Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 212155

Area Forecast Discussion
National Weather Service Tucson AZ
255 PM MST WED SEP 21 2016

.SYNOPSIS...A few showers around the area today with some drying
tonight. A few storms are possible with a frontal boundary late
Thursday into Friday with a strong drying trend and cooler air for
most of the area behind the front.


.DISCUSSION...Water vapor imagery shows a weak trough moving through
southern Arizona/northwest Mexico at this time, with a swath of mid
to upper level dry air covering the eastern Pacific waters, northern
Baja, southern California and into western Arizona. Meanwhile, the
ridge currently extends from southern Baja across north central
Mexico and into north central Texas. In addition, a major trough is
currently approaching the Pacific Northwest.

IR and visible satellite imagery shows extensive cloudiness across
northern Baja, southern California and mostly western and northern
Arizona. Partly cloudy skies across northern parts of Sonora Mexico
and into parts of southern Arizona as a cumulus field has developed
this afternoon. The 21/12Z KTWC sounding revealed a moist airmass in
place with a PW of 1.59 inches. CIRA LPW Total imagery still shows
high values across the Desert Southwest, northern Mexico and the
coastal waters of the Baja peninsula. The bulk of the moisture is in
the low to mid levels as indicated by the sounding, but also the 18Z
and 21Z CIRA layered imagery, as well as surface observations which
show dewpoint temperatures in the lower 50s to the lower 60s as of
21Z (2 PM MST).

Radar mosaic from around the region shows isolated showers have
developed across the region, mainly from around the Tucson metro
area and eastward toward the New Mexico border. These showers are
moving fairly quickly to the north. In addition, isolated to
scattered showers are also occurring over the far western deserts,
west of Ajo toward Yuma and these are also moving north. We should
continue to see isolated to scattered showers and possible
thunderstorm development through the remainder of this afternoon and
into the early evening hours, but activity should rapidly die off
after sunset.

By early Thursday another major trough will be over northern
California and will move into the Great Basin by late afternoon. As
this occurs we should see a `line` of showers and possible
thunderstorms develop ahead of this feature and march across our
forecast area from west to east. At this point in time it is
expected that the bulk of this activity should be to our north with
the `line` being more scattered in nature farther to the south. So,
for now I just have isolated to scattered type POPs for my forecast
area Thursday and this threat will continue into Friday as the low
will be over the northern Rockies late in the day with a vort lobe
extending from the low through central Colorado, northwest New
Mexico and northern Arizona, but by that time only expecting
isolated showers and/or storms.

By late Friday into Saturday as the low continues to progress east
and northeast into the northern Rockies/Plains, we will see a large
swath of drier air making its way into the Desert Southwest from the
west behind the trough. This will result in a dry forecast for
Saturday. However, by Sunday models show a vort max dropping south
on the back side of this trough into western New Mexico and the
winds will become more easterly late in the weekend and early next
week, so we could see a slight threat for showers along the border
with New Mexico during that time. Although even that could change if
the high pressure to the west moves a bit farther east and pushes
moisture farther to the south and east.

For Tucson, high temps will be near normal Thursday, then several
degrees below normal Friday through Monday, although the coolest day
will likely be Friday when highs will range from 6 to 8 degs below
normal. For Tuesday and Wednesday of next week, highs will be near
or slightly above normal. Low temps will range from 1 to 3 degs
above normal Thursday and Friday mornings, then 6 to 8 degs below
normal over the weekend and only slightly below normal for the early
to middle of next week.


.AVIATION...Valid through 23/00Z.
Isold -SHRA/-TSRA with cloud decks generally from 9k-12k ft msl into
the evening then clearing. SCT -SHRA/-TSRA dvlpg once again aft
22/19Z with cloud decks generally 10k-12k ft. Surface wind will be
variable in direction mostly less than 12 kts thru Thursday morning
then becmg SW 11-16 kts Thursday afternoon. Aviation discussion not
updated for TAF amendments.


.FIRE WEATHER...There will be a few showers and thunderstorms this
evening then clearing.  A few storms are possible with a frontal
boundary late Thursday into Friday with a strong drying trend and
cooler air for most of the area behind the front this weekend. 20-
foot winds will be mainly terrain driven less than 15 mph through
tonight, then 10-20 mph west to southwest breezes will occur
Thursday and Friday afternoons.






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