Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 191138

Area Forecast Discussion
National Weather Service Phoenix AZ
438 AM MST Thu Jul 19 2018

Periodic chances for isolated to scattered showers and thunderstorms
will continue for the next several days before a decrease in
activity is expected for Sunday through early next week.
High pressure is expected to increase across the region by later
this weekend ushering in a strong warming trend and excessive heat
for early next week.


After an active period of storms night last night and early
Wednesday, followed by a less active Wednesday evening, radar is
showing no precipitation across the region. Satellite shows mostly
clear skies across our area with water vapor imagery showing a dry
slot over S NM, W Texas, and N Chihuahua Mexico edging west. 00Z
soundings for Phoenix and Tucson had PWs near 1.59-1.65 in.
respectively with dry southeasterly flow aloft and westerly flow at
the surface and boundary layer. Soundings are now drier through the
mid levels with mid level caps present and CAPE values near 1.4-1.5K
j/kg. SPC mesoanalysis also depicts SBCAPE in the 1-2.5Kj/kg across
the area with typical nighttime CIN/inhibition. Moderate to breezy
westerly winds continue across the area associated with the
continuing Gulf of California sea breeze/seep.  This is helping to
maintain elevated dewpoints in the mid/upper 60s and low 70s with a
dewpoint of 70 degrees currently being reported at Sky Harbor.

From today through Saturday expect continued periodic chances of
isolated storms across the lower deserts of S Central AZ, also
including SE CA and SW AZ, with scattered storms possible for the
higher elevations to the N-NE-E-SE and S. However, due to a general,
unsteady and gradual drying trend, our POPs will be lower compared
to the last 10 days across the lower deserts and Phoenix--ranging
from about 5-15 percent. The banana shaped high pressure system to
our north will begin to reposition and reshape itself beginning
today through early next week. Its core will slowly slide to the
southeast until it arrives near the TX Panhandle by the middle of
the weekend. During this time period its southwesterly periphery will
be in a good position to continue to provide a convenient path for
inverted shortwaves, disturbances, and Monsoon jet maxes aloft from
N Mexico and the Gulf of Mexico to provide us with our continued
lower grade Monsoon storm chances through the middle of the weekend.

Then from Sunday through the early to mid part of next week a major
warming trend and a plummeting of POPS takes place across the
deserts. During this time the high builds back towards the west
towards AZ so that by early to mid next week, the ball shaped high
pressure is parked over AZ. Models are also in good agreement on
stalling the high over our region for a couple of days. The GEFS
ensemble members and the ECMWF show the H5 high pressure heights
center at 598-600 dam. In addition to the supremely dangerous late
July heatwave that is expected by early next week, the high`s new
southerly position will largely cut off our optimal Monsoon moisture
pipeline resulting in very low POPS.  Our Excessive Heat Watch thus
continues with good chances that it will be upgraded to an Excessive
Heat Warning fairly soon. It is believed that when the models
predict high pressure systems of this magnitude, that they are
rarely wrong.


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

Westerly surface winds will prevail through the majority of the
overnight hours with KPHX likely staying west through Thursday
afternoon. Any showers and storms for the rest of the evening and
tonight will remain out of the lower deserts with little chance of
any storm outflows affecting the terminals. Storm chances for
Thursday evening will likely be a bit better, especially across
the high terrain. This should at least send easterly outflows
into the Phoenix area sometime Thursday evening.

Southeast California/Southwest Arizona including KIPL and KBLH:

Little in terms of aviation concerns for the next 24 hours as no
thunderstorms are expected to impact the terminals. Winds will
typically favor the southeast at KIPL and the south at KBLH with
speeds mostly 12kt or lower. A few high based afternoon cumulus
are possible otherwise just variable amounts of mid/high cloud

Aviation Discussion not updated for amended TAFs.


Saturday through Wednesday: Isolated showers and thunderstorms will
remain possible Friday, especially across the higher terrain north
and east of the Phoenix area. However, moisture values will undergo
a gradual decrease into the weekend, and expecting only a slight
chance of any thunderstorms area-wide by Saturday. Daytime minimum
relative humidity values will also fall closer to 15%. A significant
increase in heat is advertised for Monday through Wednesday, with
high temperatures above 115 degrees expected over the lower deserts.
The winds will be generally light and follow typical diurnal trends.
Only very isolated afternoon and evening thunderstorms are expected
early next week over higher terrain east of Phoenix, mainly east of


Spotters should follow standard reporting procedures.


AZ...Excessive Heat Watch from Monday morning through Tuesday evening
     for AZZ532-535>537-539>544-546-548>551-559.

     Excessive Heat Watch from Tuesday morning through Tuesday
     evening for AZZ530-531-533-534-538-545-547-552>556-560>562.

CA...Excessive Heat Watch from Tuesday morning through Tuesday
     evening for CAZ560>570.



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