Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 280330 AAA

Area Forecast Discussion
National Weather Service Phoenix AZ
830 PM MST Thu Jul 27 2017

.UPDATE...Updated aviation discussion.

After a couple dry days, moisture will begin increasing late
tonight ushering in an increased chance of storms across much of
the region Friday through the weekend. Subsequently, high
temperatures decrease as well...most noticeably over south-central


The reprieve from monsoon storms is basically over as winds aloft
have already begun to shift to a more favorable southeasterly
direction. Though we are not anticipating any storms across the
lower deserts today, increased thunderstorm activity is likely
across southern Gila county late this afternoon and evening. High
Res CAMs indicate storm activity will approach the lower deserts
later this evening, but weaken considerably or completely fall
apart. A significant inverted trough will move into southeast
Arizona late tonight and slowly progress into south-central
Arizona during the day Friday. It is possible we may have what
should mostly be showers develop along the leading edge of the
inverted trough just before sunrise, possibly affecting areas as
far west as Phoenix sometime Friday morning.

The aforementioned inverted trough will be a slow mover across the
region as it remains nearly stationary across southern Arizona
possibly through the entire weekend. This will allow for a
persistent flow of moisture out of Mexico with PWATS already
creeping toward 2.00 inches by late Friday night across parts of
southern Arizona and up to 1.75 inches across southeast
California. The combination of the excessive moisture and support
from the mid level inverted trough will promote periods of
showers and thunderstorms at any point from Friday night through
Sunday across our entire CWA. Instability will be somewhat limited
due to extensive cloud cover, but modest CAPE values resulting
from the excessive moisture will still be enough for some
thunderstorm activity, especially if any heating is realized
Saturday or Sunday afternoon. Mostly chance PoPs are in the
forecast for now through Sunday, but some tweaking into the likely
category is expected once confidence grows.

Heavy rainfall and localized flooding will be the main weather
hazards going through this weekend, similar to that of last
weekend. It seems very possible that Flash Flood Watches may be
needed at some point this weekend. Cooler conditions will return
starting Friday with highs several degrees below normals,
especially across the eastern half of Arizona.

Looking beyond the weekend, we should see storm chances linger at
least through early next week, but support aloft will wane as the
inverted trough shifts to the west. A drying trend is likely for
the second half of next week as models are showing the upper level
high center shifting to the northwest over the Great Basin. This
would bring a drying northeasterly flow and likely result in less
storm activity, especially across the lower deserts. If this
drier air sets up over the region, temperatures will climb back to
above seasonal normals.



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

Outflows from decaying thunderstorms have already turned the winds
to an easterly direction at all of the Phx area terminals in the
last couple of hours. These easterly winds are expected to persist
through the rest of tonight and through early Friday. There is a
remote chance that leftover light showers could affect the taf sites
later tonight or very early Friday, as debris from thunderstorms
that are now over SE AZ move into the Phx area, but confidence is
far too low to include any mention of SHRA in the tafs at this time.
Thunderstorms are once again expected to develop to the east of the
Phoenix area in Friday afternoon, but confidence is also too low to
include mention of them in the tafs at this time.

Southeast California/Southwest Arizona including KIPL and KBLH:

No real chance at any storms at the terminals through the taf
period. Generally, winds will remain light outside of some afternoon
gusts. KBLH will maintain mostly southerly winds through the taf
period. After a period of westerly winds later tonight, KIPL will
experience a shift back to the south by daybreak on Friday.

Aviation discussion not updated for amended TAFs.


Sunday through Thursday:
Thunderstorms chances will peak on Sunday and will hang around
throughout the forecast. The best chance for rain in the lower
desert is Sunday, but the higher terrain north and east of Phoenix
has a decent chance nearly every afternoon. Minimum relative
humidities will stay above 15 percent across the board with great
overnight recoveries. High temperatures will be mostly below
seasonal normals this weekend, then gradually climbing to slightly
above normal levels by Wednesday as storm chances dwindle and
moisture decreases. Winds will otherwise be light outside of any
thunderstorm related winds.


Spotters should follow standard reporting procedures.




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