Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 270425

Area Forecast Discussion
National Weather Service Phoenix AZ
925 PM MST Sun Feb 26 2017

Cooler than average temperatures will persist the next several days
as two fast moving weather disturbances move into the region. The
first system today will produce only minimal showers while a second
disturbance Monday night and Tuesday will have an excellent
potential for spreading widespread rain across the area. Dry
conditions and much warmer temperatures will return for the latter
half of the week.


Latest satellite imagery shows a band of moisture associated with a
nearly zonal jet streaming over the Pacific and into Baja California
.  This moisture is expected to interact with a trough that is
dropping down from the Pacific Northwest and into our area tomorrow
which is expected to bring precipitation to the deserts and
mountains. Latest model guidance suggests light showers could begin
in Imperial county early tomorrow afternoon and in the Phoenix metro
by late afternoon/early evening with amounts possibly approaching
three quarters of an inch around Phoenix.  Additionally, surface
pressure gradients are expected to tighten tomorrow ahead of the
approaching trough and may create some breezy conditions over
portions of Imperial and Yuma counties. This system moves out on
Tuesday with dry weather expected through the extended. Everything
looks on track with no changes made to the forecast.


Forcing for ascent will increase tomorrow morning as an upper-
level jet intensifies across the southern Great Basin. Given the
orientation of the moisture plume, the most favorable area for
initial precipitation development appears to be across far
southeast California into western Arizona tomorrow afternoon. This
should quickly spread eastward to encompass most of Arizona by
tomorrow evening, with a high likelihood for widespread rainfall
occurring in most locations overnight. Typically favored upslope
areas within southerly or southwesterly flow regimes are expected
to receive the highest precipitation totals. QPF amounts have been
increased slightly in the latest forecast update in line with WPC
guidance, with storm total amounts expected to generally range
from 0.6-0.9 inch around the Phoenix area, with over an inch
likely north and east of Phoenix. Lighter amounts (less than 0.5
inch) are generally expected over southwest Arizona into southeast

The shortwave trough and an associated surface frontal wave will
begin accelerating eastward on Tuesday, with a rapid reduction in
precipitation chances from west to east occurring during the day.
Showers should linger into late Tuesday afternoon across eastern
Maricopa and southern Gila counties before clearing the forecast
area by Tuesday evening. A secondary wave embedded within large-
scale cyclonic flow over the western U.S. will move through early
Wednesday, with the only notable effect on sensible weather being
to keep temperatures cool for one more day.

Broad ridging will envelope most of the west into late next week,
but mid-level heights will remain generally suppressed as the
storm track remains active across the Pacific Northwest.
Regardless, drier conditions and a substantial warming trend
appear probable into the weekend, with temperatures likely to
reach the low-80s across many locations Saturday and Sunday.


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

East winds tonight are expected to gradually veer to the south and
southwest tomorrow afternoon in response to an approaching system.
Relatively low (~6kft) FEW/SCT clouds may stream into the area early
tomorrow morning well in advance of any precipitation.   Latest
model guidance suggests precipitation will hold off until late
afternoon/early evening on Monday before heavier showers move in.  A
challenging forecast on tap for Monday night/Tuesday morning where
heavier showers and ample low level moisture could bring ceilings
temporarily down to MVFR (~2500ft) criteria. For now, confidence
is not high enough and have left these out of the TAF.

Southeast California/Southwest Arizona including KIPL and KBLH:

A weather system is expected to move in early tomorrow morning with
mid-level clouds moving in late tonight.  Showers will begin to
develop over SW California early afternoon on Monday which will
persist into the evening. Additionally, south winds will increase in
the morning and may become gusty in the afternoon.  Ceilings should
remain in VFR categories tomorrow with a very slight chance they may
drop briefly to MVFR in the heavier showers.  Winds subside and
skies clear as the system moves out Monday night.

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday...
A Pacific low pressure system will bring lingering rain mainly to
eastern Arizona on Tuesday followed by considerable drying
starting Wednesday. Below normal temperatures are expected
through Wednesday with a significant warming trend likely late
in the week. Breezy conditions are expected on Tuesday most areas
with breezy conditions persisting down the Lower Colorado River
each afternoon through at least Thursday. Increased humidities
with minimum readings between 25 to 40% on Tuesday will dry out
starting Wednesday with afternoon readings closer to 15% over the
deserts through the rest of the period.


Spotter reports will not be needed through the weekend.




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