FXUS65 KPSR 091455

Area Forecast Discussion
National Weather Service Phoenix AZ
755 AM MST Sun Dec 9 2018

Drier weather with steadily warming temperatures will grace the
region through the first part of the week. A fast moving, weak
weather system will brush through the area Tuesday and Wednesday
bringing thicker clouds, but just a very slight chance for
precipitation. Clearing conditions will exist later in the week
though temperatures should hover near or slight below average.


Upper level high pressure ridge continues to build into the desert
southwest, with 500mb heights up to around 580dm across the southern
deserts per the latest 12z plot data. Dry subsident northwest flow
aloft continues in place across the area, with variable amounts of
high clouds moving through the ridge and into Arizona. IR satellite
imagery clearly shows the high clouds moving in from the west, and
they will continue to thicken up later today and tonight as the
ridge axis moves east and into central Arizona. The generally clear
skies and some lingering low level moisture southeast of Phoenix led
to another episode of patchy dense fog over portions of the east
valley and down into the Queen Creek and Casa Grande areas. Dense
fog appears to be a bit less expansive in areal coverage compared
to 24 hours ago, and should dissipate by mid morning when sunshine
and mixing increase. Do not expect another fog issue tomorrow
morning given the high overcast conditions and lack of radiational
cooling. For the rest of today expect increasing high clouds along
with high temperatures that should reach into the low 70s over the
warmer lower deserts.

Increasing ridging and anti-cyclonic flow can be seen in early
morning WV imagery with objective analysis indicating H5 height
rises spreading into the SW Conus. While synoptic scale subsidence
and warming of the atmospheric column will be the theme for the next
couple days, a large slug of upper level subtropical moisture (quite
apparent on IR satellite) will also stream into the region
potentially hindering what otherwise could be a couple of abnormally
warm mid December days. Regardless, there is excellent model
agreement suggesting highs in the upper 60s/lower 70s for lower
elevation communities through Tuesday.

Near term concerns once again hinge on morning fog potential though
several components and indicators are not nearly as favorable as
Saturday morning. Objective analysis and perusal of forecast
soundings indicate a more pronounced dry NE flow just above the
boundary layer; and sfc dewpoints have already been falling well
before nearing saturation. This evolution matches closely with
forecast NAM BUFR soundings. Time lagged ensemble forecasts are also
not nearly as bullish with minimal probabilities of dense fog, and
even modest likelihood of 3-5 mile haze. Kept some mention of
patchy fog included in the forecast for the most susceptible river
valley and agricultural areas of northern Pinal County.

Operational and ensemble members still show a sharp, elongated
Pacific trough sweeping through the western Conus Tuesday and
Wednesday. There is still notable forecast spread in the exact
orientation and evolution of this system though given the enhanced
southern stream jet and closer proximity of a stronger upstream
kicker, feel the faster, more progressive open wave depicted by the
00Z GFS is more likely to come to fruition. Nevertheless given the
overland trajectory and dearth of moisture transport surrounding the
shortwave, precipitation looks very unlikely with this system (at
most squeezing out a few very light showers over high terrain of
southern Gila Co).

The aforementioned kicker system will dig through the Great Basin
during the middle of the week entering a rapid deepening phase over
the southern plains. This will result in an overall amplification to
the western Conus flow pattern forcing strong sfc pressure rises
through the northern and central Rockies. While some of the colder
low level airmass associated with the sfc high will bleed into
Arizona, the larger synoptic pattern will remain progressive
providing just a glancing blow of the cold air to the forecast area.
Latest model guidance shows temperatures likely not as cold as
previously indicated, yet still hovering in a near normal range as
low amplitude waves and a healthy zonal Pacific jet keep heights
aloft hovering around typical winter season levels.


South-Central Arizona including KPHX, KIWA, and KSDL:
Primary short-term aviation concern will remain the potential for
fog this morning, especially in outlying areas surrounding
Phoenix. Satellite and observational data already show patchy fog
forming southeast of Phoenix into Pinal County, and expect it to
gradually become more widespread through daybreak. KIWA is the
most likely terminal to be affected, with visibilities down to 4
SM the most likely outcome, but still some potential for even
lower visibilities after 12Z. KPHX and KSDL are not currently
expected to have fog affect terminals, though some patchy vicinity
fog is possible. Otherwise, high clouds will be streaming in from
the west throughout the period, with cigs lowering to 15-20
thousand feet early Monday morning. Winds will be light and follow
typical diurnal patterns.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation impacts expected as light diurnal winds and mostly
clear skies will persist through Sunday under high pressure. High
clouds will increase throughout the day, and especially later this
evening with ceilings approaching 20 thousand feet overnight.

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday:
Dry weather with very seasonal temperatures will be common over the
districts during the middle of the week. Somewhat cooler conditions
will spread into the region through the latter half of the week with
just a very slight chance of precipitation in the higher elevations
of Gila County. Most locations should not expect to see any rain or
snow. Some locally breezy north winds will be possible Tuesday
afternoon through SE California, and then again Thursday across all
districts. Minimum afternoon humidity levels will likely not fall
below 25-30% throughout the week with good to excellent overnight


Spotters should follow standard reporting procedures.


AZ...Dense Fog Advisory AZZ548-550-551-553 until 9 am MST.



NWS PSR Office Area Forecast Discussion