Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 220506

1100 PM MDT Mon Jul 21 2014

Update this evening to trim areas of higher precip probabilities
and tweak winds. Showers over Blaine County have been diminishing
this evening, with other areas over southwest Montana. One area
from near Anaconda to Townsend will persist this evening. A
disturbance over Idaho will continue to push into southwest
Montana overnight and during the early morning hours on Tuesday.
This will continue isolated showers/thunderstorms over southwest
MT overnight. db


VFR conditions will prevail through the next 24 hours. Scattered
showers and thunderstorms across Southwest Montana continue until
about 09Z. MVFR conditions are possible near showers and
thunderstorms. Mid level ceilings remain after 12Z. Unsettled
conditions remain over Southwest Montana and a chance of showers and
thunderstorms will return Tuesday after about 18-20Z.


/ISSUED 600 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. Waranauskas

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.


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 /  60  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 /  20  30  20  30
LWT  56  86  58  90 /  50  30  20  30



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