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
FLIGHT CAT: VFR - IFR.
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
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...
Flood Watch through Monday morning for Dawson...Garfield...