Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Tucson Arizona
832 am MST Thursday Dec 18 2014
Synopsis...the final weather system in this latest series will move
across the region today with another round of showers developing
later this morning and continuing into the afternoon. The showers
will end this evening with areas of fog developing for Friday
morning. Dry conditions are then expected into the middle of next
week with temperatures gradually warming to slightly above normal
levels by Wednesday.
Discussion...last in the series of impulses that has brought showers
to the area is moving into the area this morning. Seeing some
clearing on the west half of the system with dense cloud cover
east. The trailing end of the cloud cover will makes its way to
Tucson over the next hour or so. However...expect clouds to fill
back in some as showers spread during the afternoon hours. The
main area for showers today nevertheless looks to be Tucson
eastward...with a west to east clearing in the late afternoon.
Expect most activity to end by late evening. This afternoon there
looks to be enough instability in place for some convectively
enhanced showers to develop...which will bring locally heavier
amounts of precipitation. The main area of concern for this
convection will be again near the mountains around Tucson. There
is less instability east but still could see some enhancement.
As things clear out tonight the concern will be fog development
especially in the eastern valleys. This is currently handled well
in the forecast...though will be give extra attention with
afternoon package. Otherwise...things will begin to dry out for
the remainder of the forecast...with the exception of Christmas
day and into the weekend. This is due to another upper level
system prognosticated to slide down the western states. Stay tuned.
Aviation...valid through 19/12z. Isolated to scattered valley
Rain/Mountain snow showers will occur mainly east of ktus through
mid-morning. Scattered to numerous valley Rain/Mountain snow
showers will then develop around 15z-17z today and continue into
this afternoon. Areas of MVFR conditions will occur especially
across the higher terrain and with heavier precipitation.
Thereafter...valley Rain/Mountain snow will decrease from west to
east this evening followed by dry conditions late tonight into
early Friday morning. Cloud decks today will generally range from
3-6k feet above ground level...though bases less than 1k feet above ground level will occur. Expect
occasional visibilities reduced to less than 1sm in fog through this
afternoon. Gradual clearing will occur this evening into early
Friday morning. Surface wind will mostly be less than 10 kts.
Aviation discussion not updated for taf amendments. Francis
Fire weather...unsettled weather will continue today in the form
of valley rain and mountain snow. Snow levels will be as low as
6000 feet with several inches of snow accumulations above 7000 feet.
The valley rain and mountain snow will decrease from west to east
this evening...then dry conditions will prevail late tonight
through next Wednesday. Expect generally light terrain driven
winds during much of the period into the middle of next week.
However...some northwest breezes may occur Monday afternoon across
the upper Gila River valley including Safford. Daytime
temperatures will be well below normal today followed by a gradual
warming trend Friday into the middle of next week. Francis
Previous discussion... /issued 215 am MST Thursday Dec 18 2014/
Discussion...just a few isolated showers around this hour with
some fog developing here and there. This will change as the morning
progresses...especially after sunrise. Final shortwave currently
rotating around the bottom of the longwave trough will move across
the region through the day resulting in precipitation blossoming
over the central part of the County Warning Area later this morning then expanding
as it moves off to the east through the afternoon. Around middle
afternoon expect showers to be occurring from the Tucson area
eastward with snow falling above about 6500-7000 feet. By
sunset...showers will be confined to Cochise...Graham and Greenlee
counties as it pushes east and ending all areas by/around midnight.
I raised the quantitative precipitation forecast a bit from what was in the grids earlier with the
focus south and east of Tucson for a general 0.10-0.30 over the
valley areas and and 0.25-0.50 over the mountains. For snowfall...I
would expect an additional 2-4 inches over most of the mountain
areas with local accumulations pushing 6 inches today into the
After the precipitation ends later tonight the skies will begin to
clear out which after 48 hours of wetness...should allow for areas
of fog to form late tonight into Friday morning so I tweaked the
grids from patchy valley fog to areas of valley fog. Potential for
some locally dense fog in the favored colder valleys east of Tucson
so tossed in patchy dense fog amongst the areas of fog. With the
fog and cold air mass still in place Friday will have below normal
high temperatures for another day.
Then Saturday a rather weak shortwave will drop southeast across the area
but this feature should only produce a few middle and high clouds as it
passes. However...this will transition the flow aloft to northwest from
Sunday through Wednesday with heights rising resulting in mostly
clear skies and warming temperatures reaching a few degrees warmer
than normal by Tuesday and Wednesday.
Beyond that...long range models beginning to advertise the potential
for another decent trough to move through or nearby the area next
Thursday and/or Friday for another chance of precipitation.
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