Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 211740

Area Forecast Discussion
National Weather Service Great Falls MT
1130 AM MDT Wed Mar 21 2018

Aviation Section Updated.


Forecast remains in good shape this morning. A minor update was
issued to freshen hourly forecasts with respect to observations.



Temperatures will warm nicely today across most of North Central
and Southwest Montana, with the exception of the Hi-Line where a
substantial snow pack still resides. For areas along the Hi-Line,
highs in the 30s can be expected today, with the mid 40s to mid
50s expected elsewhere. Rain and snow showers begin to develop
this afternoon across the mountains of Southwest Montana, expanding
in coverage tonight. Precipitation spreads further through Thursday
night across all of Southwest and North Central Montana as an upper
level disturbance moves towards the Northern Rockies.


Updated 1730Z.

VFR conditions prevail this afternoon. Mid- and high-level clouds
increase from southwest to northeast tonight. Mountain obscuration
is likely this afternoon/tonight across Southwest Montana due to
showers and lowering ceilings. Fog is possible overnight through
early Thursday at KCTB/KHVR/KGTF. Uncertainty remains, but it does
appear that fog could produce MVFR and IFR conditions. Moldan/PN


/ISSUED 515 AM MDT Wed Mar 21 2018/

Today through Thursday...main forecast concerns over the short
term are temperatures and precipitation chances.

Broad H500 ridge over the Western CONUS this morning, will amplify
further as it slides east across the US/Canadian Rockies today and
into the US/Canadian High Plains during the morning/afternoon hours
on Thursday. All the while this occurring, H500 closed low situated
within a broad trough over the Gulf of Alaska this morning, will
deepen further as it digs south towards the Pacific Northwest by
Thursday evening. This will result in increasingly moist H700-H500
southwest flow, beginning today across Southwest Montana and along
the Continental Divide. By Thursday afternoon, PWATs climb to 1-2
standard deviations above normal across all of North Central and
Southwest Montana thanks to this moist southwest flow, with portions
of Southwest Montana rising as high as 2-3 standard deviations above
normal for PWATs. Strong warm air advection will commence today
across the Northern Rockies, with H700 and H500 temperatures peaking
at 1 to 2 standard deviations above climatological norms across
Southwest and North Central Montana on Thursday. Thickness values on
the order of 535-540dam this morning, will rise to 540-550dam by 00z
Thursday and 545-555dam by 00z Friday across North Central and
Southwest Montana respectively. Low level flow across North Central
Montana begins to back to the southeast and east tonight, in
response to the approach of the of the upper level trough.

For today, high temperatures will climb near or slightly above
climatological norms, mainly along and southwest of a Cut Bank, to
Great Falls, to Lewistown line. For these areas, highs in the mid-
40s to mid-50s can be expected, with the mountains rising into the
upper 20s and 30s. Areas northeast of this line can expect highs in
the 30s for the most part, with the coolest readings across Hill and
Blaine Counties where the 2017-2018 Winter "mini ice block" still
resides. Rain and snow showers chances begin to increase this
afternoon across Southwest Montana, especially across the mountains.
These showers then expand in areal coverage this evening, possibly
affecting some valley/plain locations across Southwest and North
Central Montana. In addition to the precipitation chances tonight,
snowmelt from today across North Central Montana, combined with
light easterly low level flow and mostly clear skies, will yield the
potential for the development of fog. On Thursday, low level
easterly flow continues across North Central Montana, with expansive
mid- and upper level cloudiness. Despite anomalously warm
temperatures aloft and ideal thickness values, believe that
temperatures across North Central Montana (i.e. Cut Bank, Havre,
Great Falls, and Fort Benton to name a few) will be cooler than on
Wednesday. Across the remainder of the region, temperatures will be
similar or slightly warmer than on Wednesday. - Moldan

Thursday night through Tuesday...Confidence in a rain on snow event
continues to increase across parts of the area, especially far SW
MT. The well-advertised atmospheric river event across the West is
underway. The anomalously high moisture associated with this plume
of moisture will reach Montana by Thursday. Ensemble guidance
continues to suggest precipitable water values will reach or exceed
the 99th percentile for this time of year along the MT/ID border.
Values look to be in the 90th percentile for the remainder of the
forecast area. The higher moisture combined with an extended period
of upslope flow and large scale synoptic lift should allow
significant precipitation amounts to be realized in the mountains of
SW MT, including the West Yellowstone area. The key here is that not
all of this will fall as snow. Forecast soundings on Thursday
suggest snow levels may rise to 7500-8500`, creating a rain on snow
scenario there for at least several hours. With time, snow levels
will begin to fall as a large trough moves inland, allowing a
transition back to snow. At lower elevations, the precip will likely
fall as mostly rain THU/FRI, although precip amounts in the valleys
look to be not nearly as impressive as in the mountains.

In the SW MT mountains, it looks like 0.10-0.25" of precip will fall
as all rain before the transition back to all snow (possibly a bit
higher at mid slopes, though). Around 0.10" of an inch of rain is
expected in the valleys (slightly higher south of I-90). The rain on
snow plus temperatures staying above freezing (6000` and below)
Thursday night continues to suggest a locally enhanced flooding
potential and we will continue the Flood Watch. North of I-90, an
increased risk of flooding could develop as well, but is likely more
dependent on how much snow is left to melt out at low/mid
elevations. Because of this uncertainty, we`ll hold off on extending
the Flood Watch north for now.

The other concern with this system is the potential of some mixed
precip, mainly along the Hi-line. Model guidance has come in with
higher precip amounts there, possibly due to a more pronounced
overrunning scenario aided by the increased/anomalous moisture. Even
so, significant winter-related impacts are not expected, but this
will continue to be closely monitored. MARTIN


GTF  52  25  45  32 /   0  10  30  50
CTB  43  22  35  25 /   0  10  30  70
HLN  54  33  54  33 /   0  10  50  60
BZN  49  34  53  34 /   0  20  50  60
WEY  39  28  42  26 /  40  50  90  90
DLN  51  31  54  32 /  10  20  60  60
HVR  36  18  35  28 /   0  10  20  80
LWT  49  30  50  33 /   0  10  20  50


Flood Watch from late tonight through late Thursday night


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