Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
523 am MST sun Oct 4 2015
a strong and cold low pressure system from western Canada will
develop over western states today and Monday...then move into
northern Arizona Tuesday. In response...gusty afternoon south winds
are expected today...with a slight chance of afternoon and evening
thunderstorms mainly over portions of south central Arizona. The
best chance of showers and thunderstorms will occur Monday and
Tuesday...but mainly across portions of central and eastern Arizona.
Clearing skies are expected Wednesday...with mostly clear skies and
warmer temperatures toward the end of the week.
Note...a dramatic change in the weather pattern is expected to begin
this afternoon...but particularly Monday...as remnant monsoon
moisture surges north into Arizona. There is slight chance for strong
afternoon thunderstorms from Phoenix eastward today...but storms are
likely on Monday with possible isolated severe thunderstorms...
strong winds...and hail.
Today through Tuesday...
A strong and relatively cold Canadian low pressure system will
continue to deepen south toward central California today...then into
Southern California by Monday morning. Models are doing a good job with
forecasting the track of this system through Tuesday.
A couple of observations noted in Saturday evenings upper air data
that could make a difference in our forecast area today and Monday
are. 1) a number of smaller shortwaves/vorticity areas were
developing and rotating around the larger Canadian low pressure
system. This usually signals a slower moving trough...and may perhaps
delay enough for more appreciable remnant monsoon moisture to surge
north and west into Arizona later today and Monday. Recent concerns have
been how far west the low level dry/wet line would set up over
southwest and south central Arizona...I.E. West of Phoenix or more
generally into parts of southwest az? This could make a big
difference where thunderstorm alley so to speak...sets up over south
central Arizona and its proximity to the populated greater Phoenix area.
And...2) dry air from the west continued to flow across central and
northern Baja California...and parts of far northwest Arizona on Saturday...so
significant moisture advection north into south central Arizona may be a
little slower to develop this afternoon.
Otherwise we will continue to forecast a slight chance late
afternoon and evening thunderstorms over south central Arizona today...but with
increased convective potential on Monday. Moderately strong
southerly winds should develop this afternoon for a windy day across
most areas...gusts to 30 miles per hour...with patchy blowing dust on the lower
As stated above...based on Saturday evenings upper air data...we
expect a moisture surge north into parts of southwest but mainly
south central Arizona Monday for a good chance of strong fast moving
thunderstorms. Developing vertical wind shear profiles over the area
indicate the possibility of isolated severe thunderstorms with hail.
In fact the Storm Prediction Center day 2 convective outlook puts much of Southeast
Arizona...and parts of central Arizona in a marginally severe storm category
due to increasing c.A.P.E. And vertical wind shear.
On Tuesday the center of the low pressure system is forecast to move
into northern Arizona...continuing the chance of showers from Phoenix
The northern Arizona cold core low is forecast to deepen southeastward
into east central Arizona/west central New Mexico. A slight chance of
early afternoon showers/thunderstorms is therefore forecast over our mountainous
zone 24...southern Gila County. Mostly clear and dry conditions are
expected elsewhere across southeast California...southwest and south central
Thursday through Sunday...
Dry and stable northerly flow aloft will provide mostly clear skies
along with a warming trend. Lower Desert temperatures along the
Colorado River and in southeast California may approach the 100 degree mark
again Friday and the weekend.
south-central Arizona including kphx...kiwa...ksdl...and
southeast California/southwest Arizona including kipl and kblh...
Increasing moisture will lead to low ceilings this morning over the Phoenix
area before scattering out this afternoon somewhat. Storm chances
return to the forecast for later today into early next week...mostly
for the phx area terminals...however the initial round of activity
is expected to stay mostly east of the terminals today.
Aviation discussion not updated for amended tafs.
Tuesday through Saturday...
vigorous low pressure system will continue to track through the
region...continuing unsettled conditions with periods of strong
gusty winds...the potential for some strong thunderstorms along with
showers while also providing increased humidities and cooler
temperatures. The focus of any additional precipitation activity for
Tuesday...and possibly Wednesday...will likely be east of the Colorado
River valley. The upper low will make a slow exit to the
east...keeping northeasterly flow over the fire districts during the
midweek period into Thursday. Drier conditions will gradually return
to the region with daytime humidity percentages in the 20s and good
to excellent overnight recoveries. Temperatures will also warm
slightly over their readings from earlier in the week.
Spotter information statement...
spotters are encouraged to report based on Standard operating
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