Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 191544
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
844 AM MST Sat Aug 19 2017

.SYNOPSIS...Expect scattered thunderstorms during much of the
upcoming week. The greatest coverage of thunderstorms should occur
east to south of Tucson, but the lower deserts won`t be completely
left out. Daytime temperatures will be close to seasonal
averages.

&&

.DISCUSSION...We`ll be fighting through debris cloud left over
from the convection just south of the border last night. Solar
insolation will be inconsistent at best this morning. Great
moisture trends though. Surface dew points have jumped into the
lower to middle 60s, up about 15 to 20 degrees over the past 24
hours. A nice moderate gulf surge is underway in Yuma, up to
around 4k feet now, and a 77 dew point is nothing to sneeze at.
Satellite estimates show a tight moisture gradient with around .8
to .9 near Tucson, rapidly climbing to around 1.6 inches just
south of the border in northern Sonora. We`re going to put up an
18Z release and check on that progress. With less surface heating
expected, we`re probably still on the right track as far as
limited thunderstorm potential during the day today. This evening
and overnight may be another story. The next several days look
favorable with a better pattern for importing and keeping deeper
moisture.

&&

.AVIATION...Valid thru 20/18Z.
Isolated to scattered -TSRA/-SHRA mainly east to south of KTUS
this later this afternoon into tonight, with better chances
Sunday. Otherwise, VFR conditions with surface wind variable in
direction mainly less than 12 kts. Aviation discussion not updated
for TAF amendments.

&&

.FIRE WEATHER...There is a chance of thunderstorms mainly southeast
of Tucson this afternoon and evening. Scattered thunderstorms will
then prevail especially east to south of Tucson through about next
Thursday. A gradual reduction of thunderstorm coverage may occur by
next weekend.

&&

.PREV DISCUSSION...A weak low pressure system aloft was centered
early this morning near the southern California coast. Meanwhile,
high pressure aloft was centered east of this forecast area over
central Texas. The upper pattern is progged to exhibit little
change through about next Wednesday.

In the short term or through Sunday morning, the GFS depicted the
driest scenario versus the NAM/ECMWF/CMC, and generally kept
measurable rainfall across far southeastern sections of this
forecast area. The NAM and several HRRR solutions were considerably
more robust versus the GFS/ECMWF regarding precip potential into
Sunday morning.

However, the main theme was similar in that the bulk of showers/
tstms starting early this afternoon and continuing into Sunday
morning should occur mostly east to south of Tucson. Scattered
showers/tstms will likely prevail across much of the area Sunday
afternoon and evening, with more showers/tstms to occur west of
Tucson especially if the ECMWF is reality. The official forecast
depicts a gradual daily reduction of showers/tstms across western
sections Mon-Wed, but scattered showers/tstms should prevail east of
Tucson mainly during the afternoon/evening hours.

Thereafter, the deterministic GFS/ECMWF were fairly similar with
depicting a potentially active period for showers/tstms to occur
late Wed into Thur. This appears to be in response to the weak upper
low near the southern California coast progged to fill and move
eastward into far western Arizona as a weak upper trough. Per
coordination with neighboring WFO`s, for this forecast issuance have
maintained scattered showers/tstms Wed night-Thur east of Tucson,
and just isolated showers/tstms for the Tucson metro area. However,
precip chances may need to be raised for Wed night-Thur if
subsequent model solutions remain similar to the 19/00Z GFS/ECMWF.
Upper level ridging next Friday should limit showers/tstms mostly to
locales east of Tucson.

Forecast high temps today thru at least next Wednesday will likely
exhibit only very minor daily changes, and will remain quite close
to seasonal normals. Slightly warmer temps are on tap next Thur-
Fri.

&&

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

&&

$$

Meyer/Francis

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