Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 221714
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1114 AM MDT FRI JUL 22 2016

Updated Aviation Discussion

.UPDATE...

Satellite imagery this morning shows a fairly vigorous/negatively
titled shortwave trough across WA with increasing SW flow aloft
downstream over the Northern Rockies and MT. Some mid-level
cumulus also noted in place across SW MT this morning, where radar
also shows some weak returns that are likely sprinkles or not
reaching the ground. Pacific cold front associated with the upper
level trough will move east across the Continental Divide early
this afternoon and continue east across central MT late this
afternoon. Increasing winds aloft and mixing ahead of the front
will lead to the development of gusty SW winds early this
afternoon, shifting to W/NW with the frontal passage and
continuing through this evening as pressure gradient behind the
front remains moderate. See discussion below for specific impacts
to fire weather. Moisture somewhat lacking across much of the
forecast area, particularly in the low levels across all but
eastern portions of the forecast area. a low coverage of
thunderstorm activity is expected with the frontal passage with
best coverage likely across Glacier county and vicinity as well
as downstream of the Little Belts across Judith Basin and Fergus
counties. Shear is not lacking and a few of the storms that do
develop could become marginally severe, particularly for strong
winds due to dry low levels and stronger wind aloft. Hoenisch

&&

.AVIATION...
Updated 1714z.

Increasing SW flow aloft across the region today with upper level
trough and surface cold front moving east across N-Central and SW MT
this afternoon through early this evening. SW winds will increase
ahead of the front early this afternoon and shift to W/NW with the
frontal passage late this afternoon/eve and remain somewhat gusty
through much of tonight as pressure gradient remains moderate across
the Rockies behind the exiting surface low. A few thunderstorms are
possible this afternoon, primarily near the KCTB/KLWT and KBZN
terminals and vicinity. Strong and erratic wind gusts will be the
primary threat with any convection along with some possible MVFR
cigs and local Mtn Obscuration. Otherwise, VFR conditions will
prevail. Hoenisch

&&

.FIRE WEATHER...
Deep layer mixing and increasing southwest winds this afternoon
will cause minimum relative humidity to fall below 15 percent over
the southern districts. A Red Flag Warning is in effect for MTZ118
this afternoon where fire danger ratings have reached high to very
high due to drying of fine fuels, especially grasses. A cold front
expected to move through the region late this afternoon will bring
cooler temperatures and higher humidity, but gusty west winds will
continue through early Saturday. Isolated dry lightning may be
possible over the higher terrain of southwest Montana where
moisture is increasing aloft while remaining dry at the surface.

&&

.PREV DISCUSSION...
/ISSUED 530 AM MDT FRI JUL 22 2016/

Today through Sunday...Water vapor imagery reveals upper level moisture
spreading into the region ahead of a shortwave trough presently
crossing the WA/OR coast. Temperatures remain around 10 degrees
above average today. Surface pressure falls and deep layer mixing
will produce strong gusty winds this afternoon while lowering
afternoon relative humidity below 15 percent for much of central
and southwest MT. Red Flag fire weather warnings are in effect for
this afternoon as discussed below. Forecast soundings reveal
increasing instability and shear today, but with CAPE limited by
high LCLs due to the dry boundary layer. Nonetheless, short term
models consistently support showers and thunderstorms over
northern portions of the Continental Divide - including Glacier NP
- and extending east through our northern counties as the
shortwave trough and associated cold front cross the region. These
forecasts are consistent with observations indicating higher
dewpoint temperatures. Winds turn westerly and strengthen behind
the frontal passage late this afternoon and evening. Winds will
taper on Saturday as temperatures return near to just below
seasonal averages. Fair and pleasant weather prevails on Sunday
with a weak northwest flow aloft over the region. PN

Sunday Night through Friday...High pressure ridging centered
near/slightly west of the Four Corners area will remain in place
through this period. That will keep our region under west-southwest
flow aloft, leading to very warm temperatures and mostly dry
conditions.  Weak disturbances moving through the flow will combine
with a mix of Pacific and monsoonal moisture to produce isolated
thunderstorms each afternoon, mainly over the central and Hiline
counties.  Given the spotty coverage pattern of the moisture and dry
near-surface conditions (aftn humidities in the teens and low 20s),
convective activity on Mon-Wed will likely be a mix of wet and dry
(<0.1 inch rainfall) storms.  Gusty outflow winds and possible fire
starts from lightning are the primary threats from these storms in
the first half of next week. Storm type will trend wetter for
Thurs/Fri as slightly higher moisture is expected after a cool front
passes across the region Wed night. High temperatures will run 5-8
degrees (upper 80s to low 90s) above normal before the cool front
arrives and then drop back close to seasonal values (mid 80s) on
Thurs/Fri.
Waranauskas

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  93  56  80  50 /  30  20   0   0
CTB  86  55  77  49 /  30  30  10  10
HLN  93  59  82  54 /  20  20   0   0
BZN  93  52  81  47 /  20  20   0   0
WEY  82  43  72  39 /  20  10   0   0
DLN  90  49  79  47 /  10  10   0   0
HVR  94  59  81  53 /  30  30  20  10
LWT  92  56  78  52 /  30  30  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 1 PM this afternoon to 9 PM MDT this
evening for MTZ118.

&&

$$

weather.gov/greatfalls



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