Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 180945

Area Forecast Discussion
National Weather Service Tucson AZ
245 AM MST Sat Aug 18 2018

.SYNOPSIS...Drier air and building high pressure will result in less
thunderstorm activity with higher afternoon temperatures through the
weekend. Early next week the high drifts just far enough away to
allow some moisture to return, generating isolated to scattered


.DISCUSSION...The mid level high center that has been east of our
area this week is becoming disorganized and in the process of
reconsolidating northwest of our area over the next 24 hours. Our
monsoon-favorable southerly flow has already veered around to the
west and will be dominated by a less favorable northerly
component later today. As this happens moisture will continue to
drop resulting in less thunderstorm activity by this afternoon or
Sunday. The 00Z KTWC sounding had dropped to 1.27 inches. We may
see a temporary halt to that trend with the morning sounding, but
the overall trend will continue with values near 1.1 inches or
below by Sunday afternoon. As mentioned yesterday, that puts the
well above average precipitable water we had most of the week to
below average by Sunday afternoon at the latest.

The high center will once again wander eastward in response to
larger scale synoptic features at higher latitudes as well as a
lazy pattern upstream and downstream, and will likely be nearly
overhead Sunday night and Monday. Heights and thicknesses support
temperatures 4 to 6 degrees above average as this happens with
limited moisture to mitigate the heating trend. At that point it
all depends on how far east the high center is able to shift. The
GFS favors an elongated ridge across the desert southwest in a
semi-blocking position for moisture, while the ECMWF is sharper as
it shifts it just far enough east to bring in more moisture by
the middle of next week. It`s really a subtle difference in a
forecast that far out, so we will go with a blend of the two that
favors levels near or just below climo for now. That means a week
that is not as busy as the previous week. Stay tuned as we try to
nail this down a little better.


.AVIATION...Valid through 19/12Z.
SCT-BKN cloud bases generally 15-20k ft AGL. -SHRA/-TSRA develop
this afternoon, favoring mountain locations east of KTUS with cloud
bases dropping slightly to 8-12k ft AGL. SFC winds variable less
than 10 kts this morning, becoming WLY-NWLY in the afternoon at 10-
14 kts with a few higher gusts, especially through the Gila Valley
near KSAD. Wind gusts will die down this evening and overnight into
Sunday morning. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Thunderstorm activity trends down this weekend, with
isolated storms favoring higher terrain areas east of Tucson. The
pattern then becomes more favorable at the start of the new week
allowing for increased moisture and greater areal coverage of
storms. 20-ft winds remain terrain driven at less than 15 mph,
except for in and around thunderstorms where strong outflows may






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