457 
FXUS65 KPSR 082114
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
214 PM MST Mon May 8 2017

.SYNOPSIS...
An unusually strong low pressure storm system for early May will 
continue to bring unsettled conditions for the Southwest United 
through the middle of the week. Precipitation chances will peak
early Tuesday morning into Tuesday afternoon across most of
south-central and southwest Arizona. Temperatures will be well
below-normal as afternoon highs struggle to get out of the 70s. Clearing
skies and a substantial warming trend will develop through the 
second half of the week.

&&

DISCUSSION... 
Upper low near the mouth of the Colorado River has temporarily
stalled, and is undergoing an eastward and eventual northward 
shift in motion. A long fetch of southerly flow on the eastern 
periphery of the low has advected mid-level moisture northward 
into the eastern half of Arizona, as evidenced by a thickening 
cumulus field this afternoon. The SPC mesoanalysis picked up on 
the diurnally enhanced area of destabilization with a large area 
of MLCAPE values around 250-500 J/kg across eastern Arizona, 
but this has dissipated somewhat with increased boundary-layer 
mixing. Low-level moisture is still lacking, but convective 
trends show enough instability to produce isolated showers and a 
few thunderstorms across the higher terrain north and east of 
Phoenix. In fact, with steering flow out of the southeast, it is 
not inconceivable we could see a couple showers or thunderstorms 
moving into the Phoenix area late this afternoon and evening. 
Rainfall amounts would likely be light, with the primary threat 
being localized gusty winds. As such, PoPs have been raised a bit 
through this evening across south-central Arizona.

The upper low will continue to track northward and will move into
southern Arizona by tomorrow afternoon. Precipitation chances will
increase overnight and remain relatively high through Tuesday
afternoon, and could see the development of a band of
thunderstorms with locally heavier periods of rain. However,
widespread rainfall amounts should generally remain at or under
one quarter of an inch. For the Phoenix area, rainfall chances 
increase after 06Z (11 PM MST) and will remain elevated through 
18Z, which is consistent with both the SREF ensemble plumes and 
most high-resolution model guidance. Much cooler temperatures and
increased cloud cover can also be expected Tuesday, with highs 
20-25 degrees below normal for the date. The upper low will begin 
accelerating northeastward into the southern Rockies by Wednesday,
with lingering cloud cover and weak showers persisting into 
Wednesday morning, before drier air moves into the region.

Upper ridging will build in the wake of this system, resulting in a
substantial warming trend Thursday and Friday. Temperatures are 
forecast to rise into the upper 90s Friday and Saturday. 
Fortunately, the above-normal temperatures may be short-lived as 
another storm system looks likely to take shape across the western
CONUS into early next week. 

&&

.AVIATION... 
South-Central Arizona Including KPHX, KIWA, and KSDL: 
Area of strong low pressure well south of the state will continue
to circulate clouds and moisture up into central Arizona.
Widespread TCU fields will continue over eastern Arizona, with SCT
to BKN coverage 10-12kft working into the Phoenix metro from the
east. Enough instability and moisture are coming together to
generate scattered small/light high based showers over the eastern
mountains and potential for showers into the terminals, at least
VC, will start this early afternoon and continue through sunset.
Primary threat/concern with the mostly high-based showers will be
strong gusty winds in the proximity of them, with some short-lived
30kts gusts possible. Longer lived/wetter rains and lowering CIGs
to develop overnight and for the AM traffic Tuesday.

Southeast California/Southwest Arizona Including KIPL and KBLH: 
Large area of Pacific low pressure will continue to circulate just
south of the border before beginning to lift northeastward into
Arizona Tuesday. SCT to BKN mid level clouds will drift westward
into southeast CA along with some weak, high-based showers through
the evening. Wrap-around activity along the north side of the low
will support at least VCSH mention for KIPL and KBLH for periods
tonight. Winds will vary off of northerly headings, but at time be
so light VRB may result. Any passing shower activity could
generate short-lived wind gusts, 20-25kts possible.
 
Aviation discussion not updated for amended TAFs. 

&&

.FIRE WEATHER...
Thursday through Monday...
Drier and warmer conditions are expected with a return to above 
normal temperatures. Minimum relative humidity Thursday will range
from 15-20 percent, then fall to near 10 percent Friday through 
Sunday. Afternoon southwest winds 10 to 20 mph, but increasing 
Saturday and Sunday into the 15 to 25 mph range as another upper
trough approaches from the west. Humidities to make mostly fair 
overnight recoveries.

&&

.CLIMATE...

Record cool high temperatures Sunday through Tuesday:

Date        Phoenix         Yuma
----        -------         ----
May 7th    70 in 1965     72 in 1964
May 8th    66 in 1930     64 in 1912
May 9th    67 in 1912     74 in 1922

&&

.SPOTTER INFORMATION STATEMENT... 
Spotter reports of gusty winds and blowing dust may be needed 
early this week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

Visit us on Facebook, Twitter, and at weather.gov/phoenix

DISCUSSION...Rogers
AVIATION...Nolte 
FIRE WEATHER...Nolte/Vasquez
CLIMATE...MO