Area Forecast Discussion
Issued by NWS Glasgow, MT

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FXUS65 KGGW 252003

Area Forecast Discussion
National Weather Service Glasgow MT
203 PM MDT Tue Jul 25 2017

.SHORT TERM...for northeast Montana...tonight through Wed...

Current Water Vapor imagery showing a slug of monsonal moisture
moving north through the central Rockies. However zonal flow
across the southern Prairie provinces is shunting this moisture
from coming into Montana and less so into our area. An upper ridge
is expected to build in after a vort max over Alberta moves to the
east. The upper vort max should keep any chance for showers well
to our north through Wednesday.

At the surface, weak high pressure over Alberta will slip off to
our east Wednesday, setting up a southeast flow. This will help to
draw in warmer air to the region and temperatures should reach
the mid 80s to lower 90s Wednesday afternoon. TFJ

.LONG TERM...Wed night through Tue...

The upper flow pattern over the West, starting Wednesday evening,
will consist of an upper-level ridge axis centered over the
Northern Rockies, a large trough over Alaska and the Gulf, and a
diminishing short-wave low over Northern California. This small
low pressure area will have been feeding Pacific moisture into
the southwest flow into Montana. Finally, there will be a short-
wave disturbance in the flow aloft through Saskatchewan.

What all that means for the local area Wednesday evening is there
will be a NW to SE boundary through Northeast Montana. That
frontal boundary will lie between the hot air of the southwest and
cooler air to the northeast. The upper flow from the west will
find moderate instability in our southwest corner for the
possibility of isolated evening thunderstorms. These general
T-storms may spread across the area overnight into Thursday

Thursday through Friday, the upper ridge moves fully over the
local area, establishing a return to hot (mid 90s) temperatures
and moderately unstable conditions. With continued southwest flow
aloft channeling in moisture, expect afternoon/evening
thunderstorms to develop. Due to the added moisture, some of these
T-storms may become strong. The Storm Prediction Center suggests
that the modest instability might limit the hail production. The
main threat will be the afternoon/evening damaging wind potential.
This is not good news for fire weather. Although, any T-storm
that does get going, there could be localized wetting rain.
Overnight an embedded low pressure disturbance in the upper flow
will continue to develop scattered showers through Friday and into
the evening.

The weekend could initially see a weaker version of the previous
couple of days with generally zonal flow aloft with moisture along
boundaries for more T-short activity. Then Sunday another ridge
begins to grow across Montana. But the ridge looks not to be
destined to last for our area. Models then go totally out of
phase Monday and beyond.

Edits, using FB, were limited to the period through the remainder
of the work week and Saturday.


Previous discussion -
Synoptic Setup: long term begins with a hot high pressure over
the western third of the CONUS. A ridge runs north from here
through Southeastern Idaho, central Montana, Alberta, and the
southern Northwest Territories. Farther west a large trough is
forming over the Gulf of Alaska while farther north modified polar
air is dropping into the northwest Territories.

Wednesday night through Friday: Top of the ridge will move
through the Canadian Prairies while the base will pass over
Montana. northeast Montana will remain under the right side of
the axis maintaining stable conditions through Thursday. Thursday
night the left side of the axis will pass over destabilizing the
atmosphere and with the modified polar air near the surface and
pacific moisture streaming in from the west conditions will rise
for the chance of showers and thunderstorms.

Friday night through Sunday: The blunted ridge will attempt to
rebound to the west through Idaho and southern British COlumbia
cutting off pacific moisture stream and heating up the forecast
back into the mid to upper 90s.

Sunday night into Monday: Confidence drops off at this point. A
shortwave originating from southern British Columbia blunts the
ridge and travels over the hump into the southern Canadian
Prairieswhere it may drop a weak front across the region and
trigger a few isolated showers.

Monday night onward: a rebounding ridge the the west and warm dry
conditions begin yet again.    GAH



VFR expected through Wednesday, although smoke from the Lodgepole
Complex fires may cause unpredictable visibilities at times.

The storm system will move farther east tonight. This will bring
general clearing across the north. However, with the frontal
boundary being fed with moisture from the southwest, expect SCT-
BKN mid-level clouds spreading across the south.

Wind: Mainly northwest at around 10kt today. Wind will then veer
to the east overnight, becoming southeasterly Wednesday.




Surface high pressure will allow for light winds tonight as it
crosses the area then positions itself to our east or northeast
Wednesday. This will allow winds to become light southeast and
last into Thursday. Also humidities will not drop into the teens
so critical conditions are not expected.

The next disturbance will then move in on Thursday, marked by
developing scattered thunderstorms. These thunderstorms move
through mainly Thursday Night and into the eastern zones on
Friday, but an isolated storm may show up as early as Wednesday
Night. Unlike the recent past thunderstorms, these will move
slower, so some rainfall is likely.

Otherwise, expect a return to hot and dry conditions late in the
week. TFJ


Dense Smoke Advisory until 9 PM MDT this evening for Garfield.


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