Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 131131
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
530 AM MDT Fri Oct 13 2017

Updated Aviation Discussion

.SYNOPSIS...

A cold upper level low pressure trough will keep temperatures
well below normal with unsettled conditions through Friday night.
Snow will increase over the mountains today with rain and snow
showers affecting lower elevation areas this afternoon and evening.
Drier conditions are expected this weekend with warming
temperatures early next week as gusty west winds develop.

&&

.DISCUSSION...

Today through Sunday...A deep upper level trough extending from
western Canada into the NW US will move into the Northern Rockies
today and across MT tonight. Main upper level energy associated
with this system moves by to the south of the region, however
cold air aloft and weaker shortwave energy embedded within the
trough will bring a decent chance of showery precipitation to
much of the area. Widespread precipitation will increase over
areas along the continental divide this morning with areas around
Glacier Park most likely to see several inches of snow
accumulation through this afternoon. Expect a decent coverage of
convective showers to develop over the region early this afternoon
as the airmass becomes unstable with some bursts of moderate
snow/rain/graupel at times in stronger cells with some potential
for isolated lightning as well, particularly over SW MT. At the
surface, winds have already shifted to the N/NW near the Canadian
border early this morning in response to high pressure in AB and a
weak pressure gradient currently across MT. By this afternoon
weak low pressure develops over MT with N/NW winds spreading
south. An area of enhanced snow showers may develop along the wind
shift boundary today and shift south across central MT this
evening. Airmass stabilizes tonight with some showers lingering
over the mtns into early Saturday morning. An upper level ridge
then moves onto the west coast Saturday with a drier NW flow
aloft over the region Saturday afternoon through Sunday.
Temperatures warm back to near seasonal averages by Sunday, but
moderate W/NW flow aloft and a lee-side trough of low pressure at
the surface will bring fairly gusty west winds to much of north-
central MT with strongest winds along the Rocky Mtn Front.
Hoenisch

Sunday Night through Friday...By the beginning of next week, we will
continue to see a steady warming trend thanks to a developing
western US ridge. GEFS/EPS ensembles support this with good
confidence given strongly positive height anomalies in place. As we
remain on the northern periphery of the ridge, a developing strong
westerly unidirectional flow will develop across the area. While
many will see mostly sunny skies, some terrain-enhanced clouds will
likely hang around the northern Continental Divide through the
evening hours on Monday. However, as we get into Tuesday, the
aforementioned ridge will begin to drift off to the east which will
allow for a quick-moving shortwave to progress onshore the PacNW. As
heights begin to tightly pack between 700 and 500mb during the day,
it is likely that a rather strong jet will set up, especially across
the Hi-Line with 700mb winds averaging 55-65 kts at times, it will
not take much for afternoon boundary layer mixing to deliver strong
winds to the surface across the Plains, with much stronger wind
potential for mountain ridgetops, especially across the Continental
Divide. Although large discrepancies still exist in GFS and ECMWF
solutions, a blend of either solution still supports rather windy
conditions on Tuesday, which has been highlighted in the forecast
package for generally central MT on north.

Temperatures on Monday and Tuesday will warm nicely given
progressive westerly to southwesterly unidirectional flow. Tuesday
will likely be the warmest day with widespread upper 60`s across the
Plains. As the shortwave trough across the PacNW quickly passes the
state Tuesday night/Wednesday morning, temperatures will bump down a
tad thanks to a passing dry surface cold front but wont cool down by
much. Additionally, this may become the window with the potential
for strongest winds, especially across the Continental Divide and Hi-
Line as the trough passes quickly along the US/Canadian Border.
While the likelihood of a strong wind event is looking very
possible, confidence remains on the low side given model
discrepancies so stay tuned for further updates in regards to
specific potential impacts. By late week on Thursday and on Friday,
the next broad shortwave trough swings across the Pac NW into the
northern Rockies. Look for spotty showers, especially across SW
areas on Thursday and then increasing in coverage across most of the
area by Friday with temperatures 10 to 15 degrees below normal.
KLG

&&

.AVIATION...
Updated 1200Z.

Upper level trough continues to dig into the region today spreading
an increasingly moist and somewhat unstable airmass into the region.
Snow showers will increase along the continental divide this
morning and across much of the region this afternoon as the airmass
destabilizes and an upper level disturbance moves across the area.
VFR conditions will prevail at area terminals through early this
morning with local MVFR/IFR developing at many TAF sites this
afternoon. More persistent -SN is likely at KCTB for a period this
afternoon where CIGS will also be persistently lower. High pressure
moving south from AB and weak low pressure forming across central
and SE MT will bring a gradual shift to NW winds from N to S across
the region later this afternoon and evening. Showers decrease this
evening, but low clouds may linger later into the night before more
significant drying in the low levels arrives Saturday morning.
Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  44  28  47  34 /  40  60  10   0
CTB  37  22  46  31 /  70  20  10   0
HLN  45  28  48  32 /  30  60  10   0
BZN  46  26  46  26 /  40  40  20  10
WEY  35  19  36  18 /  60  50  10  10
DLN  43  21  43  25 /  40  30  10   0
HVR  44  24  48  29 /  40  70  10   0
LWT  45  27  44  30 /  40  70  20   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight MDT tonight Northern
Rocky Mountain Front.

&&

$$

weather.gov/greatfalls



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