Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 201308

Area Forecast Discussion
National Weather Service Phoenix AZ
608 AM MST Tue Mar 20 2018

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


A warming trend continues this week with high temperatures reaching
the mid 80s to 90 by Thursday. A weak weather system moves across
the region from late Thursday through Friday bringing our next
chance of showers together with modest rainfall totals. Cooler
and drier conditions will accompany this system with near normal
temperatures expected through the weekend.


Current satellite shows mostly clear skies across region with a
large area of thick cirrus climbing over a building West Coast
ridge. Cool and dry conditions with light to calm winds are being
observed across the deserts with temperatures now in the mid 40s to
mid 50s. Low dewpoint temperatures, as low as the single digits in
the deserts, together with pronounced radiational cooling will
support another chilly morning with lows dropping into the 40s to
near 50. With mostly clear skies, expect temperatures to warm nicely
today into the seasonably pleasant upper 70s.

Plenty of WAA through the column ahead of a strong Pacific Basin
low pressure system is helping to build the west coast ridge at a
significant pace. As the warming trend continues, highs in the low
to mid 80s are anticipated for Wednesday as the West Coast ridge
axis moves across the region. Although the ridge axis shifts off
to the east by Wednesday evening, heights and thicknesses will
continue to increase over the area to near 581-583 dam and 568-569
dam respectively by Thursday. This is due to the ridge
dramatically increasing in amplitude as it moves out over the
Plains. As a result, expect the highest temperatures so far this
year on Thursday with highs reaching the upper 80s to 90 in
Maricopa and Pinal counties, and the low to mid 80s elsewhere.
The middle of the week will also feature mostly cloudy skies and
high cloud decks associated with the upper level moisture field
ahead of and with the next weather system. This may help to
moderate the forecast high temperatures on Wednesday and Thursday.

The models are now in good agreement on the next weather system
bringing chances of light rain showers late this week with the best
chances for the Phoenix area coming very late Thursday night and
early Friday morning through Friday afternoon. The best rain
chances for out west and SE CA occur on Thursday evening. This
represents a later arrival time than model runs from earlier this
week. This fast- moving, filling, shortwave disturbance from the
west is expected to produce very modest rainfall accumulations for
the region. Its associated IVT/AR weakens as it moves inland with
the rain shadow effect taking its toll. Short-lived mixing ratios
are expected to peak around 6 g/kg with PWs near 0.75-1.05 in. on
Friday morning. POPS remain near 30 percent for Friday from
Phoenix eastward and 10-20 percent elsewhere. Expect low rainfall
totals, generally less than a tenth of an inch around the Phoenix
metro. Expect much cooler temperatures on Friday as well.

By next weekend expect cool and dry conditions with near normal
high temperatures in the upper 70s. For early next week a chance
of breezy to windy conditions are expected in SE CA and SW AZ as
a low pressure system moves in.


South-Central Arizona including KPHX, KIWA, and KSDL and
Southeast California/Southwest Arizona including KIPL and KBLH:

Strong high pressure aloft will continue to build over the area
today into Wednesday resulting in continued light wind and dry
conditions. Expect increasing high cloud to spread across the area
from the west with CIGs developing at the terminals later today.
Bases to be genly aoa 20k feet but some SCT decks down to 15k feet
or so may develop over the western deserts by late afternoon/early
evening. Winds in the greater Phoenix area will be light diurnal
next 24 hours with speeds mostly below 8kt.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday... Expect warm and dry conditions today
ahead of a weather system that will pass across the region later
today into Friday with drier air returning by the end of the week.
The weather system will bring isolated showers to the western and
central deserts tonight and scattered showers to south central
Arizona on Friday. High temperatures today reaching as high as 90
over portions of south central Arizona will drop back down into the
upper 70s to low 80s by Friday. Min RH`s of 15-25% today will climb
mostly into the 20s Friday and then drier air will spread in over
the weekend, dropping minimum RH values into the 10-17% range.
Breezy southwesterly winds gusting in excess of 25kt from SE CA to
SW AZ are also expected today ahead of the approaching system.
Otherwise winds will be rather light and favoring typical diurnal
tendencies mostly favoring west to northwest during afternoon hours.
Generally fair overnight recoveries should improve to fair to good
by Wednesday night and Thursday night. Little change is expected
Monday as compared to Sunday.


Spotters should follow standard reporting procedures.




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