Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 222104
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
304 PM MDT Fri Aug 22 2014

...Accumulating mountain snow above 6500ft along Continental Divide...

...Potential for patchy morning frost Tuesday and Wednesday next week...

.DISCUSSION...An area of widespread showers and thunderstorms are
starting to fire across the southern Bitterroot and Clearwater
Mountains this hour. This activity is forming near a developing
mid-level circulation and will very slowly move to the northeast
before gradually curving to the northwest. Severe weather is
unlikely today, given the cool atmosphere and low cloud tops, so
heavy rainfall will be the key threat. This will especially be the
case near wildfire burn areas and remain closely watched through the
evening. For further details regarding the flooding potential over
the next 24 hours and the currently issued Flood Watch, please see
the below hydrology discussion.

An intensification of precipitation is expected tonight as the mid
level circulation further develops and strengthens. The focus for
this precipitation will be two fold. The developing late tonight
across the southern Bitterroot/Sapphire Mtns eastward through the
Anaconda- Pinter Wilderness towards Butte. Locations such as
Hamilton, Philibsburg, Georgetown Lake, Sula, Lost Trail Pass,
Butte and perhaps even as far north as Stevensville will
experience a period of heavy rainfall.

The next area of focused precipitation will fall along the
Continental Divide from the Canadian border southward to around
MacDonald Pass. This precipitation will be fed by an extremely
moist atmosphere across the upper plains states and further
accentuated by strong easterly winds producing up slope flow along
the leeward side of the Divide. But descending cold air from Canada
will add an additional element of complexity to the forecast and
result in the snow level lowering to around 6500ft under this area
of heavy precipitation. Thus a Winter Weather Advisory will go
into effect for Glacier National Park and the Bob Marshall
Wilderness from 10pm tonight through midday Saturday. Generally
speaking, 1 to 3 inches are expected between 6500 and 7000ft with
2 to 5 inches possible above 7000ft. The highest peaks could
potentially see upwards of 8 inches under extremely heavy
precipitation cores by Saturday morning. Those with plans to
adventure in Glacier National Park or other trails along the
Divide should plan for cold, wet conditions at all elevations with
snow expected across the higher terrain above 6500ft.

Conditions will begin to improve throughout Saturday from west to
east with most precipitation completed by Sunday morning,
revealing a cool, but pleasant Sunday.

Next week will start out with another trough over the northern
Rockies which should increase the shower chances again and keep
temps slightly below normal. High pressure then appears to build
midweek with cold mornings and warm dry afternoons. This lull in
the active fall pattern is transient as computer models continue
to show even colder systems coming down from the Bering Sea into
the Pacific Northwest for Labor Day Weekend into the first week of
September. /ss

&&

.HYDROLOGY...A Flood Watch remains in effect through midday tomorrow
given the potential for convective redevelopment overnight across
Clearwater, Ravalli, Missoula, Lemhi, Granite, Deer Lodge and
Silver Bow County. Some minor urban flooding (ponding of water on
roads) will be possible with some burn areas being threatened.
Meanwhile farther north convective precipitation is expected to
gradually transition to more light, stratiform rainfall by late
this evening. This combined with a slowly falling snow level
overnight will inhibit the potential for flooding and thus the
Flood Watch for Flathead, Lake and Powell County will be in effect
till 10pm. There may be some minor issues with stream decending
from the Divide should the maximum rate of precipitation become
incredibly intense. However these concerns should remain limited
to the east side of the Divide.

.AVIATION...Thunderstorms are starting to develop along the
terrain of southwest Montana and north central Idaho. Thunderstorm
coverage will continue to increase through the afternoon and is
expected to transition into steady precipitation tonight. Aviation
terminals KBTM, KMSO and KSMN will have the best chance to see a
thunderstorm this afternoon with the main impacts being brief heavy
rain, small hail and gusty winds. Ceilings will also significantly
lower and visibility may go below 3 miles under intense rainfall.
KGPI will see some passing showers this afternoon but is expected
to see drier conditions as breezy northeasterly winds move in this
evening. For tonight, expect steady moderate precipitation at
KBTM, KSMN and KMSO. Ceilings and visibility will steadily drop
through Saturday morning which may cause travel delays.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...LAKE WIND ADVISORY until 9 PM MDT this evening for Flathead Lake
     IN THE Flathead/Mission Valleys.

     WINTER WEATHER ADVISORY from 10 PM this evening to Noon MDT
     Saturday ABOVE 6500 FEET FOR THE Potomac/Seeley Lake
     Region...West Glacier Region.

ID...None.
&&

$$

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Dickerson/ss






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