FXUS65 KPSR 141729

Area Forecast Discussion
National Weather Service Phoenix AZ
1029 AM MST Tue Aug 14 2018

Monsoon activity will be on the increase starting today, first
affecting high terrain areas and then across the lower deserts
during the overnight hours tonight. A substantial increase in
moisture will bring good chances of showers and thunderstorms on
Wednesday into early Thursday with locally heavy rainfall
possible. Temperatures will dip to below normal midweek but warm
back to above normal as drier conditions move into the region this


Morning soundings show a substantial increase in moisture has
occurred overnight, with precipitable water values now exceeding
1.6 inches across the lower deserts of south-central Arizona.
Surface dewpoints reflect this increase -- now in the lower-mid
60s at most locations. Visible satellite imagery also showed accas
present across the area, while shallow cumulus was developing more
recently already across the higher terrain. Mid-level flow has
become southeasterly as high pressure has become re-centered over
northeast Arizona. MLCAPE values are now exceeding 2500 J/kg with
relatively little inhibition. Confidence is fairly high that
convection will become fairly widespread across southeast Arizona
and perhaps into the White Mountains and Mogollon Rim. Steering
flow will likely bring outflows and the potential for new storms
northwestward late this afternoon and evening, potentially
supporting another round of thunderstorms for Phoenix and the
surrounding areas. With weaker deep-layer flow and shear, not
expecting the level of thunderstorm organization that was observed
Sunday, but still could have the potential for isolated damaging
wind gusts and localized flooding. Blowing dust is also a concern
this afternoon and evening.



The persistent upper level low to our east that brought an
unusually long period of strong mid level northeasterly flow has
finally moved into the Central Plains leaving us under weaker
flow. Moisture levels remain fairly high across much of the region
and with good surface heating today storms will be more active
across the high terrain. Very weak steering flow should keep these
high terrain storms mostly out of the lower deserts this
afternoon and evening, but we can`t completely rule out an
isolated storm or two making into the Phoenix area.

The main forecast concern over the next several days will be a
significant moisture surge associated with an inverted trough
seen moving northward out of Mexico tonight into Wednesday. This
trough is already well evident in water vapor imagery across the
southern Baja area. Quite a bit of storm activity is likely south
of the border later today, possibly reaching into far southern
Arizona as early as late evening. Models are essentially all in
agreement the inverted trough will move into southern Arizona
tonight triggering a strong Gulf surge starting mid to late
evening. Forecast soundings do not show overwhelming CAPE and what
instability is there is elevated. Nevertheless, it is common to
see elevated showers and thunderstorms in this type of situation
and the bulk of the CAMs show activity moving into western Pinal
and southern Maricopa Counties sometime after midnight. We have
raised PoPs to reflect this thinking with 30-50% PoPs across
south-central Arizona late tonight. The main threat for tonight
will be locally heavy rainfall with slow moving storms.

The strong moisture advection into the area tonight into Wednesday
will result in PWATs mostly between 1.6-2.0" and quite a bit of
cloudiness. We may have a difficult time destabilizing on
Wednesday and forecast soundings show an inversion layer which may
keep storm activity isolated through the afternoon hours. Current
thinking is showers and thunderstorms should mainly be confined
to high terrain areas until the evening hours when the inversion
layer is shown to erode. Am anticipating scattered showers and
thunderstorms Wednesday evening through much of the overnight
hours. This will likely be the best time frame for heavy rainfall
and any potential flooding. Late Wednesday night, models show the
high center shifting well to the east with unidirectional
southwesterly flow developing. This is a change and will likely
cut back on storm activity on Thursday and Friday. Have cut back
on PoPs a bit for Thursday, but they are still between 15-30%.

The southwesterly flow will gradually bring in drier air into the
weekend with lower and lower chances for storms. Starting Friday,
it may be difficult to see any storms over the deserts under the
weak westerly steering flow and drying conditions. From Friday
into Saturday and high center quickly shifts westward from New
Mexico to off the southern California coast. This should bring in
even drier air out of the north with PWATS dropping well below
1.5". This drier air and 500mb heights around 594dm will allow
high temperatures to creep upwards late this week into the
weekend. The warmest desert locations could see 110 degrees by
Sunday with 105-107 anticipated for the Phoenix metro. Storm
chances this weekend and likely into early next week will
generally be confined to the high terrain of far eastern Arizona.


South-Central Arizona including KPHX, KIWA, and KSDL:

An active sea breeze today will maintain moderate westerly winds
across the area near 8-12kt with gusts of 15-20kt developing by
early afternoon. For late tonight nocturnal thunderstorms are
expected to move into the vicinity around 08-09Z from the southeast
or east with very breezy southerly outflow winds gusting to
20-30kt. Areas of blowing dust and reduced visibilities are also
possible near the storms although not to the extent of including
them in the TAFS. Otherwise skies will be clear to mostly clear
with FEW-SCT decks from 9-18kft for today. Then late tonight with
the storms expect BKN-OVC CIGS from 12-18kft.

Southeast California/Southwest Arizona including KIPL and KBLH:

Expect light to moderate diurnal and southerly winds for today near
6-12kt.  Breezy southerly winds are expected for KBLH by mid morning
with gusts of 16-20kt. Some breezy sundowner westerly winds can also
be expected to develop at KIPL by early this evening with gusts up
to 24kt. Otherwise skies will be mostly clear with FEW-SCT decks
from 10-15kft.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday:
Elevated moisture levels will continue into Thursday for all
districts resulting in enhanced chances for thunderstorms and
wetting rains. Some locally heavy rain will be possible,
especially throughout central Arizona. Slightly drier air will
slowly push into the region by the weekend, but afternoon storms
will still be possible over higher terrain areas. Throughout the
period wind speeds will be typical for mid summer with the usual
afternoon breezes. Temperatures will start out below average on
Thursday and then gradually warm near to slightly above normal
Friday through Monday.


Spotters should follow standard reporting procedures.





NWS PSR Office Area Forecast Discussion