Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
855 am MST sun Nov 29 2015
cool but dry weather will continue across the southwest states into
early this week. Some lower elevation desert spots will dip below
freezing during the morning hours through Tuesday...with Monday
likely the coldest. Warmer weather will arrive starting middle week and
likely lasting into next weekend. The dry weather pattern is
expected to continue through the coming week.
broad upper low remains over the interior west this am...drawing
down cold Canadian and northern Continental U.S. Air into the Great Basin and
subsequently California and the SW. That cold air has allowed for the first
freeze across the population centers throughout southern Gila County
yesterday am and also delivered the Phoenix area's coolest morning
today so far this fall with our low of 41f. Water vapor loops and
pvu model analysis plot two separate shortwave lobes rotating around
the main low circulation. The first plotted at 500mb and above is
moving across central Arizona currently and not generating much...if
any...noticeable impact by way of clouds and or elevated breeziness
over our area. It is interacting with the second trailing vorticity
lobe...which stretches from just north of the l.A. Basin into
southern Nevada...and is already promoting areas of ml clouds as low as
7kft across portions of the Colorado River valley and northern Arizona. Pv
analysis plots some pockets of energy down through the 700mb layer.
The second vorticity lobe and in actuality the main upper low/trough axis
will sweep through the four corner today into early Monday as the
flow becomes a bit more progressive and unblocked. For today...we'll
have continued cool and below normal temperatures for the region
with some periods of passing clouds. Any clouds will remain mostly
along the northern tier/border of the County Warning Area as the second vorticity lobe
cuts across northern Arizona. Current forecast grids are in good
shape...no updates needed this morning.
Previous discussion /issued at 246 am MST/146 am PST/...
the cool airmass in place will remain entrenched for another couple
days as the large upper low centered over northern Utah and western
Wyoming continues to pull down cold Canadian air. Because of
this...chilly temperatures are expected into Tuesday with overnight
lows into the 30s for many Lower Desert spots. The coldest morning
still looks to be Monday morning with at least a few colder desert
locations seeing sub-freezing temperatures. Not anticipating a large
enough area of freezing temperatures to warrant any headlines...but
there will likely be some impacts to cold sensitive vegetation. The
upper low will finally start to move eastward Sunday night into
Monday but we will only see slight warming aloft and thus only
minimal changes in lows Monday night and highs Tuesday.
A warming trend will finally pick up traction starting Wednesday as
the pattern shifts enough to allow a fairly substantial Pacific Ridge
to pass through the western United States. Highs should rebound back
to near normals on Wednesday and even climb a few degrees above
normal for late in the work week. Dry air will persist across the
region through the entire week with little sign of any change in
moisture levels. This will allow for clear to mostly clear skies each
day and still rather chilly overnight lows...but most locations will
keep lows in the 40s.
Models have been advertising some sort of a trough or upper level low
affecting the Desert Southwest Friday night into Saturday and the
past couple model runs have been backing off on the strength of the
trough and potential impacts. Timing still looks to the same...but
for now it seems little to no moisture will be advected into the
region and thus only around 10 percent probability of precipitation remain across far eastern
Gila County for Saturday morning. The trough will at least bring some
stronger winds...especially down the lower Colorado River
valley...but not looking like anything near advisory levels.
Temperatures will dip a few degrees on Saturday with the trough
passage...but likely warm back up on Sunday as another Pacific Ridge
moves into the Desert Southwest.
south-central Arizona including kphx...kiwa...and ksdl...
southeast California/southwest Arizona including kipl and kblh...
Clear skies today except perhaps for some altocumulus...few-sct at
fl100-120. Similar wind pattern today as yesterday with light east
over Phoenix area becoming west around midday/early
afternoon...northerly over southwest Arizona and southeast
California with occasional gusts of 15-20 kts in/near lower Colorado
Aviation discussion not updated for amended tafs.
Tuesday through Saturday...
High pressure will slowly build into the region through Thursday
leading to a warming trend. A low pressure system moves through the
region Friday and Saturday. Minimum humidities will be in the low-
middle teens through Thursday before increasing a bit Friday and
Saturday. Breezy conditions can be expected at times...mainly over
southeast California and southwest Arizona...favoring northerly
Spotter information statement...
spotters are encouraged to report based on Standard operating
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