Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 201207
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
605 AM MDT FRI MAY 20 2016

UPDATED AVIATION SECTION

.DISCUSSION...

TODAY THROUGH TONIGHT...The forecast for this time period remains
on track. The upper low over the Pacific Northwest coast will
gradually drift southeast to the Oregon/Nevada border area. The
resulting southerly flow aloft will be moist, unstable, and weakly
diffluent, bringing widespread showers to Southwest and North
Central Montana. Weak surface high pressure from Canada will
hinder thunderstorm development over the northwest plains areas.
The remainder of the area is under the threat for scattered
thunderstorms. The strongest temperature gradient will be along
and east of a line from Bozeman to Chester (including Great Falls,
Havre, and Lewistown), where moderate instability (600 to 1200
J/kg CAPE) may cause some stronger thunderstorms. As a result,
the Storm Prediction Center (SPC) has that area under a Marginal
risk for severe weather, so I have added mention of gusty winds
and small hail for that area this afternoon and evening. Stronger
wind gusts and/or larger hail is still possible from a few storms,
but am addressing the more widespread threat. Shower/storm
activity should decrease after midnight as the airmass starts to
stabilize somewhat. Widespread cloudiness/precipitation will
mostly keep temperatures near/below normal, but warmer highs are
likely in the east. Overnight lows will remain mild. Coulston

SATURDAY THROUGH SUNDAY...An unstable weather pattern will
continue to reside over the region through the weekend. With an
upper level trof just to our west on Saturday, a southwesterly
flow aloft will develop over the region. The airmass will become
quite unstable, especially east of a line from Havre to Stanford
on Saturday afternoon/evening. Thus a few strong/severe storms
will be possible in this region, with numerous
showers/thunderstorms elsewhere. On Sunday, the upper level trof
will continue to push eastward over the region, allowing for
scattered showers to continue. Afternoon temperatures on Sunday
will likely range from 5 to 10 degrees below normal across the
region. Brusda

SUNDAY NIGHT THROUGH FRIDAY...Large upper level low responsible
for the recent pattern change will migrate further north into
Canada for Sunday night into Monday. This will bring a more
straight westerly flow...which could provide a brief break in
precipitation away from mountainous areas due to down sloping
effects. Increased westerly winds will also be found from this
downsloping. Another piece of energy could swing through Monday
afternoon and evening...returning showers and a few thunderstorms
across the area. Large scale trough then sets up for
Tuesday...and continues through Friday. With ample moisture
trapped with this trough...an active weather pattern will likely
be found for mid to late next week. Widespread showers and
daytime thunderstorms will be found through the period. Not a lot
of cold air will be found with this system. Snow levels could
lower to around 6000 feet on Monday morning...then raise
gradually through out the week...and by the end of the week only
lower to about 7500 to 8000 feet. These are still low enough to
bring some snow at times...but only to the highest peaks of west
central MT...and little snow accumulations. Below normal
temperatures will be found Monday and Tuesday...with gradual
warming to near normal temperatures by mid to late next week.
Anglin

&&

.AVIATION...
UPDATED 1205Z.

A moist and unsettled southerly flow aloft will keep a threat of
showers with associated low VFR/high MVFR conditions and widespread
mountain obscuration across Southwest and North Central Montana
through at least 03Z. The airmass will become unstable enough
between 18Z and 03Z to cause scattered thunderstorms as well for the
area along and southeast of a line from KHLN to KHVR. Some storms
along and east of a KBZN to KHVR line could become quite strong with
wind gusts to 40 kt, small hail, and brief heavy rain that may
reduce visibility. The disturbance responsible for these
showers/storms will move north through the area after 03Z, causing
activity to decrease overnight and allow conditions to improve.
Coulston

&&

.HYDROLOGY...
Overall, widespread showers with brief heavier showers associated
with thunderstorms will cause rivers, creeks, and streams across
North Central and Southwest Montana to rise and run fast.
Although, most waterways are expected to remain within their
banks, Clear Creek in Blaine County may exceed flood stage, at
least briefly, Friday evening. Therefore, a Flood Watch is in
effect for a portion of Clear Creek near Chinook through this
weekend. A warning may be issued if the exceeding of flood stage
is imminent.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  59  45  63  42 /  50  60  40  50
CTB  55  42  54  40 /  40  90  50  60
HLN  61  45  64  43 /  80  70  50  40
BZN  65  44  63  39 /  70  50  40  30
WEY  58  40  48  33 /  60  40  50  30
DLN  59  43  57  36 /  80  50  40  30
HVR  69  52  69  44 /  50  60  60  70
LWT  66  47  68  41 /  30  60  60  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch for Clear Creek in Blaine County through Sunday night.

&&

$$

weather.gov/greatfalls



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