Area Forecast Discussion

FXUS65 KPIH 102053
Area Forecast Discussion
National Weather Service Pocatello ID
153 PM MST Sun Dec 10 2017
.SHORT TERM...Tonight through Wednesday. A ridge of high 
pressure was anchored over the West early this afternoon and 
numerical models show some retrogression westward toward the coast 
through Wednesday. Thus we are expecting little change to the daily 
weather cycle which features valley fog/low cloud development 
overnight and cold inverted temperature profiles during the 
least until it breaks sometime Friday afternoon (more on that 
below). Model Output Statistics (MOS) temperature guidance has been 
running too warm and thus we will continue to undercut guidance. 
.LONG TERM...Wednesday night through next Sunday night. Finally 
some light at the end of this foggy tunnel eastern Idaho has been 
in. 500mb longwave forecast is for the omega-shaped blocking high to 
de-amplify by Fri morning, and by late Sun the flow becomes zonal in 
the longwave pattern. Factoring the shortwaves, the partial 
breakdown in the longwave pattern on Fri is timed perfectly with a 
significantly moist snow storm that arrives during the day and 
continues to bring snow through the weekend, letting up some Sun 
night. The heaviest precipitation appears to be on Sun, for both the 
GFS and ECMWF guidance. With the approach of this weekend storm, the 
cloud cover kills the radiational cooling for the fog and cold 
overnight lows as early as Thu night. The flattening of the ridge 
also allows much warmer afternoon highs as early as Thu, with valley 
highs approaching 40. This weekend storm could also bring windy to 
very windy conditions, especially Sat. Messick
.AVIATION...Clear skies aloft and light wind make the visibility and 
low level stratus the only forecast problems. The NAM is very 
optimistic until Mon morning hours for all but KBYI, and then it is 
only KIDA and KPIH with challenges. The GFS is in its unusual 
position of being more pessimistic tonight for KPIH and KIDA. Since 
last night was no repeat of the night before, it seems unlikely for 
a persistence forecast to succeed. So have brought back in stratus 
and fog for KBYI, KIDA, and KPIH, following the pessimistic outlook 
from the GFS guidance, since little has changed aloft believe these 
three airdromes will struggle with being open in the morning. KSUN 
and KDIJ appear to be finished with the fog, but that forecast is 
only moderately confident. Messick
.AIR QUALITY...Inversions continue to hold and reduce air quality, 
particularly for Franklin County where an Air Stagnation Advisory 
remains in effect. Little change is expected until the end of the 
week. Huston\RS