Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 151014

Area Forecast Discussion
National Weather Service Tucson AZ
315 AM MST Mon Jan 15 2018

.SYNOPSIS...Very warm mid-January afternoon temperatures and
generally dry conditions will persist through the upcoming week. A
pattern change late in the week into next weekend should bring much
cooler temperatures and the chance of valley rain and mountain snow
Friday night and Saturday.


.DISCUSSION...The upper weather pattern tonight is defined by a high
amplitude ridge along the western states (extending from Northern
Baja Mexico into British Columbia Canada), and a broad trough of low
pressure churning out in the Eastern Pacific.

The upper ridge is currently in the process of flattening as the
upper trough tracks east into the Great Basin and Pacific Northwest.
We will see a small shortwave split and spill over the weakening
ridge late tonight into Wednesday. There will be a general lack of
low-level moisture available for this feature. In general, we will
see the atmosphere try to moisten from the top down Tuesday night,
especially over the White Mountains. This will result in a few light
showers mainly to the higher elevations to the northeast of Tucson.

Otherwise, the main forecast concern over the next week remains a
quick moving trough of low pressure that will move through the
forecast area Friday night and Saturday. The models remain in good
agreement with the strength and evolution of this feature. The main
impact we will see with this system will be the cooler temperatures,
with afternoon high temperatures dropping 15-20 degrees between
Friday and Saturday.

This system will be rather moisture starved, so heavy precipitation
amounts are not expected. The best chance of valley rain and
mountain snow showers for Southeast Arizona will be early Saturday
morning as the lower-level cold front moves through. The upper
trough axis/cold core will lag behind, bringing with it a second
wave of mountain snow showers to the White Mountains during the day
Saturday. I would expect only a few inches of snow to the White
Mountains with this system.


.AVIATION...Valid thru 16/12Z.
SCT-BKN clouds between 20-25k ft AGL will persist into tonight.
Expect SFC winds to remain diurnal in nature and less than 12 kts
into this evening. Then, SFC wind generally to the east of Tucson
will become ELY/SELY 15-20 kts. Aviation discussion not updated for
TAF amendments.


.FIRE WEATHER...Warm and dry conditions will continue today. A
passing disturbance will create gusty easterly winds Tuesday, as
well as a slight chance of light showers mainly in the White
Mountains Tuesday night into Wednesday. Things will dry out for
Thursday before another system moves through Friday night and
Saturday, bringing with it cooler temperatures, increased
westerly/southwesterly winds, and a better chance of valley rain and
mountain snow showers.





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