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Office: TFX

FXUS65 KTFX 130548
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1040 PM MST Tue Dec 12 2017

Aviation Section Updated.

.UPDATE...

Previous update mainly focused on the High Wind Highlights and
clouds. Took another look at clouds and Wednesday's snow event for
this evening's update. Those looking to view the Geminid Meteor
Shower tonight are still looking for abundant cloud cover across
the south (mainly from Helena south). Satellite trends showing a
brief clearing for central and northern portions early in the
early overnight hours. An upper level disturbance will then bring
increased clouds late tonight through the day Wednesday. This
mentioned disturbance could still bring scattered light snow to
the southern half of the CWA. This evenings models appear snow
start time may delay until after 6 AM Wednesday morning.
Widespread light snow is then possible for these areas through all
of Wednesday. Accumulations still look to be light...with around
an inch at lower levels..and 1 to 4 inches across the mountains.
Anglin


&&

.SYNOPSIS...
An upper level ridge of high pressure will be over the region
through Friday, bringing mostly quiet weather to the region. A
weak, fast-moving system will bring light snow to southwest
Montana during the day Wednesday, with snow accumulations up to 1
inch. Another system arrives for Friday and will bring increasing
chances for rain or snow to the region.

&&

.AVIATION...
Updated 0540Z.

Remaining clouds over far southwest MT this evening will yield to
clear skies overnight. Clouds then increase in coverage from the
northwest Wednesday morning. Areas of light snow will develop across
southwest Montana between 16Z and 00Z, causing mountain obscuration
and occasional MVFR conditions. The snow will exit the area from the
northwest after 03Z. VFR conditions prevail over North-central MT. PN

&&

.PREV DISCUSSION...
/ISSUED 435 PM MST Tue Dec 12 2017/

Tonight through Thursday...

Winds will diminish through the evening as a passing disturbance
responsible for today's strong winds and fire weather threat moves
away from the area. Expect some clearing especially over northern
and eastern parts of our area, but some clouds will remain along
the Continental Divide and much of southwest Montana. Models seem
to be catching up to the next fast-moving system in northwest flow
that will move across western and southwestern Montana during the
day Wednesday. Decent dynamics combined with good (but not
excellent) moisture in the dendritic snow growth layer suggest up
to a quick inch of snow for most of the mountains and valleys of
Southwest MT, with localized higher amounts to 3 inches on
northwest-facing slopes. This system will move through primarily
during the daytime hours, so there will likely be at least some
travel impacts. Yet another system will come immediately on its
heels, dropping south from Alberta and Saskatchewan over north-
central and northeast Montana. Models suggest very little moisture
to work with despite decent dynamics, so precipitation should be
limited to minimal amounts and primarily over the higher
elevations such as the Bears Paw and possible the Little Belts
Wednesday night. High pressure at the surface and aloft move
overhead Thursday, resulting in continued strong inversions. Fog
will be likely especially over the southwest valleys Thursday
morning, especially if there is fresh snow from Wednesday's
expected minor snow event. CC

Thursday night through Saturday...A very slow change in the weather
pattern is expected to begin, as the upper-level high pressure ridge
will start to break down. Because of how strong the ridge is, it is
going to take a few disturbances to knock it down. Strong winds are
possible on Friday ahead of the precipitation. Expect snow over much
of the CWA Friday into Saturday, with the potential for some rain to
mix-in at lower elevations. Minor snow accumulations remain possible
as a shortwave trough traverses the region Friday evening into
Saturday morning. Thereafter, quieter weather is expected as another
high pressure ridge aloft builds-in from the west.

Sunday through Tuesday...Both the ECMWF and GFS continue to depict a
transition day on Sunday, with breezy southwesterly winds developing
as the aforementioned ridge is broken-down by another disturbance
that looks to remain north of Montana. These downslope winds will
warm temperatures a bit over North-Central Montana and allow only a
small chance for a passing shower over the plains. For Monday into
Tuesday, the aforementioned forecast models still differ on when the
next storm system will arrive from the Gulf of Alaska region. Thus,
while forecast confidence remains low during this time period,
precipitation probabilities may increase over much of the CWA.
Brusda/Jaszka

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  32  44  23  46 /  10  20  10   0
CTB  29  46  20  45 /   0  10  10   0
HLN  23  33   5  25 /  20  40  10   0
BZN  22  29   0  23 /  20  60  20  10
WEY  13  24  -6  18 /  20  40  20   0
DLN  24  35   3  28 /  20  40  10   0
HVR  30  47  19  45 /   0  10  10   0
LWT  31  42  21  41 /   0  10  20  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

http://www.weather.gov/greatfalls



Office: MSO FXUS65 KMSO 122134 AFDMSO Area Forecast Discussion National Weather Service Missoula MT 234 PM MST Tue Dec 12 2017 .DISCUSSION...The valley inversions are still the main story for a bit longer. Patchy to areas of freezing fog and low stratus continues to be the biggest impact for the next 12-18 hours. A weak disturbance moves over the top of the ridge Wednesday morning bringing some light precipitation to portions of west central Montana into southwest Montana. The aforementioned area could receive 0.5" to an 1.00" of new snow. A concern exists that this weak feature could become a seeder/feeder into the trapped valley inversions, so a few locations could receive even higher amounts. Confidence is low on this seeder/feeder scenario. The ridge rebounds Wednesday night into Thursday, keeping the valley inversions entrenched. This inversion pattern will finally come to an end on Friday. A cold front will move out of British Columbia and bring light to moderate snowfall back to the mountains of north central Idaho and western Montana. Impacts will be mostly confined to mountain pass roadways crossing Lost Trail, Lolo, Lookout, and Marias passes. Valleys may see up to an inch of snow through Saturday, but the better chance for valley snowfall will come late Sunday through Tuesday. During this time, the entire western United States will undergo a longwave pattern change to a much more active, moist weather pattern. Valley snowfall, cold temperatures, and abundant mountain snow will all be more prevalent as a result throughout the week next week. In fact, long term forecast models have shown good agreement this far out for a potentially significant winter storm to develop on or around December 20th. Again, this is quite a ways out in the forecast, but one thing is certain: inversions are leaving us, and snowfall is returning! && .AVIATION...The strong valley inversions persist tonight, with low stratus and freezing fog being the biggest impact to the region's terminals. A weak disturbance will be moving through the ridge Wednesday morning, providing an opportunity for light snow showers (especially to west central and southwest Montana). This feature moves out by the afternoon and the strong ridge rebounds with continued strong valley inversions. && .MSO WATCHES/WARNINGS/ADVISORIES... MT...None. ID...None. && $$
Office: BYZ FXUS65 KBYZ 130134 AFDBYZ Area Forecast Discussion National Weather Service Billings MT 634 PM MST Tue Dec 12 2017 .UPDATE... Water vapor imagery shows NW flow aloft with a shortwave dropping thru southern Alberta and Saskatchewan. Plenty of high clouds exist over our cwa and have raised sky grids accordingly for the evening. We will see clearing skies overnight. Dry conditions will prevail outside of a possible sprinkle or flurry over the mountains. As for wind, winds have decreased along our western foothills as pressures are now rising east of the divide. Have cancelled all wind highlights, with expectation of 30-40 mph gusts for the rest of tonight, highest this evening. We will see an increase in frontal winds at Sheridan this evening, and in fact that is already starting to occur with gusts in the 20s. Could see some 40 mph gusts in exposed areas along the eastern slopes of the Bighorns, but low level lapse rates do not appear steep enough to cause more of an issue, and mid level will begin to decrease by late evening. Will see gusty NW winds develop across our eastern plains as well, especially over the hills. JKL && .SHORT TERM...valid for Wed and Thu... Shortwave responsible for the very warm and windy conditions this afternoon was moving SE into southeast Alberta this afternoon. Pressure rises were slowly nosing down the Rockey mountain front and were lessening the winds along the mountain foothills northwest of our region in locations like Great Falls. These will push through to our mountain foothills later tonight which should put an end to the very strong winds in Livingston, Big Timber, and other foothill locations. In and east of Billings and towards the Dakotas border, these winds will take a little time to subside as a strong push of northwest winds along and ahead of the shortwave keeps conditions breezy through 3am tonight. Fire concerns will slowly alleviate through the night as relative humidities recover and winds slowly diminish. Cooler conditions are anticipated Wednesday afternoon as heights lower across the region. Another weak shortwave will move through western Montana through the day. The best chance for precipitation with this system will be in the mountains and nearby foothills from Big Timber west and over the Beartooth mountains. Expect the low levels to see rain and the mountains could see around an inch of new snow above 6/7000 feet. High temperatures across the region will be in the mid to upper 40s. Thursday will be another relatively quiet day with high temperatures in the mid to upper 40s again. Could see another increase in our winds along the mountain foothills into Friday morning as pressure gradients increase along our mountain foothills. Dobbs .LONG TERM...valid for Fri...Sat...Sun...Mon...Tue... No major changes to the extended. The main time period of concern is the precipitation mainly on Saturday as a shortwave begins breaking down the ridge. The GFS, with a higher resolution, is still showing more limited precip, mainly in the higher terrain. The EC is still going with a more widespread precip event. Have continued the higher PoPs in the mountains with mainly chance PoPs elsewhere. Sunday looks like another potential windy day for much of the area behind the departing shortwave. Winds around Livingston still are potentially in wind advisory range. Temperatures will continue to be warmer than normal through the period. Farther out the general trend has continued with the pattern changing to colder and snowier pattern. The EC is still not bring as much cold air into the area. The GFS continues to show temperatures dropping below 10 degrees for highs. Will continue to monitor these changes as we approach the Christmas travel time. Reimer && .AVIATION... VFR conditions will continue into Wednesday across the area. Expect breezy conditions around KLVM, K6S0, and KBIL, with gusts of 20 to 30 knots possible overnight. Winds around KMLS and KSHR remain breezy overnight. AAG && .PRELIMINARY POINT TEMP/POPS... Wed Thu Fri Sat Sun Mon Tue ----------------------------------------------------------- BIL 036/046 031/046 032/053 031/042 025/046 031/046 028/045 00/B 10/U 00/B 33/O 11/B 11/B 12/O LVM 034/039 028/043 031/047 030/038 025/041 032/043 031/044 04/O 10/U 01/N 43/O 11/N 22/O 32/O HDN 033/047 028/044 027/054 029/042 023/046 028/047 025/047 00/B 11/U 00/B 43/O 11/U 12/O 12/O MLS 033/044 028/038 025/051 029/041 023/044 028/043 025/045 00/N 32/S 10/B 23/O 11/U 11/B 11/E 4BQ 035/044 029/039 025/053 029/041 023/045 029/045 025/046 00/N 32/S 10/B 23/O 11/U 11/B 11/B BHK 030/037 025/033 021/047 025/037 020/041 025/041 022/041 01/N 43/S 00/B 22/O 11/B 11/B 11/E SHR 034/045 027/042 024/055 027/040 020/045 025/047 023/047 00/N 22/S 00/B 34/O 21/U 12/O 11/B && .BYZ WATCHES/WARNINGS/ADVISORIES... MT...None. WY...None. && $$ weather.gov/billings
Office: GGW FXUS65 KGGW 130542 AFDGGW Area Forecast Discussion National Weather Service Glasgow MT 1042 PM MST Tue Dec 12 2017 .DISCUSSION... 9pm Update: Dropped the fire weather highlights has relative humidity has slowly increased over the past hour. Wind remains strong across the region for the overnight hours. A wind gust to 57 mph was recorded in Sheridan county this evening. Very few changes needed to the ongoing forecast. Proton Previous Discussion: An upper ridge near the West Coast will continue to reign supreme through Thursday while disturbances drop southeast in the northwest flow into the Northern Plains or the Great Lakes. Since this storm pattern is to our east, we'll remain above normal in the temperature department, but it will be a bit cooler than today. The first clipper this afternoon is dropping into southern Manitoba with a Pacific cold front sweeping through the area this afternoon/evening. Mid-level winds (850 mb) will be about 50-55 kts (with 700 mb winds higher). Depending on maximum temperatures this afternoon shape up will determine how much mechanical mixing takes place this evening. Winds look to miss the mark with high wind criteria as we are experiencing the shortest days of the year. Expect winds to decrease later tonight as cold front and surface low moves away. Temperatures will be held up with clouds, expect lows in the 20s to low 30s, close to normal daytime highs. Wednesday, clouds will increase with increasing 850/700 mb moisture and approaching disturbances. Temperatures will only reach the mid 30s to mid 40s. 15-20 degrees cooler than today, but still about 10 degrees above normal. TFJ Previous Discussion... The next wave on NW flow aloft drops SE across the region Wednesday Night with another cold front. Another wave comes through late Wednesday night. Again windy conditions are possible over eastern Montana, and some spotty precipitation. The system may have enough moisture for a chance of rain and/or snow depending on temperatures. That system moves to the southeast Thursday morning. Thursday will be mainly dry and cooler with highs in the 30s in most areas. The upper ridge builds back into Montana Thursday and Thursday Night which will bring warmer air to the forecast area on Friday. However, a shortwave trough will flatten the ridge on Friday and move through the state Friday Night and Saturday with a cold front Friday Night. Models differ at this point on where to track the moisture with this system. Could see a chance of rain and snow depending on temperatures Friday Night and Saturday morning. Models are pointing towards a shortwave ridge on Sunday with dry weather and continued above normal temperatures for mid December. NW flow aloft prevails Monday and Tuesday. A cold front is expected Sunday Night or Monday. Then, the models differ on whether to keep the front to the south of the forecast area on Tuesday or bring it back east as a warm front. Could see a chance of rain or snow at times. Forrester && .AVIATION... Flight Condition: VFR. Winds: NW winds will ramp up this evening with some LLWS continuing. Winds will diminish after 3 am. Otherwise, expect some passing high clouds tonight. Scattered low VFR clouds will huge eastern Montana from North Dakota Wednesday. && .FIRE WEATHER... Wind continues strong across the region late this evening, however humidity levels have moved up into the 30 to 40 percent range as temperatures slowly drop. The red flag warning has been allowed to expire at 10 pm. && .GLASGOW WATCHES/WARNINGS/ADVISORIES...NONE. && $$ weather.gov/glasgow