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000
FXUS65 KTFX 180510
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1100 PM MDT Fri May 17 2013

...UPDATE TO AVIATION...

.DISCUSSION...
Update forthcoming. Have made some minor adjustments to the grids to
better align with neighboring offices. Upper trof continues to move
toward the area with plenty of moisture and instability moving into
the state ahead of the trof. Models continue to indicate some breaks
in the showers from time to time but timing of these breaks will be
difficult. Unsettled showery conditions will persist through the
weekend. Overnight temperatures look good with overcast conditions
keeping readings from dropping precipitously. Emanuel

&&

.AVIATION...
UPDATED 05Z.
Moist unstable flow aloft will continue across the region for the
next 24 hours as the upper-level trough slowly progresses into the
Western US through the TAF period. Showers will remain in the area
throughout the TAF period with some breaks between individual
showers...however widespread precipitation is expected throughout
the time frame.

&&

.PREV DISCUSSION...Issued 255 PM MDT Fri May 17 2013

Tonight through Sunday...Rain and isolated embedded thunderstorms
will continue to develop this evening as an upper-level trough of
low pressure pushes into the Northern Rockies. Short-range models
continue to show differences in the track of each shortwave moving
across the forecast area. While precipitation will continue to
develop over north-central Montana, across southwest Montana
precipitation will taper tonight. Partial clearing in this area
may allow for fog to develop, however confidence is not high given
the amount of cloud cover expected. There will be breaks in
precipitation throughout the weekend with the greatest
concentration of showers and thunderstorms during the afternoon
when instability increases. Cooler, Canadian air will keep snow
levels lower Saturday and Sunday, especially along the Rocky Mountain
Front, though any accumulating snow will be well above pass level
and precipitation should remain all rain at most mountain passes.
The only exception is for Kings Hill and Chief Joseph passes where
a mix of rain and snow showers are possible, but little or no snow
accumulation is expected. The main trough axis will move over the
Northern Rockies on Sunday and as better lift and northerly flow
wraps in on the northeast side of the trough, precipitation will
become more widespread and steadier over north-central Montana.
Over southwest Montana, precipitation will remain scattered.
Temperatures will remain below seasonal averages through the
period. MLV

Monday night through Saturday...Medium range models are in fair
agreement overall through next week in the large scale, depicting a
relatively unsettled and highly amplified pattern with one closed
upper level low drifting from the central Rockies into the upper
Midwest Sunday night and Monday while another drops south along the
west coast, impacting the region in the middle to end of next week.
The circulation around the broad upper low drifting east from WY to
the Dakotas Sunday night/Monday will maintain a moist northerly flow
over the region with widespread precipitation likely Sunday night,
especially over portions of North Central Mt along and adjacent to
the Rocky Mtn Front and Central MT Mtns. Weak/transient upper level
ridging between systems drifts across the northern Rockies and MT
Monday night through Tuesday, bringing a period of relatively
dry/quiet conditions. While medium range models agree on the idea of
a deep and slow moving upper low along the west coast during Wed
thru Friday of next week, gradually working inland next weekend,
details of shortwave energy placement and timing are not yet
resolved. The first shortwave rotates up through western MT Wed/Wed
night, bringing a good chance for precipitation across much of the
forecast area. Later in the week into next weekend, the overall
pattern looks to be conducive to increasingly active convective
shower/thunderstorm development as flow aloft turns southerly with
with low level east to southeast flow importing moisture into North
Central and eastern MT. Hoenisch

&&

.HYDROLOGY...
A broad, upper level low pressure trof will bring cooler
temperatures to the region and this will moderate the melting of
the snowpack. However, the increased chance of precipitation will
also increase the chance for further rises in rivers and streams
across much of north central and southwest Montana through the
upcoming weekend. In addition, rain falling on recent burn areas
can cause rapid runoff and possible debris flows.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  45  64  44  60 /  70  80  70  70
CTB  43  62  42  60 /  60  70  70  60
HLN  44  65  43  61 /  70  80  70  70
BZN  40  65  40  60 /  80  80  80  70
WEY  40  57  37  55 /  60  80  70  60
DLN  40  62  38  59 /  80  80  80  60
HVR  47  69  47  66 /  60  70  70  70
LWT  44  61  43  57 /  70  80  80  70

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

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