890
FXUS65 KMSO 180248
AFDMSO
Area Forecast Discussion
National Weather Service Missoula MT
848 PM MDT Fri May 17 2013
.UPDATE...Canceled the snow advisory for Lost Trail pass.
&&
.DISCUSSION...The snow level has risen high enough that snow on
the pass is not a concern this evening.
&&
.AVIATION...Rain-showers will be on the increase this evening and
through the night for KGPI and nearby areas. Rain will begin to
decrease in coverage later this evening for aviation sites KBTM,
KSMN, and KMSO. Rain will occasionally reduce visibilities and
ceilings as well as obstruct mountain terrain all across western
Montana and Central Idaho. A few thunderstorms will also be
possible in central Idaho as well as northwest Montana this
evening.
&&
.PREV DISCUSSION... /ISSUED 252 PM MDT Fri May 17 2013/
DISCUSSION...
A complex upper level closed low pressure system is slowly moving
across the Northern Rockies and central Idaho. Significant rainfall
amounts are likely through Saturday afternoon
IMPACTS:
25"-.50" through Saturday morning in the valleys of:
West Central & Southwest Montana
East Central Idaho.
25"-.50" through Saturday afternoon in the valleys of:
Northwest Montana (locally higher amounts)
50"-1.0" through Saturday afternoon in the mountains of:
Northwest Montana including Glacier NP.
(locally 1.5" in the highest elevations of Glacier NP.)
Rainfall will be greatest in Glacier NP as the upper low basically
stalls. This will be conducive for a good period of steady rain
through Saturday morning or early afternoon.
The mid & upper level closed low finally moves into Canada midday
leaving behind showery conditions through the afternoon hours in
most of the Northern Rockies. Showery conditions with no major
impacts are expected Sunday in between upper closed lows.
The next closed low dives down from British Columbia along the
Eastern Pacific. Models have slowed this down a bit. However the
potential still exists for a warm and moist southwest flow will be
ushered into the Northern Rockies. This combined with increasing
instability will produce an increasing confidence for thunderstorms
Tuesday afternoon. Strength of thunderstorm are somewhat uncertain.
Confidence remains high for the closed low to impact the Pacific
Northwest in the middle of next week, and confidence is increasing
that Wednesday northwest Montana will be a focal point for
precipitation. Northerly flow on the west side of this system will
bring in very cold air from Canada, while southerly flow will keep
warmer air and rain on the east side of the system. The most
uncertain part of the forecast is where the boundary between the
warm and cold air will line up, which will be a focal point for
precipitation. The most likely scenario for Wednesday with the
current forecast data is for snow in far northwestern Montana and
rain in Glacier National Park. The main concern with this system
is the potential for streams and small river flooding, though
travel over some mountain passes may also be difficult.
Beyond Wednesday, confidence if the forecast drops off quickly.
There is a lot of uncertainty with how it will evolve and interact
with a jet streak moving south of the low pressure center. Some
solutions rapidly move the system out while others cause the low
to split into two spiraling lows that could linger and cause
repeated precipitation events.
AVIATION...Rain-showers will be on the increase this evening and
through the night for KGPI and nearby areas. Rain will begin to
decrease in coverage later this evening for aviation sites KBTM,
KSMN, and KMSO. Rain will occasionally reduce visibilities and
ceilings as well as obstruct mountain terrain all across western
Montana and Central Idaho. A few thunderstorms will also be
possible in central Idaho as well as northwest Montana this
evening.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&
$$
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