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fxus65 ktfx 210550 

Area forecast discussion
National Weather Service Great Falls Montana
1140 PM MDT Wed Mar 20 2019

Aviation section updated.


Only some minor updates were made this afternoon, mainly focusing
on fog development over parts of northcentral and southwest
Montana overnight tonight into early tomorrow morning. There is
higher confidence that Helena will see some fog again early Thursday
morning, and there remains some potential for Bozeman, Havre, and
Ennis to see some patchy fog as well. -Tp



Primarily fair weather and warming temperatures continue through
Friday as a high pressure ridge drifts eastward. However, a
disturbance approaching from the Great Basin should allow some
scattered precipitation to overspread southwest Montana Friday
afternoon and evening. A cooling trend and more widespread
precipitation are expected this weekend, when multiple disturbances
and an eventual Canadian cold frontal passage impact our area.


updated 1140 PM MDT Wed Mar 20 2019

VFR conditions are expected to prevail over the majority of the
terminals throughout the taf period, with the exception of kbzn,
khln, and khvr. Fog development is expected to develop over khln
and kbzn during the overnight/early morning hours again, with khvr
now being added to the list of sites who can see fog develop. -Tp



Warm temperatures this week has led to snowmelt and runoff across
the region, but most notably over the plains of north central
Montana. With the ground still frozen, ponding of meltwater has
been noted in low lying areas. The additional runoff of this
meltwater into creeks and streams could increase the potential
for ice jams.

A disturbance will bring precipitation to the region this weekend,
with a rain/snow mix expected at lower elevations and snow in the
mountains. At this time, most locations over the plains of north
central Montana (where the rain/snow mix is expected to occur)
could see precipitation totals of 0.2" to 0.5", which would only
exacerbate the snowmelt and subsequent runoff into area streams
and creeks.


Previous discussion...
/issued 535 PM MDT Wed Mar 20 2019/

Rest of this afternoon through Thursday night...primary forecast
concerns throughout the period is fog development tonight and high
temperatures on Thursday.

H500 high was centered over north central Montana/southern Alberta
this afternoon, all the while a closed low off of the California
coast was drifting east southeast and weakening. The upper level
high will continue to drift southeast through Thursday night, while
the previously mentioned closed low and associated trough slides
east into the Desert Southwest and begins to pinch off the upstream
flank of the ridge. Subsidence over the region tonight from the
upper level high will provide clear skies, with light and variable
surface winds. Snowmelt in the valleys of southwest Montana will add
additional moisture to the boundary layer beneath a stubborn
temperature inversion, which combined with ideal radiational cooling
will promote the formation of fog yet again tonight. Those valleys
most likely to see fog overnight are the Big Hole, Centennial,
Gallatin, Missouri, and Helena, in addition to the milk River Valley
in north central Montana. While visibilities may fall to a quarter
mile in some locations tonight and Thursday morning, widespread
dense fog is not anticipated. Mid-level cold air advection during
the day on Thursday will likely lead to better mixing in the valleys
of southwest Montana, which will allow most of these valley
locations to see their warmest day of the week so far. Further
north over the plains of north central Montana, high temperatures
in the mid-40s to low 50s are expected on Thursday. - Moldan

Friday and Friday night...a trough of low pressure will continue to
move in from the southwest with its axis extending southeastward
from near far-southwest Montana to Colorado by late Friday. This will force the
high pressure system to drift eastward and weaken, allowing for some
rain and/or snow showers to move into southwest Montana on Friday, with
minor accumulations expected.

Saturday and Sunday...the aforementioned high pressure system begins
to stretch and become cutoff over the western Canadian prairies,
while disturbances cross through Montana in an west to east fashion.
At the surface, a Canadian cold front is expected to move through
our region from the north or northeast on Sunday. Accordingly, a
cooling trend and much more widespread precipitation is anticipated
this weekend. Precipitation should primarily fall as snow in the
mountains, while lower elevations experience mainly rain (snow)
during the day (night). Some uncertainty exists regarding how much
rain/snow will fall due to the possibility of some additional
moisture from the Central Plains becoming entrained into the
cyclonic circulation. The situation will need to be monitored
closely given the current hydrologic conditions and a possible shift
to a wetter solution. Winter weather headlines may also be
considered by future shifts, primarily for higher elevation areas
such as the northern Rocky Mountain front.

Monday through next Wednesday...precipitation chances diminish on
Monday and Tuesday as high pressure passes through the region. Winds
may increase along the Rocky Mountain front on Tuesday as a
disturbance passes on the northwest periphery of the high pressure
ridge but winds speeds appear to be below high wind criteria at this
time. By next week Wednesday precipitation probabilities increase
once again as general troughing moves in from the west.


Preliminary point temps/pops...
gtf 26 46 26 50 / 0 0 0 0
ctb 24 49 25 53 / 0 0 0 0
hln 11 38 14 49 / 0 0 0 0
bzn 10 42 12 46 / 0 0 0 0
wey -2 56 4 49 / 0 0 0 20
dln 15 41 16 44 / 0 0 0 0
hvr 21 48 23 53 / 0 0 0 0
lwt 26 53 25 53 / 0 0 0 0


Tfx watches/warnings/advisories...

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