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fxus65 kpsr 152342 
afdpsr

Area forecast discussion
National Weather Service Phoenix Arizona
442 PM MST sun Dec 15 2019

Update...aviation discussion.

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Synopsis...
cooler temperatures and windy conditions are expected today
behind a cold front that has moved through the region. The coldest
temperatures so far this winter are expected tonight and tomorrow
night as they drop to around freezing over portions of Gila
County and Joshua Tree National Park in southeast California.
Another dry frontal system will pass through the region on
Wednesday into Thursday before temperatures increase several
degrees above normal for Friday into next weekend as high pressure
briefly builds back in over the Desert Southwest.

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Discussion...
cooler temperatures have arrived as advertised behind a cold front
as most Lower Desert locations across south central Arizona are
warming up into the lower to mid 60s, with upper 60s over
southeast California and southwest Arizona. Wind gusts up to 25
mph will continue over the lower deserts for the next few hours
with gusts up to 30 mph over Gila County until gradually
subsiding after sunset during the evening and overnight hours.
Although there will be some mixing overnight, clear skies combined
with this cooler air mass will allow most Lower Desert spots in
south central and southwest Arizona to cool into the upper 30s to
near 40 degrees, with lower to mid 40s over southeast California
and Yuma. High temperatures tomorrow will only warm up into the
lower 60s under clear skies and lighter winds than today.

Most of the region will experience the coolest temperatures of
this winter season so far by Tuesday morning as clear skies will
help promote efficient radiational cooling. However, model
guidance has stopped trending colder for Tuesday morning, most
likely due to enhanced mixing overnight associated with a fast-
moving secondary shortwave moving in from the north-northwest as
the broader longwave trough exits the region. This will help
encourage an enhanced surface pressure gradient on the fringes of
a 1052 mb surface high over northwest Colorado that should keep
northeasterly winds elevated over at least south central Arizona.
This will result in a downsloping component over northern and
eastern Maricopa County that should keep those spots above
freezing in the mid to upper 30s with just enough mixing to most
likely keep most rural Lower Desert locations from experiencing a
freeze. Although a freeze is possible for isolated Lower Desert
locations, particularly in La Paz and northern Pinal counties, it
will generally be confined to higher terrain locations in Gila
County and Joshua Tree National Park in southeast California.

A brief period of ridging aloft will help temperatures increase a
couple degrees on Tuesday and Wednesday before yet another fast-
moving shortwave and dry front moves through the region Wednesday
into Thursday. However, cloud cover and enhanced mixing associated
with this disturbance should prevent a drop in temperatures on
Wednesday night, with high temperatures on Thursday only
decreasing a couple degrees. Relatively strong ridging is expected
to develop aloft behind this shortwave on Friday into the weekend
as a relatively deep trough over the northern Pacific Ocean digs
south. This will allow temperatures to warm back up above normal
by Friday and several degrees above normal for the weekend with
highs most likely in the lower to mid 70s for most Lower Desert
locations. Ensemble model guidance continues to show increasing
chances of precipitation at some point during the first half of
next week. However, the timing of the best precipitation chances
has been pushed back to Christmas eve or Christmas for now as most
ensemble clusters depict a longer storm track associated with a
progressive cutoff low developing off the coast of Southern
California and Baja California before ejecting northeastward.

&&

Aviation...updated at 2342z.

South-central Arizona including kphx, kiwa, ksdl, and kdvt:

Breezy conditions this afternoon with gusts upwards of 20-25 kts
will continue through a little after sunset before speeds begin to
weaken below 10 kts. A typical diurnal switch to the east-
southeast isn't expected this evening, however, a north-northeast
pressure gradient will develop across Phoenix terminals, resulting
in a north-northeast wind direction sometime around or just after
15z. By tomorrow afternoon, winds will begin to favor the west-
northwest again with speeds generally below 6 kts. Low ceilings
around 6 kft this afternoon will begin to diminish after sunset,
with mostly clear skies expected overnight and through tomorrow.

Southeast California/southwest Arizona including kipl and kblh:

Wind directions at both terminals will favor the north-northwest
through the taf period. Although gusty winds will begin to
subside at kblh after sunset, speeds are still expected to remain
elevated close to 10-15 kts through the remainder of the taf
period. At kipl, wind speeds will be lighter, generally below 10
kts. Skies will remain mostly clear at both terminals through the
taf period.



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Fire weather...
Tuesday through saturday:
overall, conditions will remain dry. Relative humidity values will be the lowest
Tuesday and Wednesday, 15-25%, and improve to 25-35% thereafter.
Overnight recovery will struggle Tue/Wed into the 35-50% range but
will be much better in the 50-75% range the rest of the week.
Winds will generally be light although Tue/Wed may see a few
afternoon and evening breezes between 10-20 mph. Directions will
generally favor typical diurnal trends.

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Spotter information statement...
spotters should follow Standard reporting procedures.

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Psr watches/warnings/advisories...
Arizona...none.
California...none.
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