Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 242040 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Great Falls MT
240 PM MDT Thu May 24 2018



A weak weather disturbance and lingering moisture will result in
scattered showers and a few thunderstorms over southwest Montana
this evening. However, warmer and drier conditions are expected on
Friday. Showers and thunderstorms will increase across western
portions of Montana late Friday with more widespread and
potentially severe thunderstorms expected Saturday.



Tonight and Friday...Upper level ridge continues to build over the
Pacific NW tonight, then shifts eastward across the Northern
Rockies and Montana on Friday. Only isolated showers or possibly a
thunderstorm can be expected over north-central and Central
Montana late this afternoon through this evening, but scattered to
numerous showers and a few thunderstorms can be expected in far
southwest Montana. Friday still looks to be the warmest and
driest of the next several days with most lower elevation sites
seeing afternoon temperatures approach or exceed 80 degrees.

Friday night through Saturday night...Pacific shortwave energy
undercutting the upper ridge into CA Friday night carves out
another closed low over the great Basin this weekend, which is
slow to lift out to the NE through early next week. The first
piece of energy lifts north out of the Great Basin across the
Northern Rockies Friday night, bringing a chance of evening and
overnight convection, primarily to western portions of the
forecast area. Saturday still looks to have the greatest potential
for more widespread and potentially severe thunderstorm activity
as a surface trough shifts eastward across the region under moist
and unstable southerly flow aloft. Timing and location of the
surface trough will be critical to the location of any severe
thunderstorms activity with latest models continuing to suggest
eastern portions of the forecast area will have the best
potential from mid afternoon through the evening.

Sunday through Thursday...The Great Basin upper level low slowly
trudges east-northeast Sunday and Monday for continued mostly
diurnal showers and thunderstorms. Latest models suggest deepest
moisture and greatest coverage of precipitation will be focused
over southern and eastern portions of MT with some potential for
drier air to move into northern portions of the forecast area
which would help limit the coverage of precipitation there. Models
spread increases with the eventual exit of the upper level low
through the middle of next week and arrival of the next upstream
trough. Hoenisch


Updated 1735Z.

Widespread VFR conditions are expected to prevail through late
Friday morning and beyond as high pressure aloft develops. A weak
disturbance moving through the ridge will keep scattered to broken
mid level cloudiness over the area through 02Z. Weak instability
will also likely develop this afternoon, bringing a chance of light
showers and weak thunderstorms to much of the area. However, breezy
westerly downslope winds will hinder cloud/shower development along
the east slopes of the Rockies. Clouds and winds will decrease after
02Z. Coulston/mpj



Additional showers and thunderstorm activity should be light
through Friday so the immediate threat for rapid minor flooding
has lessened. However, snowmelt continues to fuel rises on many of
the mainstem rivers. Flood Watches have been upgraded to warnings
for the Missouri River at Toston and the Gallatin River near
Logan. Additionally, the Flood Watch for the Sun River has been
pushed back into early next week.


GTF  52  82  54  81 /   0   0  30  50
CTB  51  81  52  76 /   0   0  20  50
HLN  51  80  53  79 /   0  20  40  60
BZN  47  78  48  78 /  20  20  20  50
WEY  37  71  38  69 /  50  10  10  30
DLN  44  75  45  73 /  30  40  30  50
HVR  55  84  55  85 /   0   0  10  40
LWT  50  77  52  79 /  10  10  10  50


Flood Watch from Saturday evening through Tuesday afternoon

Small stream flood advisory for Meagher County until 615 PM

Flood Warning in effect until 300 PM MDT Sunday for the Big Hole
River in Beaverhead and Madison Counties.

Flood Warning in effect until 145 AM MDT Monday for the Jefferson
River near Three Forks.

Flood Warning in effect until 1000 PM MDT Saturday for the
Missouri River near Toston in Broadwater County.

Flood Warning in effect until 1015 PM MDT Saturday for the
Gallatin River near Logan.


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