655 
fxus65 kgjt 200551 
afdgjt 


Area forecast discussion 
National Weather Service Grand Junction Colorado 
1151 PM MDT Friday Mar 19 2010 




Update...as the low pressure center shifts southeast of the San Juan 
Mountains late this evening...activity will continue to wind down 
across the area. Satellite images indicate that the enhancement that 
moved west of The Divide late this afternoon/early evening has waned 
as northwest flow spread in from the northwest. While snow will 
continue to be likely beyond midnight along The Divide...it generally 
will be light and will continue to taper off. Little to no additional 
accumulations are expected. The exception to this will be over the 
San Juan Mountains which are in closest proximity to the low. Snow... 
heavy at times...will continue through late tonight. Therefore except 
for the San Juans...zones 18 and 19...I will drop all other hilites 
at midnight. I will let hilites for zones 18 and 19 continue for 
now. 




&& 


Update.../issued 506 PM MDT Mar 19 2010/ 
reports of heavy snow falling across the northwest San Juans 
continue to filter in with amounts well over one foot near Ouray 
since 5 am this morning. Spotters indicating heavy snow continuing 
to fall and radar supports additional activity through at least middle- 
evening as convection becomes a big player in the snowfall. 
Silverton avalanche center also reporting snowfall rates near 3 
inches per hour in The Gorge between Ouray and The Summit of Red 
Mountain Pass. As a result...have upgraded the advisory for the 
northwest San Juans to a warning and also included the southwest 
San Juans into an advisory as convection tops the higher mountains 
and moves south into this area. Remainder of the forecast remains on 
track. 


&& 


Previous discussion.../issued 351 PM MDT Friday Mar 19 2010/ 


Short term...tonight through Sunday... 


The upper low is dropping into New Mexico this evening. Jet 
support is over southeast Utah this early evening. Convection this 
afternoon is most widespread under this upper divergence even 
producing a few lightning strikes. These showers will likely 
continue into the evening hours...so have extended the advisories 
for the central Colorado Mesa and plateaus until midnight. 
Otherwise snowfall continues to favor the central mountains. 
Cyclonic flow will slowly shift to more orographic north overnight. 
This will favor the north-facing slopes closer to the Continental 
Divide. Even The Flattops and Park Range will likely see some new 
snow accumulations. 


The 500/700mb -32c/-14c cold core will be nestled over the Park Range 
by sunrise Saturday. Went below guidance for low temperatures 
tonight. And snow ratios will increase to 20 or 30:1 late tonight 
for nice right-side up ski snow Saturday morning. Morning fog is 
possible where significant showers occur this afternoon and evening. 


High pressure begins to build in from the west on Saturday but 
isolated to scattered snow showers will continue along Colorado 
north-facing slopes. Snow accumulations will be light as the 
instability layer shrinks from top down through the Day. 


Valley inversions will become strong Saturday night into Sunday with 
warming aloft providing the cap under the building ridge. 


Long term...Sunday night through Friday... 
high pressure will be over the region Sunday night...but the ridge 
axis will be to our east providing a nearly flow aloft. The ridge 
quickly translates ewrd Monday as the next short wave approaches the 
region from the Pacific northwest. Main weather effect will be increasing southwesterly 
gradient winds. This short wave deepens and closes over southern Idaho 
Monday night as jet energy increases. Its associated baroclinic 
band moves into NE Utah overnight and approaches extreme northwest Colorado. 
Current scattered mountain probability of precipitation look reasonable. Tuesday this closed low 
will sink southward into Utah at the same time a weak short WV over Arizona 
looks like it will pull moisture northward into our County Warning Area. All the while 
dynamic forcing increasing as proximity of upper low gets closer. 
Expect showers to be more widespread and have adjusted probability of precipitation up 
some...realizing subsequent model runs likely to be different with 
timing/intensity. The upper low then moves to near the 4 corners 
region Tuesday night providing the best chances of precipitation over our County Warning Area. 
Showers will continue Wednesday in nwrly flow and wrap around moisture 
favoring the northern and central mountains near The Divide. 


Drying is expected in a nearly flow Thursday as a weak ridge builds to 
our west. The ridge shifts ewrd Tuesday only to get flattened by 
storm energy moving into the Pacific northwest. 


Temperatures are expected to be near seasonal normals for all but 
Tuesday and Wednesday when above mentioned storm system will effect 
the region. 


&& 


Aviation... 
convective instability will produce locally heavy showers and 
IFR ceilings/visibilities through 03z. Then VFR conditions will become more 
widespread over eastern Utah and far western Colorado...with local 
MVFR ceilings/visibilities in showers until 12z. Showers will linger 
vicinity of the Continental Divide for MVFR local IFR ceilings/visibilities 
becoming isolated by Saturday afternoon. In addition patchy valley 
morning fog is possible especially in the I-70 corridor for IFR 
visibilities through 15z. 


&& 


Gjt watches/warnings/advisories... 


Colorado...Winter Weather Advisory until 6 am MDT Saturday for zone 019. 
Winter Storm Warning until 6 am MDT Saturday for zone 018. 
Utah...none. 


&& 


$$ 


Update.......jdc/eh 
short term...Joe 
long term....mc 
aviation.....Joe