to 20-25 knots 
expected at KBLH with weaker winds expected at KIPL. 


Tuesday through Saturday: 
Dry conditions will prevail under high pressure aloft. Minimum
humidity levels initially will be around 30-40% early in the
period, but will fall to 25-30% beginning Thursday. Overnight
recovery will still be good to excellent. Winds through the entire
period with be fairly light with little to no breeziness.


Spotters should follow standard reporting procedures.


AZ...Flash Flood Watch through Tuesday afternoon for AZZ556.



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