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fxus65 kgjt 190522 
afdgjt 


Area forecast discussion 
National Weather Service Grand Junction Colorado 
1121 PM MDT Thursday Mar 18 2010 


Update...last few model runs have shown the 700 mb low tracking from 
north to south along the eastern edge of the forecast area tonight 
through Friday afternoon. As a result...a strong east flow will 
develop in its wake. This flow very unfavorable for heavy snow 
accumulations in the park and Gore ranges...and in The Flattops. 
Still expect good accumulations late tonight as the cold front 
moves through...with snow production decreasing after sunrise. 
Moisture still remains abundant...and instability good as the cold 
core moves overhead later Friday. But feel that warning criteria will 
not be reached. Therefore have lowered snow amounts...and downgraded 
the Winter Storm Warning to an advisory for zones 4 and 13. The 
activity has been a little slow to get underway over the southern 
zones...but feel that things should begin to pick up there within a 
couple of hours. Therefore will leave the present forecast for the 
rest of the area. 


Previous discussion... /issued 332 PM MDT Thursday Mar 18 2010/ 
short term...tonight through Saturday... 


The 12z GFS and 12/18z NAM are in excellent agreement with 
synoptic and mesoscale features...with 18z NAM accelerating its 
frontal timing to match the GFS. At 3pm...the cold front was 
south of Salt Lake and Rock Springs. It will be near Vernal at 6pm 
and near the I-70 corridor at midnight. Then it will weaken 
considerably as it works into the San Juan Mountains at sunrise. As the 
cold front drops into our northern forecast area this afternoon... 
isolated thunderstorms are possible that could produce gusty erratic 
winds. Best shot of snow will be between midnight and sunrise from 
Rabbit Ears SW to Telluride. Grand Junction will likely see some 
sunrise snowfall that may stick to the vegetation...with light accums 
in Craig-Meeker-Rifle-Eagle. 


One of the Keys to this storm is the 700mb low that forms over 
Steamboat at midnight then drops to west of Vail Pass at 6am and 
south of Crested Butte during the day on Friday. This will produce 
gusty east winds for the park/elkheads/Flattops late tonight-Friday 
and will limit snowfall there to the east side of Flattops and the 
Park Range ridge...mainly south of Rabbit Ears as per the nam12. Will 
leave warnings active there but don't expect most of the Park Range 
or Flattops to get big accumulations. Another effect of the dropping 
700mb low is a baroclinic band over the elks and west elks...with the 
area between McClure and Monarch passes probably getting the highest 
snowfall through Friday. 


North moist cold-advection flow Friday-Friday night may also bring 
enhanced snowfall from Cerro Summit SW to the Highway 550 gorge. Raised 
snow amounts for Ouray-Red Mountain Pass to 10 inches or so mainly Friday 
night as 700mb temperatures drop to -12c after midnight. 


Saturday...a cold morning in store especially along the Continental 
Divide. Added patchy morning fog to the northern and central zones. 
Light north flow will back to northwest in the afternoon. Only light 
instability showers will linger along The Divide. 


Some highlight details...extended the northwest San Juan mountain advisory 
through Friday night when The Gorge will see significant snowfall. 
Also extended the eastern Uinta Mountains until midnight Friday. 
For other highlights...expect the Park Range to see their best 
snowfall along the ridge and south of Rabbit Ears Pass. The heavier 
snowfall will favor the NE slopes of The Flattops...and the eastern 
portions of the Roan and Uncompahgre plateaus and Grand Mesa. 


Thanks to pub and slc for coordination. 


Long term...Saturday night through Thursday... 


High pressure will build into the region from the west beginning 
Saturday night. However with the ridge axis to our west we will 
remain under a nwrly flow aloft...but no precipitation is expected. Warming 
and additional drying is expected Sunday as the flow aloft 
transitions to a more westerly flow. This occurs as ridge is 
flattened by energy moving into the northern rockies. 


Some minor differences begin to show up in the medium range models by 
Monday...however sensible weather about the same with both models 
showing showers breaking out along our border with Wyoming during the 
afternoon. Model differences become more apparent Monday night resulting 
in decreasing forecast confidence. That said...trend seems to be a slight 
uptick in showers over the northern mountains as northern branch of split flow shows a 
short WV over Wyoming. This short WV feature then expected to drop southward into 
northwest Colorado Tuesday. This will result in showers becoming more widespread so 
increased probability of precipitation a little. This short WV lifts to the east Wednesday 
being replaced by a drying nwrly flow aloft. High pressure will 
rebuild Thursday for a warming trend. 


&& 


Aviation... 
breezy southwest pre-frontal winds and scattered showers are 
expected this evening with an isolated thunderstorm possible in 
advance of the next storm approaching from the north. Mostly VFR 
conditions are expected this evening. Then as the cold front moves 
into northwest Colorado later this evening showers will become more 
numerous and increase in intensity. Coincidentally VFR conditions 
will deteriorate to MVFR to localized IFR conditions. As deeper 
moisture moves into the region early Friday morning expect widespread 
MVFR ceilings and visibilities...with scattered IFR conditions. Some improvement 
is expected from the west Friday afternoon. 


&& 


Gjt watches/warnings/advisories... 


Colorado...Winter Storm Warning through 6 am Saturday morning for 
zone 10. 
Winter Weather Advisory through 6 am Saturday morning for 
zones 4/13 
Winter Storm Warning from midnight tonight through 6 am 
Saturday morning for zone 12. 
Winter Weather Advisory midnight MDT tonight through 6 PM 
Friday for zones 3/9/17. 
Winter Weather Advisory midnight MDT tonight through 6 am 
Saturday for zone 18. 
Utah...Winter Storm Warning through midnight Friday for zone 23. 


&& 


$$ 


Update...eh 
short term...joe+eh 
long term....mc 
aviation.....Mc