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Area forecast discussion 
National Weather Service Tucson Arizona
912 PM MST Tuesday Jan 27 2015

Synopsis...dry conditions with well above normal daytime
temperatures through Thursday. Valley rain and mountain snow will
then occur Thursday night into Saturday as a storm system moves
across the area. Expect much cooler daytime temperatures with this
system. Dry conditions will return early next week along with a
warming trend.

&&

Discussion...the area will be in between weather systems tomorrow
and part of Thursday which will result in more well above normal
high temperatures. This will change by late Thursday...though...as
models showed a system entering the region and bringing mountain
snow and valley rain to Southeast Arizona mainly Thursday night into
Saturday. Pwater values were prognosticated to be well above normal for
this time of year by Friday. Sref pqpf values suggested the
possibility of significant valley rain from Tucson to Nogales
eastward...along with heavy snowfall in the White Mountains likely.
Current forecast looked on track for now so no updates necessary
this evening. Please refer to the additional sections for more
details.

&&

Aviation...valid through 29/06z.
Expect a few to broken clouds generally around 5-9k feet above ground level and
surface wind less than 10 kts through Wednesday afternoon. Patchy fog
may occur from ktus vicinity nwwd into southern Pinal County between
09z-15z Wednesday. Aviation discussion not updated for taf
amendments.

&&

Fire weather...dry conditions will prevail through Thursday
morning. A storm system will then bring valley rain and mountain
snow late Thursday through Saturday. Expect significant rainfall to
occur late Thursday night into Friday night. Significant snowfall
should then occur late Friday into Saturday. The White Mountains and
Mount Graham should receive the most snowfall with lesser amounts
across the Catalina and Rincon Mountains near Tucson. Dry conditions
will return early next week.

&&

Previous discussion...visible satellite imagery indicated clouds were
gradually decreasing across Southeast Arizona during the past 1-2 hours in
response to subsidence behind a departing middle-level trough axis.
Expect mostly clear skies to partly cloudy skies tonight. There is
enough near-surface moisture to justify patchy fog late tonight into
early Wednesday morning from Tucson westward into the tohono o'odham
nation...and nwwd into southern Pinal County. Dry conditions will
continue into Thursday morning...although clouds will increase
especially Wednesday night and Thursday ahead of a deepening low pressure
system west of the area.

Various 27/12z nwp model solutions point toward a significant precipitation
event for Southeast Arizona starting Thursday night and continuing into Sat. Deep
layer moisture transport is prognosticated to occur from well SW of Baja California
California northeastward into Southeast Arizona. 27/12z GFS total precipitation water value for
Friday morning is prognosticated to be about 1.10 inches. This precipitable water value is 3-4
Standard deviations above normal. A 559 dm low is prognosticated via the
GFS/European model (ecmwf) to be centered near Yuma Arizona midday Friday. There is the
potential for some lightning strikes to occur Friday given the GFS cape
values...and the negative-tilted system structure. However...have
precluded the depiction/mention of thunder for now. The upper low
will then move very slowly eastward into Southeast Arizona Sat. The upper low is then
prognosticated to fill and move south to southeast of the area sun.

For this forecast package...probability of precipitation were increased late Thursday night into
Friday night with the expectation this will be the period when
widespread rainfall will occur. Similar rainfall coverage is quite
likely to occur Sat...though with markedly lesser amounts. Snow
levels by midday Friday are forecast to generally be 8000-8500 feet...
with snow levels falling to around 6000-6500 feet by Sat evening.

Preliminary storm total liquid amounts will range from 0.50 - 1.25
inches in the valleys...with locally heavier amounts possible.
Preliminary storm total snow accumulations may approach 2 feet in
the White Mountains above 7000 feet...with 12-18 inches of snowfall
above 7000 feet further southwestward across the Pinaleno Mountains including
Mount Graham. Lesser snow accumulations should then occur across the
Catalina/Rincon Mountains near Tucson. A few lingering showers may
yet occur sun particularly east to south of Tucson. Thereafter...dry
conditions will prevail Sun night-Tue.

High temperatures across the area Wednesday-Thursday will average about 5-10 degrees f
or so above normal. Expect nearly 10-15 degrees f of cooling Friday
followed by slightly cooler temperatures Sat. A warming trend will then
occur sun-Tue.

&&

Twc watches/warnings/advisories...none.

&&

$$

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Previous discussion...Francis

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