Area Forecast Discussion
Issued by NWS Tucson, AZ

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749
FXUS65 KTWC 200345
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
845 PM MST MON SEP 19 2016

.SYNOPSIS...Moisture associated with decaying tropical system Paine
will bring scattered showers and isolated thunderstorms through
Wednesday. A drying trend will begin Thursday and continue into this
weekend. However, enough moisture should exist for a slight chance of
showers and thunderstorms east of Tucson.

&&

.DISCUSSION...KEMX WSR-88D depicted mainly 30-45 dBZ moving
northward 10-15 mph across western Pima County, the Tohono O`odham
Nation, and south central Pinal County at this time. The rest of
southeast AZ was mostly void of precip echoes. Rainfall amounts thus
far have been limited to trace amounts or just a few hundredths of
an inch.

Regarding Hurricane Paine, the National Hurricane Center advisory
number 9 issued at 8 PM PDT this evening stated that Hurricane Paine
was located at 23.8N/116.7W, or southwest of the Baja California
spur. Hurricane Paine was moving to the north-northwest at 13 mph.
Hurricane Paine was beginning to decrease in intensity, as maximum
sustained winds had decreased to near 80 mph. Rapid weakening is
forecast during the next day or so, and Paine is expected to weaken
to a tropical storm by Tuesday morning, then become a remnant low on
Wednesday.

Meanwhile, back across southeast Arizona, IR satellite imagery
depicted cloud tops to generally be cooling during the past hour or
so. Dewpoints at lower elevations valid 8 PM MST ranged from the
lower-mid 40s across eastern sections, and from the 50s-lower 60s
from the Tucson metro area westward into western Pima county. The
dewpoint jump at KTUS occurred after the 20/00Z KTWC sounding was
completed. The sounding total precip water value at that time was
0.89 inch, and increase of nearly one-quarter of an inch versus 12
hours earlier. The profile exhibited a nearly saturated column above
500 mb, though was still quite dry in the surface-600 mb layer.

Several HRRR solutions and the 20/00Z NAM12 suggest the best chance
of measurable rainfall the rest of tonight will be west of Tucson.
This scenario seems plausible given radar trends. However, still
believe that rain chances will gradually spread eastward by Tuesday
morning.

For this forecast update, only very minor adjustments were made to
PoPs through Tuesday afternoon. However, the gridded data weather
fields and lightning activity levels were adjusted to depict less
coverage of thunderstorms versus the previous forecast. Thus,
isolated to scattered showers mainly from Tucson westward the rest
of tonight along with a slight chance of thunderstorms. Scattered
showers and isolated thunderstorms to then prevail area-wide by
Tuesday afternoon and continuing into Tuesday evening. The threat
for widespread heavy rainfall appears to be very low. High temps
Tuesday are forecast to be mainly 5-10 degs cooler versus today.

Please refer to the additional sections for further detail.

&&

.AVIATION...Valid through 21/06Z.
Scattered -SHRA mainly west of KTUS the rest of tonight, then
scattered -SHRA spreading eastward across southeast Arizona Tuesday
into Tuesday evening. A few -TSRA will also occur, though less
coverage versus the -SHRA. Otherwise, cloud decks lowering to
generally 8k-12k ft msl by daybreak Tuesday and continuing into
Tuesday evening. Surface wind the rest of tonight mainly wly/nwly
10-15 kts, then variable in direction at 10-15 kts Tuesday. Aviation
discussion not updated for TAF amendments.

&&

.FIRE WEATHER...There is a chance of showers with a few
thunderstorms through Wednesday. A drying trend will start Thursday
and continue into this weekend. However, moisture will be sufficient
for a slight chance of showers and thunderstorms east of Tucson. 20-
foot winds will be mainly terrain driven less than 15 mph through
Wednesday, then westerly to southwesterly breeziness developing
Thursday and Friday afternoons.

&&

.PREV DISCUSSION /219 PM MST/...Moisture associated with tropical
system Paine located well west of the tip of Baja early this
afternoon will continue to stream northward into Arizona tonight and
Tuesday. Clouds have already been on the increase with regional
radars detecting rainfall just south of the International border.
Recent model solutions suggested that showers will start to develop
and spread into southern sections of the CWA this evening and tonight
with the initial moisture advection. Deeper moisture will move into
the region Tuesday, with more widespread showers and isolated
thunderstorms developing. Lingering moisture will interact with the
tail end of the system moving into the Great Basin around Thursday.
This will keep a chance of rain in the forecast through at least
Friday, mainly east of Tucson. Both the ECMWF and GFS models drop
this system farther south into Arizona over the weekend with rain
chances possible into early next week along with cooler
temperatures.

&&

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

&&

$$

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