Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 261614

Area Forecast Discussion
National Weather Service Phoenix AZ
914 AM MST Fri May 26 2017

Dry and tranquil weather will persist into the weekend with
afternoon high temperatures very typical for the end of May into the
90s and lower 100s. Temperatures will warm somewhat into the
beginning of next week with some scattered mountain storms possible
by the middle of the week.


A decent drop in temperatures for today will be realized as weak
troughing continues to slide through the region. The 12Z Tucson
raob showed a 3C temperature drop at 850mb over the past 24hrs
which matches well with model output. Morning surface temperatures
are also running roughly 5F cooler than yesterday. Even with the
cooler airmass, full sun and good mixing today will allow for
highs this afternoon to easily reach into the lower 90s across the
deserts. No updates to the current forecast are needed.


General mid continent troughing and a series of shortwaves over the
Rockies will keep H5 heights locally dampened in a 576-582dm range
today with only modest increases through the weekend as the core of
lower heights aloft translates into the Great Lakes. A more split
flow regime will predominate over the western Conus early next week
as high amplitude ridging through the Pacific Northwest is undercut
by an active subtropical jet into the Southwest. The end result will
be temperatures oscillating not horribly far from the seasonal
average, yet potentially better moisture availability than would be
expected at the end of May.

The majority of height falls and cooling aloft has locally
stabilized as the primary troughing circulation spins over the
Rockies and the trailing trough axis becomes less defined into the
Southwest. As a result, cooling around the H8 layer will be
maximized today with afternoon highs some 4F-8F cooler than
Thursday. Low level thermal profiles will rebound rather quickly
through the weekend with most lower elevation communities once again
breaching the 100F threshold (and very close to the seasonal normal)
by Sunday. Otherwise, any remaining strong winds will abate with the
weakening pressure gradient this morning; and only the more typical
afternoon gusts will prevail through the weekend.

Through the first half of next week, a more active subtropical jet
will bring at least one lower amplitude disturbance into the
Southwest domain though the details regarding timing, intensity, and
moisture availability remain varied among ensemble member output.
Ahead of this wave, heights and temperatures aloft will temporarily
spike and pockets of afternoon highs in a 105F-110F range will be
common Monday and Tuesday. Taking a GFS/ECMWF blend (which also
approximates a full ensemble blend), a quasi-cutoff feature will
drift into the Baja/northern Mexico allowing substantial backing of
winds in the H8-H7 layer through the four corners. Most model output
suggests notable moisture advection up the Rio Grande Valley and
juxtaposed with weakly forced ascent associated with the shortwave,
isold/sct thunderstorms will be possible along the Rim, White
Mountains, and portions of southern Gila County.

The forecast for the second half of next week remains highly
uncertain with model spread growing abruptly in a partially blocked
flow pattern. Have mostly ignored the operational 00Z GFS and its
follow-up deeper shortwave and closed low as an outlier versus
almost all other ensemble members. The 00Z ECMWF looks far more
agreeable when compared to other medium range guidance keeping an ill
defined trough lingering towards the four corners and yielding a
forecast very close to climatology.


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

Breezy west winds will develop area-wide once again this afternoon,
but will be a bit weaker than yesterday. Still could see gusts to
20 kt at KPHX/KIWA, and 25 kt at KBLH/KIPL. A few high clouds will
be possible throughout the day, generally above 18 thousand feet.
Otherwise, aviation concerns are expected to be low.

Aviation discussion not updated for amended TAFs.


Sunday through Thursday:
A return to above normal temperatures will occur Sunday and
especially Monday as high pressure builds across the Great Basin.
Dry conditions will also persist though winds will generally be
light. A weak Pacific low pressure system will develop and move
eastward towards the Southwest and could result in a slight chance
for thunderstorms across the higher terrain of east-central
Arizona, including southern Gila County, Monday through Wednesday
afternoons. Any associated rainfall totals would likely be very
light, and there is some potential for isolated dry lightning


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




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