Area Forecast Discussion

FXUS65 KGGW 221019
Area Forecast Discussion
National Weather Service Glasgow MT
419 AM MDT Thu Mar 22 2018
Synoptic Setup: Ridge is meandering across the western two-thirds
of the CONUS with a crest over the southern Canadian Prairies.
Coldest air lies bottled up over Baffin Island with a trough 
running southwest through the Northwest Territories and spilling 
through British Columbia and off shore of the Pacific Northwest 
where it gains a Pacific marine attribute. Farther west another 
ridge is developing south of the Aleutians. 
Today through tonight: Ridge will be in full control with highs 
rising into the upper 30s and 40s. This will lead to further snow
melt and more run off for rivers. It will also lead to a moist 
surface layer for fog overnight. However, as the crest of the 
ridge moves through overnight a warm front will develop and begin 
taking advantage of extra surface moisture as well as a pacific 
stream of moisture aloft. This will bring developing mixed 
showers after midnight.
Friday: Showers across the area are leading to a much more complex
forecast than is usual in this period. NAM Forecast soundings are
clearly pointing towards a sleet to snow transition in their
profiles across western zones from 03Z to 09Z. While in the east 
rain to snow is more likely. Complicating this is that in the 
last 12 hours snow ratios and QPF totals have gone up 0:1 to 12:1.
Leading to potentially greater than 6 inches of snow for Sheridan
county. But the increase of snow ratio in one run of the forecast
means that confidence is low in specifics of transition and 
therefore QPF totals on a whole. For this reason, am upgrading to 
advisories in any places where greater than 2 inches of snow is 
present. But, have held off from emphasis of sleet/freezing rain 
potential, and also held off on upgrading Sheridan County to a 
winter storm where higher totals may be possible. 
IF... forecast topples back towards more rain with current QPF and
the system moves 100 miles south, there is an outside chance for 
flash flooding due to rain over snow pack falling into what are 
already rising rivers. This chance is currently outside/minimal 
and should NOT be hyped, but does exist. Thankfully, most rivers 
that are facing ice jamming already are not in the places where 
bulk of QPF will fall.
Friday night through Sunday night: Flow will become southwest as
the trough off the PCNW moves inland and into western Montana
during this time. Quick weak snow and rain showers will be
possible in this flow regime. Saturday currently looks the most
impactful in terms of precip. 
Monday into Wednesday: A trough followed by Northwest flow is
expected at this time. Conditions should be drier with the 
pacific flow shut off. Confidence is average. 
Wednesday night onwards: Ridge moves onshore and takes control. 
Expect dry and further warming conditions. Small possibility 
exists for a back door front cooling one day or two so in the 
middle of this warming trend. Confidence is average. GAH
SYNOPSIS: Light MVFR/IFR fog will be possible at KGGW and KOLF 
through the morning but should clear by the afternoon to VFR. 
Tonight after 06Z a developing frontal passage will bring 
increasing rain to sleet to snow to the northern terminals. 
Visibility will decrease to IFR again with the snow transition in
this passage which is expected to not end until late Friday 
WIND: east at 5 to 10 kts in the morning... increasing to 10 to 20
gust 30 kts by the afternoon... further increasing to 15 to 25
gust 35 kts tonight. Becoming light and gaining a northerly flow
friday mid-day. 
Winter Weather Advisory from midnight tonight to 6 PM MDT Friday 
for Central and Southern Valley...Daniels...Eastern Roosevelt...
Northern Phillips...Northern Valley...Richland...Sheridan...
Western Roosevelt. 
Flood Watch through Monday morning for Dawson...Garfield...