Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 201648
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
948 AM MST Sun Aug 20 2017

.SYNOPSIS...Expect isolated to scattered thunderstorms during much
of the upcoming week. The greatest coverage of thunderstorms should
occur east to south of Tucson, but the lower deserts will not be
completely left out. Daytime temperatures will be close to seasonal
averages.

&&

.DISCUSSION...Well, sufficient moisture for thunderstorms is back
in place with precipitable water up to 1.35 inches on the 12Z
KTWC sounding. Surface dew points are in the middle 50s to lower
60s. A couple of problems to inhibit thunderstorm development
today though. First, we again are fighting through a lot of debris
cloud from overnight convection that will inhibit solar insolation
in many areas. Second, that overnight convection was a result of a
weak mid level circulation that is currently lifting north and
northeast of our area. The resulting weak subsidence in the wake
of this feature should somewhat inhibit storm development. Overall
an uninspiring below average storm day and evening for the second
half of August. Please see the previous discussion below for
details beyond first and second period.

&&

.AVIATION...Valid thru 21/18Z.
Scattered -TSRA/-SHRA east to south of KTUS through the valid
period, and isolated -TSRA/-SHRA possible west of KTUS this
afternoon into tonight. Otherwise, VFR conditions with cloud decks
8k-12k ft AGL into Monday morning. Surface wind variable in
direction mainly less than 12 kts, although brief gusts to near 30
kts vicinity TSRA. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...Expect isolated to scattered thunderstorms favoring
locales east to south of Tucson this week. Normal diurnal wind
trends will occur aside from any gusty and erratic thunderstorm
outflows. Daytime temperatures will remain close to seasonal
normals.

&&

.PREV DISCUSSION...Forecast confidence regarding precip potential
across southeast Arizona later today and into tonight is fairly
low given the disparity amongst various NWP model solutions. For
instance, the 20/00Z Univ of AZ WRF-NAM and WRF-GFS that the
ongoing showers producing light rainfall will continue until
around 10 am MST or so, then much of southeast Arizona would be
precip-free this afternoon into tonight. The 20/00Z GFS
deterministic solution is fairly close to this scenario. The Univ
of AZ WRF-NAM and WRF-GFS suggest that the aforementioned cyclonic
circulation will move northeastward into southwest New Mexico
later today resulting in a subsident area across southeast
Arizona.

However, the 20/06Z NAM12 and 20/00Z ECMWF suggest that greater
coverage of showers/tstms will occur mainly this afternoon, with
some showers/tstms to develop west of Tucson. The ECMWF then
depicted precip-free conditions most sections tonight, but the NAM12
suggests that some rain may continue tonight especially north of
Tucson.

At any rate, for this forecast issuance have continued with
scattered showers/tstms east to south of Tucson into tonight, and
isolated showers/tstms elsewhere. The same scenario generally
applies Monday into Thursday. The exceptions occur Tuesday and
Thursday with dry conditions depicted across at least western Pima
County. A gradual daily reduction in shower/tstm coverage is on tap
Friday and Saturday. Thus, expect isolated to scattered showers/tstms
mainly east of Tucson Fri-Sat as high pressure aloft strengthens
over the area.

Daytime high temps today into next Saturday will generally be within
a couple of degrees (above or below) of seasonal normals.

&&

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

&&

$$

Meyer/Francis

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