339 
fxus65 ktwc 182143 
afdtwc 


Area forecast discussion 
National Weather Service Tucson Arizona 
242 PM MST Friday Jul 18 2008 


Synopsis...high pressure aloft will reside over The Four Corners 
region this weekend...while a large upper level low moves slowly 
west through Texas. These two features...combined with plentiful 
monsoon moisture spreading into Arizona...will lead to a significant 
upswing in thunderstorms this weekend...some of which may be strong 
and produce heavy rain. High pressure will remain over the central 
rockies through the middle of next week...which will keep Southeast 
Arizona in an active monsoon flow regime. 


&& 


Discussion...to sum everything up below...it is going to be a busy 
weekend...and there are no real changes with this package other than 
to accentuate the trends we have been carrying for days. In the 
short term...kemx VAD wind profile and cell motions over SW nm 
confirm the high has moved into position over The Four Corners... 
while the periphery of the large S Texas upper low is increasing upper 
level divergence just to our east. Scattered thunderstorms over the whites 
and Gila Mountains will continue to drift south-southwest into the east County Warning Area late 
this afternoon/evening where dewpoints are only in the 40s. These 
are too low to support much in terms of areal coverage...but they do 
highlight the potential for an isolated microburst or two. We will 
leave inherited...low end scattered probability of precipitation alone out east...and adjust 
the low pop timing a bit later around ktus since it will take a few 
hours for any outflows to arrive. 


The surge arrived in Yuma around 18z...and as is typical for a 
daytime arrival...the initial push is mixing out with temperatures near or 
above 110f. Once the boundary layer decouples after sunset...the 
surge will likely increase in intensity per model and MOS guidance. 
That same guidance advertises notable S winds out there tonight... 
which will carry in our west zones. Now that the surge is definite and 
underway...will raise already high probability of precipitation another 10-20 percent Sat 
afternoon into sun. One tricky thing is that a strong Gulf surge 
like this also cools the boundary layer. As a result...both NAM and 
GFS bufr soundings show a cap around 10kft over the valleys for much 
of Saturday while elevated thunderstorms try to develop. Thus the reason why 
we are not raising probability of precipitation too high from ktus west during the 12z-00z 
period. Then both models break the cap sometime between 21z and 
03z...with convective available potential energy somewhere between 1500 and 2000 and about 25kts of 
east shear. We like the 5 percent severe risk area Storm Prediction Center introduced in their 
18z outlook...and if I was more confident about sunshine tomorrow I 
would probably go with severe wording. But that is a wild card...so 
will stop just short. By Sat night/sun...with precipitation water 
approaching 2 inches...we will exchange a risk of severe thunderstorms with 
the risk of heavy rainers. We normally see a down day after a big 
mesoscale convective system evening...however all of the models reload US for Sun 
afternoon/night while we maintain a decent east-southeast flow south of the 
upper high. Once again...the models have been indicating this for a 
couple of days...so we'll merely keep the trend going. 


See no real reason to back off from a middle/high grade monsoon 
forecast through at least day 5. European model (ecmwf) and GFS both keep the upper 
low stalled over Texas while the upper high meanders between the 4 
corners and Colorado. We will probably see some cycling between up 
and down days by then...but this far out...climatology probability of precipitation plus 10 percent 
are the way to go...along with slightly below normal temperatures. 


&& 


Aviation...scattered thunderstorms dropping SW off the White Mountains... 
and isolated thunderstorms developing over NE Sonora...will place areas 
along the nm border...including kdug...under the greatest risk of 
thunderstorms and rain this evening...with cumulonimbus bases down to 060-080 and gusts near 
40kts. Between 02z and 06z...outflows from thunderstorms to the east and southeast 
may push across ktus and kols with gusts to 35kts and isolated -tsra 
with bases 090-100. Expect altocu and cirrus decks to develop over 
Southeast Arizona overnight with perhaps a shower or two. Outside of thunderstorms... 
light diurnal winds this evening may be overwhelmed by increasing S 
flow overnight into Sat morning. Aviation discussion not updated for 
taf amendments. 


&& 


Twc watches/warnings/advisories...none. 


$$ 


Pytlak 


Weather.Gov/Tucson