Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 072054
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
154 PM MST Wed Dec 7 2016

.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...

Visible satellite imagery today shows scattered to broken cirrus
moving across the desert southwest. The clouds are associated with a
weak trough located over Arizona in the very zonal westerly flow
aloft. Large low pressure centered over the great lakes is bringing
westerly to northwesterly flow aloft to the southwestern third of the
U.S.

Temperatures are near normal levels. They should inch up just a bit
over the next 48 hours as heights move up slightly.

Models indicate our region will stay in this zonal pattern of
westerly to northwesterly flow for the next seven days with little
to no chance for precipitation. Well out in the extended, 7-10 days
we see a setup of a blocking ridge over the southwest U.S. This
should keep weather conditions benign and potentially increase
temperatures. But, that`s still more than a week out so much could
change before then.

&&

.PREVIOUS 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:

Winds will remain very light, aob 6 kts, through the period and
follow typical diurnal headings. Current scattered to broken low
clouds, will gradually lift and decrease in coverage between 21Z
today and 06Z tomorrow morning. Thus, conditions will only improve
from this point forward, with cloud decks aoa 15 kft by midnight.

Southeast California/Southwest Arizona including KIPL and KBLH:

At KBLH, winds will be out of the northwest, with speeds close to 10-
15 knots through 00Z tomorrow, then decreasing to 5-10 kts. At KIPL,
winds will be primarily out of the north through 06Z tomorrow, then
shifting more to the northwest. Speeds will be light, aob 10 kts.
Mid to high level clouds will exist over both terminals through the
period, with few-sct clouds near 10 kft, and sct-bkn clouds near 20
kft.

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...Waters/Nolte
AVIATION...Hernandez
FIRE WEATHER...CB



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