Area Forecast Discussion
Issued by NWS Tucson, AZ

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839
FXUS65 KTWC 180420
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
920 PM MST Thu Aug 17 2017

.SYNOPSIS...A moisture increase from the south will gradually
bring isolated to scattered thunderstorms back to southeast
Arizona this weekend and especially early next week. High
temperatures will remain within a few degrees of normal through
the weekend.

&&

.DISCUSSION...A painfully dry sounding for mid August this
afternoon. KTWC was showing about 1/2 inch precipitable water.
We`re actually seeing some surface dew points in the 30s this
evening. We`ve seen some overall improvement over the past 24
hours though. That monster complex merger over Sinaloa last night
helped fire a lesser convective complex further north over
southern and central Sonora during the afternoon. Precipitable
water values have pushed up to near 1.6 inches in central Sonora,
and the impulse supporting this activity will help organize the
storms that will push convection to near the border tomorrow
evening. Our current forecast has great trends with the return of
monsoon moisture and thunderstorms over the weekend into next
week.

&&

.AVIATION...Valid thru 19/06Z.
Patchy cirrus from Mexico at times, otherwise just a few CU in
the afternoon. A few buildups are possible near the border late
Friday afternoon and early evening. Expect normal diurnal wind
trends less than 12 kts. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...Dry weather will continue into Friday. Isolated
to scattered showers and thunderstorms will return this weekend
into next week. 20-foot winds will generally be terrain driven at
less than 15 mph apart from any thunderstorm outflows.

&&

.PREV DISCUSSION...Focus of the forecast is the return of
moisture and thunderstorms to southeast Arizona.

Current surface obs show dew points in the 30s and 40s with a few
cumulus clouds over mountains. Better moisture is south of the
border with thunderstorms occurring from central Sonora/Chihuahua
southward. Upper trough/low over California will not move a lot
through this weekend, but the upper ridging/high pressure
stretching from nrn Mexico to the Gulf of Mexico will become
reoriented farther north into Texas. This will allow increased
southerly flow between the low to our west and high to our east
and a resultant moisture transport northward into SE AZ.

Initially, it looks like the axis of deepest moisture will be over
New Mexico or perhaps far eastern AZ, but storms should form on
the western edge of this axis this weekend, triggered by a series
of disturbances in the the southerly flow. Southern and eastern
areas will have the best chances for rain. As moisture continues
to stream in, the axis of best moisture should shift westward
early next week with 1.5+ inch precipitable water values indicated
over SE AZ Monday night through Wednesday. Thus, rumors of the
demise of our monsoon have been greatly exaggerated.

A relatively large difference between daytime highs and nighttime
lows will only continue another day or two for most areas before
the diurnal range narrows as is typically the case during an active
monsoon time frame.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

Meyer/Drozd

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