Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 182027
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
227 PM MDT Fri May 18 2018

.SYNOPSIS...

Widespread rain or rain showers will continue to affect much of
north central and southwest Montana this afternoon and into the
evening hours as moisture circulates into the region around a
broad low pressure system. Drier conditions develop this weekend
along with a warming trend.

&&

.DISCUSSION...

A broad upper level trough remains over the western US with several
embedded shortwaves slowly ejecting east across MT this afternoon
while an upper level ridge builds into BC. Upper ridge in BC this
afternoon gradually shifts SE into the central Conus over the
next several days while additional energy moves into the western
US to maintain a weak but persistent upper level trough. Easterly
mid level flow this afternoon transitions to
Southerly/southwesterly late this weekend, then back to
east/southeasterly by mid to late next week as upper level
troughing reorganizes to the SW of MT. Cool surface high pressure
has spread south across the region and will slide SE through the
weekend with broad low pressure developing across the interior
Western US through next week.

Deep/moist easterly flow will maintain scattered to at times widespread
precipitation across much of the forecast area with steadiest
precipitation through the afternoon hours generally along the
mountain/plains interface over central and western portions of
north-central MT before precipitation gradually winds down
tonight. Additional precipitation amounts through tonight of 0.15"
to 0.35" are likely over areas W/SW of a line from Cut Bank to
Loma to Lewistown up to the Rocky Mtn Front and Central MT Mtns
where E/NE upslope flow will be most effective. Snow levels could
fall as low as 6500 at times through this evening, though any snow
accumulation would likely be limited to locations above 7500 ft.
Temperatures begin to rebound on Saturday then warm to above
seasonal averages Sunday through much of next week as flow aloft
becomes more southerly. Saturday and Sunday look relatively dry as
the forecast area will be under weak ridging in between
shortwaves to the east and west but light flow aloft and afternoon
instability could still produce a few isolated showers. Warming
next week and upper level troughing developing to the
west/southwest will bring increased instability and chances for
precipitation as next week progresses. Models indicate somewhat
better coverage of showers and thunderstorms over SW MT, but
convective nature of the precipitation and degrading model
agreement on specifics of the upper level pattern by late next
week lead to lowering confidence. - Hoenisch/Moldan

&&

.AVIATION...
Updated 1722Z.

MVFR/IFR/LIFR conditions with mountain obscuration will continue
through at least much of the afternoon for the forecast area. A low
pressure system combined with northerly upslope winds will continue
widespread rain and mountain snow across the area for much of today,
with widespread rain gradually becoming more scattered through the
evening hours and extending into early Saturday. Conditions will
very gradually improve to VFR for much of the area overnight as the
system starts to move east out of the area. However, favored upslope
areas (KCTB KGTF KLWT) will keep MVFR ceilings, despite the decrease
in areal coverage of precipitation. CC/Coulston

&&

.HYDROLOGY...

Lewis and Clark County...Flood Warning continues for the Lincoln
area for continued snowmelt runoff, in addition to recent rainfall
runoff from the past 24-36 hours. Radar estimates of around a half
of an inch of rain has fallen across the Lincoln Valley over the
past 24 to 36 hours, with upwards of one to one and a quarter inches
falling in the mountains. Tenmile Creek near Helena remains high but
below flood stage with additional snowmelt and rainfall runoff
keeping levels high and a flood advisory continues, though we are
not currently anticipating significant impacts across the Helena
Valley.

The Flood Warning for the Sun River affecting the communities of
Simms, Fort Shaw, Sun River, and Vaughn has been cancelled. While
isolated pockets of heavier precipitation have occurred over the
past 48 hours within the Upper Sun River Basin, amounts across the
remainder of the basin have been lower than what was forecasted
earlier this week. These lighter observed precipitation amounts have
subsequently brought the forecasted river levels down below flood
stage for both the Simms and Vaughn forecast points through the
weekend and into early next week. While flood stage is now not
currently anticipated to be reached over the weekend or during the
first half of the upcoming work week on the Sun River, the river
will remain high if not bankfull at most locations. Future flooding
on the Sun River to end the month of May is possible as additional
releases from Gibson Dam occurs due to melting snow in the mountains.

The Flood Watch continues across portions of North Central Montana
until 18z Saturday. Light to occasionally moderate precipitation
through the evening hours today combined with runoff from
snowmelt/recent rainfall will likely bring rises to some waterways
as well as local ponding of water in poor drainage areas.

The Flood Watch for the Big Hole River in Southwest Montana
including the counties of Beaverhead and Madison has been cancelled.
While isolated pockets of heavier precipitation have occurred over
the past 48 hours across portions of the Big Hole River Basin, the
river is no longer expected to go above flood stage this weekend.
Future rises on the Big Hole River, potentially to flood stage, may
occur over the latter half of May as temperatures warm and the
remaining snowpack begins to melt and runoff into the river.

Looking ahead to next week, warming and potential for precipitation
could bring increases to the Missouri upstream of Canyon Ferry lake
and its tributary rivers including the Jefferson and Gallatin Rivers
where latest river forecast guidance has levels near flood stages by
mid to late next week. - Moldan/Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  40  58  42  73 /  90  40  10  20
CTB  40  56  38  72 / 100  60  10  10
HLN  41  60  41  73 /  70  60  10  40
BZN  38  58  39  70 /  70  60  20  50
WEY  33  56  33  62 /  70  90  50  50
DLN  38  59  38  68 /  60  50  30  50
HVR  42  64  44  76 /  60  30   0   0
LWT  37  54  39  70 / 100  60  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Flood Warning in effect until 3:45 PM MDT Saturday for the
Lincoln area of west-central Lewis and Clark County.

Flood Advisory in effect until 11:15 AM MDT Saturday for the
Helena Valley.

Flood Watch until 6 PM MDT this evening Cascade...Eastern
Glacier...Eastern Pondera...Eastern Teton...Judith Basin...
Meagher...Northern Rocky Mountain Front...Southern Rocky
Mountain Front.

&&

$$

http://www.weather.gov/greatfalls



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