Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
300 am MST Friday Dec 6 2013
a large and cold low pressure system will continue to cover the
western states through early next week. A couple of weather
disturbances...rotating through the large western region trough...
will bring a chance of valley rain and mountain snow later Saturday
and Saturday night. Rather cold night-time temperatures are expected
through Monday morning...with clear skies and a warming trend by the
middle of next week.
GFS and European models are nearly identical in their solutions for
the western states through early next week. Generally these models
continue to forecast a large cold trough over the western states
through early next week...with several shortwaves/disturbances
rotating through the trough into Arizona through this period. However...
we have noticed a slight change in the models since yesterday. It
appears that because of a jetstream break through under or south of
the Gulf of Alaska Omega block...the shortwave trough forecast for
late Saturday through Sunday morning will move a little farther to
the north or slightly less deep over southern Arizona. This has some
implications to sensible weather Saturday night which is explained
Today through early Saturday...
this period will be in-between shortwave troughs. Dry westerly flow is
forecast with mostly clear skies but continued unseasonably cool.
Afternoon temperatures today will be a repeat of Thursday.
Saturday afternoon and night...
the next cold shortwave trough will approach southeast California and
southwest Arizona during the afternoon. Since this is a cold system...
moisture will be limited to the low levels...generally at or below
700 mb which was mechanically lifted into the boundary layer from
wind driven rough seas. Interesting though...the plume of boundary
layer moisture advected under 20-25 knots west/southwesterly winds...
overruns a much colder airmass to the north along the 289k Theta-E
surface by late afternoon. In other words isentropic lift in cold
temperatures will be shallow...between 2 and 8 thousand feet. The best
chance of showers will exist over the higher terrain of Joshua Tree
north... of Riverside County...and La Paz County away from the
strongest rain shadow areas.
Saturday evening and overnight...
low level moisture will quickly spread into south central Arizona...
vicinity Phoenix and nearby mountains to the north and east...where
a good chance of showers are forecast. However...as mentioned
earlier...since the trough will move a little father to the north...
the airmass aloft is not as cold as predicted by earlier model
solutions...and will likely result in the diminished potential for
precipitation amounts. In other words...a few hundredths of an inch of
precipitation is possible over the greater Phoenix area and surrounding
deserts...with about 1-3 inches of dry snow in southerly Gila County
above 4500 feet. Snow levels Saturday night will remain relatively
high between 4500 and 5000 feet across the entire forecast area...
southeast California to south central Arizona.
Sunday and Sunday night...
since the low level moisture plume and chance of precipitation will move
quickly across the area...only lingering rain/snow showers are
expected in southern Gila County through early afternoon. Otherwise
clearing skies are expected across the entire forecast area Sunday
afternoon and night.
Monday and Tuesday...
Monday morning temperatures will be cold again under clear skies and
much drier air. In other words...strong radiational cooling. However
Tuesday mornings low temperatures will be a little tricky due to a
very shallow inversion and increased northeast winds just above the
inversion. In other words...periodic wind mixing in the shallow
inversion could result in warmer night-time temperatures.
Wednesday through Friday.
Mostly clear skies with temperatures finally approaching normal.
south-central Arizona...including kphx...kiwa...and ksdl...and
southeast California/southwest Arizona...including kipl and kblh...
Clear to mostly clear skies...and light winds generally 10kt or less
expected at all taf sites through at least Friday evening.
Aviation discussion not updated for amended tafs.
Saturday through Wednesday...
another low pressure system will move through the forecast area this
weekend. Best chance for precipitation will be Saturday
night...mainly over south-central Arizona. Amounts will be
light...less than a tenth of an inch on the lower elevations and up
to a quarter of an inch liquid equivalent over the higher mountains.
Snow levels will be 4000-4500 feet Saturday...lowering several
hundred feet Saturday night. Accumulations of 2-3 inches above 5000
feet are possible. Windy conditions can be expected Saturday
afternoon and evening over southeast California...breezy elsewhere.
Clearing can be expected Sunday. Dry conditions with north and
northeast breeziness can be expected Monday into
Wednesday...especially over the lower Colorado River valley. After a
cold start Monday...temperatures will slowly increase.
Arizona...freeze warning in effect tonight through this morning for
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