Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 211532
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
932 AM MDT Sat Apr 21 2018

.UPDATE...

Lowered temperatures due to increased cloud cover a few degrees
primarily from Great Falls north and west given latest model
consensus. Also raised winds and wind gusts in the grids, though
the existing hazard statements for High Winds looks plenty
sufficient for the time being. No other significant changes were
noted. CC


&&

.SYNOPSIS...

A fast moving Pacific cold front will move through the region this
afternoon and evening, producing strong winds from the Rocky
Mountain Front east across the plains of North Central Montana. In
areas where flooding is occurring, these strong winds may lead to
increasing water levels on the eastern edges of flooded areas due
to wave activity, which may overtop sand bags and or levees.
Otherwise, high temperatures today will climb into the upper 50s
to low 70s.

&&


.AVIATION...
Updated 1125Z.

A compact shortwave will quickly moves from the Pacific Northwest
and into Southern Alberta by this evening. The main impacts that
this disturbance will have on the terminals of North Central and
Southwest Montana will be a period increasing high clouds during the
mid/late morning hours through the afternoon, in addition to strong
winds during the afternoon hours across North Central Montana. Winds
gusts in excess of 50kts will be possible this afternoon along the
Rocky Mountain Front and over the immediate plains east of the
Front, especially at the KCTB terminal. Frequent wind gusts in
excess of/near 35kts are then expected at the KHVR, KGTF, and KHLN
terminals this afternooon. Mountain wave induced turbulence is
possible, especially near the Rocky Mountain Front. Mountain
obscuration is likely across the Continental Divide due to low
ceilings and precipitation. - Moldan

&&

.HYDROLOGY...
Updated 300 PM Friday
Areal flood warnings continue for Toole, Liberty, Hill and Blaine
counties.

Areal flood advisories continue for Glacier and Pondera counties.

Numerous point flood warnings continue for the Milk River
and its tributaries across Hill and Blaine counties where additional
snowmelt and inflow from upstream tributaries will maintain flooding
through the next several days.

All flooding over North Central MT will be slow to diminish over the
next few days, as snowpack from Southern Canada begins to melt, and
the runoff works its way southward into MT, particularly from the
Cypress Hills area. Thus Blaine County will be the hardest impact
area, with flooding expected to continue through all of next week.

A storm system will bring rain/snow to Southwest MT Sunday night
through Monday. Some of the heavier precipitation amounts of 0.40 to
0.60 inches will be possible over portions of Southwest and Central
MT. Brusda

&&

.PREV DISCUSSION...
/ISSUED 525 AM MDT Sat Apr 21 2018/

Today and tonight...primary impacts/concern is the potential for
High Winds across portions of North Central Montana.

Compact shortwave over the Pacific Northwest this morning, will
quickly race east/northeast towards Southern Alberta by this evening.
At the surface, a developing and strengthening area of low pressure
over Central Alberta will slide east towards Saskatchewan today, all
the while dragging a cold front from west to east across North
Central and Southwest Montana this afternoon/evening. The
combination of anomalously strong H700 and H850 winds associated with
the approaching shortwave combined with strong diurnal mixing,
passage of the cold front, and decent pressure rises (behind the
front) will lead to the potential for strong winds from late this
morning and through the evening hours today across portions of North
Central Montana. For this reason, have decided to upgrade the High
Wind Watch to a High Wind Warning for the Rocky Mountain Front and
Eastern Glacier County. In addition, decided to expand the High Wind
Warning east across Eastern Pondera/Teton, Toole, Liberty, Chouteau,
and Hill Counties as some favored locations will likely see wind
gusts to 60 mph this afternoon/evening. Along with the chances for
scattered power outages, the antecedent saturated soils will present
the opportunity for some trees to be blown over. Finally, in areas
where flooding is occurring, these strong winds blowing over areas of
decent fetch will create the potential for waves. These waves may
increase water levels on the far eastern edges where flooding is
occurring, which may top levees and/or sand bagged areas.

Sunday through Friday night...primary impacts over this timeframe is
the potential for mountain snow and valley rainfall across Southwest
and Central Montana Sunday night and through the day on Monday.

After a quiet day on Sunday, the next upper level disturbance to
move east from Pacific Northwest will begin to affect the Northern
Rockies by Sunday night. The potential exists with this disturbance
for QPF amounts to exceed 0.5" across an expansive area of Southwest
and portions of Central Montana, with some mountains (i.e. Tobacco
Root, Madison Range, Gallatin Range, Bridger Range, Castle, and
Crazy`s) nearing 1" of QPF. Snow levels Sunday night of 7000ft to
9000ft, will fall to around 5000ft to 6000ft by Monday morning.
Snowfall totals by Monday evening of 6" to 12" will be possible for
elevations above 7000ft, with 2" to 6" possible between 6000ft and
7000ft. Given the high snow levels to start off this event,
precipitation will begin as rain, which is concerning as this rain
will be falling on a rotting snowpack (especially at mid-slope
elevations). This event will need to be watched closely by future
shifts, as it may begin the next wave of flooding due to snowmelt
runoff from these mid-slope elevations.

Transitory ridging quickly builds in behind this departing
disturbance on Tuesday, before a weak clipper system moves in late
Wednesday. This clipper system will bring light precipitation
chances, but little to no impacts to the CWA. Ridging then build in
to end the work week, with temperatures rising well above
climatological norms. These warm temperatures will push freezing
(melting) levels into the 8500ft to 9500ft range to end the work
week, which could lead to increased runoff from these mid-slope
elevations. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  64  32  57  33 /  10   0   0  40
CTB  55  31  54  30 /  30  10   0  20
HLN  64  36  60  37 /   0   0   0  60
BZN  67  34  61  37 /  10  10  10  80
WEY  54  28  52  33 /   0   0   0  70
DLN  65  32  60  35 /   0   0  10  80
HVR  64  32  56  32 /  10   0   0  20
LWT  68  33  57  34 /  10  10   0  60

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 9 PM MDT this evening Chouteau...Eastern
Pondera...Eastern Teton...Hill...Liberty...Toole.

High Wind Warning until 9 PM MDT this evening Eastern Glacier...
Northern Rocky Mountain Front...Southern Rocky Mountain Front.

Flood highlights continue for portions of North Central MT.

&&

$$

http://www.weather.gov/greatfalls


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