Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 280315

Area Forecast Discussion
National Weather Service Phoenix AZ
815 PM MST Sat May 27 2017

Very typical Memorial Day holiday weather will persist through the
weekend with afternoon high temperatures climbing into the 90s and
lower 100s. Temperatures will peak on Monday and Tuesday before
minor cooling starts beyond the middle of next week. Some scattered
mountain storms will also be possible by the middle of the week.



A low amplitude high pressure system will continue to build into the
region, resulting in mostly clear skies and light wind. Warmer days
are also forecast. Current dry and warmer forecasts through Monday
look ok. No short term updates necessary.


Dry southwest flow continues across far southeast California and
southern Arizona early this afternoon; very typical conditions for
late May in Arizona. 12z plots this morning showed very little
change in the 500mb heights and not surprisingly temperatures at 2
pm this afternoon were running roughly the same as they were
yesterday. We can expect high temperatures today to top out in the
mid to upper 90s with a high around 97 in Phoenix. Gradients have
been decreasing and as such winds have been typical for late May
with some afternoon gusts at times favoring the southwest to west
less than 25 mph.

Operational models as well as ensemble guidance has changed little
over the past couple of runs and very little has changed in the
forecast from the previous midnight shift. For the most part the
deserts will remain dry through next work week and any threat for
rain will be limited to high terrain areas of far eastern Arizona;
any threat in our forecast area should stay mostly east of Globe.
POPs have changed very little and remain close to the latest NAEFS
ensemble POP trends. Flow backing ahead of the first weak wave
approaching the Baja coast will lead to a warming trend; thus
Monday/Tuesday will be the warmest days of next week with desert
highs above 100 and topping out near 104 in the greater Phoenix
area. Rest of the discussion is from the previous mid shift; again
forecast thinking remains pretty much the same.

Through at least the first half of next week, an active subtropical
jet will bring several lower amplitude disturbance into the
Southwest domain. The details regarding depth/intensity and moisture
availability remain varied among operational and ensemble member
output. The 00Z operational GFS remains on the more aggressive side
bringing lower heights and somewhat stronger forcing mechanisms
further south. Ensemble means more closely resemble the 00Z ECMWF
and CMC though impacts on end result sensible weather forecasts are
minimal and limited.

Regardless, ahead of these series of waves, substantial backing of
winds in the H9-H7 layer will occur through the Rio Grande Valley
and Four Corners. Almost all model output suggests notable moisture
advection up the Rio Grande Valley and juxtaposed with weakly forced
ascent associated with the shortwaves, isold/sct thunderstorms will
be possible along the Rim, White Mountains, and portions of southern
Gila County. The largest uncertainty lies with the areal extent and
westward push of the best quality moisture plume. Initial look at
SREF probabilities suggest good chances of storms, however
deterministic model QPF is quite underwhelming. This may be a case
of more elevated echoes and lightning versus accumulating rainfall.

The forecast for the latter half of next week still remains quite
uncertain with model spread growing in a partially blocked flow
pattern. The operational 00Z GFS still remains a more aggressive
outlier though has backed off the most unlikely outcome of a deep
closed low. The 00Z ECMWF and GEM continue to look far more
agreeable when compared to other medium range guidance keeping an
ill defined height weakness and troughing signature lingering
through the SW Conus. Moisture will mostly be relegated to New
Mexico in persistent westerly flow (i.e. little to no chance of
rain), though the general weak cyclonic flow pattern will favor
temperatures hovering close to climatology.


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

A high pressure system is forecast to build into the region through
Sunday evening, resulting in mostly clear skies and light and
variable wind under 8 knots.

Aviation discussion not updated for amended TAFs.


Tuesday through Saturday:

Generally warm with mostly dry conditions for the period. However, a
weather system will bring a slight chance for isolated thunderstorms
Tuesday and 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. High temperatures will run near to
slightly above seasonal normals. Minimum relative humidities will
bottom at 10 to 20 percent with fair overnight recoveries.


Spotter reports will not be needed through the middle of next week.




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