Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 191101

Area Forecast Discussion
National Weather Service Phoenix AZ
401 AM MST Thu Oct 19 2017

.UPDATE...Updated aviation and fire weather discussions.


A few isolated light showers will be possible across eastern
Arizona through this afternoon as a weak disturbance moves
through the region. Drier and cooler air will settle into the
region into this weekend before high pressure rebuilds over the
region leading to well above normal temperatures by early next


A weak shortwave trough centered near Las Vegas early this morning
will slowly move east northeastward today with a trailing trough
axis moving through the southern half of Arizona. Modest moisture
levels still exist across much of the region, but the bulk of the
moisture exists in the mid to upper levels of the atmosphere. The
best chance for light showers today will be focused across the
higher terrain northeast and east of Phoenix. The combination of
weak difluence aloft and the passage of the trough axis this
morning into early this afternoon should result in at least a few
light showers over the high terrain. Any showers that develop over
the south-central Arizona deserts should only result in sprinkles
at best due to the very dry sub-cloud layer below 14k feet. The
trough axis and the chances for showers will exit into New Mexico
by late afternoon. Temperatures today will again be well above
normal, but some slight cooling will be seen across southeast
California and southwest Arizona. Behind the exiting trough, drier
air aloft will begin to gradually filter into the region.

For Friday into the weekend, a deeper upper level trough passing
by to our north will begin to influence the Desert Southwest.
Upper level heights and temperatures will fall modestly Friday
into Saturday with the main trough axis passing through Arizona
early Saturday. Highs Friday will drop back into the upper 80s
across most lower desert locales with another degree or two drop
on Saturday. Skies are expected to be clear to mostly clear under
drying conditions. Strong surface high pressure will build across
the Great Basin Friday into early Saturday resulting in a modest
pressure gradient setting up across the northern half of Arizona
and southern California. Westerly surface winds will become breezy
on Friday across southern Arizona and windy at times across
portions of southeast California. Near-advisory level winds
across Imperial County on Friday may result in some patchy blowing

Winds will weaken into Saturday and turn northerly across the
Lower Colorado River Valley as the surface high shifts further
eastward into the Central Rockies. This turn to northerly winds
will bring rapid drying in the low levels on Saturday dropping
surface dew points into the teens across the Lower Colorado River
Valley and into the 20s and 30s across southern Arizona.
Eventually, the eastward shifting surface high will set up an
easterly flow across the majority of southern Arizona beginning
Saturday night, lasting into early Monday.

An unseasonably strong ridge of high pressure is forecast to build
over the Pacific just west of the Baja peninsula Friday into
Saturday and eventually shifting northward into the Desert
Southwest Sunday into early next week. The NAEFS ensemble mean
forecast shows record or near record climo 500mb heights late
Sunday into Monday across southern Arizona, but this doesn`t last
very long as the ridge center shifts well to the northwest by
Monday night. Either way, we are looking at a quick turn-around in
temperatures starting Sunday. Highs Sunday should easily make it
back into the lower 90s followed by middle to upper 90s for Monday
and Tuesday. Though forecast highs fall short of records on
Monday by several degrees, the forecast high of 97 degrees for
Phoenix on Tuesday would top the previous record of 96. Some
gradual cooling is likely going through the middle of next week as
the upper level ridge weakens, but temperatures should still
remain several degrees above seasonal normals.



South-Central Arizona including KPHX, KIWA, and KSDL:

A weak disturbance moving through this morning will bring
isolated areas of virga to mainly the east valley with perhaps a
few sprinkles reaching the ground. This threat will quickly end by
late morning as the disturbance moves off to our east. Beyond
this, winds will follow typical diurnal tendencies this afternoon
with no other aviation concerns.

Southeast California/Southwest Arizona including KIPL and KBLH:

Westerly winds will continue at KIPL today and tonight with even
stronger westerlies this evening. Winds will favor the southwest
at KBLH today and remain at or below 10 kts. Only a few passing
high clouds are expected at both terminals today.


Saturday through Wednesday:
Temperatures hovering near average over the weekend will rebound
back close to 10F above normal on Monday and Tuesday. High
pressure and dry air will become locked over the districts
ensuring no chance of rain. Afternoon minimum humidity levels in a
10-20% range over the weekend will fall into the single digits
next week while overnight recovery deteriorates from good down to
poor to fair levels. Breezy northwesterly winds are likely in SE
California and along the Lower Colorado River Valley on Saturday,
but speeds will remain below critical thresholds. Winds are
forecast to weaken by Sunday.


Spotters should follow standard reporting procedures.




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