Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 230015
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
615 PM MDT Fri Aug 22 2014

.UPDATE...
Have decided to issue an areal Flood Watch for the Rocky Mountain
Front through Saturday night and for the eastern portion of north
central Montana through Sunday. Upslope enhancement of rainfall
along the Rocky Mountain Front will likely cause an increase in
the flow of water out of the mountains via creeks and streams. In
the east, the Fort Belknap area has already received in excess of
1 inch of rain from the first wave of showers, causing minor
basement flooding. The expected additional rainfall there will
only exacerbate the situation. Am generally thinking that mainly
creek and stream flooding is possible, but cannot rule out that a
river or two could approach flood stage.
Coulston

&&

.AVIATION...
UPDATED 2355Z.
Periods of rain with low ceilings and mountain obscurations will
continue through the period. Scattered nature of precipitation
Friday evening limits confidence in timing of rain arriving at the
terminals. Showers transform to widespread rain tonight through
Saturday over north-central Montana. Areas of heavy rain early
Saturday will produce IFR conditions. Isolated thunderstorms will
continue over southwest MT Friday evening.

&&

.HYDROLOGY...
A Flood Watch is in effect through Saturday night for the Rocky
Mountain Front and through Sunday for the eastern portions of
north central Montana. Rainfall across much of north central
Montana has saturated soils in some areas, and the expected
additional rainfall through the weekend will likely cause at a
minimum ponding on roadways and flooding of low lying areas. Some
creeks and streams may flow also out of their banks. Although
rivers are expected to rise through the weekend, they are
generally expected to remain within their banks.
Coulston

&&

.PREV DISCUSSION...
/ISSUED 555 PM MDT Fri Aug 22 2014/

Tonight through Sunday...A vigorous mid-upper level trough
currently centered over the Inland Pacific NW will continue to
strengthen as it digs SE into ID tonight, then lifts NE through NW
WY and Eastern MT Saturday and Saturday night. Moist and unstable air
in the mid-levels to the east of the circulation will continue to
produce scattered showers and thunderstorms across much of the area
this evening. Steadier precipitation associated with a lobe of
shortwave energy currently over N-Central MT will lift north into
Canada this evening, however convection currently developing over
SW MT and eastern ID will lift north into central and N-central MT
later tonight and continue, moderate to heavy at times through mid-
day Saturday. A second lobe of energy and deep moisture, currently
located over Colorado will be drawn north ahead of the Northern
Rockies trough with much of its moisture wrapping around the upper
level circulation into north central MT Saturday night and Sunday.
Total precipitation amounts through Sunday will be heaviest over
north-central MT, where 1 to 3 inches of total precipitation is
expected while SW MT looks to receive 1/2" to around 1". As the
upper low approaches and deepens tonight through Saturday,
temperatures aloft will cool, allowing snow levels to fall to
around 6500-7500ft tonight along the Rocky Mtn Front/Glacier Park
area and remain around that level through Saturday night. Snow
will become likely across the higher elevations of Glacier
National Park and the Bob Marshal Wilderness area where general
accumulations of 1-3" are expected at elevations above 6500 ft
with localized higher amounts possible on the higher peaks above
8000 ft. Therefore a winter weather advisory has been issued to
highlight snow and cold/wet conditions, though Logan pass will be
the only pass impacted and only light accumulation is expected
there on the road surface. Will need to monitor hydrologic
conditions closely across much of north central MT through the
weekend as the precipitation add up. Fortunately, most rivers and
streams are running at relatively low levels and are expected to
handle the additional runoff, but small streams and urban areas
could be at risk for minor flooding. Hoenisch

Sunday night through Friday...A broad trof will remain over Central
Montana and a disturbance will move through the back side of this
trof and drop across Southern Idaho and Nevada Sunday night. Most of
the dynamics will stay well south of the zones. However, the air
mass will remain unsettled with the best chances for precipitation
over the Rockies and Southwest Montana. The trof will remain over
the zones until moving east Monday night but a high pressure ridge
will be over the Pacific Northwest by Tuesday and pressure aloft
will be rising over Central Montana. This will dry the air mass and
begin a period of cool overnight lows and warming day-time temps.
The high pressure ridge will move east of the zones Thursday and
this will allow moisture to once again move across the Rockies from
the west. Models are in general agreement until beginning to diverge
Thursday. The GFS brings a low pressure trof over the Pacific
Northwest Thursday and moves this over Central Montana Friday. This
would bring another period of cooler temps and precipitation into
the area. The approach of this system would also develop a low
pressure low over Alberta with brisk surface winds across the
Rockies and adjacent plains Thursday night and early Friday morning.
The ECMWF and GEM leave this low well over the Pacific and leave the
zones underneath rather zonal flow. Will shade towards the ECMWF and
GEM solutions which will keep the temps warmer and the chance of
precip less. Zelzer

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  52  55  45  60 / 100 100  90  70
CTB  47  53  43  60 / 100 100  70  30
HLN  52  59  46  65 /  90  90  70  30
BZN  51  61  42  64 /  80  60  60  30
WEY  40  58  36  59 /  60  60  50  30
DLN  47  60  40  64 /  70  60  60  30
HVR  54  58  47  60 /  90 100  90  80
LWT  52  55  43  57 / 100 100  90  80

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
FLOOD WATCH through late Saturday night Northern Rocky Mountain
Front...Southern Rocky Mountain Front.

WINTER WEATHER ADVISORY from 10 PM this evening to Noon MDT
Saturday FOR ELEVATIONS ABOVE 6500 FEET FOR Northern Rocky
Mountain Front...Southern Rocky Mountain Front.

FLOOD WATCH through Sunday afternoon Blaine...Chouteau...
Fergus...Hill...Judith Basin...Liberty.

&&

$$

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