Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 241625

Area Forecast Discussion
National Weather Service Tucson AZ
924 AM MST Tue Jan 24 2017

.SYNOPSIS...Isolated showers east of the Tucson area will continue
to diminish this morning as drier air moves into the area. This will
result in cold temperatures tonight and again Wednesday night with
many locations seeing near or below freezing temperatures. A gradual
warming trend is forecast Thursday through early next week along
with dry weather and mostly sunny skies.


Mid morning water vapor imagery continues to reveal longwave
troughing across the western CONUS, with a weak shortwave trough
noted just west of California. This morning`s precipitation has
largely moved east although there are still a few light rain/snow
showers in the higher elevations/favored upslope areas east of
Tucson per regional radar imagery. We haven`t seen much in the way
of noteworthy accumulations in the higher elevations since late last
evening/this morning and given dry advection forecast later this
afternoon, I would expect this trend to continue. Locally run hi-res
models as well as the past several runs of the HRRR and NCAR
ensemble also agree with this assessment.

Forecast concerns shift toward cold temperatures tonight and again
Wednesday night. Freeze Warnings have been issued for many lower
desert locales tonight and this looks spot-on at this point. Will
take a closer look at Wednesday night`s temps once all 12Z
deterministic and probabilistic guidance arrives to see if any
locations will approach Hard Freeze criteria. At this point it
appears that won`t be the case. Otherwise I only made minor
observational changes to the short term portion of the forecast.


.AVIATION...Valid through 25/12Z.
Scattered -SHRA and mountain -SHSN this morning, then scattered
-SHSN mainly across the White Mountains northeast of KSAD this
afternoon and evening. Otherwise, cloud decks into this evening
generally 3k-7k ft AGL before clearing occurs by Wednesday morning.
Surface wind into this evening wly/nwly 8-18 kts with gusts to 25
kts. Surface wind variable in direction less than 12 kts after
25/06Z. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Scattered valley rain and mountain snow showers east
of Tucson this morning, then any snow showers will mainly be
confined to the White Mountains this afternoon and this evening. Dry
conditions will prevail area-wide Wednesday into early next week.
Expect occasionally gusty west to northwest winds today. 20-foot
winds will be terrain driven less than 15 mph Wednesday into Friday
followed by occasionally gusty east winds Saturday into Monday.


Scattered valley rain and mountain snow showers were ongoing across
Greenlee, Graham, Cochise and eastern Santa Cruz Counties east-to-
south of Tucson early this morning. These showers were only
producing light rain amounts and minimal snow accumulations.
Otherwise, IR satellite imagery and surface observations depicted
clear skies or mostly clear skies west-to-northwest of Tucson, and
partly cloudy to cloudy skies elsewhere. Surface winds were
generally from the west, and speeds had diminished to less than 15
mph across much of southeast Arizona as of 4 a.m. MST.

A broad and deep was over the western CONUS early this morning, and
a sharp ridge axis was west of the Pacific coast near 130W-135W.
Scattered valley rain/mountain snow showers will continue across
eastern sections this morning, then a drier mid-level wly flow
regime will limit any snow showers of significance this afternoon
and evening mainly to the White Mountains. Any additional rain
amounts and snow accumulations will remain minimal into this
evening. Occasionally gusty west-to-northwest winds will also occur
into this evening before winds diminish most sections this evening.

The 24/00Z GFS/ECMWF/CMC were similar with depicting the upper
trough to slowly move eastward into the central CONUS Wednesday as
the eastern Pacific upper ridge moves eastward into the Pacific
Northwest. The resultant northwesterly flow aloft will promote
precip-free conditions area-wide Wednesday into Thursday.

Thereafter, the GFS/ECMWF/CMC depicted an upper low to develop over
northern Arizona Friday as the upper ridge continues to amplify
adjacent the West coast. The position of this upper low is a sharp
departure of several previous GFS/ECMWF/CMC solutions which depicted
the development of this upper low to be much further south, or
generally over Sonora Mexico. The ECMWF in turn was more robust with
moisture associated with this system versus the drier GFS, and even
generated a few hundredths of an inch liquid accumulations across
far southeast sections late Friday night and Saturday.

For this forecast issuance and per coordination with neighboring
WFO`s, have maintained precip-free conditions Friday into Saturday.
Thereafter, the GFS/ECMWF/CMC were quite similar with moving the
upper low southward into central Sonora Mexico or the central Gulf
of California midday Sunday as high pressure aloft builds eastward
into the Great Basin region. A very dry regime should prevail across
southeast Arizona Sunday into Monday as the upper low becomes quasi-
stationary well south of the area, and a high pressure ridge remains
over the Great Basin.

High temps today will be nearly 10-20 degs cooler versus temps
achieved Monday. Expect no significant change in daytime temps
Wednesday, then highs Thursday will be about 5-10 degs warmer versus
Wednesday. The upper low will temper any warming of significance
Friday into Saturday before additional warming occurs Sunday into
next Monday.


    Freeze Warning from midnight tonight to 9 AM MST Wednesday
      for AZZ502-504>506.




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