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000
FXUS65 KTFX 181748
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1148 AM MDT Sat May 18 2013

Aviation Section Updated

.UPDATE...

Very few updates this morning as current forecast is on track.
There will be breaks in the showers today with better coverage of
showers and isolated thunderstorms this afternoon, but overcast
skies will keep temperatures slightly cooler than seasonal
averages. MLV

&&

.AVIATION...
UPDATED 1748Z.
An upper level trough extending from the Great Basin area into the
Northern Rockies will make only slow eastward progress today...
keeping central and southwest MT under BKN-OVC skies. Light
rainshowers will gradually redevelop across the area this
afternoon...starting first in the vicinity of higher terrain then
becoming widespread. There is also a slight chance for isolated
thunderstorms this aftn, but given their low probability and spotty
coverage did not include mention of any TS at primary TAF sites
today.  Ceilings and visibilities should remain VFR but brief
MVFR/IFR conditions are possible near any precipitation areas.
Shower activity will diminish after 19/06Z. Another wave generates
light rain for the north central plains from east to west early
Sunday. Winds will remain light at all stations. Nutter/MLV

&&

.HYDROLOGY...
Two upper level trofs, one moving through our region this weekend
and the other becoming stationary over the PACNW coast for much of
next week, will bring increased chances for rainfall over the next
several days.  Though cooler temperatures will reduce snowmelt
slightly, the primary concern is that rain on the melting snowpack
will cause enhanced rises in stream/river levels. Most area rivers
are currently well below flood stage but a handful of sites are near
or approaching bankfull, so rainfall totals and river stage
forecasts will be closely monitored for significant changes and
impacts.  Waranauskas

&&

.PREV DISCUSSION...
/ISSUED 525 AM MDT Sat May 18 2013/
Upper level trough extending from the Pacific NW through the Great
Basin will continue a slow progression east into the Rocky Mtn
region today, keeping the forecast area under a moist and somewhat
unstable SW flow aloft. Main focus for precipitation this morning
will be along an old/weakening deformation zone along the Hi-line
and over far SW MT ahead of the next incoming shortwave embedded
within the broader upper trough. Elsewhere, precipitation will be
limited to scattered light showers with patchy fog/drizzle through
early this morning. Daytime heating and the approaching shortwave
from the south should bring an increase in the coverage of showers
this afternoon and evening to most of the forecast area and an
isolated weak/brief thunderstorm or two cannot be ruled out,
however instability parameters are rather marginal. Sunday and
Monday, the upper trough emerges east of the Rockies with the
upper level circulation closing off as the trough undercuts an
upper ridge over central Canada. This results in a shift to more
northerly flow across the forecast area as the upper low center
tracks from Nebraska slowly northeast into the upper Midwest
through early next week. This should shift the focus for
widespread precipitation to north central MT, however all areas
will have some chance for showers through the period, especially
during the afternoon/evening time-frames. Temperatures will remain
close to seasonal averages today with slightly cooler conditions
on Sunday as the flow aloft turns to the north. Snow levels at or
above 8000 ft will limit any snow accumulation to the higher
peaks. Hoenisch

Monday Night through Saturday...Extended forecast period begins with
mostly quiet conditions as the upper level system that will cross
Montana this weekend will be moving into the Upper Midwest states
and a well-amplified high pressure ridge begins to settle in over
ern Montana and the Dakotas.  Meanwhile, a second upper level trof
is forecast to swing through western Canada and form a closed low
pressure center over the PACNW coast by Tues eve.  With the large
ridge remaining anchored thru the middle of the country, the Pacific
low essentially becomes stationary, drifting along the coast and
occasionally moving inland during the latter half of next week.
Minor shortwaves and accompanying moisture rotating around the
central low will bring intermittent periods of showers and isolated
thunderstorms each day from Wed onward.  Forecast models are in
decent agreement that most parts of central/southwest MT will see
rain during this time, but precip amounts still look to be generally
light (0.10-0.15 inch or less) over the plains and valleys.
Rainfall totals are slightly higher (0.20-0.50 inch) over the
melting snowfields of the Rocky Mtn Front and Little Belt Mtns,
which gives concern for increased streamflows in creeks/rivers in
those areas.  Persistent southerly flow aloft will allow
temperatures to stay close to typical seasonal values with highs in
the mid-60s to the low 70s each day, though that will be modified
quite a bit by a fair amount of cloud cover and the timing of the
shortwave passages.
Waranauskas

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  64  45  59  45 /  80  60  80  80
CTB  62  44  60  43 /  80  50  80  80
HLN  64  45  60  45 /  70  60  60  70
BZN  64  43  59  42 /  80  70  60  60
WEY  55  36  53  34 /  80  70  60  40
DLN  61  40  58  40 /  80  60  50  50
HVR  69  49  66  47 /  90  80  90  80
LWT  61  45  57  43 /  80  80  80  80

&&

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

$$

weather.gov/greatfalls