Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 182046

Area Forecast Discussion
National Weather Service Phoenix AZ
145 PM MST Sun Mar 18 2018

Cooler than normal temperatures will continue Monday, but conditions
will warm quickly back into the 80s by the middle of the week. A
weather system will sweep through the region later in the week
resulting in a slight opportunity for rain and temperatures cooling
back towards average.


Within a larger western Conus trough, a notable PV anomaly has
lifted into western Colorado this afternoon while the attendant
frontal boundary has swept well into New Mexico. Pronounced
subsidence and a dryslot has also punched through the majority of
the state with essentially only fair weather Cu filling in the
trailing cold core. Modest height rises and slow warming of the
atmospheric column will begin Monday though sfc temperature will
still lag slightly below normal.

More substantial ridging aloft will build over the SW Conus through
the middle of the week ahead of deep cold core low diving south
through the east Pacific basin. Model guidance remains extremely
consistent and in excellent agreement depicting H5 height spiking
into a 576-579dm range during the Tues-Wed time frame. This is not
terribly unusual for this time of year and will easily yield
afternoon highs in the 80s at lower elevations. The largest
uncertainty will be the expanse and thickness of high cirrus
spilling east from the aforementioned Pacific low. It is conceivable
thicker cirrus could restrain near sfc lapse rates and overall
temperatures on either of these days limiting better forecast

Model agreement deteriorates somewhat during the latter half of the
week as the Pacific low propagates eastward and eventually phases
with the next cold core system dropping southeast from the Gulf of
Alaska. Undoubtedly, the central and southern California coast will
be inundated by some measure of an atmospheric river; however, the
timing and ability for the frontal boundary to be maintained inland
yields a lower confidence forecast. Heights and thicknesses should
only modestly begin to drop Thursday with the remnants of the
frontal boundary, residual moisture swept over the coastal range,
and shearing vorticity center passing through the forecast area
Thursday night and Friday. Much like the past several systems, much
of the area may be rain shadowed though forecast soundings suggest
some possibility of a brief window of 8 g/kg moisture advecting along
the frontal boundary. At this time, no ensemble member indicates a
stronger event, but blended ensemble means don`t highlight better
spatial or temporal chances yet either.

Some measure of drier westerly flow will return to the region over
the weekend though height rises will likely not be that large
considering the next wave descending through the bulk of California.
A notable cyclonic jet through the east Pacific will likely bring
periods of thicker cirrus into the forecast area limiting some of
the heating potential and probably keeping temperatures near the
seasonal average.


South-Central Arizona including KPHX, KIWA, and KSDL:
Skies will remain mostly clear this afternoon, but high clouds
will overtake the region this evening and persist into Monday.
Westerly winds this afternoon and early evening will be a bit
breezy with gusts to around 15kts. After winds diminish this
evening, downslope easterly winds will develop with more typical
wind patterns taking over.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation impacts through Monday as high clouds increase this
afternoon and mainly stay BKN through Monday. Winds will be on the
light side and mostly out of the north at KBLH and west at KIPL.

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday...
Expect warm and dry conditions ahead of a weather system that will
pass across the region on Thursday with drier air returning by the
end of the week. The weather system on Thursday will bring a chance
of light rain from SE CA to AZ. Seasonably warm daytime highs on
Tuesday will increase to the mid 80s by Wednesday and then drop back
down into the upper 70s to low 80s on Thursday. Min RH`s near 10%
range on Tuesday are expected to spike up to the 22-28% with
Thursday`s system. Breezy southwesterly winds gusting to 17-22kt
from SE CA to SW AZ are also expected with Thursday`s system.
Generally fair overnight recoveries should improve to fair to good
by Wednesday night and Thursday night.


Spotters should follow standard reporting procedures.





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