Area Forecast Discussion
Issued by NWS Tucson, AZ

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000
FXUS65 KTWC 201000
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
300 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...A weak upper low remained centered just west of the
southern California coast early this morning, and high pressure
aloft was centered over south Texas. Meanwhile, IR/water vapor
imagery depicted an expansive area of cloudiness between these two
systems extending from eastern Sonora/much of Chihuahua Mexico
northward across eastern Arizona and into New Mexico. The coldest
cloud tops during the past several hours have been across far
eastern sections of this forecast area and far southwest New Mexico.
Water vapor imagery also depicted a cyclonic circulation centered
over northeast Sonora Mexico.

KEMX WSR-88D has also depicted generally 30-40 dBZ echoes during the
past several hours across eastern Cochise County and particularly
in the vicinity of the Chiricahua Mountains. The Long Park gauge on
the Chiricahuas has recorded at least 0.16 inch in the past 6 hours,
and KDUG has recorded just a trace of rain since midnight.

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.

&&

.AVIATION...Valid thru 21/12Z.
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.

&&

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

&&

$$

Francis

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