Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 191135
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
535 AM MDT Sat Apr 19 2014

UPDATED AVIATION DISCUSSION

.DISCUSSION...
Today through Monday...An upper ridge will move into western Montana
this morning and to central Montana this afternoon. This should keep
the forecast area dry and skies should be mostly sunny. Winds will
be much lighter than those of Friday. The next upper air disturbance
will move into western Montana later tonight and then into eastern
Montana Sunday afternoon. The associated cold front is expected to
reach Cut Bank and Drummond around midnight and then should be east
of a Lewistown to Bozeman line by late tonight. Behind the cold
front gusty winds will develop. With the quick-moving cold front and
winds aloft becoming moderately strong confidence in high winds over
the Rocky Mountain Front is moderate so have issued a high wind
watch which will start this evening and last into Sunday morning.
Farther east confidence in high winds is low so did not issue a high
wind watch. Models are in agreement in not forecasting much moisture
with the disturbance/cold front and expect no more than a few
showers east of the Rocky Mountain Front. After the disturbance
expect generally dry conditions which will be aided by the next
upper ridge which will move over western and central Montana Sunday
night. On Monday southwest flow aloft will develop and expect an
increase in high level moisture. However low levels will be quite
dry and even though afternoon temperatures will be on the warm side
expect little if any instability. Due to this and also models
forecasting little if any lift..have dropped the mention of
precipitation for Monday. Blank

Monday night through Saturday...Large scale upper level trough moves
onto the west coast Monday night and progresses east into the
interior western US Tuesday, crossing the Rockies Tuesday
night/Wednesday. Models are in fair agreement through Tuesday night
with SW flow aloft ahead of the trough bringing warming to the
region on Tuesday before the initial shortwave and upper level jet
energy associated with the trough approach the Northern Rockies late
Tuesday afternoon. Inverted trough of low pressure at the surface
and cold front will shift east across the forecast area Tuesday
night in conjunction with good mid-upper level forcing associated
with the trough, bringing widespread precipitation to most of the
forecast area. Enough warming and destabilization occurs Tuesday
afternoon ahead of the trough for a chance of thunderstorms late
Tuesday afternoon and evening, mainly over central and eastern
portions of the forecast area. Models then diverge with the
evolution of the trough Wed/Thurs with GFS model continuing to
depict a closed/stacked system that slowly drifts from eastern MT
into the Dakotas, resulting in continued wrap around precipitation
and northerly/moist upslope flow through at least Thursday morning,
while the ECMWF model moves the system through much quicker with
drying and weak ridging moving over the region late Wednesday
through Thursday. With a downstream upper level ridge over the
central US/Canada depicted by both models, am leaning toward a
solution slower than the ECMWF but not as wet/prolonged as the GFS
and have maintained higher pops through Wednesday evening with
gradual drying Wednesday night/Thursday. Cooler temperatures are
expected Wednesday and Thursday as the upper trough moves over the
region with snow levels lowering to around 5000-6000 ft Wednesday,
then as low as around 4000 ft Wednesday night. By Friday, models
come back into agreement, at least at the large scale, with weak
upper level ridging shifting east across the region before the next
deep trough in the eastern Pacific/Gulf of AK moves onto the west
coast next weekend. Hoenisch

.HYDROLOGY...
Expect increased snow melt Monday through Tuesday as snow levels
range from 7000 to 8000 feet in the north to 8000 to 9000 feet in
the south. Confidence remains high that most locations should see at
least a quarter of an inch of precipitation late Tuesday and Tuesday
night with central portions of the forecast area likely to see
around a half inch of precipitation during this period. Convective
nature of precipitation Tuesday evening could result in localized
heavier amounts. Confidence remains low however for additional
precipitation amounts Wednesday and Thursday due to model
differences. The threat for flooding will increase early next week
with increased snowmelt followed by widespread precipitation Tuesday
night. For Wednesday and Thursday, cooler temperatures and lower
snow levels should reduce runoff from snowmelt at higher elevations.
Hoenisch/Blank

&&

.AVIATION...
UPDATED 1200Z.
VFR conditions will prevail across north central and southwest MT
through this evening under clear to partly cloudy skies with breezy
southwest winds developing this afternoon. A cold front associated
with an upper level weather disturbance moving east along the
Canadian border tonight will move quickly across the region between
06z and 10z Sunday morning. Main impact from the front will be an
abrupt increase in west winds with gusts 30-40 kts. A brief period
of broken mid level cloud cover is likely around the time of frontal
passage late tonight, however precipitation and any lower level
cloud-cover will be confined to areas along and west of the
Continental Divide. Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  64  41  59  35 /   0  10  10   0
CTB  61  37  57  30 /   0  10  10   0
HLN  67  39  61  37 /   0  10  10   0
BZN  67  33  62  32 /   0  20  10   0
WEY  55  33  55  29 /   0  20  10   0
DLN  66  34  61  34 /   0  10  10   0
HVR  64  38  65  32 /   0  20  10   0
LWT  62  36  57  32 /   0  10  10   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WATCH from this evening through Sunday morning
Northern Rocky Mountain Front...Southern Rocky Mountain Front.

&&

$$


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