Area Forecast Discussion
Issued by NWS Phoenix, AZ

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755
FXUS65 KPSR 071149 AAA
AFDPSR

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
450 AM MST Wed Dec 7 2016

.UPDATE...Updated aviation and fire weather discussions.

&&

.SYNOPSIS...
A weak storm system will pass to our north through Wednesday helping
increase thicker clouds and cool temperatures slightly. Dry weather
with seasonably comfortable temperatures and occasional passing high
clouds will return Thursday and continue through the weekend.

&&

.DISCUSSION...
Persistence upper level flow pattern remains over the CONUS with
broad trough heights sweeping through the lower 48. Deep Canadian
upper low system has shifted east towards the Great Lakes with
several shortwave circulations embedded in the longwave continental
flow. One such shortwave skirted by the area to the north yesterday
and is just clearing the Rocky Mtn Front Range as another shortwave
and associated jet streak are staging themselves upstream of the
forecast area. Influx of LL moisture that occurred during the day
Tuesday has kept the boundary layer rather moist, with surface
dewpoints jumping 20 to 30 degrees in a few hours time. Thick cirrus
shield overnight is slowly start to erode, especially for the western
zones. Insulating effect of the clouds kept temperatures rather warm
across southeast CA with readings as warm as the 60s in the last
hour. Gradual clearing should jump start some sfc cooling, but a warm
morning nonetheless is expected for many across the forecast area.

Weakly induced isentropic upglide and slight upslope terrain
orientation has already allowed and will continue areas of scattered
to broken stratocu fields to develop over the south-central AZ
deserts into the late morning hours. Humidities begin to decline
later into the afternoon and flow turns more zonal through the K
surfaces as a weak cool front sags through the area from the north.
As with the last shortwave, the best jet dynamics and
forcing/synoptic ascent are removed much farther to the north.
However enough saturation could occur through the LL cloud layer to
support virga and even a few sprinkles reaching the ground for some
limited locales around the Phoenix metro and the higher terrain
north/east of town. Meanwhile the western CWA forecast zones will see
skies slowly clear through the day and dry advection transition in
from the north as sfc high pressure slides out of the northern
Rockies. Some more noticeable northerly breezes may develop into the
day, but given the displacement of the sfc high to the northeast vs.
true north speeds will be under those observed over the past weekend.

After today`s shortwave passage and jet streak exit from the region,
flow aloft remains persistently low amplitude across the CONUS
heading into the weekend. Boundary layer humidities will dry and ML
height fields remain rather steady state in the 570dams to low
580dams through early next week. Low amplitude pattern yields high
confidence and predictability through the extended forecast period
with daytime temperatures at or slightly above seasonal normals and
periods of passing high level clouds. Confidence is very good that H5
heights will oscillate in a 576-582dm range with the primary jet core
displaced through the northern tier of states.

&&

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

Low level southerly moisture feed associated with disturbance
passing to our north will keep areas of low based CU or SC in place
over the central deserts/greater Phoenix area into the middle or
latter portion of the morning hours. Expect mainly FEW or SCT decks
with bases between 2500 and 4500 feet with BKN decks at times;
clouds to start thinning by around 15z become mainly FEW by late
morning or early afternoon. Otherwise variable amount of high clouds
with bases mostly aoa 20k feet today into tonight. Winds to favor
the east less than 10kt into mid afternoon then weakly turning west.

One last thing; given the low bases associated with the CU/SC decks
and high surface dewpoints, it is possible to see a few light
showers or sprinkles in the area through the morning hours but areal
coverage and confidence too low to bother mentioning VCSH in the
TAFs at this time.

Southeast California/Southwest Arizona including KIPL and KBLH:

Expect variable amounts of high clouds today into tonight with SCT
to BKN decks of cirrus with bases genly aoa 20k feet. May see a few
lower decks, possibly CU/SC with bases below 8k feet but the deeper
lower level moisture should basically stay east of the CO river.
Otherwise winds will tend to favor the north, especially at KBLH
where gusts to 20kt or so are possible mainly during late morning
and afternoon. Winds to taper off after sunset.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...

Friday through Tuesday: Slightly above normal temperatures and
mostly dry conditions are likely through the period with periods of
high clouds moving across the area at times. Overall the weather
patterns shows little day to day change with a zonal, westerly flow
aloft over the area. Winds each day will mostly stay on the light
side with typical afternoon breeziness favoring the southwest or
west. Humidities will be elevated through the five day period with
minimum RH values mostly between 20 and 30 percent each day. There
are no significant fire weather concerns for the next five days.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters activation will not be needed this week.

&&

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

$$

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DISCUSSION...Nolte
AVIATION...CB
FIRE WEATHER...CB



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