Area Forecast Discussion
Issued by NWS Glasgow, MT

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FXUS65 KGGW 230148

Area Forecast Discussion
National Weather Service Glasgow MT
748 PM MDT Thu Mar 22 2018


Evening Update (745pm MDT): The two main concerns tonight are the
winter storm and the river flooding.

Precipitation is making its way into Phillips, Petroleum, and
Garfield counties this evening a couple of hours ahead of what was
previously forecast, so increased pops accordingly. The NAM seemed
to have a good handle on this. Precipitation is likely falling as
rain to start, at least from the Malta area and south where
temperatures are in the mid 30s to near 40 degrees. This system
will continue through northeast Montana and precip will transition
to a wintry mix and then snow through the overnight hours. Areas
from Jordan south may continue to see rain throughout the event.
This will finally push out of the area Friday afternoon. Winds are
another concern with this storm, with winds generally in the 20s
and 30s but over 40 mph in a few locations. Blowing snow will be a
concern where temperatures drop low enough for precip to change
over to snow as well as where there is still decent snow cover.

Flooding is still a concern throughout the area. Substantial ice
jamming upstream and sharp rises in river levels has prompted the
issuance of a flood warning for the Yellowstone River in Dawson
County. We are continuing to monitor this situation.  Hickford

Morning Update (2pm MDT): Focus for the update was tonight`s storm
system. Generally the story remains the same, and confidence
continues to be moderate to high that a modestly narrow band of
freezing rain and sleet will mix in with rain and/or snow during
the overnight hours. Winds continue to be quite blustery across
the region through tomorrow, and will lead to the potential for
blowing and drifting snow, especially in the northeastern areas of
the region. Thus, Sheridan County was upgraded to a Winter Storm
Warning while the rest of the Advisory area was relatively left
alone. The forecast beyond tomorrow was left mostly alone and in
good shape. Bigelbach

PREVIOUS DISCUSSION: 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

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: Tonight an approaching storm system will bring
increasing chances for a mix of precipitation types to the
northern terminals. Visibility will decrease to as low as IFR
conditions as precipitation transitions to snow and could
fluctuate in and out of IFR levels through much of tomorrow before
the system exits the region tomorrow evening.

WIND: Expect east to southeast winds of 20-30 kts through midnight,
and then decreasing to 10-20 kts and turning from the east to the
northwest after sunrise tomorrow morning. Expect gusts mainly to
be 20-30 kts through the period, though this evening through
midnight GGW and OLF could see 30-40 kts.



A Flood Watch remains in effect along the Yellowstone River for
Prairie and Dawson Counties and the Musselshell River between
Petroleum and Garfield Counties, both of which are in effect due
to the possibility of continued ice jam flooding. The watch
remains in effect through noon on Monday.

In addition, a Flood Warning remains in effect through this
evening for the Musselshell River from the Garfield and Petroleum
county lines north to Crooked Creek Recreation Area. Ice jamming
is occurring at the outlets of multiple tributaries along the
length of the Musselshell between Petroleum and Garfield Counties
and could lead to continued flooding issues along the River.

A Flood Warning is also in effect through much of Friday for the
Yellowstone River in Dawson County. Ice jamming is occurring from
the confluence of the Powder and Yellowstone rivers west, which is
slowly moving downstream.

Continued daily high temperatures around or above freezing will
lead to further rotting and breaking up of ice along the rivers.
Adding precipitation overnight tonight will increase the water
loading on the river and also add to the ice jam flooding

To monitor river stage readings, go to our Advanced Hydrologic
Prediction Service (AHPS) page at then
click on "Rivers and Lakes" then "River Observations". Data for
this page are provided by the USGS.


Winter Weather Advisory from midnight tonight to 6 PM MDT Friday
for Central and Southern Valley...Daniels...Eastern Roosevelt...
Northern Phillips...Northern Valley...Richland...Western

Flood Watch through Monday morning for Dawson...Garfield...

Winter Storm Warning from midnight tonight to 6 PM MDT Friday
for Sheridan.


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