Area Forecast Discussion
Issued by NWS Phoenix, AZ

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619
FXUS65 KPSR 180200
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
700 PM MST Tue Jul 17 2018

.SYNOPSIS...
The potential for isolated to scattered showers and thunderstorms
will continue for the next several days before a steady decrease in
thunderstorm activity heading into the weekend. Stronger high
pressure will build across the region later this week resulting in a
warming trend and return to above normal temperatures by early next
week.

&&

.DISCUSSION...
The most obvious feature on evening WV imagery is a well defined
easterly wave and embedded vorticity center propagating into
northern Sonora. Regionally intense effective shear acting upon
MLCape in excess of 2000 J/kg has supported tremendous severe
thunderstorm development along and south of the international
border. Further to the north, one consequence of this easterly wave
has been advection of drier air from west Texas and New Mexico into
eastern and central Arizona. 00Z KPSR sounding data revealed mixing
ratios in the sfc-800mb layer have fallen to 10 g/kg while dropping
below 8 g/kg at and above the top of the boundary layer in the H7-H5
layer. Subsequently with meager midlevel lapse rates lingering, CinH
has grown substantially through central Arizona. As a result, have
reduced and/or stripped out POPs for much of the area given all the
observational trends.

Elsewhere, clustered thunderstorms over northwest Arizona were
progressing into an area of stronger inhibition while also losing
larger scale synoptic support. While still within a region of better
overall moisture profiles, some storms could still percolate in this
area and send outflows and a few showers into western Arizona,
albeit not the most likely outcome. Have based much of the evening
and overnight forecast on a combination of NSSL WRF and NamNest
forecasts which suggest an organized complex developing from the
massive northern Sonora storms, eventually sending a combined
outflow/Gulf surge through SE California and the Lower Colorado
River Valley later tonight. The magnitude of this outflow may be
sufficient to spark a few storms late tonight/early Wednesday
morning around the El Centro/Yuma area, however mean layer
convective inhibition is quite strong in the area, and more likely
only elevated showers would be the outcome. Regardless, have added
areas of blowing dust considering the strength of the outflow and
lack of recent rainfall in these areas.

&&

.PREVIOUS DISCUSSION..
/149 PM MST Tue Jul 17 2018/
The convective-allowing models are still in good agreement showing
storms developing in two areas. The first is along the Rim and
higher terrain across Northern Arizona, and eventually making way
into La Paz County. The second area for convection is most likely to
be across Pima and southern Maricopa County later this afternoon in
association with the aforementioned trough where CAPE values will
near 2000 j/kg with ample daytime heating to destabilize the
atmosphere. Thunderstorms and showers may have enough energy to
sustain themselves into the Yuma-El Centro areas late tonight and
very early tomorrow morning. The greatest weather threat for the
Phoenix area will be gusty outflow winds and blowing dust from the
south during the evening.

For Wednesday, the inverted trough will continue to progress
westward as the ridge reestablishes itself over the Southwest. The
atmosphere will begin to dry somewhat but there will be enough
moisture for convection tomorrow afternoon, primarily across the
higher terrain. Thunderstorm activity will continue to decrease the
next few days as the center of the ridge builds over Arizona by the
end of the weekend.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
Timing of wind shifts and a lowering potential for outflow
boundaries are the main aviation challenges through Wednesday
afternoon. It is still possible outflows from distant storms in Pima
county could send a boundary with southerly winds into the Phoenix
terminals late this evening, though trends are suggesting weaker
winds and less chance of dust. After a brief period of east winds
Wednesday morning, directions should switch to westerly fairly
quickly early Wednesday afternoon with any storms remaining well
removed from aerodromes through Wednesday evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
While decaying storms could send a boundary into KBLH later this
evening, the more significant aviation impact may be a strong
outflow S/SE outflow/Gulf surge into SE California overnight. Models
are becoming more insistent of very strong winds and blowing dust
impacting terminal during the late night hours. Confidence is high
enough to include a period of MVFR visibilities prior to sunrise.
More tranquil VFR conditons should exist Wednesday afternoon.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday:
Isolated showers and thunderstorms will remain possible Friday,
especially across the higher terrain north and east of the Phoenix
area. However, moisture values will undergo a gradual decrease
into the weekend, and expecting only a slight chance of any
thunderstorms area-wide by Saturday. Daytime minimum relative
humidity values will also fall closer to 15%. A significant
increase in heat is advertised for Sunday and Monday, with high
temperatures above 110 degrees expected over the lower deserts.
The winds will be generally light and follow typical diurnal
trends.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.

&&

$$

DISCUSSION...MO/Deemer
AVIATION...MO
FIRE WEATHER...Rogers/Deemer



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