Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Phoenix Arizona
1022 am MST Wednesday Dec 4 2013
..very cold weather system moving through the Desert Southwest...
a large and very cold low pressure system will move across the
region today...and the low will persist the remainder of the week.
This weather system is expected to bring a chance for light rain and
mountain snow to the region today and Thursday...and much cooler
temperatures to the Desert Southwest through the upcoming weekend.
today through Friday...
infrared satellite imagery shows a more substantial band of middle-upper
level clouds associated with the upper level trough streaming to
the northeast across Arizona this morning...the surface front
positioned east of the lower Colorado River in a line extending from
Yuma to Prescott. Due to the Continental trajectory of the low...it
has failed to tap into hefty amounts of moisture...but it still has
enough to work with as the frontal boundary continues its trek
eastward as we go through the day. Radar is starting to show some
returns... but for the most part...precipitation amounts will be
rather light...the peak time for light rain showers and lower cloud
decks over the central deserts will be through middle afternoon...probability of precipitation
dropping off behind the front and lingering over our eastern zones
near Globe and Roosevelt Lake into the evening. Temperature-wise...
diurnal temperature curves will now be offset...as we have already
reached forecast highs at many desert locations...with Phoenix
hitting 63 degrees as of 0930 hours this morning. Temperatures will
continue to fluctuate a degree or two throughout the day...but for
the most part little change is expected. Over all a considerable
change from yesterdays observed maximum temperatures -- a good 8-10 degrees
cooler. Other than some adjustments to the near term digital
forecast grids...namely pop/wx/sky...the forecast is in good shape
and on track. More to follow with the afternoon package.
snow levels plummet today and tonight as the cold air rushes in.
However...the only accumulations anticipated for our area will be
tonight and Thursday morning above 5000 feet with half to one inch
possible. Winds will trend down over southeast California later this
morning but will be ramping up elsewhere before weakening late
afternoon. With the cold air moving in...the freeze watch for Joshua
Tree National Park for tonight...Wednesday night/Thursday
morning...was converted to a warning. The freeze watch for the next
night...Thursday night/Friday morning...was expanded to the lower
elevations east of the lower Colorado River valley and includes
Phoenix. Warmer locations like Sky Harbor are not expected to reach
freezing but a lot of other desert locations including portions of
the metropolitan will likely dip below freezing Friday morning.
Thus...Friday lows should be the coldest of the season so far. Even
though skies will be clear...highs on the lower elevations Friday
will be in the 50s.
Saturday through Tuesday...another low pressure system moves through
the region over the weekend. This one digs even further south and
will have even lower snow levels. This second one may track a bit
further west but will still be mainly overland and thus will not tap
into Richer moisture from lower latitudes. However...there will be
better precipitation efficiency due to the colder air involved. Like
the first system...the precipitation will come mainly with the frontal
zone. Thus...probability of precipitation increase from west to east during the day Saturday
and continue overnight Saturday night before tapering off from west
to east Sunday. At this time...best opportunity for precipitation over our
area with this second system appears to be Saturday night over
south-central Arizona. Its looking like precipitation amounts will be a bit
better with the second system. Snow levels bottom out between 2-3
thousand feet Sunday. Rough guess on snow amounts is 5-10 inches
above 5000 feet with a sharp drop off as elevation decreases. The
system begins moving eastward Monday as an eastern Pacific Ridge
expands eastward. Temperatures Saturday morning could be quite
cold...especially where winds stay light and skies stay clear.
Sunday morning not expected to be as cold with cloud cover and
mixing but Monday morning could be quite cold with clear skies.
Anticipate a slow warmup since the ridge doesnt build in strongly.
south-central Arizona...including kphx...kiwa...and ksdl...
ceilings already lowering to around 5-6kft across the Phoenix metropolitan as
of 17z and will remain in that ballpark through late afternoon. A
few showers are possible at sdl but phx/iwa should remain dry today.
West winds already in place with occasional gusts to 20kts expected
through late afternoon. Conditions should rapidly improve after
00-02z with winds becoming light/vrb around 12z.
Southeast California and southwest Arizona...including kipl and
cold front has already moved through southeast California and
conditions should steadily improve through the afternoon. Gusty
northwest winds around 20-25kts can be expected this afternoon but
should relax quickly after 00-02z. Ceilings will also thin out quickly
this afternoon and evening with only a few high clouds expected
Aviation discussion not updated for amended tafs.
Saturday through Wednesday...
decent model agreement showing another trough moving through the
area on Saturday...with a chance of valley rain and mountain snow
showers Saturday into Sunday. A couple inches of snow may accumulate
at elevations above 3000 feet. Otherwise...mainly dry but unusually
cold conditions will prevail over the weekend and early next week.
Much of the area will see overnight low temperatures come close or
fall below the freezing mark. Humidity values will remain quite
elevated through the period with minimum humidities mostly between
20 and 30 percent...and good to excellent overnight recovery. Winds
will generally be light with some occasional afternoon upslope
gustiness...and directions favoring between west and north.
Arizona...freeze watch for Thursday night through Friday morning for
California...freeze Waring for Wednesday night through Thursday morning for
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