Damaging Winds in Arizona, June 29, 2016

Area Forecast Discussion
National Weather Service Phoenix AZ
252 PM MST WED JUN 29 2016

.UPDATE...To Aviation and Fire Weather Discussions...

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.SYNOPSIS...
An increase in monsoon moisture will bring chances for showers and
thunderstorms the remainder of week. As moisture increases over south
central Arizona, high temperatures will fall with some locations
dropping below 100 degrees on Friday. There will be a return to drier
southwest flow aloft over the weekend and into early next week which
will limit thunderstorms to higher terrain areas east of Phoenix by
Sunday. High temperatures will steadily climb late this week and
early next week as the atmosphere becomes drier.

&&

.DISCUSSION...
Rest of today...
Debris cloudiness has kept portions of our forecast area significantly
cooler than yesterday - most noticeably over Yuma county where
cloudiness was thicker. Temperatures have also been lower further
west where there has been full sun but with high dew points.
However, that means muggy conditions. In fact, in the Imperial Valley
some heat index readings have already reached 110. Thus, held on to
Excessive Heat Warning for Imperial County. The Gulf surge
responsible for the low level moist advection over southwest AZ and
far SE Ca has weakened considerably though southerly winds continue
through about 5 kft per KYUX wind profile data. Storm activity has
begun in the usual places early this afternoon - even over southeast
Arizona where cloud cover had been inhibiting surface heating. SPC
mesoanalysis for 20Z has high CAPE values over southeast AZ
extending into central AZ. There is another bullseye over Imperial
County but CIN values are highest there as well. This suggests
potential for storms to produce both strong winds and localized heavy
rain. The latter more so for southeast AZ. Experience indicates that
the SPC automated mesoanalysis tends to be overly optimistic.

Hi-res models are a mixed bag in terms of how they depict storm
activity for our forecast area today/tonight. Our local WRF-ARW and
the UofA WRF-GFS are the most aggressive as they depict storms to the
north and south of Phoenix producing outflow and subsequent right
over the metro. The latest HRRR depicts storms moving westward from
Gila County and eastern Pinal but struggling to propagate over the
lower deserts. Even that is somewhat of an uptick compared to
previous runs. Larger scale models are also not all that enthusiastic
(except for Canadian/GEM). It's not surprising given the cooler
boundary layers and a bit of warming above 500mb. They are all
reluctant to depict storm activity west of the Lower Colorado River
Valley (except for latest HRRR which may be initializing some
isolated convection over the Baja mountains south of the Mexico
border). However, there is sufficient CAPE for storms over the higher
terrain and associated outflows. Thus in turn there is potential for
storms (mainly east of the Lower Colorado River Valley). Also, there
is an inverted wave in the mid levels slowly moving westward across
Arizona.

Thursday and Friday...
Much of what happens Thursday will be predicated on what happens
tonight. There is good agreement on something of a classic monsoon
flow with a southerly component to the steering flow and the presence
of deep moisture (e.g. 1000-700mb mean mixing ratios of 10-12 g/kg).
If activity turns out to be quiet tonight then that would bode well
for better destabilization Thursday. The converse would be true for
an active night tonight. The inverted trough/wave appears to break up
with a portion of it slowly tracking northward through AZ during the
day Thursday followed by the other piece Thursday night. It is unclear
what kind of contribution this will have. On Friday, deep moisture
will be in place but the steering flow takes on a westerly component.
With the moisture and debris clouds, temps continue to trend
downward - even dipping below normal over south-central AZ.

Saturday through Wednesday...
Guidance, including GEFS ensemble spaghetti guidance, calls for weak
troffing to develop over the PAC northwest and along the central CA
coast by Saturday and this will lead to continued southwest steering
flow over the weekend and into the first part of next week. We will
see moisture steadily decrease from west to east and by Sunday the
afternoon/evening thunderstorms will be confined to the higher
terrain areas east of Phoenix. We will then see mostly sunny or sunny
days and clear nights from Phoenix west during the first half of next
week. As the airmass dries we can expect high temperatures to climb
back up to, and then above, seasonal normals. By next Monday much of
the lower deserts will see highs near or even exceeding the 110
degree mark with little change Tue-Wed.

&&

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

Anticipate south/southeasterly surface winds to last for much of the
day with westerly winds struggling to develop this afternoon.
Thunderstorms will continue to develop north through southeast of
the Phoenix metro through 00Z, with outflows from distant storms
likely affecting some portions of the Phoenix metro this evening,
most likely after 01Z. Confidence in exact temporal and areal
coverage precludes a TEMPO or prevailing TSRA group, however, the
TAFs do reflect the scenario of nearby storms causing gusty outflow winds
primarily from the east this evening. We will be monitoring the situation
closely this evening and appropriate TAF amendments will be taken.
There are some indications that lingering shower/storm activity could
continue through late this evening, so will keep VCSH after 30/09Z.
Debris clouds will linger well past sunrise, with light southeasterly
surface winds.

Southeast California/Southwest Arizona including KIPL and KBLH:

Surface winds will favor south and southeasterly directions through
the TAF period, with a few gusts up to 20kts. Expect scattered to
broken high clouds to remain through the period. Although
thunderstorms should stay well east/southeast of the area, cannot
rule out an outflow from distant thunderstorms causing abrupt wind
shifts with some gustiness overnight.

Aviation discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday...Enhanced humidities and storm chances
will continue on Saturday before trending downward, with minimum
humidities in the lower deserts ranging from 20-30 percent on
Saturday to 10-20 percent on Wednesday. Similarly, overnight
recoveries will also be trending downward as we start to dry out
this weekend. Thunderstorm chances on Saturday will be greatest
north, east, and southeast of Phoenix. Sunday through Wednesday,
thunderstorm chances will greatly decrease and be confined to the
higher terrain of south-central AZ. Apart from strong erratic winds
associated with thunderstorms, winds will primarily have a southerly
component in southeast California and southwest Arizona through the
period with some afternoon gustiness up to 20 mph. Over south-central
Arizona, winds will following typical upslope and downslope
directions with winds around 5 to 15 mph.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is not expected.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...Excessive Heat Warning until 8 PM PDT this evening for CAZ033.

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