Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 222332
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
532 PM MDT Tue Jul 22 2014

.DISCUSSION...
This Afternoon through Thursday Evening...Atmospheric conditions
continue to take shape for an array of weather activity over central
and southwest MT through the midweek period.  Much of that activity
will center around the passage of the strong low pressure trof
currently located off the BC coast.  Broad southwest flow ahead of
the trof has been pushing plumes of moisture through our region
generating one band of showers that is presently over the Hiline
counties and initiating new thunderstorms over southwest MT.  The
southwestern storms should increase in coverage and spread to the
northeast this evening, then dissipate toward sunset.  The threat of
severe storms is the primary weather feature for Wed, as forecast
models maintained good continuity and agreement on the development
of strong instability, wind shear, and jet stream support necessary
to fuel strong-to-severe storms generally along and north of a
Butte-to-Lewistown line. However, there is one wild card in the
forecast: the evolution of the moisture field ahead of the trof.
Models are still showing run-to-run changes in the placement and
extent of the main moisture plume, but overall latest dew point
analysis shows favorable moisture should be broad enough to generate
a wide area of storms. Large hail and strong/damaging outflow winds
will be the main threats from these storms, which should begin early
Wed aftn around Butte/Helena and expand north and east into Wed
evening.

The main portion of the upper-level trof lifts into southern BC/
Alberta by Wed night and then tracks due east just north of the
US/Can border on Thurs and Fri.  The strong pressure gradient from
the upper low and the accompanying surface cold front crossing east
of the Continental Divide on Thurs morning will make for very windy
conditions through central MT on Thurs.  The high wind watch for the
Northern Rocky Mtn Front and eastern Glacier/Toole counties remains
in effect beginning late Wed night.  The same gusty winds will also
combine with a drier airmass behind the cold front to raise the fire
weather threat for Chouteau/Fergus/southern Blaine counties, so a
fire weather watch is in effect there for Thurs aftn/eve (see
additional info in the Fire Weather section below).
Waranauskas


Thursday night through Monday...The main message during the period
will be warming temperatures and dry conditions as a strong ridge
aloft develops over the western United States. Thursday evening
will start off with an upper low over southern Alberta. Associated
cool air could bring some very light snow to the mountains of
Glacier Park.At this time snow levels could range between 7500 and
8000 feet. Also Thursday evening strong westerly flow aloft
associated with the upper low could result in high winds over
Glacier..Toole..and Pondera Counties. Dry westerly flow aloft is
expected Friday with highs near 80. Over the weekend into Monday
the development of the strong ridge will occur. Even as
temperatures warm...a relatively dry airmass and the upper ridge
should result in no precipitation over the weekend. Early next
week monsoonal moisture will move into the forecast area but
models differ on its timing. At this time have opted to keep the
weather warm and dry on Monday though isolated showers and
thunderstorms are not out of the question for far southwest
Montana.
Blank

&&

.AVIATION...
UPDATED 2330Z.
A pair of upper level disturbances moving through the area will
cause scattered thunderstorms this evening followed by scattered
showers towards morning. The air mass will become quite unstable
Wednesday afternoon with showers and thunderstorms again developing.
Some thunderstorms may be severe. The best chance for thunderstorms
will be over the Rockies late Wednesday afternoon and evening,
moving northeast across the Hiline towards midnight. Local MVFR
conditions are expected near thunderstorms along with erratic and
gusty winds. Elsewhere VFR conditions will prevail through the next
24 hours.

&&

.FIRE WEATHER...
Wednesday and Thursday both days of concern for fire weather
conditions.  Wednesday will bring a chance of rain showers and
thunderstorms over southwest into central Montana, with showers and
thunderstorms likely over central and north-central Montana. At this
time expect very warm temperatures and relative humidity dropping to
the 15 to 25 percent southwest...25 to 35 percent central and
north-central. Wind, however, expected to be relatively light and
chance of wetting rain is generally less than 40 percent. With
afternoon and evening thunderstorms, could see some lightning
starts. Things become more active Thursday with a seasonally strong
cold front pushing across the area. Warm temperatures ahead of the
front will combine with relative humidity dropping to 15 to 25
percent and sustained winds 20 to 30 mph. These conditions coupled
with recent lack of precipitation enough to prompt the issuance of a
Fire Weather Watch for Fire Zone 115 in its entirety and Fire Zone
113 for the southern third of Blaine County from noon until midnight
Thursday afternoon and evening. Conditions in Fire Zone 117 marginal
at this time, and will re-evaluate with later model runs.
Loss

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  62  89  62  76 /  30  60  60  10
CTB  56  87  55  69 /  30  50  70  10
HLN  60  93  60  79 /  40  60  40  10
BZN  54  91  54  81 /  20  30  20  10
WEY  43  82  44  75 /  10  20  10  10
DLN  54  87  53  77 /  20  40  30  10
HVR  59  94  60  81 /  20  30  60  20
LWT  59  92  59  81 /  30  40  20  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
FIRE WEATHER WATCH from Thursday afternoon through Thursday
evening Blaine...Chouteau...Fergus.

HIGH WIND WATCH from Thursday morning through Thursday evening
Eastern Glacier...Eastern Pondera...Toole.

HIGH WIND WATCH from late Wednesday night through Thursday
evening Northern Rocky Mountain Front.

&&

$$

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