Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 280610 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Great Falls MT
1150 PM MDT FRI MAY 27 2016

Aviation Section Updated.

.UPDATE...Pacific cool front has pushed through much of the Great
Falls forecast area this evening. Scattered/numerous showers and a
few thunderstorms are still expected over eastern portions of
central Montana overnight and into Saturday morning. Some showers
and thunderstorms are also possible over far southern portions of
southwest Montana. However, much of the remainder of the forecast
area will likely see only isolated light rain showers or no
precipitation at all as a drier airmass moves into the region.
Have updated forecast PoPs/weather/QPF to take the latest model
runs and observations into account. Have also made minor tweaks to
max and min temperatures as well as the hourly diurnal temperature
curve based on latest short term model updates. mpj



Drier air is advancing eastward following a Friday afternoon frontal
passage. Skies have cleared along the Rocky Mountain Front. For
eastern portions of central Montana, rain showers will be slow to
dissipate overnight. Hence MVFR conditions prevail at KHVR/KLWT
through early morning. VFR conditions prevail at all terminals
through the day Saturday. PN


/ISSUED 555 PM MDT FRI MAY 27 2016/

Tonight through Sunday...Water vapor pix show a low pressure trof
moving over the Rockies. The airmass has become slightly unstable
and radar shows isolated convective activity across most of the CWA.
A surface cold front is moving through the region...wind shift is
already into Eastern Montana...and cooler, drier air is moving in
from the west. However, convection will linger into the evening
across the southwest and eastern zones. The airmass will become
slightly unstable again Saturday and Sunday afternoons across
portions of Southwest Montana but the change to drier air will keep
most of the region dry through Sunday. West winds aloft will bring
stronger winds across the slopes of the Rockies and adjacent plains
Saturday and Sunday afternoon although, in general, winds will
remain light. Temperatures will remain below seasonal normals behind
the cold front but trend slowly upwards towards seasonal normals by
Sunday. Zelzer

Sunday night through Friday...The period starts with a fairly potent
shortwave trough digging SE into the Pac NW. At this time, the
models still differ on the exact evolution of this feature. The
GFS/CMC/GEFS are stronger and more moist while the ECMWF/ECMWF
ensembles keep the wave open/weaker and more progressive with less
moisture. For now, planning to keep with previous forecasts and WPC
and lean towards the more progressive/slightly drier solution seen
in the ECMWF. Regardless of model choice, it appears that the axis
of heaviest precipitation with this system through Monday will be
across the eastern half of our CWA (ie. along and east of a Bozeman
to Havre line). In this area, the potential exists for up to an inch
of QPF for some locations, even if the drier solutions pan out.
Hydrology-wise, it will be something to keep an eye on given recent
rainfall in that area, especially for small streams/creeks, such as
Clear Creek. With Monday`s system, snow levels look to drop down
into the 7-8k ft range, but could drop a bit lower if the shortwave
slows down/closes off.

Once the Memorial Day system moves out, the models are in decent
agreement with building an upper level ridge across the western US,
something we haven`t seen in awhile. This should bring a period of
warmer/drier conditions. Of note, though, the GFS isn`t quite as
strong with the ridge and actually brings a few weak shortwaves
through the area. Will keep low chance pops in later next week, but
at this time it looks like the best chance of seeing a shower or
storm will be across the higher terrain. Martin


GTF  41  63  41  69 /  20  10  10  10
CTB  37  61  39  65 /  10  10  10  10
HLN  42  64  42  70 /  20  10  10  20
BZN  40  63  39  71 /  50  20  10  10
WEY  34  54  32  60 /  60  30  20  10
DLN  38  62  38  68 /  40  20  10  10
HVR  46  66  42  70 /  80  30  10  10
LWT  42  63  41  68 /  90  30  10  20



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