Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 241530

Area Forecast Discussion
National Weather Service Phoenix AZ
830 AM MST SUN JUL 24 2016

An increase in moisture and cloud cover over the region is expected
to take some edge off the heat, and provide somewhat better chances
of showers and thunderstorms through the middle part of the upcoming
workweek. The best opportunity for rainfall will occur over the
higher terrain north and east of Phoenix, with lesser chances heading
west into southeast California. A better chance for showers and
thunderstorms is then forecast for the end of the week and into the
upcoming weekend, as deeper moisture begins to move into the region
from the south and southeast.



One thing noticeable from the regional weather balloon data was that
monsoon moisture levels in southern AZ have now crept back up to
those seen last week, July 17,18, and 19th, when we had isolated
severe storms and flooding in south central AZ, including parts of
Phoenix. Monsoon moisture has surge dramatically into southern AZ
from Mexico this weekend due to a strong push of humid convective
outflow boundaries. This morning was no different. A large convective
complex south of the border last night generated northward moving
outflows which hit the lower Colorado River Valley at Yuma with
southerly gusts of 43 mph after 4 am. Gusts to 43 mph were still
ongoing at 745 am. South and southeasterly wind gusts to 31 mph at
Imperial CA and 30 mph at Blythe started at 8 am. As moisture and
winds from southwest AZ recurve east, Phoenix may be the recipient
of westerly breezes and dewpoint jumps later this morning.

The large convective MCS outbreak in northwest Mexico last night was
located on the difluent west side or leading edge of upper level
inverted trof that came out of south Texas. Streamline analysis
derived from satellite imagery, including satellite high density
winds and satellite wind computations of divergence fields, highlight
the upper level storm forcing. And, this inverted trof seen in
the model guidance yesterday, is forecast by last nights models
to slow and weaken before moving into southeast and portions of south
central AZ later tonight through Monday afternoon.

For today. Monsoon moisture in southern AZ is not excessive
nor minimal either, just pretty much at typical levels. From the
weather balloon soundings thermodynamic convective trigger
temperatures remain fairly high, toward late afternoon hours.
Therefore in our forecast area of south central AZ we are expecting
a late afternoon start to convection, despite increased boundary
layer moisture, with most tstms relegated to the mountains
north through southeast of Phoenix. Since moisture is at elevated
levels, convective outflow boundaries coming off the mountains, or
moving in from southeast AZ could spark secondary and even tertiary
storms this evening. A low grade monsoon forecast is expected
today and this evening, meaning a slight chance of showers and
thunderstorms, with storms favoring the mountains.

For now we will wait and see how the incoming models handle the
Mexican inverted trof into southeast AZ later tonight and
Monday, since a good initialized data set across northern Mexico is
expected (i.e. Chihuahua, Guaymas, and other Mexican balloon
soundings were available this morning). For now no forecast updates
planned, but adjustments to the late tonight and Monday forecast
period may be forth coming based on the approaching inverted

.Previous Discussion...440 AM MST...

Today through Thursday...

After a relatively quiet today today, a return to more southerly
flow aloft is expected to bring a bit more moisture into the region,
starting tonight and continuing into Tuesday, with pwats across
South-Central AZ rising from around 1.10-1.20 inches up to around
1.50 inches. The first impact from this increase in available
moisture will likely be an increase in cloudiness and perhaps even
some light showers and a few thunderstorms late tonight and early
Monday as debris from thunderstorms that are expected to develop
over northern Sonora, Mexico and southeast AZ this afternoon/evening
move into the region. Available moisture levels are then expected to
remain pretty much steady through Tuesday, before decreasing
slightly on Wednesday and Thursday as ridging aloft rebuilds over
the region.

Although moisture levels will be decent (around average for this
time of year) through Tuesday, convective activity will likely be
limited by rather warm air aloft (500mb temps in the -5c to -6c
range), the fact that most available moisture will be at 600mb or
higher, and very light winds through the entire column. The impact
of the warm mid-level temps, lack of moisture below 600mb, and light
winds aloft can be seen in the latest NCEP SREF 3h-MUCAPE and 3h-
MLCAPE plumes, with the mean values remaining mostly aob 500 J/KG
across the Phoenix area through Thursday. Thus, it will be hard to
trigger any convective activity across the lower deserts, unless
significant lift can be provided by strong outflows from storms over
the higher terrain, or from weak mid-level disturbances, which are
hard to forecast/time. Thus, have currently gone with mostly slight-
chance pops across the lower deserts through Tuesday, with even
lower pops on Wednesday and Thursday, as somewhat drier air moves
back into the region. On the other hand, chances for locally gusty
winds/blowing dust appear to be better, as outflows from storms over
the higher terrain of South-Central AZ and SE AZ (which are much
more likely to develop than over our lower deserts) move into the
lower elevations each afternoon/evening. Along with the low-end
pops, have kept highs above normal through Thursday, with many lower
desert locations likely once again reaching/exceeding 110 degrees on
Wed/Thu as drier air/warmer air aloft moves back into the region.

Friday through Sunday...

There is still indications that this could be a more active period
weatherwise as the main 500mb high center weakens a bit and shifts
northward to near the AZ/UT border, allowing stronger easterlies to
develop over the region, and more moisture to work its way into the
region from the south and southeast. The GFS is showing PWATS as
high as 2.00 inches moving into the region by Saturday. This
increase in moisture, along with stronger winds aloft, and an
easterly wave moving across northern Mexico/extreme southern AZ
would lead to a rather substantial increase in convective activity,
if these model runs prove to be correct.


South-Central Arizona Including KPHX, KIWA, and KSDL:
Moderate confidence exists regarding limited aviation impacts
today. Any easterly winds this morning will be weak and will likely
give way to an earlier than normal westerly switch mid to late
morning. Storm potential over the high terrain should pick up today
which may push some outflows into the lower deserts this evening.
Confidence in an early switch over to easterlies from outflows is
not high enough to currently include in TAFs.

Southeast California/Southwest Arizona including KIPL and KBLH:
Strong south to southeast prevailing winds will be the most likely
outcome through this afternoon. Occasional gustiness will once again
be likely this afternoon and early evening. Some mid/high clouds
will be possible along with dust trapped in the lower atmosphere.

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday...
Monsoon moisture working its way back into the region will bring
chances for afternoon and evening thunderstorms each day. The best
chances for wetting rains will be over the higher terrain east of
Phoenix with lesser chances over the lower deserts of south-central
and southwest Arizona and only slight chances over southeast
California. Some increase in convective activity is expected on
Friday as deeper moisture begins to move into the region.
Temperatures to remain slightly above normal through the entire
forecast period. Minimum humidities should fall in the 15-30 percent
range through Thursday and rise into the 20-35 percent range for
Friday and Saturday.


Spotter activation is not expected.




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