980 
fxus65 ktwc 220425 
afdtwc 


Area forecast discussion 
National Weather Service Tucson Arizona 
925 PM MST sun Mar 21 2010 


Synopsis...high pressure aloft will bring dry and warm conditions 
Monday. A low pressure system will then bring a chance of valley 
rain and mountain snow showers Tuesday afternoon into midday 
Wednesday along with cooler daytime temperatures. Expect dry and 
seasonally warm conditions Thursday and continuing into next weekend. 


&& 


Discussion...latest satellite trends show significant cirrus deck 
overspreading much of the southwestern United States this evening in 
response to weak shortwave in upper level flow downstream. Weak 
upper level ridging will become more zonal overnight over Southeast Arizona in 
advance of our next system on Tuesday. Please refer to previous 
discussion about this approaching system. Cirrus will stick around 
tonight and much of the day tomorrow. Inherited forecast has low 
temperatures across the area a few degrees warmer than last night...which 
fits well with the addition of high cirrus overnight reducing 
radiational cooling. More Ely/southeasterly winds aloft will result in warm 
air advection and warmer daytime temperatures tomorrow. Short term forecast 
looks good so no changes needed. 


&& 


Aviation...high level clouds increasing through Monday as a Pacific 
trough approaches Southern California. Ceilings at or above 20k feet 
above ground level. Surface wind generally less than 10 kts across Southeast Arizona into 
Monday. Surface wind increasing on Tuesday late morning and 
afternoon to 10-15 kts with gusts to 25 kts. Aviation discussion not 
updated for taf amendments. 


&& 


Fire weather...a series of systems are expected to affect Arizona 
this week. The main affect of each system will be an increase in 
wind and a slight chance of showers. Timing on these systems is 
variable...but Tuesday and Saturday are the most likely days given 
in 
the models. The general weather pattern is beginning to transition 
into the classic springtime pattern where low pressure troughs give 
US wind but not much precipitation. 


&& 


Previous discussion...upper ridge to flatten and move eastward tonight and 
Monday. 
Fairly thick cirrus over Southern California and far western Arizona 
to move across this forecast area tonight and Monday as the ridge moves 
eastward. For now will opt for partly cloudy skies tonight and Monday... 
though cirrus may eventually be sufficiently thick to merit mostly 
cloudy skies late tonight and Monday morning. Focus of attention to 
then turn toward shortwave trough prognosticated to be over the Pacific northwest 
late Monday...as well as upper low in the southern stream off the northern Baja California 
coast. Systems expected to phase and form a consolidated upper 
trough over the southwestern Continental U.S. Tuesday. 21/12z GFS/European model (ecmwf) solutions have 
trended somewhat drier with this trough versus previous solutions. 


At any rate...will still maintain a slight chance of showers across 
the White Mountains Tuesday morning...then a slight chance of showers 
areawide Tuesday afternoon. There will be a slight chance of valley rain 
and mountain snow showers for the Tucson metropolitan area Tuesday night...as 
well as a chance of valley rain and mountain snow showers across eastern 
and southern sections. Snow levels by daybreak Wednesday expected to be around 
6000-6500 feet...with only minimal accumulations above 7000 feet. 
Storm total liquid amounts should be mostly less than one-tenth of 
an inch at valley locales. 


Shortwave ridge to move eastward across the area Thursday...then flow aloft 
to become increasingly westerly/southwesterly Friday ahead of the next shortwave 
trough entering the Pacific northwest. This system is prognosticated to move 
across the Great Basin region Friday night-Sat...with the European model (ecmwf) slower 
with the progression of the system versus the GFS. Main impact from 
this system may be some breezes especially north of Tucson late Friday. 
Low amplitude ridge to then prevail late next weekend for continued 
dry conditions. High temperatures Monday will be nearly 10 degrees f or so above 
normal. Cooler Tuesday and Wednesday with high temperatures a few degrees below normal. 
Warmer Thursday then above normal daytime temperatures Fri-sun. 


&& 


Twc watches/warnings/advisories...none. 


$$ 


Minnick/jd 
previous discussion...bf 


Weather.Gov/Tucson