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Area forecast discussion
National Weather Service Great Falls Montana
902 am MDT sun may 20 2018

Update...

A few minor updates need this morning. Low clouds and patchy fog
continue to linger across a few areas this morning...mainly across
southwestern areas and the Cut Bank area. Feel daytime heating
should diminish this low cloud deck by early afternoon. Should get
some decent heating then this afternoon...with highs in the low to
mid 70s. Models showing some decent instability developing this
afternoon...with cape values over 1000 j/kg in some areas that
might see elevated dewpoints. Shear is a bit low and there is a
general lack of lift across the area today. Still updated some
chances for thunderstorms later this afternoon into the evening
hours...and expanded slight chances slightly northward into the
Great Falls to Lewistown area...although southwest Montana still has
the best chances for today. Any thunderstorm could produce brief
heavy rain...small hail...lightning...and gusty winds. Those with
outdoor interests today should keep an eye for developing
thunderstorms. Anglin

&&

Synopsis...

Much warmer temperatures are expected today with a slight chance of
showers and thunderstorms, mainly across southwest Montana late
this afternoon and evening. A low pressure system moving slowly
out of the Great Basin this week will bring a chance of showers
and thunderstorms each day, especially in the afternoon and evening
periods with temperatures remaining above seasonal averages.

&&

Aviation...
updated 1115z.

Scattered high MVFR/low VFR ceilings over the mountains of southwest
and central Montana will gradually lift to all VFR through 16z.
However, through 16z, patchy MVFR fog will occasionally move over
kbzn through 16z, while areas of MVFR to LIFR fog moves over kctb.
Otherwise, weak high pressure aloft will keep skies partly cloudy
with VFR ceilings through the period. Weak instability may cause a
few thunderstorms to develop over southwest Montana between 20z and
02z, with erratic wind gusts to 35 kt being the main threat. Coulston

&&

Hydrology...

The Flood Watch has been upgraded to a Flood Warning for the Big
Hole river in Beaverhead and Madison counties. The river gage at
Melrose has risen to flood stage overnight and may stabilize for a
period later today before additional rises are expected throughout
the upcoming week as warming temperatures result in increased
high elevations snowmelt.

A Flood Warning continues for the Lincoln area with no
significant changes expected to the ongoing flood situation there.

The next location of concern is the Jefferson river at Three Forks,
which may approach minor flood stage later today and is forecast
reach moderate flood stage later this week. Additional locations in
the Missouri headwaters area are expected to approach or exceed
flood stage in the coming days, including the Missouri River near
Toston and the Gallatin river near Logan. Hoenisch

&&

Previous discussion...
/issued 515 am MDT sun may 20 2018/

A broad upper level trough along the West Coast will gradually
progress inland over the next several days. A closed upper low is
forecast to develop over the SW US Monday within the larger scale
trough, then open/weaken while lifting out to the northeast Tuesday
through Thursday, slowed by an upper level ridge in place across
western Canada. Models are in fairly good agreement on the overall
pattern evolution through this week with an upper level ridge likely
to follow by early next weekend.

Light S/SW flow aloft over the region will bring significant
warming to the area today with generally dry conditions through
early this afternoon. By late this afternoon/evening, moisture
from a disturbance over the Great Basin will begin to move north
into SW Mt, bringing a threat for widely scattered showers and
isolated thunderstorms.

Additional moisture and shortwave energy lifting NE out of an upper
low in the Great Basin will bring increased coverage of showers and
thunderstorms Monday and Tuesday, mainly during the afternoon/evening
periods. This continues on Wednesday and Thursday as the Great Basin
upper low weakens and lifts out across Montana. Wind shear is generally
weak Monday and Tuesday, limiting the potential for strong
thunderstorm organization, but relatively light flow aloft and above
normal available moisture (anomalous pwat) raises some concerns for
slow moving storms with locally heavy precipitation. Model spread
increases some with the specific track of the weakening low from the
Great Basin across the region Wed/Thursday, leading to somewhat
lower confidence in coverage of precipitation those days with
southern and eastern portions of the forecast area most likely to
see a greater coverage of precipitation Wed/thurs. Temperatures
remain generally above seasonal averages through the upcoming week
with slight cooling Tuesday/Wed followed by additional warming late
this weekend into early next weekend as an upper level ridge becomes
established over the interior western US. Hoenisch

&&

Preliminary point temps/pops...
gtf 73 48 75 51 / 20 20 50 50
ctb 73 46 75 48 / 10 10 40 50
hln 73 47 76 50 / 20 30 50 60
bzn 69 44 75 47 / 20 20 50 50
wey 63 33 65 34 / 20 20 40 30
dln 68 43 69 44 / 30 40 60 40
hvr 77 48 79 53 / 10 10 20 50
lwt 71 47 74 49 / 10 10 30 60

&&

Tfx watches/warnings/advisories...

Flood Warning in effect until 345 PM MDT Monday for the Lincoln area
of west-central Lewis And Clark County.

Flood Warning in effect until 300 PM MDT Monday for the Big Hole
river in Beaverhead and Madison counties.

&&

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