Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 281554

Area Forecast Discussion
National Weather Service Phoenix AZ
855 AM MST Sun May 28 2017

Very typical Memorial Day holiday weather will continue with
afternoon high temperatures climbing into the 90s and lower 100s.
Temperatures will peak on Monday and Tuesday before some cooling
starts during the middle of the week. Some scattered mountain storms
will also be possible Tuesday and Wednesday.


Very tranquil weather this morning across the desert southwest; high
amplitude upper ridge is in place to our north over the Pacific
Northwest with a weak split in the flow leaving weakly cyclonic flow
through Arizona and NW Mexico. Plots and Raob data show dry mostly
westerly flow in place across the area and 8 am IR data depicted
sunny skies area wide. 12z 500mb plots indicated either little
change or slight (10m) falls over the past 12 hours but despite this
the boundary layer has been warming and should be up 2-3C by this
afternoon as compared to this time yesterday. Thus under full
sunshine we should see several degrees of warming today and the
preponderance of the guidance calls for high temperatures to reach
back into the triple digits today. Phoenix should at least hit 100
and may well hit our official forecast of 102. Overall today we will
see sunny skies, light winds and warming temps. Forecasts look to be
in good shape and no updates are needed.

Dry quasi-zonal flow continues across the forecast area on the
trailing end of a departing trough axis. Regional height rises will
remain rather modest however, despite an amplifying and blocked
upstream flow pattern. Slow moving wave features (and in particular,
a series of poorly defined vorticity centers) will be common
throughout the week over the SW Conus yielding temperatures not
terribly far from average, yet with more elevated moisture and
mountain storms than would otherwise be expected for late May/early

Forecast confidence for the remainder of the holiday weekend remains
very high as all model output indicates H5 heights hovering in a 582-
585dm range. With clear skies and persistent dry westerly flow,
efficient deep mechanical mixing will support temperatures in a
slightly above normal range, however short of the most dangerous or
hazardous heat levels (historically, the greatest heat risk days
start appearing around Memorial Day).

Beginning as early as Monday afternoon, increasing cyclonic flow
aloft associated with an active subtropical jet undercutting
northern stream ridging will herald the pattern of several lower
amplitude disturbances rotating into the Southwest. This pattern
configuration will favor backing winds in the H9-H7 layer and
elevated moisture pulled northwest up the Rio Grande Valley. There
is still considerable uncertainty regarding the quality of moisture
return, in addition to the breadth at which the moisture plume is
pulled westward towards the midlevel circulation center. SREF
probabilities still suggest good chances of deep convection and
thunderstorms over higher terrain areas such as the White Mountains
and Mogollon Rim, however most deterministic model QPF is quite
underwhelming. Pattern recognition, time of year, and interrogation
of moisture in BUFR sounding data suggests an outcome of scattered
mountain storms but little rainfall accumulation (i.e. high concern
for lightning caused fire starts).

Somewhat better confidence exists regarding the forecast for the
latter half of the week. The operational 00Z GFS still remains a
more aggressive outlier with respect to lower H5 heights and height
falls enveloping the forecast area. The 00Z ECMWF and GEM continue
to more closely mirror the ensemble mean depicting a weaker, more
convoluted, and messy cyclonic flow persisting through much of the
week. There is some limited chance residual moisture hangs into the
state yielding more mountain convection though almost all guidance
relegates better moisture into New Mexico. Otherwise with a
preference towards somewhat higher heights aloft, temperature
forecasts towards the end of the week were weighted more towards the
median and upper half of the guidance envelop.


South-Central Arizona including KPHX, KIWA, and KSDL, and
Southeast California/Southwest Arizona including KIPL and KBLH:

Few aviation weather concerns Sunday for any of the terminals
with clear skies and light winds favoring typical diurnal

Aviation discussion not updated for amended TAFs.


Tuesday through Saturday:
Generally warm with mostly dry conditions for the period with high
temperatures at or slightly above seasonal normals. However, a
weather system will bring a slight chance for isolated
thunderstorms Wednesday in the higher elevations to the north and
east of Phoenix. Rainfall amounts with any storms looks minimal
but a threat of dry lightning will exist for both days. Outside of
thunderstorms, the winds will be the breezy Wednesday and Thursday
with gusts up to 20- 25 mph, especially along the Colorado River.
Minimum relative humidities will drop to 10 to 20 percent with
fair overnight recoveries.


Spotter reports should not be needed this week.




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