Area Forecast Discussion
Issued by NWS Great Falls, MT

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243
FXUS65 KTFX 190957
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
357 AM MDT Sat May 19 2018

.SYNOPSIS...

Lingering moisture circulating around a broad low pressure system
will keep cloudy and cool conditions across the region today.
Scattered showers will decrease today and come to an end this
evening with drier and warmer conditions moving in for Sunday.
Warm but unsettled conditions with a chance of afternoon and
evening showers and thunderstorms is anticipated through much of
next week.

&&

.DISCUSSION...

A blocking weather pattern continues with an upper level ridge
located over BC/AB this morning and weak trough to the south over
the NW US, extending SE into the central US Plains. The block
breaks down with the western Canada ridge progressing east over
the next several days but the US west coast trough deepens with a
low cutting off in the CA/NV vicinity early next week, setting up
another high-over-low block with SW US energy lifting out across
MT around next Wed/Thurs. Models have come into fairly good
agreement on the evolution of the upper level pattern next week,
leading to a higher degree of confidence in the overall forecast.

Lingering moisture in E/NE low-mid level flow will keep cloudy
conditions over much of the area this morning with scattered showers
along with some areas of fog and drizzle early. By this afternoon,
some partial clearing is possible, but this this will act to
destabilize the airmass some for additional isolated to scattered
showers and possibly a weak thunderstorm, mainly over SW MT.

Sunday looks to be the driest day of the upcoming period with
southerly flow aloft bringing significantly warmer temperatures. Weak
shortwaves ejecting out of the west coast upper trough will bring
a chance of showers and thunderstorms Monday and Tuesday
afternoon/evening with more widespread coverage of showers and
thunderstorms Wed/Thurs as flow aloft backs to the E/SE and the SW
US upper low begins to lift out across the region. Hoenisch

&&

.AVIATION...
Updated 0535Z.

A low pressure trough over the Pacific Northwest and into the
northern Rockies will continue VFR/MVFR ceilings with areas of fog
and light rain/mountain snow through 15Z. Some more favored upslope
areas (KGTF KLWT) could have IFR ceilings at times. As the trough
lifts north after 15Z, ceilings should generally rise to VFR, but
weak instability between 18Z and 00Z could cause showers with brief
MVFR ceilings. The airmass could become unstable enough in far
southwest Montana (KEKS, possibly KBZN) to cause isolated
thunderstorms. The airmass will stabilize after 01Z, for decreasing
clouds and showers.  Coulston

&&

.HYDROLOGY...

No changes to flood products overnight. A flood watch remains in
effect though this morning for portions of north central MT where
moderate precipitation fell in the last 48 hrs, though
precipitation is ending and no reports of flooding have been
received so far. A flood warning continues for the Lincoln area
and flood advisory for the Helena valley area with no significant
changes expected to the ongoing flood situation in those areas.

Several SW MT rivers bear watching over the next several days as
temperatures warm and high elevation snowmelt resumes. The Big
Hole River near Melrose remains just below flood stage and could
reach flood stage temporarily this afternoon/evening with a more
sustained rise above flood stage expected later this weekend into
next week as temperatures warm. Several stations in the Missouri
headwaters are forecast to approach or exceed flood stage by the
middle of this upcoming week and will be monitored closely. Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  59  41  73  48 /  40  10  20  20
CTB  56  37  73  45 /  30  10  10  10
HLN  60  41  74  48 /  50  10  30  30
BZN  57  39  71  44 /  30  10  40  40
WEY  55  33  63  34 /  40  40  40  40
DLN  59  39  68  43 /  40  20  40  50
HVR  63  44  76  48 /  10   0   0   0
LWT  54  40  70  45 /  40  10  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 12 PM MDT Saturday for 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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