Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 221025

Area Forecast Discussion
National Weather Service Phoenix AZ
325 AM MST SAT OCT 22 2016

Updated Aviation and Fire Weather sections.


Dry weather with mostly clear skies will continue across the Desert
Southwest today. Temperatures will be unseasonably warm.
By Sunday, a large Pacific low pressure system will move into the
western states. This will draw a considerable amount of subtropical
moisture into the region with threat of showers starting Sunday
afternoon and continuing through Monday night. The best chance of
showers will occur Monday over south central Arizona, with afternoon
thunderstorms possible. Drier and warmer conditions will return over
the middle of next week.


The strong upper level high pressure ridge responsible for mid to
upper 90s high temperatures over the lower deserts over the past
couple of days has shifted eastward a bit allowing more of a south-
southwest flow aloft to set up over the area. As seen in water vapor
imagery, this flow has started to tap increasing amounts of
subtropical moisture, with the latest blended total pwat imagery
depicting a plume of nearly 0.75 inch pwat now spreading into the
far southwest Arizona deserts. This moisture will not be of
significance today, as we will see one more day of well above normal
high temperatures under generally sunny skies, but it signals the
start of a significant moistening trend that will really set into
the area Sunday and continue into early next week.

During the day on Sunday, a large upper trof setting up just off the
west coast will continue to spread a plume of deep subtropical
moisture with pwat over 1 inch into the lower AZ and southeast CA
deserts. Moisture values will be 2-3 standard deviations above
climo, and pwat values will climb well above the 90th percentile
compared to climo values. Initially we will see thick mid and high
cloud overspreading the area, and then as UVV fields increase the
atmosphere will wet up from the top down leading to increasing
chances of showers with time. By Sunday afternoon the best threat of
showers will occur over western deserts - to the west of the greater
Phoenix area - as POPs climb to around 20 percent. There will be
close to a 10 percent chance in the greater Phoenix area as well.
Despite the thick cloud cover spreading in, high temperatures will
likely stay in the low 90s over the warmer lower deserts due to
brisk lower level southerly flow giving some warm air advection

Sunday night into Monday, QG forcing increases and spreads into
south central Arizona as a short wave moves into the area and with
deep moisture persisting over the area POPs will steadily climb
eventually reaching around 50 percent in the south central deserts
Monday afternoon and into the likely category over the higher
terrain of southern Gila County. Our forecast POPs continue to climb
and will largely follow the NAEFS POPs trends which look to be
reasonable if slightly overdone. Instability and wind shear increase
on Monday as well with CAPE values rising in excess of 400 j/kg over
the central deserts. As such there will be a chance for
thunderstorms, although the storms will likely be more on the
isolated side, embedded in more significant areas of light to
moderate showers. High temps will fall into the mid to upper 80s
over the lower deserts given the increase in cloudiness and showers;
the forecast high in Phoenix on Monday is now 87 degrees which is
just 2 degrees above the normal of 85. Depending on how significant
and widespread the showers are, the high could certainly lower to
near or even below that normal value.

Scattered to numerous showers and along with a few embedded storms
will persist into the evening Monday but as the short wave quickly
lifts off to the northeast, a clearing and drying trend will take
place from the west and by Tuesday morning there will be just a
lingering slight chance of showers over the higher terrain east of
Phoenix with mostly sunny skies developing over the central and
western deserts. High pressure will build over the area from the
west later Tuesday and continue through the day on Wednesday for dry
conditions along with warming temperatures. Highs over the lower
deserts Wednesday will climb back into the low to mid 90s with a
high of 92 forecast for Phoenix.

For Thursday into Friday, operational models really diverge and the
GEFS spaghetti ensemble members area all over the map showing a very
chaotic pattern. It looks to be a somewhat unsettled pattern as the
upper ridge breaks down a bit as short wave energy moves into the
ridge and then lifts off towards the northeast. The ECMWF is MUCH
drier and more or less gives the lower deserts a brush by whereas
the GFS brings a VERY wet and dynamic system into the state. NAEFS
POPs align closely to the ECMWF forecast and will be followed; we
will add a slight chance of showers mainly to the deserts west of
Phoenix - especially over far southeast California - with POPs
generally 15 percent or lower.


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

Expect lighter winds today compared to the past two days. Easterly
directions will be favored through midday before a slow turn toward
westerly directions this afternoon. There may be a period of
southerly directions between 18Z-22Z but speeds will be less than 10
kts. Winds will trend toward light and variable during the evening. Skies
will be clear except for some scattered cumulus/altocumulus
developing later today with bases AOA FL100.

Southeast California/Southwest Arizona including KIPL and KBLH:

Light and variable winds this morning will become primarily southerly
this afternoon with some local gusts of 15-20 kts by late afternoon
(weakening during the evening). Skies will be clear except for some
cumulus/altocumulus later today over southwest AZ (bases AOA FL100)
and some very thin cirrus.

Aviation discussion not updated for amended TAFs.


Monday through Friday...
Considerable cloudiness can be expected Monday along with a chance
of showers and thunderstorms. Precipitation chances begin to taper
from west to east overnight Monday with only a lingering slight
chance Tuesday over the higher terrain of south-central Arizona.
Temperatures will be cooler but because the brunt of the system stays
west and north, there won`t be a lot of cold air moving in. High
pressure strengthens again Wednesday into Thursday for warmer
temperatures and lower humidities but overnight recovery remains
good. Another Pacific system brings a slight chance of showers again
on Friday to southeast California and southwest AZ. Winds throughout
the forecast period will be light overall except on Monday if
thunderstorms develop which will create the potential for strong
erratic winds in localized areas.


Spotters should follow standard reporting procedures.




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