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Area forecast discussion 
National Weather Service Riverton Wyoming 
240 am MDT Sat Mar 20 2010 


Short term...today through Monday night 
convective snow is lingering across portions of central Wyoming on 
both sides of The Divide. Not all the convection died after sunset 
as the reinforcing vorticity maximum that was due to track south-southeast across Wyoming at 
00z today is keeping enough of a negative lapse rate to keep snow 
showers going. They have been enough to make roads slick after the 
sun went down where they occurred...especially at bpi. Very little 
if any snowfall will be around by sunrise today as temperates will 
be rising at 500mb and will be increasing at 700mb after 
12z...effectively shallowing out the instability. 


Decided to remove fog from the forecast this morning and delay it 
until Sunday morning after we have given the inversions a chance to 
establish themselves. With a 1035mb surface high ovhd and a positively 
tilted flattening ridge building into the area from the west expect 
mainly dry and warming conditions through the weekend. A surface Lee 
side trough will develop tonight with southwest winds increasing 
from the Red Desert to Casper tonight and Sunday. 


The next strong vorticity center now spinning over 40n/140w will 
track northeast up and over the long wave ridge over the western 
United States and then track across southern British Columbia and then head south-southeast 
across eastern Idaho just like the feature that is now exiting the 
area...another potential eventual Lander show. The latest GFS/NAM 
runs appear to track this system across southwest Wyoming Tuesday 
morning...closer to the rest of our County Warning Area then previous runs. The ec 
splits this system more. Monday is shaping up to be a transition day 
with precipitation breaking out in the northwest as the Pacific front 
tracks southeast. Lifted indices will be negative ahead of the front 
so will include isolated lightning. Will leave likely probability of precipitation for the northwest for 
now. This next system will not have as a strong a thermal ribbon 
with 700 mb temperatures dropping to -9c by 12z Tuesday as opposed to -15c with the 
exiting system. 


Long term...Tuesday through Saturday 
the upper trough digs into Utah on Tuesday and into New Mexico Wednesday. 
Cold front moves across the area as well Tuesday. Will maintain the 
precipitation chances Tuesday. Tuesday night will see precipitation decreasing from north 
to south and have adjusted forecast to show this trend. There may be 
lingering precipitation in the south zones Wednesday so went with slight 
chance. Dry weather expected elsewhere Wednesday. Dry Wednesday night and 
Thursday for most places. A few rain and snow showers may spread 
into the northwest mountains and northern Bighorn mts Thursday afternoon as 
moisture increases ahead of the next weather system. GFS is faster 
than the European model (ecmwf) model with this precipitation on Thursday PM. This weather 
system moves across Montana Thursday night and Friday. The 00z GFS 
is about 12 hours faster than the 00z European model (ecmwf) at 00z Sat with the 
system moving across Montana. Will keep precipitation chance across the far 
north Thursday night and Friday siding with European model (ecmwf) model here. Have brought low 
precipitation chance to the western mts Friday afternoon as moisture returns 
to that area ahead of another storm system. Will keep the forecast 
dry on Friday for the rest of the area. Next upper trough is to impact 
the area next Friday night and Saturday. The European model (ecmwf) is stronger with 
the trough than the GFS. Will side with the European model (ecmwf) model for now as 
it has been better at showing these troughs digging into the Great 
Basin over past few weeks while the GFS catches up with time. As for 
temperatures...colder highs on Tuesday then warmer highs Wednesday and Thursday. Slightly 
cooler Friday and next Sat with the impending unsettled weather. 


&& 


Aviation... 
final disturbance in a series finally slipping south of the state 
early this morning. The tail end of another disturbance swinging 
through the northern plains has helped to initiate continued cloud 
cover and very light snow showers in the central basins last night 
and early this morning. Both of these systems will exit the area and 
not be players in the weather story today. Drying middle levels and 
ridge nudging in from the west will limit cloud development today. 
Gusty west to southwest wind generally 15-25kts to kick-in during 
the morning hours at krks and kcpr...with similar speeds from the 
west-northwest at kbpi and kpna by 20z. These winds should 
diminish to around 8-13kts around sunset. Elsewhere...little wind 
and VFR conditions to prevail through at least 12z/Sunday. 


&& 


Fire weather... 
drier weather is in store for the weekend along with 
a warming trend. A temperature inversion swill establish itself in 
the basins and valleys Sunday morning. Southwest winds will increase 
from the Red Desert to cpr tonight and Sunday coincident with a 
developing Lee side trough. Another Pacific weather system will 
approach Wyoming from the northwest Monday with precipitation 
breaking out in northwest Wyoming. The airmass will become unstable 
Monday along with increasing wind. Precipitation will overspread the 
rest of central and western Wyoming Tuesday. 


&& 


Riw watches/warnings/advisories...none. 


&& 


$$ 


Short term...lipson 
long term...Arkansas 
aviation...cnj 
fire weather...lipson