Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 171759
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1058 AM MST Wed Dec 17 2014

UPDATED AVIATION DISCUSSION

.UPDATE...
Have continued to decrease the area of the freezing fog advisory.
Have also lowered high temperatures where the freezing fog is
persisting and already keeping locations cooler than previous
forecast. Otherwise no significant changes to the forecast this
morning. Suk


&&

.AVIATION...
UPDATED 1758Z.
Light snow will be ending at KHVR and KLWT by late afternoon with
conditions improving to VFR. At KCTB LIFR conditions in freezing fog
will also improve to VFR by late afternoon. Local MVFR visibilities
in freezing fog over southwest Montana will end by mid-afternoon.
For tonight through Thursday morning expect scattered snow over the
Rocky Mountain Front. For tonight into early Thursday morning for
KCTB and KLWT some guidance suggests IFR ceilings and/or
visibilities in freezing fog could return but think there will be
enough wind to prevent this from happening so did not mention in
their TAFs. At KHVR IFR ceilings and/or visibilities in freezing fog
could also return but think there is a better chance there relative
to KLWT/KCTB so included in their taf. But some guidance keeps
conditions VFR at KHVR. For late tonight think southwest winds will
dissipate the freezing fog at KHVR but should southwest winds not
develop IFR conditions could linger into Thursday morning.

&&

.PREV DISCUSSION...
/ISSUED 807 AM MST Wed Dec 17 2014/
FXUS65 KTFX 171507
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
807 AM MST Wed Dec 17 2014

UPDATE...
Dense freezing fog is no longer expected over Cascade and Judith
Basin Counties so the freezing fog advisory for those counties has
been canceled.

&&

.PREV DISCUSSION...
/ISSUED 510 AM MST Wed Dec 17 2014/

Tonight through Friday...Central Montana is underneath an area of
weak cyclonic flow aloft. Precipitation associated with a
disturbance in this flow will move east of the zones today. A weak
trof over over the Eastern Pacific will move over the Pacific
Northwest today and the zones will be underneath the southwest flow
ahead of this system. Flow aloft and dynamics will remain weak,
however. Models show very little warming of the airmass yet raise
temperatures from yesterday. With cloud cover and, possibly, areas
of fog, not convinced that all of that warming will occur and will
moderate high temps somewhat. The airmass will remain unsettled
Thursday as the trof over the Eastern Pacific slowly moves east and
and approaches the zones. Little chance of precipitation is expected
with this system. The Rockies and mountains of Southwest Montana
will have the best chance for snow although accumulations will
remain low. Zelzer

Friday Night through Wednesday...A weak shortwave trough will move
through Montana Friday night into Saturday, bringing just a chance
of snow to the mountains. Forecast models continue to indicate that
the pattern will then change for Saturday night through Tuesday. An
upper level high pressure ridge will strengthen off the California
coast and a low pressure trough will deepen from Hudson Bay into the
Great Lakes. This will result in Montana being under a strong
northwesterly flow aloft. The models indicate that the strongest
winds aloft will occur Sunday into Sunday night with the passage of
a disturbance embedded in the flow. However, the models differ on
just how strong these winds will be. The ECMWF only forecasts winds
of 45 to 55 kts at 700 mb, while the GFS generally has 60 to 70 kt
winds. Have therefore used a compromise solution between these two
models with regards to surface wind strength. Despite this
difference, this set-up will bring gusty westerly downslope winds to
much of the area, but especially over the plains of north central
Montana. In fact, some areas along the Rocky Mountain Front could
approach High Wind criteria (40 mph sustained/58 mph gusts).
Confidence remains moderate that High Wind highlights may be needed
for this event. The only concern is that the northwesterly wind
direction is not as favorable for widespread High Winds. There is
also significant moisture with this disturbance, so have continued
to spread the chance for precipitation out onto the plains. Winds
aloft will decrease somewhat after the passage of the disturbance,
but the northwesterly flow aloft will remain fairly moist, so have
kept the chance for precipitation above climatological normals
across the area into Tuesday. A potentially significant weather
system originating in the Gulf of Alaska is then forecast to move
into Montana Tuesday into Wednesday, bringing a very good chance of
precipitation. Because this feature is near the end of the period,
exact timing and types of precipitation are very uncertain.
Regardless, this system will need to be monitored for potential
winter weather highlights, especially considering that it would be
moving into the area during peak holiday travel time. The breezy
winds through Monday will keep temperatures around 10 degrees above
normal. The warmest day of the period appears to be Monday, when
most lower elevation locations should get well into the 40s.
However, with the system moving into the area at the end of the
period, temperatures will likely fall back to near or slightly below
seasonal averages by Wednesday.
Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  35  24  38  26 /  20  10  10  10
CTB  26  21  37  24 /  10  10  10  10
HLN  33  17  33  19 /  20  10  20  10
BZN  28  12  30  14 /  40  10  10  10
WEY  32  16  30  18 /  30  20  20  20
DLN  31  16  33  19 /  10  10  20  10
HVR  29  19  36  21 /  30  10  10  10
LWT  36  22  41  24 /  40  10  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
FREEZING FOG ADVISORY until 11 AM MST this morning Eastern
Glacier...Toole.

&&

$$

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