Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 211738
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1135 AM MDT Mon Apr 21 2014

UPDATED AVIATION SECTION

.UPDATE...
Current forecast is on track...with only minor changes to the sky
grid. Temperatures look on track...and winds are fairly light across
the entire region today. More active weather will move into the
region by Tuesday afternoon. Brusda

&&

.AVIATION...
UPDATED 1735Z.
VFR conditions are expected to continue through at least 18Z
Tuesday. Areas of high cloudiness with ceilings of above 15000 feet
AGL will increase over the area through the period. However,
disturbances in the increasingly moist southwesterly flow aloft may
bring scattered to broken mid level clouds with ceilings of 8000 to
10000 feet AGL to the area between 00Z and 12Z, along with a few
mountain showers. Breezy southwesterly winds of 5 to 15 mph will
shift more easterly through 03Z.
Coulston

&&

.HYDROLOGY...
Mild daytime and overnight temperatures through Tuesday evening
will cause an increase in mountain snowmelt with rising water
levels anticipated on creeks and streams originating in the
mountains through the middle of this week. In addition, widespread
precipitation Tuesday afternoon and evening will occur with snow
levels between 7500 and 8500 feet, which could result in rain
falling on the mountain snowpack and increasing the rate of
snowmelt runoff. Cooler conditions and lower snow levels on
Wednesday through the weekend will reduce the rate of snowmelt.
Hoenisch

&&

.PREV DISCUSSION...(Issued at 400 AM MDT Mon Apr 21 2014)

Today through Wednesday...An upper level ridge with axis over the
Northern Rockies this morning shifts east across MT today, keeping
the region dry with temperatures warming to the upper 60s to around
70 this afternoon across most lower elevation portions of the
forecast area. At the surface, weak high pressure will maintain
relatively light winds, becoming south to southeasterly this
afternoon and evening. Dry/mild conditions will prevail through
tonight with cloud-cover increasing in SW flow aloft ahead of a
large upper level trough moving onto the west coast. Models remain
consistent in showing an inverted surface trough strengthening over
central MT Tuesday in response to the incoming upper level trough
with the surface trough and associated cold front shifting east
across central and eastern MT late Tuesday afternoon and evening.
Showers will develop by Tuesday afternoon over western and SW
portions of the state and spread N-NE across the area Tuesday
afternoon and evening as the stronger energy associated with the
upper level trough moves into the region. Still expect some
thunderstorms embedded within a more widespread region of
showers/rain Tuesday Aftn/Eve with better chances for areas east of
I15. A fairly strong surge of west winds moves in behind the surface
trough/cold front Tuesday night and this will need to be watched for
possible wind highlights. The upper level trough lifts NE across the
state on Wednesday with a mid level circulation closing off over
southern AB by Wednesday afternoon. Westerly flow south of the
circulation will bring some drying into the region Wednesday,
especially over areas along and just east of the Rocky Mtn Front,
though most/cyclonic flow will continue to produce at least
scattered precipitation through Wednesday afternoon elsewhere.
Westerly flow aloft will maintain gusty winds across much of the
forecast area on Wednesday. Total precipitation amounts Tuesday
afternoon through Wednesday look to range from from around a quarter
to a half inch at lower elevations with up to around three quarters
of an inch over the mtns. Snow levels will fall to around 4000-5000
ft by Wednesday morning with area mtn passes likely seeing an inch
or two of snow accumulation late Tuesday night into Wednesday
morning with higher amounts confined to elevations above 7500 ft.
Hoenisch

Wednesday night through Sunday...model runs from Saturday night had
precipitation moving into the north central and central Montana
Wednesday night on the backside of an upper low but latest runs have
backed off on this idea and instead keep precipitation near or north
of the Canadian border. Models are on the same page in forecasting
an upper ridge over central Montana by late Thursday night. Lowered
the chances of precipitation a little from the previous forecast for
both Thursday and Thursday night. For Friday through Saturday night
models have a moist southwest flow aloft with embedded
precipitation. Models differ on precipitation details so continued
with the idea of 30 to 40 percent chances of precipitation for the
lower elevations and 40 to 60 percent chances of precipitation for
the mountains for any one forecast period. For the 48 hour period
from 6 am Friday to 6 am Sunday precipitation totals are forecast to
range from only a few hundredths of an inch to a little over an inch
depending on the particular model used. For Sunday models diverge
with the ECMWF model forecasting a moist southeast flow aloft at
least in the north while the GFS and GEM models forecast a drier
southwest to west flow aloft. As for temperatures..confidence in
forecast values becomes low by the weekend with huge differences
between the ECMWF and GFS models. The ECMWF indicates highs only in
the 30s while the GFS has highs 55 to 65. Leaned more towards the
GFS solution with forecast highs over the weekend. Blank

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  70  44  70  39 /   0   0  70  90
CTB  67  39  66  37 /   0  10  70  80
HLN  71  44  66  36 /   0  10  70  90
BZN  70  41  70  33 /   0  10  60  80
WEY  60  36  59  28 /   0  20  80  80
DLN  68  42  64  31 /   0  10  80  80
HVR  70  40  76  39 /   0   0  30 100
LWT  66  41  70  35 /   0   0  40  80

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

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