Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 210231
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
730 PM MST Sat Oct 20 2018

.SYNOPSIS...There will be a chance of showers and thunderstorms
tonight through Tuesday as a trough of low pressure moves across the
area. Expect windy conditions to persist into Sunday morning, easing
Sunday afternoon and decreasing even further on Monday. Temperatures
will hover within a few degrees of normal through the forecast
period.

&&

.DISCUSSION...Gusty east/southeast winds continue across southeast
Arizona early this evening, with wind speeds generally at 10-20 mph
and gusts to around 30 mph. Expect these breezy winds to persist
through Sunday morning before slowly diminishing Sunday afternoon.
For peak wind speeds so far today, refer to the product PHXPNSTWC.
Otherwise, except for clear skies west of Sells, clouds are slowly
increasing across central/eastern portions of the forecast area this
evening.

The models continue to be in good agreement with increasing moisture
transport into the region from the south and east tonight and Sunday
in advance of an upper level low off the central California coast.
The UofA Regional WRF NAM/GFS runs both indicate precipitation
initiation across the forecast area between 21/09Z-11Z and then
becoming more widespread through the morning hours on Sunday. The
UofA runs show the precipitation developing across northern Sonora
and then expanding northward through Nogales and into the Tucson
metro area during the morning hours Sunday, which matches up well
with current pop grids.

Overall, the current forecast package looks to be in good shape. No
updates this evening. For more information on the latter periods of
the forecast, please refer to the PREV DISCUSSION section below.

&&

.AVIATION...Valid through 22/00Z.
FEW-SCT clouds at 6k-9k ft AGL, with locally BKN clouds east of a
KSAD to KDUG line. Clouds increasing in areal coverage late tonight,
with SCT-BKN clouds at 6k-9k ft AGL by 21/12Z along with isold -SHRA
developing. Expect scattered -SHRA and isolated -TSRA by 21/18Z,
especially west of a KSAD-KFHU line.

SFC wind generally be ely/sely at 10-20 kts with gusts to near 30
kts through the period. The highest speeds will be at KTUS and KDUG
and over higher terrain. Aviation discussion not updated for TAF
amendments.

&&

.FIRE WEATHER...Gusty east to southeast winds will continue through
Sunday with decreasing easterly winds on Monday. Winds then become
terrain driven at less than 15 mph through the end of next week. A
slowly moving upper level trough will bring a chance of showers and
thunderstorms to the area starting late tonight and continuing
through Tuesday. Dry conditions can be expected the second half of
the upcoming week as high pressure aloft becomes established across
the southwestern United States.

&&

.PREV DISCUSSION...Mostly clear to partly cloudy skies prevail this
afternoon across southeast Arizona with the greatest cloud coverage
across the higher terrain. Larger scale pattern features a mid/upper
level low off the California coast with south to southwesterly flow
aloft across southeast Arizona. Meanwhile, lower level flow is east
to southeasterly as a tight pressure gradient sets up across the
area with high pressure building across the southern Rockies.
Current sustained winds of 15 to 25 mph with gusts of 30 to 35 mph
are being observed across the region with breezy to windy conditions
expected to continue into Sunday morning before gradually
decreasing. Latest GOES Total Precipitable Water imagery shows
values around 0.7 inches which is above average for this time of
year. The latest CAMs have backed off slightly on developing showers
primarily west of Tucson late this afternoon and evening where the
best area of low level convergence lines up with weak instability.
However, a few showers remain possible this evening and will
maintain slight chance PoPs tonight.

The aforementioned upper level low and associated mean trough will
gradually shift eastward but this will take through Tuesday to
occur. Multiple disturbances/shortwaves are expected to round the
base of the trough Sunday through Tuesday resulting in several
rounds of showers and thunderstorms as lower level south to
southeasterly flow continues to transport anomalously deep moisture
with PWAT values getting close to 1 inch at times Sunday through
Tuesday. We`ve continued to make upward adjustments to the PoPs,
especially on Sunday when fairly widespread areal coverage of
showers (especially across central portions of the forecast area,
including Tucson) with some embedded thunderstorms are expected to
occur. The unsettled weather will continue right into Tuesday with
the best chances of showers with embedded thunderstorms from Tucson
eastward. Rain amounts of 0.10 to 0.50 inch on average through
Tuesday for much of southeast Arizona with the exception of the
western deserts which will see lesser totals. We can`t rule out a
few spots with 1 to 2 inches where some of the heavier
showers/thunderstorms occur and across the higher terrain. This
scenario is shown in both the 12z WRF UOFA-NAM and GFS solutions.

Drier air will filter in Tuesday especially west of Tucson, but
enough moisture remains for a chance of showers mainly from Tucson
eastward before drier westerly flow moves in for Wednesday when only
a slight chance of showers is possible near the New Mexico border. A
more typical late October pattern is expected to end the week with
northwesterly flow as ridging builds in with dry conditions
prevailing.

Temperatures will remain within a few degrees of normal through the
upcoming week.

&&

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

&&

$$

DISCUSSION...Zell
PREV DISCUSSION...GL/74

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