Area Forecast Discussion
Issued by NWS Tucson, AZ
FXUS65 KTWC 271619
Area Forecast Discussion
National Weather Service Tucson AZ
919 AM MST SAT AUG 27 2016
.SYNOPSIS...Drier air associated with a weather disturbance passing
through Arizona will limit thunderstorm activity today into Monday.
With drier air in place, daytime temperatures will warm a few
degrees today through Tuesday. Thereafter, deeper moisture needed to
support daily thunderstorms will return the middle of the
week with high temperatures lowering to more seasonable readings.
.DISCUSSION...Water vapor imagery and 27/12Z upper-air plots show a
trough of low pressure currently across Utah, southern Nevada and
northwest Arizona. Plenty of dry air associated with this system
this far south. The dry mid to upper level air covers much of the
eastern Pacific waters where a large area of high pressure is well
west of the southern Baja peninsula. This dry air extends east
across southern California, much of northern Mexico and into
Visible and IR satellite imagery shows extensive cloudiness
associated with the trough mostly over southern Utah, and northern
Arizona. Some mid level clouds also extend south and cover mainly
eastern portions of my forecast area from Tucson to the New Mexico
border and areas east. All of these clouds are moving to the north
The 27/12Z KTWC sounding revealed a PW of 1.10 inches, which is down
three tenths of an inch from 24 hours ago. The bulk of this moisture
was in the surface to 525MB layer. Above that, it was very dry. Some
of the sounding parameters indicated Cape ranging from just under
1000 J/KG to near 1500 J/KG, and a lifted index of minus 3 or 4
depending on the method in which the parcel was lifted.
The current radar mosaic from around the state shows most of the
showers and thunderstorms associated with the trough are occurring
north of of the Mogollon rim, generally along interstate 40, with
scattered storms south of I-40 and east of I-17 toward New Mexico. A
few showers and storms currently over parts of Graham, Greenlee and
northern parts of Cochise county in my forecast area, but all
of this activity is moving northeast.
The Univ of AZ WRF/NAM & GFS models show most of the convection
today occurring along the New Mexico border or well north of my
region. The HRRR was showing a similar solution. That said, the
inherited POP forecast for today has the focus between Tucson and
New Mexico. As the day progresses we should dry even further, so not
expecting much activity from Tucson south and west. Will continue to
monitor satellite and radar trends throughout the morning and will
make adjustments to the POP forecast as needed.
As of 16Z (9 AM MST), temperatures ranged from the mid 70s to the
lower 80s and these readings seem to be on track with the
anticipated afternoon highs, so no changes planned at this time.
See previous discussion below for details beyond today.
.AVIATION...VALID THRU 28/18Z.
Sct-bkn clouds with bases at 8-12k ft agl. Isolated to scattered
-TSRA/-SHRA developing aft 27/17z, mainly east of KTUS to the New
Mexico border. Brief MVFR conditions possible with the stronger
storms. Partial clearing expected aft 28/04z. Surface wind generally
less than 12 kts thru the forecast period, except for strong and
erratic outflow winds possible with the thunderstorms. Aviation
discussion not updated for TAF amendments.
.FIRE WEATHER...Isolated to scattered afternoon and evening
thunderstorms mainly east of Tucson this weekend and during the
upcoming week. 20-foot winds into next week will be terrain driven
and less than 15 mph in most locations. Expect the potential for
strong and gusty winds near any thunderstorms that do develop.
.PREV DISCUSSION...A weak trough of low pressure will spread
some drier air into southern Arizona as it passes through the state
today and Sunday. This drier air will result in fewer showers and
thunderstorms today compared to yesterday. Limited thunderstorm
activity can be expected Sunday into Monday as the deeper moisture
shifts east of the area. Eventually, the models showed a pronounced
low pressure trough along the west coast with high pressure
amplifying over the central sections of CONUS. This results in a
southerly flow with deeper moisture spreading back into the area
around the middle of the week. Thus, will advertise an increase in
thunderstorm activity by Thursday. By next weekend, the GFS model
shifts the west coast low pressure trough inland with much drier air
spreading into the Desert Southwest. This should translate to a more
prolonged decrease in rain chances. Daytime temperatures will warm a
few degrees through Monday with drier air in place. Thereafter,
lower to more seasonable readings around Wednesday.
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