Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
211 PM MST Sat Jul 23 2016
Update...to aviation discussion...
some relief in desert heat is forecast for Sunday and next week, as
more monsoon moisture and variable amounts of clouds move into the
region. There is a chance of afternoon showers and thunderstorms each
day, mainly over the central and eastern Arizona mountains, with a
slight chance of storms spreading toward the Colorado River valley
A few days ago we knew this excessive heat watch, then warning for
the lower deserts would be a little tricky because of an uncertain
amount of remnant monsoon moisture that could retard the diurnal
heating curve. And we were at the mercy of the numerical models that
were consistently forecasting a drying trend. Atmospheric thickness
values and a much warmer airmass aloft certainly support excessive
heat, however we now realize we could never dry out enough. For
example to our surprise this morning, an old and distantly traveled
convective outflow boundary moved out of old Mexico and surfaced in
southern Arizona between 4 am and 6 am MST. A weak boundary moved through
Phoenix between 530 am and 6 am (locals noted the suspended dust
this morning) sending surface dewpoints into the middle 50s. And
between 4 am and 6 am 44 mph southerly gusts hit Yuma, with 38 mph
gusts at Blythe, all contributing to more surface humidity. This is
the last day of the excessive heat warning, and higher humidity
levels with a cooler airmass should reduce afternoon temperatures
slightly and under excessive heat criteria.
Otherwise, the passing westward moving inverted trof in Mexico and
its associated thunderstorm outbreak, has contributed to a much
deeper moisture profile on the Tucson weather balloon sounding.
However with a 300/250 mb weakly convergent flow developing in the
wake of the passing Mexican inverted trof today, any thunderstorm
development should be thermodynamically driven, and focused in the
mountains north through southeast of Phoenix for later this
afternoon and evening.
Sunday and Monday...
On Sunday the pattern gets a little more interesting especially for
the evening and night-time hours. An inverted trof, weakly seen in
south Texas this morning, is modeled to move northwestward an
approach Southeast Arizona later Sunday evening. The 12z GFS model
(current run) is now a little slower with moving this feature into
southeast AZ, and any forcing in our forecast area in south central
Arizona including Phoenix comes overnight. Therefore a slight chance of
showers and thunderstorms are forecast for a large portion of south
central AZ, including Phoenix Sunday night into early Monday morning.
Tuesday through Saturday...
This period contains alot of uncertainty as the 300/250 mb flow
pattern transitions from southeasterly (and containing a few
disturbances from mexico), to north and northeasterly Thursday
through Saturday. With continued elevated boundary layer moisture,
afternoon thermodynamics, and small hard to time perturbations in
the upper flow, its best to go with a broad brush forecast of slight
chance showers/thunderstorms this period just about everywhere in
our forecast area.
south-central Arizona including kphx, kiwa, and ksdl:
Decent low-level Gulf surge and outflow movement overnight brought
dust and an earlier westerly wind shift to the Phoenix terminals. In
spite of the Gulf surge, moisture will be thin and storm chances very
low (not non-zero, but less than 10 percent) for Phoenix this
evening. Even the threat of storm outflow winds is low this evening,
with west to northwest winds to persist overnight. Could see another
Gulf surge overnight towards early Sunday am, but storm activity over
Sonora from last evening shouldn't get going as strong and will not
include any outflow shifts or sfc vsby restrictions in the current
tafs. Slantwise vsbys in the am may continue to be an issue as with
past mornings as the airmass won't change out overnight. Few to scattered
layers should remain at or above 12kft today, if not higher around 20kft.
Southeast California/southwest Arizona including kipl and kblh:
strong outflow and outflow driven Gulf surge will persist strong and
at times gusty south to southeasterly winds for kblh and kipl. Dust
from the outflow will also continue to impact longer range vsbys,
leaving hazy conditions and occasional slant wise vsby impacts for
aviators throughout the evening. Debris clouds from last night's
Mexican storms will continue to drift to the west, resulting in scattered
to broken mid level clouds throughout the evening.
Aviation discussion not updated for amended tafs.
Tuesday through Saturday...
monsoon moisture working its way back into the region will bring
chances for afternoon and evening thunderstorms each day through
Thursday. The best chances for wetting rains will be over the higher
terrain east of Phoenix, will lesser chances over the lower deserts
of south-central and southwest Arizona, and only slight chances over
southeast California. Some increase in convective activity is
expected on Friday, as deeper moisture begins to move into the
region. Temperatures to remain slightly above normal through the
entire forecast period. Minimum humidities in the 15-30 percent range
Monday-Thursday to rise into the 20-35 percent range on Friday.
Spotter information statement...
spotter activation is not expected.
Arizona...excessive heat warning until midnight MST tonight for azz020>023-
California...excessive heat warning until midnight PDT tonight for caz031>033.
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