Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 210243
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
843 PM MDT FRI MAY 20 2016

.DISCUSSION...

The upper level low pressure system to the west will keep some
showers in the area. With the moist conditions, fog will continue
across mainly the central and northern portions of the CWA.
Updated to freshen POPs, WX, and QPF. Zelzer

&&

.AVIATION...UPDATED 2350Z.

An approaching low pressure system will keep moist, unsettled
conditions across Central and Southwest Montana through Saturday.
Areas of MVFR/IFR CIGs are expected tonight with VFR conditions
prevailing again by mid-morning. The airmass will become unstable
Saturday afternoon and showers and thunderstorms are expected.
Northeastern portions of the region, which includes Hill, Chouteau,
Blaine, and Fergus counties, could see very strong thunderstorms
with three quarter inch hail and gusts to 50 kts. This includes KHVR
and KLWT.

&&

.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.
At this time, most waterways are expected to remain within their
banks.

&&

.PREV DISCUSSION...
/ISSUED /

Tonight through Sunday... Periods of rain with afternoon
thunderstorms will continue to rotate through the forecast area.
A closed upper level low centered over northern CA places central
MT under a moist and weakly diffluent flow aloft. A cold front has
penetrated through the north central plains with temperatures holding
in the 40s to near 50. Although stable for now, the southerly flow
aloft appears to be eroding the inversion as thinning clouds are
noted on visible satellite imagery. A thunderstorm crossing the
MT/ID border reveals increasing instability further to the south.
Short-term model guidance anticipates a band of precipitation
developing late this afternoon over central MT, which then lifts
north through the evening. CAPE values are rather low due to the
later timing. While thunderstorms are expected, do not presently
anticipate severe weather for the rest of today and tonight.
Further north, along the Rocky Mountain Front, a period of
moderate upslope rain is possible overnight as 700 mb winds turn
easterly.

Precipitation ends for much of the forecast area early Saturday as
subsidence and drier air associated with a right jet exit region
passes over the region. This could allow for warmer temperatures
Saturday with a period of partly sunny skies. Surface winds turn
southeasterly and converge along a trof setting up through
central MT. The upper level jet shifts to the east, providing more
favorable support for ascending air. Hence, the airmass will
become quite unstable, especially east of a line from Havre to
Stanford on Saturday afternoon/evening. 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 with steepening mid-level lapse rates. PN

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  46  62  43  58 /  70  50  40  40
CTB  42  54  40  53 /  60  50  70  40
HLN  45  63  43  57 /  60  40  30  40
BZN  44  63  38  56 /  50  30  20  40
WEY  41  48  32  44 /  60  40  30  50
DLN  43  56  37  53 /  50  40  20  40
HVR  52  70  44  62 /  60  70  60  30
LWT  47  68  41  60 /  50  60  50  40

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

weather.gov/greatfalls



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