Area Forecast Discussion
Issued by NWS Phoenix, AZ
FXUS65 KPSR 291642
Area Forecast Discussion
National Weather Service Phoenix AZ
942 AM MST WED JUN 29 2016
High pressure will move somewhat north and east of Arizona this week
allowing warm temperatures to prevail, but also bring a couple of
weather disturbances through the state. An increase in monsoon
moisture will accompany these weather systems, providing a threat of
afternoon and evening 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.
Last night`s storm activity put a dent in the boundary layer
temperature profiles as seen in the 12Z soundings from the Arizona
stations. It was even more active south of the Mexico border. Outflow
from storms over Sonora spread out over the Gulf of California and
has pushed northward into the lower deserts of the Lower Colorado
River Valley and Imperial Valley. This is seen in surface obs which show
a jump in winds speeds and dew points. VAD wind profile from KYUX
also shows strong south-southeasterly winds in the lower 4 kft. As
the outflow spreads further from the Gulf, it will thin out. There
will also be mixing with daytime heating so the surface dew points
will trend down even at places like Yuma and El Centro where they are
running in the upper 60s to around 70. Meanwhile, there is
considerable debris cloudiness around. These factors will influence
temperatures and some adjustments to the highs will need to be made
(mainly over southwest AZ and southeast CA). Latest hi-res models are
depicting storms encroaching on the Phoenix area a bit before 00Z (from
both north and south). The development over southeast AZ looks a bit
overdone - or at least early - given the lower level cooling and
lingering cloudiness. Latest RAP/HRRR recognizes the outflow surge
but holds off on convection over SE CA and SW AZ (except La Paz Co.).
This makes sense given the lower level cooling associated with the
marine tropical air plus debris clouds. Anticipate Gila and Yavapai
Counties to be active but SE AZ to be slower to develop. With
steering flow becoming southeasterly, storm activity over south-
central AZ deserts will tend to be at the margins/foothills before
00Z but more active after that. Current PoP/Wx forecasts reflect
this. Other than temperatures, no significant changes at this time.
.PREVIOUS DISCUSSION...Issued 325 am MST/PDT...
Strong high pressure aloft remained situated across the desert
southwest early this morning with the main high center located near
the four corners towards far north central Arizona. Typical monsoonal
circulation has been established with southeast flow spreading
increasing moisture westward across the lower deserts out towards the
lower Colorado River valley. At 2 am considerable showers and
isolated storms persisted in the Tucson area, with scattered mostly
light showers pushing westward across the central/southwest deserts.
Flow aloft is not especially divergent/difluent and no significant
inverted trof features are in the area so much of the convection is
likely being driven by continued boundary interactions.
For the next few days the monsoonal circulation will remain in place
across the area keeping moisture flowing across the lower deserts.
Occasional minor disturbances will be embedded in the flow around
the high, helping to enhance thunderstorm development from time to
time, however for the most part upper wind fields do not appear to
be very strong over Arizona today through Friday. Mean 300-700mb
wind fields are mostly at or below 15 knots over most of the area,
and upper streamline fields tend to be more anticyclonic and
laminar, not exhibiting significant deformation or difluent fields.
With weaker winds and more modest support aloft, we will fall back
towards more of a typical diurnal and climo-based convective pattern
every day with the best chances for storms developing over higher
terrain then moving into the lower deserts overnight. Moisture does
look to be impressive however with PWAT values between 1.5 and 1.8
inches across most of the area from the lower Colorado River
eastward each day through Friday. One feature that the progs have
been trying to latch onto is an inverted trof the pushes north and
into south central Arizona Thursday night into the day Friday. As
this feature approaches and then moves across the area, PWAT value
climb to around 2 inches over portions of south central AZ. As such
we may see the potential for heavy rains developing at times
Thursday night into the day Friday. POPs have been raised 5-10
percent at times mainly Thursday and Friday to account for this with
chance numbers in place across most of the central deserts as well
as the higher terrain east of Phoenix. It is very possible that our
POPs will need further raising later this week.
With cloud cover and moisture increasing from east to west this
week, the temperature trends will be downward, with portions of the
central deserts falling well below seasonal normals by Friday.
However, there is one issue with expected heat and that will be over
the far western deserts and into southeast California today. With H5
heights around 594dm and the airmass still a bit on the dry side,
there is potential for some locations, especially the Imperial
valley, to reach or exceed 115 degrees today. Imperial reached 117
yesterday and we expect similar conditions today. There is potential
for debris clouds current over the AZ deserts to spread into SE
California today and cool things a bit but to be on the safe side we
will be issuing an Excessive Heat Warning for CAZ033 -Imperial
County - from 10 am until 8 pm today. It should cool below warning
thresholds tomorrow so this will be just a one day warning.
Despite copious moisture in place Friday over deserts east of the
lower Colorado River valley, upper streamlines and mean steering
winds are forecast to start turning to the south/southwest. This will
not dry out the area for Friday or Friday night, but this
directional shift in the winds will play a role for the extended
portion of the forecast. 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 and upper streamlines 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 latter
portion of the extended. 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.
South-Central Arizona including KPHX, KIWA, and KSDL:
Any remnant shower activity thru the early morning will stay well to
the W/SW of the Phx area terminals with BKN-OVC debris clouds also
clearing to the west. Morning winds to continue with their easterly
headings, with slight lower speeds than the outflow induced ones from
overnight. With skies clearing enough to provide heating and support
storm initiation this afternoon, will keep at least VCTS mention in
the TAFs for this evening, around 30/02z. Gusty outflow winds,
broadly from the east, are possible along with storm development
along outflow intrusions. Winds in the wake of outflow and storm
development will become more erratic, with several outflow/storm
wind interactions causing more variable headings.
Southeast California/Southwest Arizona including KIPL and KBLH:
Morning debris clouds from thundershower activity across AZ will
cross the CO River, generating SCT-BKN coverage AOA 12kft this
morning for both KIPL and KBLH. Little aviation impacts through
Wednesday afternoon with storms remaining well east of SE CA. While
surface winds will favor a south and southeasterly direction, outflow
boundaries from distant storms may cause abrupt wind shifts, or
variable winds overnight.
Aviation discussion not updated for amended TAFs.
Friday through Tuesday...
Humidities, dewpoints and storm chances will peak Friday before
trending downward through the late weekend. Elevated humidities and
dewpoints will linger for Saturday, with slight chances for storms holding
over the south-central AZ deserts and better chances for the higher
terrain north/east/southeast of Phoenix. Gradual drying trend will
commence Sunday and into early next week with lower desert minimum
humidities drier than 15 percent (and possibly lower) by Monday.
Thunderstorm chances will be confined to the higher terrain of
south-central AZ Sun-Tue and be only slight. Apart from strong
erratic winds associated with thunderstorms, southerly breeziness can
be expected over southwest AZ and southeast CA Thu-Sat(less so over
south-central AZ). Winds will favor southwesterly directions next
.SPOTTER INFORMATION STATEMENT...
Spotter activation is not expected.
CA...Excessive Heat Warning until 8 PM PDT this evening for CAZ033.
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