Area Forecast Discussion
Issued by NWS Phoenix, AZ

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000
FXUS65 KPSR 240446
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
945 PM MST Sun Jul 23 2017

.SYNOPSIS...
Thunderstorm activity will continue today with activity expanding
into southeast California by Monday morning. Frequent cloud cover
and a moist atmosphere will lead to cooler temperatures through
Monday. This will be followed by drier southwesterly flow aloft
leading to warmer temperatures along with fewer storms across the
region Tuesday through Thursday. More active monsoon conditions
return by next weekend.

&&

.DISCUSSION...

The upper high remained centered to our north today giving southern
Arizona a deep east/southeast steering flow. Moisture remains very
high; afternoon surface dewpoints again were mostly in the 60s and
latest PWAT values from the evening soundings ranged from about 1.8
inches at Phoenix to around 2 inches at Tuscon. Plenty of CAPE is
present in the environment so it will not take much to trigger
convection. During the early afternoon a line of strong storms
formed to our north, from the central mountains southeast through
southern Yavapai county and down into southern Gila County.
Additional slightly weaker storms developed across much of southeast
AZ. Steering flow sent the rim storms towards the southwest and all
of the storms generated significant outflow boundaries. During the
late afternoon multiple strong boundaries converged on the greater
Phoenix area from the north, northeast, east and southeast and
subsequent boundary mergers forced a strong MCS over the central
deserts. Significant heavy rains resulted and there were multiple
instances of small stream flooding as well as flash flooding in
areas such as the San Tan Valley.

Latest GOES16 IR imagery did a great job showing the impressive cold
cloud tops with prominent overshooting tops nearing -80C, and as the
MCS developed the very cold tops quickly expanded outward, minute by
minute, as heavy rains developed at the surface. The Flash Flood
Watch for south-central Arizona remains in effect through midnight
and it worked out well given the widespread rains that occurred and
the instances of flooding/flash flooding across the
east/southeast/south portions of the greater Phoenix area. Guidance
continues to suggest that storms will continue to dissipate this
evening, but the presence of an inverted trof moving into southern
Arizona overnight will lead to redevelopment of showers over much of
the lower central/western deserts overnight. Expect considerable
mid/high level cloud cover across the area overnight as well.
Forecasts look to be in good shape and no imminent updates are
planned.

.PREVIOUS DISCUSSION...

 As of this moment, some of the strong storms with the heavy rain
have already occurred near Flagstaff and in southeast Arizona.
Currently, there are a few storms being monitored over Gila County.
Coverage of storms is expected to increase through the afternoon.

Surface analysis this morning indicated great low level moisture
with dew points in the upper 60s and into the low 70s throughout
the area. The visible satellite showed a pretty thick cloud deck
over south-central Arizona left over from yesterday`s activity,
but early clearing is serving well to allow atmospheric
instability to increase. The 12Z Phoenix sounding revealed a
whopping 2 inches of precipitable water which leads to concerns
for heavy rainfall in the afternoon.

The 250 millibar level streamlines depict the upper high situated
over west-central Arizona while several circulations and
perturbations can be seen circulating the upper high in the
GOES-16 experimental water vapor imagery. A little ways to our
south, an inverted trough is protruding into southern Arizona,
which will be a key feature for our weather over the next day or
so. As the trough progresses westward, the low level winds will
become more south to southeasterly which will enhance storm
development over the higher terrain north and east of Phoenix with
a more upslope wind. Model consensus shows storms percolating off
the higher terrain towards Phoenix and other desert locations.
Western Arizona and southeast California locations will see a
better chance for showers very late tonight through early Monday
morning.

Given the excessive moisture, the forecast offices covering Arizona
have coordinated a Flash Flood Watch. For our area, the place of
greatest concern is Gila County, the lower mountains northeast of
Phoenix, and portions of Pinal County where storms are likely to
initiate. The WPC`s mesoscale precipitation discussion #0543
covers the eastern portion of our CWA where storm coverage is
greatest, and notes hourly rain rates up to 2 inches or greater
are possible. While the flash flood watch may not cover the entire
CWA, that does not the flood threat is isolated. Monsoon storms
can cause deadly flooding anytime.

Monday`s weather will depend greatly on what happens today. The
location of the inverted trough and cloud coverage will play large
roles into what happens tomorrow, but the unpredictability of the
monsoon leaves me cautious to really say whether tomorrow will be
a down day or not.

Tuesday through Sunday:

We start to see a break in monsoon activity Tuesday through
Thursday as dry southwesterly flow starts to situate itself,
pushing moisture north and east. This will keep thunderstorm
chances confined to the higher terrain north and east of Phoenix
each afternoon. 500 mb high begins to situate itself near the four
corners Thursday evening, transporting moisture back into our
region. This will result in a gradual increase in POPs Thursday
night into Sunday.

&&

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

Thunderstorms, triggered largely by the intersection of numerous
outflow boundaries, have largely dissipated over the central
deserts, but an approaching disturbance will likely keep at least
isolated showers in the picture for the rest of the night and into
the morning on Monday. Expect mostly BKN to OVC mid/high cloud decks
rest of the night and thru late morning with bases mostly aoa 10k
feet. May see a few lower decks at times. Will keep VCSH in the TAFs
thru around 18Z. Thunder is not out of the question tonight but
confidence is too low so we will stick with the VCSH. Winds to favor
the east the rest of tonight and they should return, albeit weakly,
to the west by afternoon tomorrow. Expect another chance for mainly
evening storms on Monday, which may affect the terminals after 01Z.
Will not be mentioning thunder that late in the TAFs however.

Southeast California/Southwest Arizona including KIPL and KBLH:

Do not expect precip this evening for the most part, just thickening
mid/high clouds spreading in from the west ahead of an approaching
upper disturbance. Expect SCT-BKN mid/high decks rest of tonight and
by Monday 12z skies to be mostly cloudy with BKN-OVC decks mostly
above 14k feet. May see a few showers after 10Z in the KBLH area.
Winds will continue to favor the southeast at KIPL and the south at
KBLH with speeds up to 16kt at times.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Tuesday through Saturday:
A drying trend starts Tuesday with a significant drop off in
humidities and storm chances through Wednesday and leveling off
Thursday. Max temperatures trend upward as well to near or
slightly above normal readings. At the end of the week, a return
of Monsoon conditions begins with moisture and storm chances
increasing from east to west.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch until midnight MST tonight for AZZ541-545-547-
     549-552-553-555>558-560>563.

CA...None.
&&

$$

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DISCUSSION...CB
PREVIOUS DISCUSSION...Deemer/Hernandez
AVIATION...CB
FIRE WEATHER...AJ



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