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fxus65 kpsr 210336 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
835 PM MST Thursday Nov 20 2008 


Synopsis... 
high pressure aloft will continue to weaken with somewhat lower 
temperatures Friday...then little change by Sunday. High cloudiness 
will gradually increase across the entire area tonight through 
Saturday. Strong surface high pressure building over New Mexico will 
result in breezy to windy weather from the east in southwest and 
especially south-central Arizona including the greater Phoenix area 
late tonight and Friday. 


&& 


Discussion... 
variable amounts of Pacific high clouds were moving through the 
forecast area (southeast California to south central az) at 03z...a function 
of increasing westerly flow aloft as several Pacific storms move 
well to the north of Arizona. The only noticeable change in the weather 
pattern for much of south central Arizona (including the greater Phoenix 
area) Friday morning and afternoon will be gusty southeast winds. 
Peak gusts of 30 or 35 miles per hour are possible (starting around 10 am local 
time and continuing through 6 pm)...along with local areas of 
blowing dust (especially in Pinal County around Coolidge and Casa 
grande). Gusty winds will be influenced by a cold Canadian airmass 
dropping south into central and eastern New Mexico...sort of a 
backdoor cold front into eastern Arizona so to speak. 


Otherwise periods of high clouds are expected off and on into the 
weekend with continued above normal temperatures. Current dry 
forecasts through the weekend look OK. No updates planned. Previous 
discussions below still apply. 


(Note: some thoughts for the possible abrupt weather change 
Thanksgiving day. Both GFS and European medium range models are 
coming around to similar but still different timing/intensity 
solution on a massive low pressure system forecast to develop about 
600 miles west of San Diego by next Tuesday evening. A strong and cold 
system so far south like this...has the potential to pick up and 
send a large plume of subtropical moisture...possibly copious 
moisture...into southwest and south central Arizona. A quick look at some 
current medium range model forecast parameters...including strength 
of low level upslope wind...850/700 mb dewpoints...and 700 mb 
temperatures...and matching them with previous analog heavy rain 
studies...show a potential (shaded toward the high side) from a 1/2 
inch of rain at Sky Harbor Airport to nearly 2.00 inches over the 
mountains just north of Cave Creek/Carefree (bordering Yavapai 
county)...starting from late next Wednesday night through Thanksgiving 
afternoon. We know this is looking way ahead (7 days out in 
fact)...and upstream Pacific wave to wave interactions have the 
ability to derail model timing and intensity patterns...but keep 
these values in mind if the current medium range models continue to 
remain consistent and play out on Thanksgiving day). Again...just 
some random but exciting thoughts using current forecast parameters 
and some local heavy rain studies. 






Previous discussion... 
strong surface high pressure building over New Mexico will be 
responsible for the forecast increase in breezy to windy weather 
from the east in southwest and especially south-central Arizona 
including the greater Phoenix area late tonight and Friday. This 
could lead to patchy areas of blowing dust south and southeast of 
Phoenix Friday. 


A very weak low pressure system was located west of northern Baja California 
California near 135 degrees west this afternoon. High cloudiness 
ahead of the low was moving east toward the Southern California 
coast this afternoon. The low is expected to move slowly east toward 
the northern Baja California California coast through Friday...then northeast 
toward Southern California Saturday and Sunday. High cloudiness from 
the southwest ahead of the low pressure system is expected to slowly 
increase over our area tonight through Saturday. The high cloudiness 
is then expected to decrease from the west Saturday night with a 
return to more sunshine Sunday. 


Extended forecast... 
there is uncertainty about the extended forecast in terms of how 
close or far away from the Southern California coast a Pacific low 
pressure system will be during the middle of next week. There is a 
significant possibility that the low will stay off the coast then 
move mainly south of our area. Thus...it may remain dry and stay 
warmer than in current forecast during the middle of next week. With 
so much uncertainty...will leave forecast as is for now...and let 
midnight shift look at situation so as not to flip flop forecast. 


Previous discussion from 300 am MST... 
for the extended period...Monday through Wednesday...the GFS and 
European have come into surprising agreement with the forecast 
solution and as such confidence is growing that we will be seeing a 
significant change in the weather pattern by the middle of next 
week. Both forecasts forecast an upper low to develop off the Southern California 
coast by Monday afternoon...and this low is then forecast to slowly 
push east and approach the California/northern Baja California coast by Wednesday. No 
weather is expected with this system Monday through early Tuesday...but 
we should see variable middle/hi clouds spread across the area ahead of 
the low. By Tuesday afternoon...GFS advertises a frontal band to move 
into southeastern California ahead of the main low...with quantitative precipitation forecast spreading into our 
western County Warning Area. Have added slight chance probability of precipitation from the Colorado River west on 
Tuesday...then spread the threat for showers into the southwestern Arizona 
deserts overnight Tuesday night. Although the upper low is forecast to 
remain just offshore through Wednesday afternoon...it looks as if a 
substantial subtropical moisture tap will develop and spread 
considerable moisture into our County Warning Area ahead of the main low on 
Wednesday. As such have raised probability of precipitation quite a bit Wednesday...into the 20-30 
percent range...for the entire County Warning Area. Also lowered maximum temperatures several 
degrees to near or a bit below seasonal normal values. It appears 
that the most significant precipitation and cooling will occur beyond the 7 
day forecast period...when the main low finally moves inland across 
Arizona. 


&& 


Aviation... 
phx area... 
light surface wind with occasional high clouds are expected through 
16z Friday. However...increasing lower atmospheric winds...roughly about 
120 degrees at 20 to 325kt in the 1 to 3 thousand foot layer...and 
above the surface temperature inversion...will increase after 10z and 
continue through 15z. As a result...low level wind shear (llws) is 
expected at both kphx and kiwa airfields Friday morning. After 17z 
Friday...the surface temperature inversion will break and gusty 
southeast winds 15 to 27 knots are expected between 18z and 22z 
Friday...diminishing somewhat after 22z into the 15 knot range through 
02z Sat. 


Southeast California/SW Arizona... 
scattered-broken high clouds and light winds will be common into Friday 
afternoon (18z). The exception is at knyl where easterly breezes in 
the 10 to 15 knot range are possible between 16z and 21z Friday. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Saturday through Wednesday... 
as an upper level low pressure system develops off the California coast...we 
will begin to see changes in the weather pattern. Cloud cover will 
increase...temperatures will begin to cool into the 70s and the 
chances of precipitation will begin to increase starting in 
southeastern California then spreading eastward into the southwestern Arizona 
deserts by Wednesday morning. As a result...humidity levels will 
begin to trend upward. Winds will remain light for the beginning 
portion of the week. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


Discussion...vasquez/Ellis/cb 
aviation...vasquez 
fire weather...paddock 

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