Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 230202

Area Forecast Discussion
National Weather Service Phoenix AZ
700 PM MST Sat Jul 22 2017

Thunderstorm activity will continue through the weekend focused
mainly over mountainous areas of Arizona. Rainfall chances will
expand into southeast California Monday. Frequent cloud cover and a
moist atmosphere will lead to seasonally cool temperatures through
early next week. Drier southwesterly flow aloft will develop next
week leading to warmer temperatures along with fewer storms across
the region.


Quite a complicated upper air pattern across the SW Conus this
evening with H5 anti-cyclones situated over eastern NV and northern
NM. In between, an elongated inverted trough extended along the
Arizona/New Mexico border with a couple vorticty spokes rotating
about the trough. Meanwhile, a singular H2 jet level high has become
parked almost directly over Phoenix providing the best divergent
fields on the far northeast and southeast portions of the forecast

Mountain convection descended towards lower elevations earlier
today, however outflows were generally not sufficiently deep or
strong to spark much more than elevated accas showers. Several
culprits likely inhibited additional storms, but most notably a
steep drop off in instability from the Phoenix metro southwest and
warming aloft. 00Z KPSR (while contaminated by convective debris)
showed the warming at the H5 level and corresponding lower midlevel
lapse rates and MUCape. Sfc-H7 mixing ratios have also dropped to 10
to 11 g/kg and without deep outflows or more robust mesoscale
ascent, storms were bound to struggle.

Evening updates primarily included increasing POPs substantially
across western Maricopa County where showers lifted behind the
outflow boundary continue to percolate tapping whatever meager Cape
remains. Have also trimmed POPs through the eastern quarter of the
CWA between the current outflow and an upstream wave propagating
through the Whites. Kept a mention of showers continuing overnight
though the latest satellite and radar loops are not too optimistic
this far south.


Today through Monday:
Thunderstorm chances will continue into Sunday and Monday,
especially as the aforementioned lurking inverted trough
continues to its westward journey. As the trough approaches,
favorable atmospheric dynamics for storms should spread over the
southern half of Arizona and into southeast California. All places
look to have some chance for rain, including Yuma and El Centro,
which have largely been left out of the monsoon activity so far.
Again, flooding looks to be the primary concern; however,
southeasterly steering winds both Sunday and Monday could lead to
blowing dust issues for I-10 corridor between Phoenix and Tucson.

Tuesday through Saturday:
We start to see a break in monsoon activity as dry southwesterly
flow starts to develop and pushes moisture north and east. This
will keep thunderstorm chances confined to the higher terrain
north and east of Phoenix each afternoon. With the steering flow
out of the southwest, it`s going to be extremely difficult for
outflows and/or thunderstorms that develop over the higher
terrain to propagate into the lower deserts.

Although still too far out for much confidence, it`s looking like
the 500 mb high starts to center itself around the four corners
late in the week. This would provide us with that moist easterly
flow we need to get an increase in monsoonal moisture and increased
chances for thunderstorms across the lower deserts again.


South-Central Arizona including KPHX, KIWA, and KSDL:
Spotty evening showers/thunderstorms and shifting winds will be the
main aviation hazards through Sunday afternoon. The main bulk of
thunderstorms have moved well south of the terminal, and NE winds
should prevail through the night. However, a secondary area of
showers may move across the PHX airspace in the 05Z-09Z time frame
with a period of variable wind direction. With moderate confidence,
a S/SE wind heading would be favored Sunday morning before switching
to westerly in the afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
Minimal aviation problems through Sunday afternoon as most
thunderstorm activity stays east into Arizona. Clouds/cigs above the
10K ft level should spread west over terminals, but only a modest
chance of sprinkles/light showers exist. Although a period of
variable winds will be possible at KBLH due to distant outflow
boundary passage, sfc winds will generally remain out of southerly
direction and could become stronger above 15kt Sunday afternoon.

Aviation discussion not updated for amended TAFs.


Tuesday through Saturday:
Shower and thunderstorm potential expands westward into southeast
California Monday with minimum humidities well above 20%. A drying
trend begins Tuesday with declining humidities and an eastward
retreat of storm chances. Overnight humidity recovery remains fair
to good. Apart from thunderstorms, winds will follow familiar warm
season patterns. Temperatures will trend closer towards seasonal
normals during this brief Monsoon respite before flow aloft
transitions back out of the east. Moisture may make a possible
return by the weekend.


Spotters should follow standard reporting procedures.





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