Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
218 PM MST sun Oct 23 2016
a low pressure system will move towards the Pacific northwest early
this week. This storm system will draw a considerable amount of
subtropical moisture into the Desert Southwest with the threat of
isolated showers and thunderstorms starting this evening and
continuing through Monday night. The best chance of showers will
occur Monday over south central Arizona, with afternoon thunderstorms
possible. Drier and warmer conditions will return by the middle of
latest streamline analysis shows a broad ridge encompassing the
southern states while a low pressure system in the eastern Pacific
approaches the Pacific northwest. Southwesterly flow between these
two systems is transporting warm and moist air from the subtropics
into the region. In the lower levels, dewpoints have jumped abruptly into
the mid 60s across the Yuma in response to the low-level southerly
flow. Further east across southeastern Arizona, latest satellite
imagery shows that isolated convection has initiated, mainly across
the higher terrain.
Steering flow is generally 10 kt or less associated with the 700 mb
anticyclone centered near Nogales and the existing activity is not
expected to move towards central Arizona. However, some cams suggest
isolated cells will drift into portions of Pima and Pinal counties
late this afternoon, though latest runs of the hrrr continue to
indicate little potential for convection this far north. Pops were
again lowered slightly from Phoenix northward, but raised across
southwestern Maricopa County, albeit to only around 20 percent. Hi-
res guidance remains in good agreement that any activity that
develops will likely dissipate this evening.
Deeper moisture will continue to stream northeastward ahead of an
approaching southern-stream short-wave trough Monday. Naefs ivt
appears to be strongest across Southern California and western Arizona during the
late morning and early afternoon with a widespread area exceeding the
95th percentile. Nearly all cams are suggesting a large shield of
clouds/light rain and sprinkles will encompass the area during the
afternoon. Global models are also indicating that areas that see
less clouds will destabilize, supporting 100-300 j/kg of SBCAPE.
Conceptually, the best chance for precipitation will actually be
during the evening across central Arizona, when forcing for ascent is
maximized ahead of the vort Max. Although organized thunderstorms are
not expected, Storm Prediction Center probabilities indicate at least a slight chance of
thunder across much of the area into the evening.
A gradual clearing and drying trend from the west is expected Monday
night into Tuesday as the last short wave moves through central
Arizona and then quickly exits east into New Mexico, followed by
building high pressure and increasing subsidence. By Tuesday morning
there should be just a lingering slight chance of showers over the
higher terrain east of Phoenix with mostly sunny skies developing
over the central and western deserts. High temps will climb slightly
Tuesday and into the low 90s from Phoenix westward due to increasing
sunshine and mixing.
For Wednesday into Thursday, strong high pressure aloft is forecast
to build back into the Desert Southwest, as another large area of
low pressure sets up just off the West Coast and pumps up the ridge
downstream and over our area. High temps will climb back into the
middle 90s, with a high of 95 forecast in Phoenix by thursday; that
would be over 10 degrees above seasonal normals. It should stay dry
for the most part, though a minor short wave lifting out of the deep
trof will brush by far southeast California Thursday leading to a
slight chance of a shower over the higher terrain of Joshua Tree np.
Latest European model (ecmwf) and GFS have come into much better agreement for the
latter portion of the extended forecast, and the gefs members also
have come into better alignment; we expect that in general the ridge
will remain rather strong over the Desert Southwest but there will
be continued short waves moving out of the trof along the West
Coast, riding across and over the top of the ridge and bringing in
some modest moisture and uvv, enough to warrant a slight chance of
showers over the western deserts - mainly over higher terrain areas
of southeast California. The central Arizona deserts should remain dry and
warm through the period however. Latest naefs pops trends also
support the relatively dry conditions during the Friday through
Aviation...south-central Arizona including kphx, kiwa, and ksdl:
southeast California/southwest Arizona including kipl and kblh:
A Pacific weather system is expected to pull increasing amounts of
moisture into the region over the next 24 hours. This will result in
increasing mid and high-level cloudiness over the phx area terminals
through the taf period. Chances for showers, and even an isolated
thunderstorm are also expected to gradually increase through the taf
period as well. However, there is not yet enough confidence in the
timing/coverage of any shower/ts activity to put anything more than
vcsh in the tafs at this time. Although winds are expected to remain
mainly on the light side (with normal diurnal patterns) through the
taf period, there is a remote possibility that stronger outflow
winds from thunderstorms could briefly affect one or more of the taf
sites, especially during the afternoon/evening hours on Monday.
However, confidence is still far too low at this time to put any
outflow wind shifts in the tafs at this time.
Aviation discussion not updated for amended tafs.
Wednesday through Sunday...
High pressure redeveloping over the Desert Southwest will bring dry
conditions and well-above normal temperatures to the region on
Wednesday and Thursday. Another weather system brushes the Desert
Southwest late Thursday through Friday for an increase in humidities
and a slight chance of showers -mainly for southeast California and
southwest Arizona. Warmer and drier weather is expected to return for
next Sunday. No strong winds are anticipated through the forecast
period. Minimum humidities stay at or above 20% on the lower deserts
for the most part and overnight recovery remains at least good.
Spotter information statement...
spotters should follow Standard reporting procedures.
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