676 
fxus65 kslc 210945 
afdslc 


Area forecast discussion 
National Weather Service Salt Lake City Utah 
245 am MST Sat Nov 21 2009 


Synopsis...a cold front will move through the region today. A second 
stronger cold front will follow late in the weekend. High pressure 
aloft will settle over Utah early next week....and remain the 
dominant feature into the late work week. 


&& 


Discussion...water vapor loop shows a trough approaching the 
northern rockies and crossing northern Nevada. The next system 
upstream is crossing the Gulf of Alaska rather quickly in flat flow. 
400-250mb acars wind observations confirms the position of the upper 
level trough...and shows a 70-105kt cyclonic jet carving the 
aforementioned trough. Much of the jet has already exited the base 
of the trough and is over the northern rockies. 00z 700mb analysis 
shows ridging has slid to the southeast across New Mexico...with 
the cold front crossing Nevada. Advection is neutral locally...with 
+3c being reported at both kslc and upstream at kdra. 


00z kslc radiosonde observation indicates 7c of warming below 750mb...3c of warming in 
the middle levels...with little thermal change aloft during the past 
24 hours. A strong veering wind profile was present. 


GOES/RUC precipitable water indicate values have dropped to between 
a tenth and a quarter inch across the state. 


Was able to cancel the Wind Advisory for northwest Utah as winds 
have diminished with the passage of the cold front. 


Not impressed with the approaching system for two reasons. 
First...prefrontal moisture is quite limited...and second...all the 
dynamical support is lifting northeast across the northern rockies. 
Current infrared satellite imagery supports this thought...with cooling 
cloud tops across northern Idaho and western Montana...and much 
warmer cloud tops traversing northern Nevada and northwest Utah. 
This precipitation shield is trailing the 700mb reflection of the 
cold front (already across north central Utah per the ruc) 
significantly due to the struggle for saturation. 


Still...chances for measurable precipitation are quite good this 
morning across the northern mountains...tapering off further south. 
Chances across the northern valleys will remain limited...as 
reflected by the chance probability of precipitation in place. Generally thinking 1-3" of 
snow across the northern mountains today. Biggest impact in the 
northern valleys will be the noticeable temperature drop today 
compared to yesterday. 


A second system looks to have more of an impact across the region 
later Sunday and Sunday night. Model suite consisting of 
NAM/GFS/Canadian/European are showing the next system arriving 
about 6 to 12 hours slower. Have removed probability of precipitation Sunday morning...with 
a cold front arriving sometime late Sunday afternoon or Sunday 
evening. Its important to point out that guidance still has not 
locked into a preferred solution for timing or track of the system. 
This continues to be a lower than typical confidence forecast in the 
48-60 hour time frame. 


The GFS which backed away from a potential system Sunday/Sunday 
night a few days ago...continues to trend back to the colder 
European/Canadian solution. By 12z Monday...the 
GFS/Canadian/European are all indicating -13c temperatures across 
northern Utah at 700mb. At 500mb...there continues to be some 
disagreement regarding how quickly the cold air aloft moves through 
and how far south. This is reflected in the quantitative precipitation forecast from these 
models...with the more unstable European yielding greater totals 
than the GFS. Should the wetter solutions verify...advisory level 
snowfall may occur in the mountains of northern Utah...with minor 
accumulations to the floor of the northern valleys. 


Given the time frame of this second system and how the models are 
starting to agree once again on the amount of cooling at 
700mb...have decided to address the lake effect scenario Sunday 
night more closely. The avhrr sensor shows the Great Salt Lake water 
temperatures have cooled to +8c. With the -13c 700mb temperatures by 
Monday morning...the 00z NAM suggests about 500 j/kg of lake induced 
cape with equilibrium levels at about 12kft. These values are not as 
favorable as the last significant lake effect setup...though are 
high enough to start increasing probability of precipitation/qpf/snowfall across the west 
end of the Salt Lake Valley. This will be seen in the point and 
click forecast online...not making into the broader text forecasts 
at this time. 


The effect of this system will wane across the region late Sunday 
night into Monday morning as the middle level system deepens across the 
plains. 


Ridging at 700mb is expected to build into the region from the 
southwest Continental U.S. Through the middle week...with the next system 
approaching from the Pacific northwest late during the work week. 
Expect to see plenty of sunshine as temperatures warm to...then 
above climatology next week. 


&& 


Aviation...passage of a cold front occurred at 09z has switched 
winds to the northwest at the kslc terminal...and they are 
expected to remain out of the northwest for the remainder of the 
day. Although elevated wind speeds are occurring now...wind gusts 
are expected to drop below Airport weather criteria of 26 knots 
between 10-11z. Ceilings are expected to lower below 6000ft above ground level 
between 12z-15z...with a 30 percent chance of ceilings remaining near 
6000ft this afternoon. VFR is expected to prevail...but a 20 
percent chance exists that brief MVFR periods will occur due to 
low ceilings and light snow showers this morning. 


&& 


Slc watches/warnings/advisories... 
Utah...none. 
Wyoming...none. 


&& 


$$ 


Public...rogowski 
aviation...Merrill 


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