Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Phoenix Arizona
915 PM MST Monday Aug 3 2015
clear skies with drier and hotter weather is forecast Tuesday and
Wednesday...with high temperatures on the deserts ranging from 112 to
115 degrees. Monsoon humidity will return mainly to Arizona Thursday
and Friday...providing a threat of afternoon and evening showers and
thunderstorms over south central Arizona...along with cooler
afternoon temperatures region wide. A drier airmass will return this
weekend...with another threat of showers and thunder storms early
Discussion... another very quiet monsoon day across our County Warning Area this
afternoon/evening. An unusually deep trough for early August
approaching the West Coast is keeping the Desert Southwest under a
dry southwesterly flow...as the main upper high center remains
displaced well off to our east...over the southern Great Plains. Surface
dewpoints today at most Lower Desert locations fell into the
40s...which is well below average for this time of year. The latest
00z GFS continues to forecast that 500mb heights will be pulled
upward by the approaching trough...to near 594dm over south-central Arizona
by Tuesday afternoon...and up near 596dm by Wednesday afternoon. The
combination of these high heights and 500mb temperatures in the -3c
to -4c range...and nearly wall-to-wall sunshine is expected to allow
temperatures to well above normal...with highs rising into the 110-
115f range across the lower deserts on Wednesday an Thursday.
Thus...the heat warning products across our County Warning Area both days still are
looking on track. As far as the forecast for the rest of this
evening and tonight...other than some minor adjustments to the
hourly temperature/dewpoint grids...inherited forecasts are holding up well.
Previous discussion... tonight through Wednesday... a stable and
drier airmass will continue to cover our forecast area...southeast
California to south central Arizona...through Wednesday...along with warmer
afternoon temperatures. Forecast highs Tuesday and Wednesday will hit excessive heat
warning criteria which is 112 degrees f for Phoenix and 114 for
Yuma. An excessive heat warning will be issued for the lower deserts
extending from southeast California to south central Arizona Tuesday and Wednesday. Clear
skies are also forecast.
Thursday and Friday...
models forecast another surge of monsoon moisture from northwest
Mexico to spread into southern Arizona Thursday. Both the GFS and European
models direct a low level moisture surge into southwest Arizona...which
spreads east into central and eastern Arizona late Thursday and Friday. At
the same time both models forecast an upper level disturbance to move
into central Arizona from the southwest direction on Thursday...renewing
the threat for afternoon/evening showers and thunderstorms over
portions of southwest and south central Arizona including Phoenix.
Saturday and Sunday...
drier and more stable airmass will spread into our forecast area
from the west this weekend...and except for a slight chance of
afternoon showers/thunderstorms over mountainous southern Gila County zone
24...the remainder of the forecast area will be dry.
models forecast a significant upper level inverted trough to move into
the southeast quarter of Arizona from Mexico Monday afternoon...which is
strong enough for a threat of widespread shower/thunderstorm activity.
However...models historically have forecast these inverted troughs to
move into Arizona a little too fast. Perhaps Tuesday may be the big day
instead. We will reintroduce a slight chance of showers/thunderstorms over a
large portion of south central Arizona Monday.
south-central Arizona including kphx...kiwa...and ksdl...
southeast California and southwest Arizona including kipl and
Quiet weather expected at all terminals through at least Wednesday.
Mostly clear skies and light /except for occasional afternoon gusts
to 20kts/ diurnally driven winds expected at all sites.
Aviation discussion not updated for amended tafs.
Thursday through Monday...
starting Thursday moisture flow from Mexico will return mainly to
central and eastern Arizona. A slight chance of afternoon and early
evening showers and thunderstorms will return to south central
Arizona Thursday and Friday while activity will be mostly limited to
higher terrain areas east of Phoenix for the weekend. Minimum
relative humidities of 15 to 20 percent range will be common for the
deserts during the period. High temperatures will be near to slightly
above seasonal normals. Winds will be light through much of the
period following typical diurnal directions except for afternoon
upslope breezes gusting at times up to 20 miles per hour.
Spotter information statement... spotters are encouraged to report
conditions according to Standard operating procedures.
Arizona...excessive heat warning from 11 am to 8 PM MST Tuesday for
Excessive heat warning from 11 am to 8 PM MST Wednesday for
California...excessive heat warning from 11 am to 8 PM PDT Tuesday for
Excessive heat warning from 11 am to 8 PM PDT Wednesday for
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