Weather Service Report

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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