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Area forecast discussion 
National Weather Service Tucson Arizona
932 am MST sun Jan 25 2015

Synopsis...seasonal daytime temperatures and easterly breezes will
prevail the remainder of the weekend. Increasing moisture from
the south will bring thickening clouds and showers to southern
Arizona Monday into early Tuesday.

&&

Discussion...prominent veil of cirrus...roughly at 20 kft...is
sitting over much of Southeast Arizona. GFS suggests the moist air
will remain in this layer through the rest of the day...with a few
breaks possible toward central Arizona. Thus...made a quick update
to the forecast to increase cloud cover for today.

BUFKIT forecast soundings for Tucson based on the NAM also shows the
moist layer aloft...generally around 200 to 400 mb. The soundings
suggest middle level moist air...around 600 to 800 mb...will advect
into the area starting late this evening. The column should then
become sufficiently saturated...almost down to the surface...to
produce some showers by Monday middle day. Sref precipitable water
values range from 0.93 to 1.14 inches...which is a similar range if
you compare the GFS...NAM and European model (ecmwf).

Current satellite imagery suggests the upper low...currently off the
Baja California peninsula...may be taking a slightly further west track than
originally expected. In addition...models are not highlighting any
significant low level forcing features to really maximize precipitation
production...so still leaning toward a high likelihood...but
generally low accumulation event...maybe quarter to half inch rain
totals...for tomorrow through Tuesday.

Jjb

&&

Aviation...valid through 26/12z.
VFR conditions will prevail through the period. High clouds will be
on the increase from south to north over the next 24 hours with
scattered middle cloud starting to filter in from the south late
today. Easterly surface winds will continue through the period.
Speeds will generally be less than 12 kts with occasional gusts up
to 20 kts through much of today. However...late tonight and into
early Monday morning speeds will increase to around 10-15 knots with
gusts to around 20-25 knots. Aviation discussion not updated for taf
amendments.

&&

Fire weather...gusty east to southeast winds will occur for much of
the area over the next couple of days. Speeds will be 10-15 miles per hour with
a few higher gusts mainly during the daytime. Dry conditions will
persist through today. An increase in moisture and a high potential
of showers will occur Monday morning through Tuesday morning. The
best chance for showers will be Monday afternoon into Monday night.
Dry weather will return Tuesday afternoon through Thursday before
another disturbance brings a threat of showers once again late next
week and into the weekend.

&&

Previous discussion....discussion...water vapor imagery and 25/00z
upper-air plots indicate that the upper low is spinning offshore of
the south central Baja California peninsula. Meanwhile...infrared satellite imagery
is showing extensive cloudiness spreading northward ahead of this
system over much of Mexico and into extreme southern Arizona and
southwest New Mexico. Last evenings ktwc 25/00z sounding only showed
a precipitable water value of 0.14 inches. The satellite derived
blended total precipitable water product showed dry air across much
of the Desert Southwest...but higher precipitable water air lurks to our south
across southern Baja California...the southern Gulf of California and southern
Sonora...with values of around 0.75 - 1.0 inches. Models indicate
that this moist air to our south will work its way northward through
early Tuesday with precipitable water values increasing to around a half to three
quarters of an inch over the White Mountains of east central Arizona
to around 1.25 inches over the western deserts of central and
western Pima County.

As the moisture spreads to the north the first isolated showers
should develop late tonight into early Monday morning over south
central Pima County and Santa Cruz County...generally along the
border from Nogales to Organ pipe. As the day begins Monday through
about midday...the showers will increase dramatically from the
southwest with scattered to numerous showers from Tucson to Nogales
and westward to Ajo and beyond. A very steep gradient of rain
chances will occur as the showers begin to develop early Monday with
very little threat for showers over the White Mountains of northern
Greenlee County.

By Monday afternoon the showers will spread toward the north and
northeast and by Monday evening numerous showers will be present
over Desert/Valley locations...with precipitation a given over most
of the mountains of Southeast Arizona. Perhaps only the mountains
along the New Mexico border will see lesser chances...but still
scattered to numerous showers in that area.

After midnight Monday night...the upper trough should lie across the
Pacific waters of the Southern California coast and into southwest
Arizona. The initial vorticity lobe that will trigger showers during the
day on Monday and into Monday evening will be over northeast Arizona
and into western New Mexico by late Monday evening. As a result the
showers will continue to make northward progress overnight into
early Tuesday as this system lifts into the Great Basin (nevada and
utah) by midday Tuesday. With such an abundant amount of moisture
available with this system...quantitative precipitation forecast values will be respectable with
storm total amounts from early Monday through midday Tuesday
generally around a half inch from Tucson to Nogales and westward
across much of central and western Pima County. Lesser amounts
expected farther east across Greenlee...Graham and Cochise
counties...but still around a third to a half inch. This system will
be a warm system...so snow levels are not expected to fall very
much. Snow levels will perhaps fall to around 9000 feet...so at this
time the only potential for any appreciable snow would be over the
White Mountains of northern Greenlee County where an inch or so is
possible.

Once this system lifts to the north and east...dry weather will
return late Tuesday through much of Thursday before another Pacific
system will likely bring another shot of precipitation to the region
beginning late Thursday and continuing through the weekend. For now
I only have chance category probability of precipitation in the forecast...mainly Friday
through Saturday. However...as this system gets closer in both space
and time...the rain chances will likely need additional
modification...so stay tuned.

For Tucson...highs today will generally be around 7 to 9 degrees above
normal. Slightly below normal highs expected Monday and slightly
above normal for Tuesday. Thereafter...highs around 6 to 8 degrees
above normal Wednesday and Thursday and a few degrees below normal for
Friday and Saturday. Low temperatures will be above normal through
the entire forecast period...with lows ranging from 8 to 12 degrees
above normal Monday through Friday mornings and 5 to 7 degrees above
normal for Saturday morning.

&&

Twc watches/warnings/advisories...none.

&&

$$

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Mollere

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