Area forecast discussion 
National Weather Service Tucson Arizona 
209 PM MST Wednesday Dec 17 2014 

Synopsis...trough of low pressure will continue to bring valley 
rain and mountain snow to the area through tomorrow night. Then 
expect dry conditions with a gradual warming trend into Christmas 


Discussion...beginning to see some enhancement in precipitation 
thanks to an impulse in the flow. The energy with this impulse 
will generally stay north of bulk of precipitation 
will be there. That being said there is enough moisture in place 
for continual off and on showers throughout the evening. 

After this impulse moves through this evening expect to see 
another lull in showers after midnight. Therefore...have continued 
the lower pop from previous package...though have slightly 
increased probability of precipitation across the northern third of the forecast area for 
this period. This is to account for the additional moisture expect 
north of Tucson and any lingering energy. Have also added some fog 
mention for Pinal County during this time. Not expecting much in 
the way of dense or major visibility reduction as radiative 
cooling will be limited. 

Then tomorrow morning another impulse will slide through the area. 
This time the bulk of the energy will be across northern Sonora 
Mexico. This will keep the best quantitative precipitation forecast values across the southern 
half of the area tomorrow. As this impulse exits the region so 
does the moisture...though gradually. By midnight Friday most of 
the shower activity will be gone though have reintroduced low end 
slight chance probability of precipitation to the higher elevations out east. 

Friday the upper ridge axis shift over head allowing for the area 
to dry out and warm up. Though...the warm up will be gradual. With 
the location of the upper high late week and into the weekend 
should keep mostly northwest flow in place. Guidance has trended 
away from the weekend shortwave energy having any impact on the 
area as they keep trending further east with it. There is some 
indication that the ridge may begin to break down over the area 
around Christmas as a Pacific trough begins to dig across the 
western states. Stay tuned. 


Aviation...valid through 19/00z. 
Isolated to scattered valley Rain/Mountain snow showers will 
continue through this evening with perhaps an isolated rumble of 
thunder. Broken-overcast clouds at 4-6k feet above ground level with multiple broken-overcast layers 
above. Expect local MVFR conditions at generally 2-3k feet especially in the 
vicinity of showers through this evening. Surface winds will be generally 
westerly/southwesterly at 8-15 kts with a few gusts up to around 20 kts possible 
in far southern areas this afternoon. Winds then becoming diurnal and 
terrain driven from late this evening through tomorrow afternoon. 
Another round of isolated to scattered valley Rain/Mountain snow 
showers will occur Thursday with sky conditions quite similar to 
today. Aviation discussion not updated for taf amendments. 


Fire weather...unsettled weather will continue through Thursday in 
the form of valley rain and mountain snow with somewhat of a break 
in the precipitation tonight. Snow levels will be as low as 6000 feet 
with accumulating snows occurring generally above 7000 feet and 
several inches possible. Breezy southwesterly winds near the 
international border this afternoon should taper off during the 
early evening hours with lighter westerly winds occurring Thursday. 
Mostly dry conditions are expected through the coming weekend with 
plenty of passing higher clouds and fairly light north to northwest 


Twc watches/warnings/advisories...none. 



fire weather...81 

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