Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 172316
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
516 PM MDT Thu May 17 2018

UPDATED AVIATION

.SYNOPSIS...

Scattered showers and thunderstorms will transition to more of a
widespread rain late tonight through Friday and Saturday.
Temperatures will continue to trend below normal through Saturday,
but will begin to trend to normal or even above normal by Sunday.

&&

.DISCUSSION...

Remainder of this afternoon through Thursday next week...

Cyclonic mid-level flow will continue to support the development
of showers and thunderstorms across our area, with thunderstorms
favoring areas where greater surface destabilization has occurred
to the south and east of a cold front across our central
mountains. This cold front will be reinforced by another surface
cold front which will push well south through the state by late
tonight. This cold push will allow for more widespread rain
development tonight into most of Friday across our entire area.
By Saturday, increasing southwest flow aloft will begin to push a
warm front back northeast across our area, which will bring a
return to a warmer temperature regime, but also the chance for
showers and thunderstorms to parts of our region for most days
next week from Sunday onwards, as weak mid-level cyclonic flow
associated with weak upper level systems meander near north-
central and southwest MT.


&&

.AVIATION...
Updated 2316Z.

Widespread mountain obscuration due to mainly MVFR to IFR CIGS is
expected next 24-hours as the two primary low pressure centers of a
complex disturbance aloft meander over/near the Pac NW. Scattered
rain showers and isolated thunderstorms this evening will give way
to widespread light to moderate rain after 06Z/Fri. Occasional CIGS
down to LIFR and VSBY down to MVFR may occur in steadier rain after
06Z/Fri.
Jaszka

&&

.HYDROLOGY...

The Flood Warning for Lewis and Clark County has been reduced to
only include the Lincoln area as of Thursday afternoon. Local
emergency officials in Lincoln continued to report that water levels
remain high/have risen in and around Lincoln from continued snowmelt
runoff, in addition to recent rainfall runoff from the past 24
hours. While flooding still continues in areas of the Helena Valley,
river gauge reading along Tenmile Creek near Helena continue to
suggest that water levels are receding. Some rises in water levels
are possible on Tenmile and surrounding creeks in the Helena Valley
over the next several days due to additional snowmelt and rainfall
runoff. For this reason, have issued a Flood Advisory to cover the
expected nuisance impacts, with significant property impacts (as
what was observed the prior week) not currently anticipated across
the Helena Valley.

For the Sun River, the river was near flood stage as of Friday
afternoon near Simms and in "Action Stage" near Vaughn. Additional
rises on the Sun River are expected, with the river cresting at 8.2
feet near Simms Saturday afternoon (which would be similar to the
crest that occurred last week) and 6.2 feet near Vaughn on Sunday
(which would be about 1/2 foot higher than the previous crest last
week).

For the Flood Watch across portions of North Central Montana, with
the potential for moderate to at times heavy rainfall later this
afternoon through Friday, some waterways could exceed flood stage.
Thus, the flood watch will continue.

For the Flood Watch concerning the Big Hole River, current river
model guidance indicates that the river near Melrose will rise above
flood stage by Saturday afternoon. Given the Big Hole`s slower rise
than anticipated over the past several days, will continue the Flood
Watch for now. Future Flood Warnings are possible over the next 24
to 48 hours for the Big Hole River. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  44  47  41  59 /  90  90  60  40
CTB  41  48  39  58 /  90  90  60  30
HLN  45  51  41  60 /  80  80  60  40
BZN  45  55  39  58 /  70  60  60  40
WEY  33  56  32  58 /  40  50  30  50
DLN  42  54  38  60 /  80  60  50  50
HVR  46  54  41  65 /  80  80  30  10
LWT  43  48  37  55 /  80  80  60  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Flood Warning in effect until further notice for the Sun River
from Simms to Vaughn.

Flood Advisory in effect until further notice for Lewis and Clark
County.

Flood Watch through Friday afternoon Cascade...Eastern Glacier...
Eastern Pondera...Eastern Teton...Judith Basin...Meagher...
Northern Rocky Mountain Front...Southern Rocky Mountain Front.

Flood Watch through Friday afternoon Beaverhead...Madison.

&&

$$

http://www.weather.gov/greatfalls


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