Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 181000

Area Forecast Discussion
National Weather Service Tucson AZ
300 AM MST Fri Aug 18 2017

.SYNOPSIS...The Monsoon retreated to Mexico earlier this week,
but it`s on the way back. A moisture increase over the next 72
hours that will bring the thunderstorms back over the weekend into
next week.


.DISCUSSION...Our precipitable water plummeted to 1/2 inch
yesterday afternoon, less than half of what we normally see in mid
August. Surface dew points are no better with values in the 30s
and 40s across the area. There is hope though.

That monster complex merger over Sinaloa Wednesday night helped
fire a lesser convective complex further north over southern and
central Sonora yesterday afternoon. Precipitable water values
have pushed up to near 1.7 inches in central Sonora, and the
impulse supporting this activity will help organize the storms
that will then push convection to near the border this evening.

Initially, the deepest moisture is shaping up for Chihuahua and
New Mexico Saturday. Sunday and Monday will see that favorable H7
theta-e axis shift deeper into our area as a high center
reconsolidates over Texas downstream, and a weak low sags into
California upstream. We should see decent thunderstorm coverage
Sunday afternoon into Monday evening or Tuesday afternoon.

After that it gets tricky. The moisture will still be here and/or
nearby, but features become more subtle. The main factor is the
weakness in the flow upstream and it`s positioning. If it`s too
close then we likely wrap in some dry air under it, with the
deepest fetch of moisture through New Mexico. For now the best we
can do is just throw in a chance of mainly afternoon and evening
thunderstorms, something close to climatology.


.AVIATION...Valid thru 19/12Z.
Scattered to broken cirrus from Mexico, and some CU around 8k-12k ft
AGL especially east of KTUS this afternoon and evening. A few -TSRA
may occur near the international/New Mexico borders southeast of
KTUS tonight. However, expected occurrence of -TSRA/-SHRA at KDUG is
too low to include in the KDUG TAF. Expect normal diurnal wind
trends less than 12 kts into early Saturday morning. Aviation
discussion not updated for TAF amendments.


.FIRE WEATHER...Dry conditions today followed by a slight chance of
thunderstorms across far southeast sections tonight. Increased
moisture will then bring scattered thunderstorms especially east-to-
south of Tucson Saturday into Tuesday. Somewhat less coverage of
thunderstorms next Wednesday and Thursday, but still enough moisture
for isolated to scattered thunderstorms mainly east of Tucson. 20-
foot winds will mainly be terrain driven less than 15 mph aside from
thunderstorm gusts. The exception appears to be Saturday when an
easterly component should prevail.






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