Area Forecast Discussion
Issued by NWS Phoenix, AZ
FXUS65 KPSR 270426
Area Forecast Discussion
National Weather Service Phoenix AZ
926 PM MST TUE JUL 26 2016
Strong high pressure over the desert southwest will keep monsoon
thunderstorm activity to a minimum and many lower desert high
temperatures at or above 110 degrees through Thursday, in spite of
lingering moisture over the region. A return to strong easterly flow
aloft as the high pressure weakens and shifts to the north will allow
deeper moisture to move into the region from Friday onward into early
next week, bringing cooler temperatures and a increase in
thunderstorm activity, with locally heavy rain and gusty winds likely
at some locations.
Mostly quiet across our area tonight with only a few weak showers
seen earlier in the day. One exception is a combo of outflow
boundaries that we are keeping an eye on moving towards the northwest
out of Pima and Pinal counties. These boundaries should enter our
warning area within the next 15 minutes and bring with them some
gusty winds with the possibility of blowing dust. Also, anytime we
see more than one outflow boundary then we have to be on the lookout
for rapid convective initiation. We are seeing a little of that in
southern Pinal county right now.
Otherwise we anticipate another couple of low (to no) monsoon
activity days until moisture picks back up Friday and especially
Saturday. This should swing us back into active monsoon activity with
flash flooding and wind damage from thunderstorms a possibility.
Latest satellite imagery shows scattered convection developing across
portions of the higher terrain, mainly across the Mogollon Rim.
Towering CU is also evident across portions of northwestern Maricopa
and Eastern La Paz counties, associated with a dissipating MCV in
Latest mesoanalysis shows that conditions remain sufficiently moist
for convection this afternoon with PWATs generally in excess of 1.5
inches. However, steering flow remains weak and unfavorable for
storms to develop in the lower deserts. Temperatures in the mid-
levels are as warm as -4C, which is yielding weak mid-level lapse
rates. Latest CAMs including the HRRR confirm that activity this
afternoon/evening will mainly be relegated to the higher terrain of
southern Gila County. Only exception will be across northwestern
Maricopa County where the CU field has been persistent. If storms
continue to organize in these areas, there is a slight chance that
low-level moisture convergence will increase ahead of an outflow
boundary dropping out of southern Gila County, ultimately leading to
additional convective initiation across northern Maricopa County.
With two mid-tropospheric high centers, one across southern NV and
the other across western NM, heights are approaching the 90th
percentile and continue to rise. Consequently, temperatures are
running several degrees above those observed yesterday afternoon.
High temperature in Phoenix will likely reach 110 degrees which
would be the 24th time this year. 33 days is the record set in 2011.
Above average temperatures are expected again overnight.
Today through Thursday...
A relatively quiet period weatherwise appears to be in the cards for
this period as strong ridging (500m heights in the 593-596dm range)
remains locked over the region. Although moisture levels will be
near average for this time of year (PWATS in the 1.50-1.70 inch
range), rather warm air aloft (700mb temps in the 25-27C range) and
light winds through the column is expected to keep the airmass
fairly stable and limit convective activity across the region,
especially across the lower elevations. Although the HRRR and NAM-
4km high-res models are showing isolated-scattered showers/storms
developing each afternoon over the higher terrain of northern and
southeast AZ, they are also showing that almost all of this
convective activity dissipates before moving into the lower deserts,
with the main impacts being gusty winds, and perhaps some blowing
dust as the outflow boundaries most across the lower elevations.
Along with the suppression of convective activity, the very warm air
aloft and mostly clear skies is also expected to allow lower desert
temperatures to reach, or exceed 110 degrees each afternoon, with
temperatures even approaching 115 degrees at some of the warmest
southeast CA locations.
Friday through Monday...
Confidence is building that this will be a rather active period
weatherwise across the region. As long-wave trofing begins to dig
southward into the central US, the strong ridging that is now over
the Desert Southwest is expected to weaken and shift northward. This
pattern change is now showing up in the operational gfs/euro/gem
model suites, and many of their respective ensemble members. This
pattern change is expected to allow more/deeper tropical moisture to
move northward into the SW US, increasing the convective activity
noticeably over our region.
It now appears that the 1st active afternoon/evening will perhaps be
on Friday, with the gfs KPHX forecast sounding now showing 15-20 kt
easterly winds between 700mb and 400mb, with northwesterly winds
above that level, which is a classic sounding for back-building
storms into the lower deserts of South-Central AZ. The soundings
that evening are also showing noticeable mid-level cooling as well,
allowing CAPES to rise into the the 1500-2000 J/Kg range, with LI`s
falling into the -3 to -6 range, which would likely result in
widespread strong-severe thunderstorm activity, if the sounding does
verify. PWATS then rise into the 1.80-2.00 inch range on Sat/Sun,
which is high enough to allow showers/storms to produce locally
heavy rains, with the risk of localized flooding. This active period
could last into early next week as well, although the models do show
a gradual decrease in available moisture as ridging attempts to
rebuild over the region.
.AVIATION... South-Central Arizona Including KPHX, KIWA, and KSDL:
With only a slight chance of thunderstorms at the terminals expect
today`s thunderstorm activity to favor the deserts southeast of the
valley and the high terrain to the east, northeast, north and
northwest. The best chance for outflows at the terminals will be
from any thunderstorms that form in northwest or north Maricopa
county during the afternoon and early evening. Otherwise expect
relatively light westerly component winds with occasional mid to
high level debris cloud cigs possible from 12 to 25K ft.
Southeast California/Southwest Arizona including KIPL and KBLH:
Expect mostly light to occasionally breezy southerly and westerly
component winds along with typical diurnal shifts. Otherwise skies
will be clear to partly cloudy with few to sct mid and high clouds.
Aviation Discussion not updated for amended TAFs.
Friday through Tuesday...
Disturbances working their way in from northwest Mexico will help to
usher in increased Monsoon moisture throughout the period bringing a
chance of scattered wetting rains from thunderstorms from the lower
deserts to southern Gila county. Above normal high temperatures on
Friday will fall to near or slightly below normal beginning Saturday.
Minimum humidities in the upper teens to mid 20s on Friday will jump
up to the mid 20s to upper 30s on Saturday through Tuesday. Breezy
south to southwest winds with gusts of 18 to 25 mph are in store for
Friday and Saturday afternoons. Overnight recoveries will be good to
.SPOTTER INFORMATION STATEMENT...
Spotter activation is not expected.
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