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000
FXUS65 KTFX 181724
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1125 AM MDT Tue Jun 18 2013

UPDATED AVIATION SECTION

.UPDATE...
Minor updates to the forecast this morning. Adjusted POP a bit
based on current model data. Still expecting convective activity
with best chances developing over terrain this afternoon.

&&

.AVIATION...
Updated 1725Z.
A trof over the Pacific Northwest will continue to influence the
weather across the area through 18Z Wednesday. Thunderstorms will
initially develop over the mountains early this afternoon and begin
to push into the plains late in the afternoon. The most widespread
thunderstorms activity will be in the KCTB area where a cold front
will approach this evening. Some of the storms cloud contain large
hail and strong wind gusts to 50 knots...mainly north and west of a
KBTM to KGTF to KHVR line. Less than VFR conditions can be expected
with some of the heavier storms. The threat for storms will be lower
in KLWT and KBZN area where lack of lift and/or drier air will limit
thunderstorm coverage. Langlieb

&&

.HYDROLOGY...
More widespread showers and thunderstorms are possible along the
Rocky Mountain Front and adjacent plains today and over most of
the plains of north central/central Montana on Wednesday. Some of
these storms may produce brief heavy rainfall. These periods will
need to be monitored for possible areal flood/flash flood
highlights. The greater area of concern is across eastern portions
of north central and central Montana for Wednesday that still
have moist soil that would not be able to absorb much rapid
rainfall. However, if the rainfall from storms today are
significant along the Rocky Mountain Front, the additional rainfall
on Wednesday will increase the flooding threat there as well.

&&

.PREV DISCUSSION...
/ISSUED 555 AM MDT Tue Jun 18 2013/

Today through Thursday...Forecast models continue to indicate that
strong to possibly severe thunderstorms are possible both today
and Wednesday. As the Pacific low pressure system moves onto the
Pacific Northwest coast today, the flow aloft over western and
central Montana will become more moist, unstable, and
southwesterly. Temperatures will also warm to around 10 degrees
above normal. These factors will help generate showers and
thunderstorms over the higher terrain of these areas. The frontal
boundary associated with this system will also move into western
Montana today to provide a good focusing mechanism for storm
development. The flow aloft will also become more diffluent, which
should enhance vertical development of storms. These factors,
along with strong wind shear (both speed and directional) and
significant instability (CAPE values of 1500 to 2500 J/kg), the
Storm Prediction Center (SPC) has put portions of western and
central Montana under a Slight Risk for severe thunderstorms, and
we have subsequently added mention of damaging winds, large hail,
and heavy rainfall for this area. This area is generally northwest
of a Havre-to- Butte line. Thunderstorms will decrease overnight,
but another round of potentially severe weather is expected for
Wednesday as the system continues to move east across the Pacific
Northwest. The most unstable portion of the flow aloft and the
surface frontal boundary will also shift east into central and
eastern Montana, where SPC has the Slight Risk for Wednesday. Am
thinking that the potential for the strongest storms will be along
and east of a Havre-to-Stanford line, as that appears to be the
favored area for wind shear and instability due to the position of
the frontal boundary. However, isolated severe storms are possible
west of this line as well, though they may be more marginally
severe. Have therefore continued the damaging wind/large
hail/heavy rainfall wording for these areas. Precipitation from
these storms may be exceptionally heavy in the aforementioned
eastern areas, adding the potential for flash flooding as well
(please see Hydrology section for details). The system will lift
northeast across the area later Wednesday night and into Thursday.
This will shift the flow aloft more westerly, but it will remain
moist, so have kept high POPs in the mountains. However, the
resulting strong westerly down-slope winds will hinder
shower/thunderstorm development over the plains. Winds aloft
become marginally strong along the Rocky Mountain Front on
Thursday, so will need to monitor this for possible high wind
highlights there. Temperatures will begin  to cool closer to
normal from the west on Wednesday as the frontal boundary moves
through the area and showers become more widespread. In the wake
of the front, temperatures will cool to around 10 degrees below
normal by Thursday, which may cause snow to fall at elevations
above 6500 feet.  Coulston

Thursday night through Tuesday...No significant changes made to
the medium range forecast. An unsettled pattern will persist
through Saturday as a closed upper low will remain quasi-
stationary over northwest Montana. Short wave energy will
continue to rotate out from the base of the low and help initiate
shower and thunderstorm development each day. The strength of
these storms will depend on availability of low level moisture
and the strength of the divergent flow aloft. The models also
continue to differ on the upper low`s position which, in turn,
will affect where and how strong thunderstorms will be. Because
of these differences, have continued to broad-brush the pops
through Saturday. By late Saturday, the models move the low to
northeast Montana and lift it out as an upper ridge begins
building over the Pacific Northwest. The ridge breaks down on
Sunday as short wave energy ripples across the area within the
westerly flow aloft. Isolated thunderstorms will remain a
possibility Sunday and Monday. By Tuesday, weak upper ridging
returns though isolated showers will persist over the southwest
mountains. Temperatures will remain below seasonal averages to
start the period due to cloud cover and periodic cold advection
from transient shortwaves. Temperatures should rise above
seasonal averages to start the week.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  85  54  76  46 /  60  60  80  80
CTB  83  51  73  45 /  70  70 100  90
HLN  86  52  75  46 /  60  50  70  60
BZN  86  47  79  42 /  30  30  50  40
WEY  78  37  70  32 /  20  20  30  20
DLN  84  48  71  39 /  40  40  60  50
HVR  86  59  83  51 /  20  50  80  90
LWT  83  53  79  44 /  30  40  60  80

&&

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

$$

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