Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 281108

Area Forecast Discussion
National Weather Service Phoenix AZ
408 AM MST Fri Jul 28 2017

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


After a couple dry days, moisture will begin increasing from the
east today and through the weekend ushering in an increased chance
of showers and thunderstorms, especially across south-central
Arizona. Subsequently, high temperatures decrease as well...most
noticeably over south-central Arizona. Next week will remain
unsettled each day with slight chances for showers and storms over
the deserts and higher chances across the higher terrain. High
temperatures will climb back to near seasonal normals.


After a couple of dry and "down" days, convectively speaking, it
appears that Arizona will returning to wet and active weather as a
large and slow moving inverted trof moves into the state and slowly
treks westward across our area today through the weekend. The trof
is already clearly visible on vapor imagery and to a lesser extent
plot data; difluence ahead of the trof is lending support to
continued showers which are currently still moving across much of
southeast Arizona and portions of south-central AZ mainly east of
Phoenix. IR imagery at 3 am showed considerable mid/high level
debris clouds spreading north across the Arizona desert, the result
of dissipating convection that formed over northwest Mexico as well
as southeast AZ yesterday.

As the inverted trof moves slowly into the area we will see moisture
continue to increase; PWAT was already over 1.5 inches at Phoenix and
Tucson per the latest 00z soundings, but we can expect readings over
2 inches across portions of the southern deserts by Saturday morning
continuing through Sunday. Difluence and deformation aloft
associated with the inverted trof will help support convection; CAPE
will be plentiful and steering flow becomes more favorable -
generally out of the southeast. All of this will lead to enhanced
chances for showers and thunderstorms today through the weekend and
we will likely see very wet mornings Saturday and Sunday. As such we
have a good chance for overnight and morning showers as well as
isolated thunderstorms in the forecast both days this weekend. With
the increase in clouds and humidity, high temperatures will be
falling off sharply over the next couple of days. Phoenix reached
110 yesterday but will likely drop close to 100 today and then into
the 90s Saturday and Sunday. Some of the increase in moisture will
be due to a modest tap from tropical weather systems now moving
westward off the southern tip of Baja CA; this moisture will
supplement that moving into the area from the dissipating MCS over
northern Mexico.

For the remainder of the forecast period, Monday through Thursday,
model guidance (such as GFS and ECMWF) are relatively consistent in
the bigger picture in terms of shifting the high center north and
into central Nevada however the guidance is really quite chaotic
with respect to the smaller scale features. Streamlines aloft are
very chaotic and deformed much of the time and confidence is VERY
low with respect to any small vort maxes or weak trofs moving around
the high. Moisture and instability remain elevated through the
period, there will be CAPE present each day, even into southeast
California and PWAT has a hard time dropping much below 1.5 inches
over the lower deserts. As such confidence is low with respect to
the details or which day might be an "up" day versus a "down" day.
We will simply keep a rather broad brushed forecast in place,
suggesting moderate grade monsoon conditions with best threats for
storms over higher terrain areas north/east of Phoenix and 10-20
percent chances over much of the lower desert each day. High
temperatures will climb several degrees Monday and then change
little with most days hovering near or slightly below seasonal



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

Easterly winds are expected to switch back to the west late this
morning or in the early afternoon. Models are more mixed than
usual on the exact timing but best guidance indicates the early
afternoon. Thunderstorms will once again form over the mountains
out east of the TAF sites this afternoon. More instability today
will bring a better chance of storms to the valley late this
afternoon and evening. However, confidence is still low enough to
not include them in the TAFs. Outflows from decaying thunderstorms
look to be a better bet with winds switching back east anywhere
from 01-04Z. Have included this in the TAF for now but timing will
likely change depending on how thunderstorm activity plays out.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation concerns for today as winds follow typical diurnal
patters for this time of year.  For the overnight hours, models hint
that weak convection could reach KBLH and possibly KIPL late
tonight (06-09Z) but confidence remains very low of that actually
happening. For now, have no mention of any weather in the TAFs.

Aviation discussion not updated for amended TAFs.


Sunday through Thursday: Thunderstorm activity looks to peak late
this weekend and into early next week but will hang around
throughout the forecast period. The best chance for rain in the
lower desert appears to be Sunday, but the higher terrain north
and east of Phoenix has a decent chance nearly every afternoon.
Minimum relative humidities will stay above 20 percent across the
board with great overnight recoveries. High temperatures will be
mostly below seasonal normals this weekend, then gradually climb
to near normal levels by Wednesday as moisture and storm chances
decrease. Winds will otherwise be light outside of any
thunderstorm related winds.


Spotters should follow standard reporting procedures.




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