Dense Fog in Arizona, February 20, 2017

Area Forecast Discussion
National Weather Service Phoenix AZ
217 PM MST Sun Feb 19 2017

.SYNOPSIS...
An exiting Pacific weather system will leave lingering showers
into early this evening and clouds through much of the night
across the eastern half of Arizona. Drier conditions will
gradually move into the region through the middle part of this
week as above normal temperatures return to the area Tuesday and
Wednesday amidst a significant warming trend. A mainly dry cold
front will move through the region Thursday, with temperatures
returning to near normal. A more potent storm system may affect
the region next Sunday.

&&

.DISCUSSION...
Scattered instability type showers across south-central and
eastern Arizona will gradually diminish starting late this
afternoon with most of the activity coming to an end early this
evening. Soggy conditions in the wake of the exiting Pacific low
will persist into tonight with low stratus likely continuing
across much of south-central and eastern Arizona. Areas west of
Phoenix through the Lower Colorado River Valley should mostly
clear out by midnight leaving a brief window before high clouds
stream in from the west. Fog formation is possible tonight with
the clearing skies, lingering boundary layer moisture, and light
winds, but thinking it will mostly be localized and situated
across southern Maricopa and Yuma counties. Have added patchy fog
mention to these areas late tonight through early Monday morning.

A substantial warming trend begins on Monday and especially
Tuesday, as midlevel ridging builds over the Southwest. High
temperatures are expected to be just shy of 80 degrees across many
lower desert locations on both Tuesday and Wednesday. A
significant, but mostly dry, cold front will move through the area
on Thursday, bringing high temperatures back into the upper 60s
to lower 70s for the remainder of the week. This will be
associated with a fast moving storm system that will glance
central Arizona.

Model guidance continues to advertise a more significant upper
trough developing next weekend near the Pacific coast, but with
significant timing/evolution differences still remaining. The
most probable solution at this point reflects the storm system
affecting southeast California as early as late Saturday evening,
and the remainder of western/central Arizona on Sunday. This storm
system does not appear to be as moisture-rich as more recent
events, but still could see an uptick in precipitation chances for
Sunday/Monday. However, given the uncertainty, only slight chance
PoPs are currently forecast. Below-normal temperatures are also
possible by Sunday, with high temperatures forecast to be in the
mid-upper 60s.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA,and KSDL:

Upper low slowly exiting to the east with scattered to numerous
light showers persisting across the central deserts. Expect light
rain/rain showers through early afternoon before dissipating and as
such will keep -RA in the TAFS thru 21z, after that may see VCSH
into the late afternoon as cold air aloft will lead to isolated
instability showers. Otherwise main concern will be low ceilings;
much guidance suggests CIGS in the 2-3k ballpark thru at least 21z
with a few lower CIGS down to around 1k feet. From mid aftn thru
early evening should still see CIGS 3-5k feet widespread. Do not
expect significant visibility issues this afternoon, maybe 5-6SM at
times in mist/drizzle but as showers decrease VIS values should
generally stay above 6SM. Any ceilings after 03z should genly be
above 6k feet. Winds not an issue, following mostly diurnal
tendencies aob 12kt.

Southeast California/Southwest Arizona including KIPL and KBLH:

Fog in the Imperial Valley should be over by 19z and no longer
affect the KIPL TAF. Otherwise expect few-sct lower cloud decks in
the 3-5k foot ballpark through the afternoon. May see some increase
in high clouds overnight tonight into Monday morning. At KBLH should
see few-sct low decks this afternoon mostly above 3k feet with
additional decks near 8k feet, clouds becoming few at best by 03z.
Again, may see SCT-BKN high cloud decks develop tonight into Monday
morning. Winds will be rather light, favoring the south at KBLH and
the west at KIPL with speeds mostly below 12kt, over the next 24
hours.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Tuesday through Saturday...Dry conditions are expected for most
areas through Saturday. Above normal temperatures can be expected
Tuesday and Wednesday. A low pressure system passing mainly to
the north Wednesday night and Thursday will lead to cooler
temperatures and locally breezy conditions. Rain chances with the
midweek system will be limited to the higher terrain of south-
central AZ. Anticipate slight warming Friday and Saturday.
Minimum humidities will trend downward through the workweek (especially
on Thursday) with most lower elevations in the 15-20% range by
Friday. Overnight recovery though will remain good.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

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DISCUSSION...Kuhlman/Rogers
AVIATION...CB
FIRE WEATHER...AJ