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000
FXUS65 KTFX 182057
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
257 PM MDT Sat May 18 2013

.DISCUSSION...

Tonight through Monday...Scattered showers and isolated
thunderstorms are developing this afternoon as instability
increases with an upper-level trough pushing through the Northern
Rockies. With the cooler air aloft, some of these showers and
thunderstorms may contain graupel. On Sunday, the upper-level low
associated with the trough will further deepen as it moves into
eastern Montana, which will shift the surface flow more northerly.
As abundant moisture wraps into this system, upslope precipitation
will develop and become steadier and more widespread, especially
during the afternoon hours. There are still some differences in
the models regarding where the heaviest precipitation will be
located, however confidence is increasing that the best precip
chances will be for areas along and east of I-15 as well as areas
along the Rocky Mountain Front. Cooler, Canadian air will also
keep snow levels lower on Sunday with snow levels between 7000 and
8000 feet. Over southwest Montana, precipitation will be
widespread but off and on through the period. The system will move
further east on Monday with drier air pushing in west to east.
High temperatures will remain below average through Sunday, then
rise to near seasonal averages on Monday. MLV

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

&&

.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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  44  61  44  66 /  60  70  60  40
CTB  42  62  42  67 /  60  50  40  30
HLN  44  62  45  68 /  60  50  50  30
BZN  41  60  41  66 /  80  60  50  30
WEY  36  53  35  59 /  70  60  50  40
DLN  38  59  40  66 /  60  50  40  20
HVR  47  68  46  71 /  80  80  70  70
LWT  44  57  43  62 /  80  80  70  60

&&

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

$$

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