Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 220445

1045 PM MDT Sun Sep 21 2014


No update to the forecast this evening as the going forecast covers
current conditions very well. Mid/upper level cloudiness continues to
stream northward into southwest MT from the Great Basin. Precip
shield remains over ID but is gradually shifting northward with
time as well. Chance PoPs overnight are sufficient. Foltz


High pressure continues to provide dry and stable conditions. A
disturbance aloft over southern Idaho will continue to spread mid
and high clouds across extreme southwest Montana and high clouds
across central Montana. Isolated showers over extreme southwest
Montana, mainly along the border with Idaho, are possible after 09z.
Winds will remain light. VFR conditions prevail.


/ISSUED 545 PM MDT Sun Sep 21 2014/

Tonight through Tuesday...Main concern in the short term period
will center around the location of an upper level cutoff low. This
system is currently over Nevada and appears to be moving north-
northeast at a leisurely pace. Model solutions move the low into
southern Idaho by midnight then weaken and shear it eastward
during the overnight period. A residual low is expected to be over
Yellowstone NP early Monday morning and will move gradually
eastward along the Montana-Wyoming border. Main impact from this
system will be isolated showers near the Montana-Idaho border this
evening with showers becoming likely over the border mountains
after midnight and continuing through the day on Monday. Isolated
to scattered showers are possible as far north as Butte by early
morning and Helena by noon. The airmass is expected to become
unstable during the afternoon with precipitable water values up to
two standard deviations above seasonal averages across all of
Southwest Montana and southern portions of Central Montana. As a
result, scattered thunderstorms are expected to develop and could
bring a risk for heavy rains during the afternoon and evening
hours. Precipitation will diminish across the southern half of the
forecast area Monday night as the upper low continues to progress
eastward. This will leave Montana in a weakly unstable southwest
flow aloft with several embedded disturbances that will push
through western Montana Tuesday afternoon. This will bring another
chance for showers and isolated thunderstorms over the western
mountains with a few storms moving out over the plains during the
late afternoon. Temperatures are expected to be comfortably cool
during the overnight hours but afternoon highs will 10 to 15
degrees above seasonal averages. mpj

Tuesday Night through Sunday...An upper level ridge will amplify
over the region early Wednesday while temperatures warm to well
above seasonal averages. The ridge axis moves away to the east on
Thursday as a deep trof moves ashore over the Pacific Northwest.
Wednesday should be the warmest day of the week ahead, and only
slightly cooler on Thursday. Isolated showers and thunderstorms will
begin to develop Thursday night with decreasing stability over the
higher terrain of western and southwest Montana.

Confidence is growing in a solution of widespread showers and
thunderstorms on Friday evening. However, model differences begin to
develop Friday night. The GFS moves the trof through the northern
Rockies more rapidly than the ECMWF and GEM...both of which split
the trof and dig it further south along the west coast. Have raised
POPs on Friday night and Saturday, but keeping below the likely
category given uncertainties in timing and position. Temperatures
will drop back into the 60s as the trof swings through over the
weekend. At this time, it appears any snow would be limited to the
highest mountain peaks Saturday night.


GTF  48  82  52  83 /   0  10  10  10
CTB  42  83  47  79 /   0   0  10  20
HLN  49  82  51  83 /  10  20  20  20
BZN  45  75  46  79 /  20  30  20  10
WEY  38  61  36  68 /  50  60  30  10
DLN  47  74  48  78 /  30  40  20  10
HVR  42  82  48  85 /   0   0   0  10
LWT  46  81  50  83 /   0  10  10  10



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