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Area forecast discussion
National Weather Service Great Falls Montana
1142 am MDT Tue Sep 26 2017

Updated aviation discussion

Update...

Area remains under a northerly flow aloft today which is moist
and slightly unstable in the low levels. This will maintain
mostly cloudy skies with some breaks of sun at times this
afternoon. Any showers will be light and primarily focused along
them Continental Divide and central Montana mtn ranges today.
Temperatures top out near or just shy of seasonal averages in most
areas this afternoon. Hoenisch

&&

Synopsis...

Isolated to scattered showers will be possible through the day
and into the evening hours tonight, mainly over the higher
elevations along the Continental Divide, north central, and
southwest Montana Mountains. Precipitation amounts with any
shower will be light, generally less than five hundredths of an
inch. Temperatures will warm through the week, with Thursday and
Friday being the warmest days. A disturbance may then move into
the northern rockies by the weekend, bringing cooler temperatures,
breezy conditions, and increasing precipitation chances.

Aviation...
updated 1742z.

VFR conditions will prevail through Wednesday morning. Area remains
beneath a north/northwesterly flow aloft with weak embedded
disturbances and lingering moisture in the mid levels. This will
result in times of broken mid-level cloud-cover today before
clearing arrives tonight. A few isolated showers are possible in the
mountains today as well. Hoenisch

&&

Discussion...

H500 ridging axis was beginning to move into the Pacific northwest
this morning, and will continue to build east towards the northern
rockies through the day on Thursday. Over much of the short term
period, the northern rockies will remain beneath northwest flow
aloft. Within this northwest flow, a very subtle and weak
disturbance will dive southeast from southern British
Columbia/Alberta through the day today. This disturbance combined
with the weak northwest upslope flow through the h700 to h500
column, will bring a chance for a few showers to portions of
central and north central Montana during the afternoon and evening
hours today/tonight (especially over the little belt and snowy
mountains). Precipitation amounts with any shower are expected to
be light, generally less than five hundredths of an inch. While
very weak instability and shear will be present this afternoon,
lifted indices remain above 0c through the h850-h500 column, so
kept any mention of thunder out of this forecast.

Following today's disturbance/precipitation chances, north central
and southwest Montana are expected to remain dry as the h500 ridge
continues to build into the region. Temperatures through the short
term will warm each consecutive day, with most locations rising
above normal values by Wednesday (save for a few locations across
southwest montana). Thursday will be the warmest day of the short
term period, with all locations across the region above normal (most
locations across north central Montana on Thursday will be some 5 to
10 degrees above normal). Would not be completely surprised if a few
locations along the hi-line and near the Missouri River valley
approach 80 degrees by Thursday given 1000mb-500mb thickness values
of 565-570dam (which local studies have shown to support
temperatures near 80 degrees). While temperatures will be warming
and precipitation chances will all but be non-existent Wednesday and
Thursday, have no fire weather concerns given recent soaking
rainfalls (for most locations), non-critical minimum relative
humidity values, and light winds. - Moldan

Thursday night through Tuesday...forecast models have come into much
better agreement with the solution through Sunday night, increasing
confidence in the forecast. However, they diverge with the solution
Monday through Tuesday. Weakening high pressure aloft will keep the
area dry with temperatures of 5 to 10 degrees above normal Thursday
night through Friday evening. A deep longwave trough of low pressure
will then move into the Pacific northwest Saturday, and the
resulting moist southwest flow aloft will bring a good chance of
showers and gusty southwest winds to the entire area, along with a
chance of thunderstorms to southwest Montana. As the trough moves
over the forecast area late Saturday night into Sunday, there will
likely be a decrease in shower activity, but temperatures will drop
to between 10 and 15 degrees below normal by Monday. Moisture
wrapping around the low pressure system and a shift to more
northerly upslope winds will likely bring another round of showers
Sunday afternoon through Monday. The forecast models have just come
into agreement with this system, so there still may be some
uncertainty in the timing and strength of these features. The snow
level will gradually fall to between 6000 and 7000 feet msl, which
will likely bring measurable snow to areas above Mountain Pass
level. For Monday through Tuesday, the models show significant
differences. The European model (ecmwf) moves the low pressure system northeast out
of the area for drying and slightly warming conditions. However, the
GFS keeps a deep trough over the western United States, which would
continue the cool and unsettled pattern. Recent history shows that
these systems have tended to be deep and slow-moving, so am going
more with the GFS solution for now, but will continue to monitor
this ever-changing situation.
Coulston

&&

Preliminary point temps/pops...
gtf 65 41 70 41 / 10 10 0 0
ctb 66 39 70 39 / 10 10 0 0
hln 63 42 69 41 / 10 10 0 0
bzn 60 35 66 35 / 10 10 0 0
wey 51 25 58 26 / 10 10 0 0
dln 56 34 65 36 / 10 10 0 0
hvr 69 42 73 41 / 10 10 0 0
lwt 62 40 68 40 / 20 10 0 0

&&

Tfx watches/warnings/advisories...
none.
&&

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