Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 212102

302 PM MDT Mon Jul 21 2014

This Afternoon through Wednesday Evening...Weak upper-level trof
continues to move across southern Alberta and northern MT
accompanied by thin plumes of Pacific moisture that are already
firing bands of low-topped, pulse-type thunderstorms along and south
of line from Butte/Helena to Lewistown.  Coverage of storms should
increase a bit over southwest MT into this evening as additional
upstream moisture arrives into the area.  Could see one or two cells
briefly reach moderate or strong intensity, but severe weather is
not expected through tonight.  Meanwhile, much of the Hiline and
central MT will remain precip-free as drier airmass spreads southward
behind the trof axis.  As the trof heads into ern MT tonight, a
shortwave ridge will move in from the west on Tues. Forecast models
are showing decent agreement that moisture along the west side of
the ridge will interact with the weak, tail-end portion of today`s
trof that will be draped over central MT, creating scattered
afternoon showers and thunderstorms mainly across the central
counties and southern Rocky Mtn Front.  That convective activity
will dissipate Tues night and quiet weather is forecast through Wed
morning. But conditions take a more serious turn Wed aftn/eve as
widespread high amounts of instability and vertical wind shear
develop over our region ahead of a strong weather system along the
Pacific Coast.  Parameters from latest forecast models all indicate
increasing potential for severe storms, but models are not quite in
synch just yet on location and timing for the worst activity.
However, at this time, there is some consensus that areas along and
north of a Lincoln-Great Falls-Havre line may be the prime location
for large hail and strong outflow winds...with some potential that
the area may spread south to around Helena and Lewistown. We will
continue to refine the threat area and timing with upcoming model
runs tonight and tomorrow.

Wednesday night through Monday...Wednesday evening a weather
disturbance aloft will be moving northeast through the area. This
disturbance is ahead of an upper low that will move into Washington
or far southern B.C. Wednesday night. Although models differ on
details of the precipitation associated with the disturbance have
increased the chances of precipitation especially towards the
Canadian border. Precipitation will come in the form of showers and
thunderstorms and there could be a risk of strong thunderstorms
early in the evening. Thursday afternoon and evening the remnants of
the upper low will swing through the forecast area. Increased low
level downslope could limit the threat of precipitation over the
lower elevations during that time frame. Thursday the flow aloft
should be strong enough for a chance of high winds over Logan Pass
with a small chance elsewhere over the Rocky Mountain Front. Behind
the upper trough a dry airmass will move over the forecast area.
Over the weekend models are on the same page in developing a strong
upper ridge over the western U.S. which could further intensify on
Monday. The upper ridge will result in warming temperatures. It
should also result in no precipitation at least through Sunday.


The rain showers across Central Montana are associated with a
shortwave that will be moving into Eastern Montana during the day.
This will allow the air mass over central and northern portions of
the area to stabilize and showers will diminish. However, flow will
remain unsettled across Southwest Montana and a chance of showers
will continue.  A second shortwave will approach Western Montana
tonight and bring a chance of showers and thunderstorms to the
region again Tuesday. Local MVFR conditions are possible near
showers and thunderstorms. Elsewhere VFR conditions will prevail
through the next 24 hours.


GTF  56  84  62  88 /  20  30  20  50
CTB  51  80  55  86 /  10  40  20  50
HLN  58  87  60  91 /  40  40  30  50
BZN  53  86  54  91 /  50  30  30  20
WEY  40  78  42  82 /  50  20  10  10
DLN  52  83  53  87 /  50  50  30  30
HVR  54  88  59  92 /  10  30  20  30
LWT  56  86  58  90 /  50  30  20  30



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