Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 261137

Area Forecast Discussion
National Weather Service Great Falls MT
537 AM MDT Fri May 26 2017

Aviation Section Updated


The upcoming Memorial Day weekend looks to be dry for most areas.
However, each afternoon an isolated shower or thunderstorm cannot
be ruled out, especially in the mountains. Temperatures will
gradually warm through the weekend, with unseasonably warm
temperatures likely by the middle of next week. A better chance of
showers and thunderstorms may return by late next week.



Today through Sunday...Through the Memorial Day Weekend, the local
area will be sandwiched between a departing upper level trough to
the east and, to the west, an upper level ridge trying to build
in. This will leave the forecast area in a NW flow aloft. Short
term models bring at least two, maybe three, weak shortwaves
through during this timeframe, but differ somewhat on the timing
and associated moisture. Basically, the takeaway for those with
outdoor plans this weekend is shower or thunderstorm activity
looks isolated/widely scattered. No all-day rain is expected, but
with steep lapse rates, afternoon heating, and some low level
moisture, it likely won`t take much to generate a few showers and
thunderstorms (mainly in the afternoon/early evening hours). Areas
in and around the mountains may tend to be favored with the NW
flow setup, but hard to say with great confidence that
valleys/plains will stay completely dry.

For today, the greatest concentration of showers will be across far
SW MT as this area will remain in a somewhat convergent flow on the
north side of an upper low swinging through southern Idaho.
Elsewhere, colder temps aloft combined with afternoon heating should
lead to isolated showers and perhaps a thunderstorm. The low levels
may be a bit drier on Saturday, limiting shower/storm activity to
the mountains. On Sunday, a weak front may drop south into the area
and stall from Cut Bank SE through Billings. For now, kept pops
mainly limited to the mountains once again. However, it is entirely
possible that there will be enough convergence/lift for isolated
showers/storms to form out over the plains along the front and this
will continue to be re-evaluated in later forecasts. With
increasingly warmer temperatures, it will likely be more unstable on
Sunday. Shear looks fairly weak, but there may be enough
shear/instability for a few stronger storms capable of gusty winds
and perhaps some small hail. Martin

Sunday night through Friday...An upper level ridge of high pressure
will reside over much of the region from Sunday night through
Thursday. Expect a few isolated showers/thunderstorms to move
through the region from Sunday night through Thursday. The best
chance for any precipitation will be in the mountains...but a
passing shower/thunderstorm can`t be ruled out at lower elevations.
A gradual warming trend to much above normal temperatures is
expected by Wed/Thu. Then main question in the later half of the
extended then becomes...does the upper level ridge continue to move
east into the Upper Midwest on Friday, allowing for a good chance
for showers and thunderstorms to develop over the region per the GFS
model...or does the ridge strengthen over the northern
Rockies...allowing for hot/dry conditions to develop by next weekend
per the EC model. For now...I blended the solutions...but
adjustments are likely to the forecast for late next week and into
next weekend. Brusda


Updated 1137z.

An upper level low passing by to the south (over southern Idaho)
will keep generally BKN skies and showers around across SW MT today.
For now, confidence is too low to include any prevailing group with
precip at KEKS/KBZN, but this will be monitored through the day.
Elsewhere, SCT/BKN mid level clouds can be expected as waves of mid
level moisture drop SE out of Canada. By this afternoon, there may
be just enough instability for isolated SHRA and perhaps a TSRA
across much of the forecast area. Overall, most SHRA/TSRA today
should be brief and significant drops in VIS/CIGS are not expected
at this time. Martin


GTF  63  44  72  47 /  20  10  10  10
CTB  64  42  72  45 /  20  10  10  10
HLN  64  44  73  48 /  20  10  10  10
BZN  59  38  68  42 /  30  20  10  10
WEY  48  29  59  32 /  50  30  10  10
DLN  59  37  67  41 /  40  10  10  10
HVR  68  44  75  46 /  20  10  10   0
LWT  59  40  66  44 /  20  10  10  10



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