274 
fxus65 ktwc 191558 
afdtwc 


Area forecast discussion 
National Weather Service Tucson Arizona 
858 am MST Friday Mar 19 2010 


Synopsis...a strong trough of low pressure will pass through 
Arizona today. Expect a slight chance of showers and thunderstorms 
areawide this afternoon...with higher chances for showers across the 
White Mountains and for areas north and east of Tucson. Noticeably 
cooler today with temperatures returning to near normal levels for 
middle March. Afternoon temperatures remain below normal on Saturday 
followed by a rapid warming trend Sunday and Monday. 


&& 


Discussion...water vapor imagery shows a shortwave trough which 
currently extends from southwestern Wyoming through Utah and into 
northwest Arizona and extreme southeast California. Visible imagery shows 
extensive middle level clouds from northeast and east central Arizona 
to southwest Arizona. Over my forecast area cloud cover is generally 
north of a line from Sells to Tucson to Safford. A few breaks in the 
cloud cover over eastern Pinal County...but this area should fill in 
quickly. Most of Cochise County is cloud free at this time. 


Radar mosaic shows a large batch of isolated to scattered 
showers/virga over the Phoenix area and extending back SW toward the 
western sections of my forecast area near Ajo. All of these 
showers/virga are moving to the east or east southeast at around 15 
to 20 miles per hour. 


The 12z ktwc sounding shows an increase in moisture from this time 
yesterday with the current reading of 0.35 inches compared to only 
0.13 inches Thursday morning. The lifted index is still on the plus 
side with a value of near 2...but this is less stable than the 5 
from yesterday. Models are showing lifted indice's of around zero to minus 2 
moving across the area in association with the trough today. With 
that said...think the pop forecast for this afternoon still looks 
good with slight chances for deserts and valleys and chance to 
likely for the mountains. Deserts/valleys could see a few sprinkles 
this afternoon...but mostly virga. 


&& 


Aviation...middle level clouds will work into western and central 
areas by middle morning and thicken through the day today and spread 
eastward across the remainder of the forecast area. Expect a slight 
chance of showers and thunderstorms this afternoon and evening as a 
cold front moves through the area. Best coverage will be over the 
White Mountains. Southerly/southeasterly surface winds generally less than 10 kts 
this morning...increasing to 15 kts with gusts over 25 kts Tucson 
eastward by afternoon and continuing through this evening. Aviation 
discussion not updated for taf amendments. 


&& 


Fire weather...winds are expected to be stronger than yesterday as 
a system moves through The Four Corners region and then eastward 
into New Mexico. This system will also bring a chance of showers 
mainly to the White Mountains...though locations that experience 
strong gusts will likely see a virga shaft overhead. Otherwise... 
high pressure aloft will bring a warming trend over the weekend and 
early next week. Another Pacific storm system will then result in 
breezy conditions and a chance of showers Tuesday and Wednesday of 
next week. 


&& 


Previous discussion...potential for some interesting weather this 
afternoon as strong trough swings through Arizona. Rather vigorous 
system stretches from near Las Vegas to near Los Angeles...showing 
up best on water vapor imagery this morning. This is a moisture 
starved storm...with only a few clouds evident on infrared imagery across 
western Arizona/Southern California near the cold front this 
morning. Trough and cold front expected to progress southeast across 
the County Warning Area through this afternoon. Evening shift wisely expanded slight 
chance probability of precipitation to include most of Southeast Arizona for today and 
included thunder as well. Built this morning's forecast on these 
trends...increasing probability of precipitation a bit more after 18z to near 20% for most 
lower elevations and well into the chance category for the higher 
terrain. Did trim inherited probability of precipitation back significantly especially 
southeast of Tucson this morning...with mostly sunny skies expected. 
This sunshine will give US a warm boundary layer to work with as the 
colder air aloft associated with the trough passes overhead this 
afternoon. Instability increases with lifted indices falling to near 
-1 or -2 by afternoon and cape of 300-400 j/kg. Convection will be 
encouraged this afternoon as the trough passes by...the question is 
what we will actually get. We are quite dry and will remain so 
through the day...with surface dewpoints near 30 at best. May end up 
with a lot of virga and sprinkles...and associated gusty 
winds/dust...more than anything else. Kept probability of precipitation (for measurable 
rain) low in the slight chance category for lower elevations...with 
higher probability of precipitation on the higher terrain where the drier boundary layer is 
less of a factor. Quantitative precipitation forecast grids follow 06z NAM...with just a couple 
hundreths at most places. An inch or two of snow likely in the 
whites today...with precipitation expected to be much more extensive near 
the Mogollon Rim/whites as the trough passes today. Thunder is 
included in all zones today as any precipitation should be 
showery/convective especially with the help of sunshine this 
morning. Increased probability of precipitation for the evening hours through 06z for the 
eastern zones as system departs. 


Noticeably cooler today as cold front and trough swing through... 
falling back to just below normal for March. Temperatures expected 
to remain below normal Saturday...behind trough departing to the 
east and under northwest flow aloft. Expect a rapid warming trend 
Sunday and Monday as ridge phases overhead...with lower deserts well 
back into the 80s again Monday afternoon. The warming trend will be 
cut short again next week...as some flavor of trough passes through 
during the Tuesday/Wednesday time frame. Recent trends suggest this 
will be more of a wind maker for US with a more northerly 
trajectory...we are getting into the time of year to expect such 
things. Still...left inherited probability of precipitation completely alone in the extended 
after coordination with weather forecast office Phoenix/Flagstaff...and will let day 
shift take another look. 


&& 


Twc watches/warnings/advisories...none. 


$$ 


Current discussion...mollere 
previous discussion...Turner 
fire weather/aviation...smr 


Weather.Gov/Tucson 

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