Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 311006

Area Forecast Discussion
National Weather Service Phoenix AZ
306 AM MST TUE MAY 31 2016

A slow moving low pressure system across the Desert Southwest will
bring near normal temperatures and dry conditions to the deserts
today, but a slight chance of showers and thunderstorms over the
higher terrain. Thereafter, strong high pressure will build over the
region resulting in a sharp warming trend. By the weekend, excessive
heat will prevail with many lower desert locations reaching the 110
degree mark.


The main forecast focus over the next week will be the expected
excessive heat (potential record highs) later this week into the
weekend, but first a weak upper low will pass through Arizona today.
This upper low is easily seen on water vapor imagery early this
morning, centered just east of Yuma. For the most part, this upper
low is innocuous, but there is some mid level moisture wrapping
around the upper low. On the back side of the low, scattered mid
level altocu is seen on IR imagery across southeast California along
with some very light returns on radar.

As the upper low slides southeastward into northern Mexico this
afternoon, mid level moisture will wrap around the northern fringe of
the low entering east-central Arizona from New Mexico. This moisture
along with some modest upper level support should allow for some
scattered showers and thunderstorms along the Mogollon Rim and over
the White Mountains this afternoon. Storm bases will be fairly high
(12-15K ft MSL) so not anticipating much in the way of rainfall over
our higher terrain spots, but there will be a gusty wind potential
due to a dry sub-cloud layer. A weak northeasterly steering flow
may bring some of these storms closer to the lower deserts by early
evening, but they stand little chance of surviving off the high
terrain. However, the gusty outflow winds have a better shot at
affecting the Phoenix area along with a small potential for some
blowing dust.

By Wednesday, the upper low moves out of the region giving way to an
impressively strong Pacific ridge. Once this ridge shifts over the
Western United States later this week, an omega block regime sets up
as the ridge continues to build in strength. Currently, 500mb heights
associated with the high center off the West Coast are around 585dm
and should maintain this strength through Wednesday as the ridge axis
shifts eastward into southern California. The ridge will then
strengthen as it moves into the Desert Southwest for later this
week, easily topping 500mb heights of 590dm by early Friday.
Temperatures during this period will quickly climb from the current
slightly below normals to highs above 100 on Wednesday and then
quickly approaching 110 degrees Thursday. Once high temperatures get
to 110 or above, excessive heat watches and warnings come into play,
and this is all but a sure bet starting Friday.

Will be keeping the Excessive Heat Watch for much of our area going
without any changes and anticipate needing to upgrade to a warning
very soon. Model deterministic and ensemble means all point to this
impressive ridge setting up later this week and strengthening into
Saturday, before starting to shift eastward on Sunday. Models over
the past several days continue to bump up heights and temperatures
and the newest 00z model suite followed suite. Highest 500mb heights
now top out around 594dm on Saturday with NAEFS mean values
indicating above 97% of climo. 850mb and 700mb temperatures also peak
at around 99% of climo on Saturday. Given these data, anticipating
widespread highs in a 110-114 range for the Friday through Sunday
period. Readings like this would be near or in record territory,
especially for Phoenix. For Phoenix to hit 110 or above during the
first week of June is actually quite rare as it has only occurred a
handful of times on any given day in early June since records began
in 1895.

The excessive heat may extend another day into next Monday, but
mainly for the south-central Arizona deserts as an upper low is still
advertised to pass through southern California. Ensemble spread is
quite a bit higher for this early next week scenario so confidence
on when and how much of a cool down is still somewhat low.


South-Central Arizona Including KPHX, KIWA, and KSDL
A weak Pacific low pressure system will move slowly through south-
central AZ this evening into southeast AZ Tuesday afternoon. For the
remainder of tonight, mostly clear skies with winds turning light and
southeasterly. Winds will take their typical westerly shift Tuesday
aftn. Although this system will be relatively dry initially, sct
high cld base/mtn tstms are expected along the Mogollon Rim and White
Mtns Tue aftn and evening. Storm activity may creep into the higher
terrain north and east of Phoenix. While rainfall/TS not expected,
gusty (sometimes erratic) winds and some dust is possible for the
evening hours in the vicinity of the Phoenix terminals.

Southeast California/Southwest Arizona Including KIPL and KBLH...
A weak Pacific low pressure system will move slowly into southeastern
AZ through the day Tuesday. Some passing SCT to BKN high level debris
clouds will drift in from the north overnight, otherwise skies to
remain clear. Light overnight winds with southerly to southeasterly
drifts less than 8kts.

Aviation discussion not updated for amended TAFs.



Record high temperatures later this week:

Date        Phoenix           Yuma
----       ---------         ------
June 2     110 in 2014       115 in 1957
June 3     112 in 2006       113 in 2006
June 4     113 in 1990       114 in 1990
June 5     112 in 1990       115 in 1957
June 6     110 in 2013       114 in 1928


Thursday through Monday...
An unseasonably strong high pressure system will build over the
region this period, bringing some of the warmest afternoon
temperatures of the season. Many desert locations will slightly
exceed the 110 degree mark. Minimum relative humidities will range
from 7 to 10 percent. Typical warm season upslope afternoon winds
are expected in the 10 to 20 mph range. Good overnight recovery is
also forecast.


Spotter activation is not expected.


AZ...Excessive Heat Watch from Friday morning through Sunday evening
     for AZZ020>023-025>028.

CA...Excessive Heat Watch from Friday morning through Sunday evening
     for CAZ031>033.



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