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Forecast Discussion for Phoenix, AZ

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FXUS65 KPSR 182240

Area Forecast Discussion
National Weather Service Phoenix AZ
340 PM MST Tue Sep 18 2018

A strong disturbance will move through the region with relatively
deep tropical moisture over the next couple days. This will result
in scattered to widespread showers and thunderstorms across much of
southern and eastern Arizona Wednesday as temperatures decrease
below seasonal normals. Slight rain chances will persist into
Thursday before much drier conditions and warmer temperatures return
for this weekend and early next week.


The strong ridge of high pressure which had held over the bulk of
the SW Conus the past several days was decaying and shifting east as
the influence of northern stream Pacific energy encroaches into the
western states. Both objective analysis and satellite derived PWAT
suggest deep quality (1.50 to 2.00 inches) moisture poised near the
international border ready to surge north into the forecast area as
a well defined inverted trough/circulation center over the Baja
peninsula steadily becomes absorbed into the northern jet flow.
Moisture should move into upper levels first tonight associated with
debris organized storms over northern Sonora, but impressive surface
moisture will not be far behind with solid 14 g/kg sfc-H8 mixing
ratios enveloping lower elevations of Arizona.

The setup remains quite good from a synoptic perspective and very
typical of a monsoon transition event incorporating tropical type
moisture with favorable jet energy. The Baja wave and associated jet
streak, in addition to the regional proximity of the right rear
northern jet entrance area should provide an upper divergence field
very supportive of deep convection. However, models still have
varying ideas of convective evolution with this significant moisture
flux. The key still seems to focus on timing of thicker clouds cover
advecting north during the day. Several models still suggest a
pronounced overcast developing early in the morning effectively
limiting any instability through both the lack of insolation and
elimination of midlevel lapse rates. Both coarser operational models
(i.e GFS/NAM) and high resolution HREF members (NMM and ARW cores)
keep oscillating between this idea of overcast/minimal
instability/mere showers and more evident cloud break/moderate
instability/more robust deep convection. In the end, something
between these two extremes will likely be the outcome with some
areas probably around or north the Phoenix metro remaining socked in
with clouds, yet breaks to the south and west of the city yielding
MLCape around 1000 J/kg and modest lapse rates.

Should greater clearing occur that would allow for more drying at
lower levels, larger DCAPE values would support stronger storms
capable of producing damaging winds, while less clearing would
probably favor more of a heavy rainfall threat over severe threat.
Either way with upstream moisture values advecting into the region
representing values several standard deviations above normal, any
deeper storm would be capable of very heavy rainfall; and have
issued a Flash Flood Watch for the SE portions of the CWA.
Conceivably, areas to the west of the watch could also experience
flooding rainfall though the areal coverage is far more uncertain at
this time. Storm activity could extend as far west as the Colorado
River, but moisture will probably be scoured out too quickly to
support any convection over southeast California.

Beyond Wednesday, the operational GFS and ECMWF along with their
ensembles are converging towards a drying trend for Thursday. Both
models keep the remnants of a potential tropical depression or
whatever develops and moves into the Gulf of California south and
east of the region. This makes sense considering this system should
quickly become extratropical as it interacts with the shortwave
trough pushing east of the region by Thursday morning. Have
generally cut POPs lower Thursday and Friday given the preponderance
of operational and ensemble guidance while also increasing
temperatures given the forecast trend of more ridging, subsidence,
and warming aloft building into the forecast area faster than
previous model iterations.


South-Central Arizona including KPHX, KIWA, and KSDL:
Low-level moisture will increase overnight ahead of an approaching
low pressure system. Showers are expected to develop generally
around midday Wednesday, though isolated activity is possible before
then. Main impact will be wet runways and CIGS as low as 6k ft in
the afternoon, however there is the potential for isolated
thunderstorms with strong winds through Wednesday evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
Increasing moisture ahead of an approaching low pressure system will
mainly affect areas to the east in Arizona. However, there is a
slight chance of isolated thunderstorms near KBLH Wednesday
afternoon. Any storms that develop have the potential to produce
brief heavy rain, strong gusty winds and localized areas of blowing

Aviation Discussion not updated for amended TAFs.


Friday through Tuesday:
Slightly below normal temperatures will linger into Friday before
rebounding over the weekend into a near normal range. Showers and
storms will likely have moved east of the districts with a drying
trend through early next week. Minimum humidity levels in a 15-30%
range late this week will fall more solidly into the teens next
week. Overnight recovery will vary widely from fair to excellent.
Winds will remain mostly light and quite typical for late
summer/early autumn.


Spotters reports may be needed later this week in Arizona.


AZ...Flash Flood Watch from Wednesday morning through Wednesday
     evening for AZZ548>555-558>563.




Previous Forecast Discussions may be found at
NWS Phoenix, AZ (PSR) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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