Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 221704

Area Forecast Discussion
National Weather Service Great Falls MT
1100 AM MDT Thu Mar 22 2018

Updated Aviation Discussion


Little change in the overall forecast thinking for today and
tonight with moist plume/decaying atmospheric river beginning to
impinge on far SW portions of the forecast area today while a
warm/occluded frontal feature shifts northeast across north-
central MT tonight. Currently snowing at WYS with T/Td at 32f with
additional warming today likely to raise snow levels to as high
as 8000 feet with a continued concern for impacts from rain on
the snowpack for areas along the ID border, and especially in the
West Yellowstone area. To the north, fog has expanded west into
the Havre area and is locally dense across portions of Blaine
county and this will persist through the rest of this morning
before likely rising to a low cloud deck this afternoon.

Models continue to depict a NW-SE oriented band of precipitation
to develop late this afternoon across N-central MT which gradually
works to the NE overnight, bringing 0.10" to 0.25" precipitation
amounts to Hill and Blaine counties. Precip type is still tricky
with plenty of warm air aloft for much of the precip to fall in
liquid form, however cold surface temperatures above the deep
snow- pack in these areas now backed up by and easterly low level
wind flow will likely bring the potential for freezing rain with
many areas in Hill/Blaine counties seeing temperatures near or
just below freezing. There are still some question if road
surface temperatures can cool enough tonight for ice to accrue on
these surfaces but elevated road surfaces will likely be
impacted. Additional impacts are expected to calving in these
areas as well due to cold/wet conditions and addition of weight
from liquid precipitation to snow covered structures may also
create issues. A winter weather weather advisory has been issued
for Hill/Blaine counties to highlight these hazards anticipated
from Tonight through Tomorrow morning. Hoenisch



A strong and moist Spring Storm will affect Southwest and North
Central Montana today and tonight. Temperatures across Southwest
Montana will be near or slightly above climatological norms, with
most of the precipitation falling as rain. Snow in Southwest
Montana will then primarily be confined to elevations at or above
8000 feet through tonight. Across North Central Montana,
temperatures will generally be below climatological norms,
especially along the Hi-Line. Rain, snow, sleet, and even light
freezing rain will be possible from the late afternoon hours and
through the nighttime hours across North Central Montana. For
those calving on the plains of North Central Montana, conditions
tonight could be extreme for newborn livestock due to falling
temperatures, breezy winds, and wet conditions.


Updated 1700Z.

Increasingly moist S/SW flow aloft continues through tonight as
large scale trough moves inland with emebedded disturbance lifting
north across MT tonight. Widespread precip and terrain obscuration
and IFR conditions will persist along ID border with freezing levels
8000 feet MSL or higher through early this evening before lowering
tonight. Showers and associated MVFR conditions spread north into
the rest of SW and Central MT this afternoon, then a broad area of
rain develops across north central MT late this afternoon and
evening, shifting east overnight. MFVR conditions will lower to IFR
at N-central MT terminals this evening with freezing rain potential
at the KCTB and KHVR terminals. Disturbance exits late tonight with
winds shifting from southeast to easterly ahead to southwest or west
overnight. Hoenisch



Flood watch continues for Southwest MT, mainly south of a line
from Lima to Big Sky. This area has the best chance to receive
over 0.50 inches of precip. Frozen culverts/ground surfaces will
create poor drainage issues in this region.

QPF amounts (some of the precipitation will fall as snow/sleet)
of around 0.20 to 0.30 inches are possible from Havre to Harlem,
which could create issues for snow loads on roofs and drainage
problems in urban areas. Widespread rural field flooding is not
expected in this region because of the deep snowpack.


/ISSUED 513 AM MDT Thu Mar 22 2018/

Today through Friday night...main forecast concerns over the short
term is precipitation and temperatures, especially with how these
temperatures affect precipitation type across Southwest Montana.

A dynamic and strong Spring Storm will eject the first of a series
of waves across the Northern Rockies today through tonight, with a
Pacific front pushing across the CWA from west to east during the
overnight hours. Strong H500 closed low of ~512-514dam will be
positioned to the northwest of Vancouver Island as of 12z this
morning, with the H500 ridge axis extending north along the Rocky
Mountain/High Plains interface. Strengthening H700-H500 southwest
flow will usher in anomalously high PWATs through the day today,
with values peaking this evening between 0.5" to 0.75". These values
for PWAT (0.5" to 0.75") are 2 to 3 standard deviations above normal
per NAEFS analysis across all of Southwest and North Central
Montana, with extreme Southwest Montana reaching 3 to 4 standard
deviations above normal. So to say the least, abundant moisture will
be in place in the atmosphere throughout the day and overnight hours
tonight across Southwest and North Central Montana. As synoptic
scale divergence and ascent overspreads the region from southwest to
northeast today, precipitation is expected to develop and expand in
coverage. For areas across Southwest Montana, precipitation in the
form of rain/snow is expected, with snow levels generally around the
8000 ft mark (eventually falling below 6000 ft by Friday morning).
For these elevations (aoa 8000ft) , snowfall accumulations across
Madison and Gallatin Counties of 5 to 10 inches is expected.
Elevations below 8000 feet will primarily see rain, however, an inch
or two of snow will be possible for elevations above 6000 feet. The
biggest story for elevations below 8000 ft across Southwest Montana
will be rainfall amounts, with some areas across Beaverhead,
Madison, and Gallatin Counties receiving a half of an inch to one
inch of liquid water.
These amounts falling on top of snow, will lead to the potential
for flooding. For this reason, have continue the Flood Watch.
Across North Central Montana, precipitation initially in the form
of rain or light freezing rain, is expected to change over to
sleet and or snow across most areas. A light glaze of ice will be
possible where light freezing rain is observed, with 1 to 2 inches
of snow being possible along and north of the United States
Highway 2 corridor. Any rain on top of snow loaded roofs will
cause further stress to these structures. For those calving,
conditions tonight could be extreme for newborn livestock due to
falling temperatures, breezy winds, and wet conditions.
Precipitation tapers off during the morning hours on Friday, with
primarily dry downsloping conditions expected across North Central
Montana. As the second wave approaches Friday evening,
precipitation will begin to redevelop. - Moldan

Saturday through Wednesday...The next shortwave, embedded within the
broad upper level trough over the West, will lift NE through the
forecast area on Saturday, eventually dragging a Pacific cold front
across the area by Saturday evening. Ahead of the front, warming
surface temps beneath colder temps aloft will allow weak instability
to develop areawide. Weak instability combined with modest low/mid
level moisture should lead to isolated/scattered afternoon rain/snow
showers. The greatest coverage may end up being from Bozeman to
Lewistown where the low/mid level flow is more convergent.
Generally, though, impacts look very minimal (and primarily confined
to sudden reductions in visibility for travelers).

In the wake of this wave, a NW flow develops which may allow a few
snow showers to develop in favored upslope areas. Early next week,
ridging tries to build back in with the potential for moderating
temperatures once again, with the only real impact being continued
snowmelt/hydro concerns. MARTIN


GTF  47  30  46  28 /  50  50  10  30
CTB  44  26  38  21 /  30  80   0  20
HLN  51  32  47  29 /  40  50  10  20
BZN  54  36  50  31 /  40  90  20  20
WEY  44  27  42  25 /  90  90  50  60
DLN  55  33  46  25 /  60  70  10  20
HVR  34  26  36  22 /  10 100  10  20
LWT  50  31  45  29 /  20  80  20  40


Flood Watch through late tonight Beaverhead...Gallatin...Madison.

Winter Weather Advisory from 6 PM this evening to noon MDT
Friday Blaine...Hill.


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