Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
935 am MST Monday Dec 9 2013
a large cold air mass across the western states will move slowly
eastward through middle week. Clear to mostly clear skies along with
the cold air mass in place will allow for cold overnight
temperatures...leading to the threat of freezing morning
temperatures across the area through Tuesday morning. A warming
trend is expected from middle week into the weekend.
strong surface high pressure continues to build over the Great Basin
this morning...with 3 hour pressure rises on the order of 3-5mb
encompassing the entire region. Skies cleared much earlier than
anticipated...and temperatures around sunrise dropped into the 20s
and 30s across the forecast area. With the pressure gradient now
just beginning to increase...mixing has remained shallow and
temperatures are running some 10f lower than yesterday morning in a
very dry airmass (dewpoints some 15f-20f lower). Given the colder
start to the day...have selectively trimmed forecast highs a degree
or two over parts of the County Warning Area.
Otherwise...the steepest pressure gradient and gustiest winds were
rapidly materializing over the Nevada/Arizona border section of the
Colorado River valley...with higher gusts just recently observed
across the California/Arizona section. The strongest north winds
will occur later this afternoon...and the advisory time and area
remains valid. Still some uncertainty regarding overnight lows
through Tuesday morning...with limiting factors still being elevated
winds above the surface inversion and some thin high clouds streaming
east from the Pacific basin. Much of the 12z guidance still suggests
areas of subfreezing lows...and will interrogate data closer for the
/445 am MST Monday Dec 9 2013/
clearing skies are underway as a weak shortwave trough traverses
through Arizona. Very dry air behind this trough will settle in for
today allowing for surface dew points to drop even
further...reaching the single digits in many Lower Desert locales.
Latest surface analysis shows an impressive cold high pressure
system across the Pacific northwest and Great Basin. This high
pressure will surge southward into Arizona today with 6 hour surface
pressure rises of between 4-7mb. A strengthening pressure gradient
along with the pressure rises will give rise to strong northerly
winds today across the lower Colorado River valley and much of
southeast California. Will likely see periodic gusts of around 40
miles per hour across the aforementioned areas and as a result...a Wind
Advisory has been issued from late this morning through the
The cold air mass that has been in place across the Desert Southwest
will get a reinforcing shot of cold air later today as this high
pressure settles in from the north. The main forecast concern and
dilemma is how cold overnight lows will be tonight. Based on local
research of past cold events...this air mass is definitely not very
cold aloft and doesn/T come close to our rule of thumb numbers.
However...forecast soundings and low temperature guidance do show a
decent chance of some areas seeing freezing conditions. Forecast
soundings show a deep inversion below 700mb and extremely dry air
below 500mb. Am not very confident we/ll see a widespread freeze
event tonight based on a couple reasons. Besides not coming close to
meeting rule of thumb aloft temperatures...there is a good chance
many areas will see periodic winds throughout the night. As the
surface high moves southward today...a strong northwest to southeast
pressure gradient sets up across the southern half of Arizona. The
question comes down to how much mixing there will be overnight and
will the inversion be strong enough to allow winds calm. Many
areas...especially lower elevations...should see near calm winds for
some time overnight...but higher elevations will likely see more
mixing and warmer overnight lows. Will hold off issuing any freeze
warnings as this definitely looks to be a marginal event...but will
keep the freeze watch going. Either way...many Lower Desert spots
will see temperatures dip into the lower 30s with the typical
coldest spots likely reaching upper 20s for a brief time.
The air mass starts to moderate on Tuesday...but will still be 5-8
degrees below seasonal normals. A developing low off the southwest
U.S. Coast will deepen later today and become closed off by early
Tuesday. Models are in good agreement with the progression of this
low as it takes it on a slow northeastward track reaching the Baja California
peninsula on Wednesday. This low will at least turn our upper level
winds out of the southwest and bring moderating temperatures. By
Wednesday...areawide highs will approach normal highs with normal
temperatures on Thursday as the low passes through Arizona. Not
expecting a ton of moisture to accompany this system...but precipitable waters do
recover...increasing to around 0.4 inches. Should see enough middle
level instability as the cold core passes through Thursday morning
along with upper dynamics to bring at least some isolated light
showers across south-central and eastern Arizona. Have added slight
chance probability of precipitation to the forecast mainly to the north and east of Phoenix.
Another more progressive shortwave is forecast to move through
mainly northern Arizona on Friday...though not expecting any chances
for precipitation. Models show a northwest upper flow set up for the
coming weekend and a moderate air mass in place. The European is on
the warm end temperature guidance while the GFS is closer to the
middle of the pack. Have trended our forecast temperatures warmer
for Friday through the weekend...but not quite buying into the lower
70s the European guidance is showing.
south-central Arizona...including kphx...kiwa...and ksdl...
through 03z Tuesday...mostly clear skies. Light variable wind through 19z...
becoming north to northeast 8 to 10 knots between 19z Monday and 03z Tuesday.
Southeast California and southwest Arizona...including kipl and
through 03z Tuesday...clear skies. Through 17z light wind under 10
knots. From 17z to 03z Tuesday...wind generally increasing from the
north 14 to 20 knots...but along the Colorado River valley becoming north 25 to 35
Aviation discussion not updated for amended tafs.
Wednesday through Sunday...
generally dry weather..except a slight chance of showers in south
central Arizona Thursday and Thursday evening. Light winds. Minimum relative humidity
generally in the 18 to 28 percent range...lower in southeast California and
higher in south central Arizona. Afternoon temperatures will be near
normal this period.
Arizona...freeze watch from late tonight through Tuesday morning
Wind Advisory until 7 PM MST this evening azz020.
California...freeze watch from this evening through Tuesday morning
Wind Advisory until 6 PM PST this evening caz031>033.
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