FXUS65 KPSR 182136

Area Forecast Discussion
National Weather Service Phoenix AZ
235 PM MST Mon Jun 18 2018

High pressure will build across the region this week resulting in a
rapid warming trend and many lower desert communities likely
exceeding the 110 degree threshold both Thursday and Friday. Some
cooling is anticipated over the weekend as a weak low pressure
system passes north of the region. There are no signs of rainfall
through early next week.


Afternoon WV imagery and objective analysis depict a deep negative
height anomaly spinning across the northern Great Basin, acting to
sweep deep westerly flow and dry air into the southwest Conus.
Regional H5 height rises in a 3-6dm range have been common and
corresponding midtropospheric temperatures have quickly jumped 2-4C
in the past 24 hours. Boundary layer temperatures have been slower
to respond and readings this afternoon are only marginally warmer
than Sunday. This will change rapidly over the next 72 hours as the
northern stream trough translates downstream into the plains and
east Pacific ridging builds strongly over the forecast area.

Overall synoptic guidance remains in very good agreement for the
remainder of the week with a deep anti-cyclone becoming established
over SW Arizona/SE California during the midweek time frame.
Ensemble means depict little to no spread with H5 heights nearing
596dm and representing levels at or above the 95th percentile for
mid/late June. While unusually warm H5 temperatures near -2C would be
near records, the vast majority of model output suggests sfc-H7
temperatures only realizing an outcome closer to the 90th
percentile. With good confidence, many lower elevation locations
should exceed the 110F threshold Thursday and Friday though well
short of records for the date (our hottest time of the year). Should
models trend stronger with the anti-cyclone or warmer within the
mixing layer, excessive heat headlines would be necessary across
several metropolitan communities (particularly central Phoenix).

Operational models continue to diverge somewhat by the end of the
weekend as at least a portion of the SW Conus ridge retreats
westward in response to another trough moving into the Great Basin.
The operational GFS depiction of the trough is still stronger than
the ECMWF suggesting more overall cooling with temperatures
returning close to normal Saturday/Sunday. This operational GFS run
is more amplified than the vast majority of NAEFS members, so
attempted to keep forecasts above the median model output. Even if
the cooling doesn`t materialize, high temperatures in the lower
deserts will be a far cry from what they were this time last year
(generally above 115 degrees). Otherwise, mid-level wind fields will
increase both ahead and behind the trough, so breezy conditions are
a good bet as well. Next week, models are in generally good
agreement that a predominant anti-cyclone will become re-established
somewhere over the Southwest. This will ultimately mean a resumption
of seasonably hot conditions along with possibly heralding an
increase in moisture, mainly southeast AZ. However, PoPs more or
less remain near 0 percent into the middle of next week.


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

No aviation concerns through the TAF period. Skies will remain
clear and winds will follow normal diurnal tendencies with speeds aob
10 kts. Overnight, very light winds speeds may cause wind
directions to become variable at times.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday:
Seasonably hot weather will return to the districts for the latter
half of the week with only modest cooling back towards more average
late June temperatures later in the weekend. Minimum afternoon
humidity values will fall into the single digits through Friday,
then closer to a 10-20% range over the weekend. Overnight recovery
will be poor to fair. Winds may become locally stronger on Saturday
with the passage of a weather disturbance to the north, but
otherwise typical afternoon upslope gustiness will be common during
the period. There are no sign of rain for the districts through
early next week.


Spotter activation will not be needed.






NWS PSR Office Area Forecast Discussion