Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 161002
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
402 AM MDT Mon Jul 16 2018

.SYNOPSIS...

Showers and thunderstorms are expected this afternoon and evening
across Southwest Montana as an upper level disturbance moves into
the Northern Rockies. These showers and storms are then expected
to move northeast into portions of Central Montana during the
morning and afternoon hours on Tuesday. Some storms today could
become strong across extreme Southwest Montana near the Idaho and
Wyoming borders, with gusty winds being the primary threat. In
addition, brief periods of heavy rainfall are possible with the
showers and storms today into Tuesday.

&&

.DISCUSSION...

Today through Tuesday night...main forecast concern is precipitation
chances across Southwest and Central Montana from the late morning
hours today through Tuesday afternoon.

An interesting H500 pattern to begin the forecast period, with an
upper level ridge axis extending well north into northern British
Columbia/Alberta and a shortwave undercutting the ridge from the
Northern California to eventually Southwest Montana. As just
mentioned, an H500 shortwave off of the Northern California Coast
this morning will lift and slide east northeast to Western Oregon/
Central Idaho by this evening, before continuing its eastward
movement over Southwest and Central Montana Tuesday morning/early
afternoon. Fairly impressive synoptic scale divergence and
subsequent ascent will overspread Southwest and then Central
Montana from this afternoon and into the afternoon on Tuesday.
This lift combined with abundant moisture (PWATs at or near the
90th percentile) and instability, will lead to the development of
rain showers and thunderstorms this afternoon and into the day on
Tuesday. While 0-6km bulk shear is fairly weak (20-30kts), given
the modest instability present today, can not rule out some storms
becoming strong across extreme Southwest Montana. The main threat
with any strong storm would be gusty winds and frequent
lightning. Brief periods of heavy rainfall are also possible today
and into the day on Tuesday.

Wednesday through Friday night...upper level ridging will gradually
setup/re-develop and amplify over the Northern Rockies, leading
to above normal temperatures and dry conditions. H700-H500 cross
barrier flow around 30kts will lead to dry downsloping winds
across Southwest and North Central Montana on Wednesday. Breezy
surface winds of 20 to 30 mph are expected by the afternoon hours
on Wednesday as deep mixing occurs. By Thursday the ridge axis
begins to move overhead, which will lead to less breezy conditions
across the region. Upper level flow then begins to increase and
turn to the southwest on Friday as a potent shortwave moves
towards the Pacific Northwest and British Columbia.

Saturday through Sunday...models are in good agreement that an H500
trough will move across Southern Canada this upcoming weekend. If
this upper pattern holds with subsequent model runs, a cool down
would be expected for portions of North Central and Southwest
Montana. At this time, precipitation chances look to stay well
north into Canada. - Moldan

&&

.AVIATION...
Updated 0500Z.

Mainly VFR conditions are expected through the 1606/1706 TAF period,
however, a disturbance aloft centered just offshore of far-northern
CA will move northeastward into central ID by 06Z/Tue. Accordingly,
isolated to scattered showers and thunderstorms are expected
over/near the higher terrain of southwest and central MT between
16Z/Mon and 06Z/Tue, with the best potential for this activity
spanning Mon afternoon and evening. Erratic surface wind gusts of 30-
50 knots and brief MVFR are possible in/near any shower or storm.
Otherwise, light and variable winds are expected at the remaining
terminals. - Jaszka/Moldan

&&

.FIRE WEATHER...

Elevated to near critical fire weather concerns exist for the day
on Wednesday as upper level ridging builds back in following
Monday/Tuesday`s disturbance. Zonal flow through the mid- to upper
levels will lead to downsloping winds across the Southwest and
North Central Montana, which will lead to warming temperatures and
dry condtions. This pattern will lead to critical minimum
relative humidity values across zones 112/ 113/114/115/116 and
northern portions of 117/118, with some areas falling as low as 8
to 15 percent. Wind gusts of 20 to 30 mph are currently expected
in combination with these dry conditions, with gusts as high as 45
mph along the Rocky Mountain Front. Depending on the state of
fuels, future Fire Weather Highlights may be needed. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  93  59  89  57 /   0  20  30  20
CTB  91  52  91  55 /   0  10  10  10
HLN  92  59  83  55 /  20  50  60  20
BZN  86  58  80  51 /  30  70  70  20
WEY  77  46  74  38 /  80  70  50  20
DLN  83  53  82  51 /  40  50  30  10
HVR  92  57  94  60 /   0  10  20  30
LWT  88  57  81  54 /  10  40  50  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

http://www.weather.gov/greatfalls



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