Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 232103
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
303 PM MDT Wed Jul 23 2014

.DISCUSSION...
This Afternoon through Friday Evening...Quiet weather right now
across the region, but conditions still looking favorable for
strong-to-severe storms later this aftn/eve, mainly over the central
and Hiline counties.  Weather balloon launched at 1:30pm from Great
Falls indicated just a small piece of stable air (or "cap")
remaining from the stable airmass brought on by the rainfall this
morning.  The cap should erode over the next hour or so as surface
temperatures climb to near 90 degrees.  Once the cap breaks, high
amounts of instability (CAPE values over 1000 J/kg) should allow for
quick initial storm development, with the updrafts straightened by
good vertical wind shear already in place.  Further support should
come from the strong jet stream (80-100 mph) current oriented
southwest-to-northeast from the central Idaho into the center of
Montana.  Primary threats from these storms continues to be large
hail and damaging winds.  Still think storms will initially fire
over parts of southwest MT (Helena area and south), then track
northeastward into central MT this evening and up to the Hiline
during mid-late evening.  A few strong storms may occur north of US
Highway 2 after midnight, but most of serious convective activity
should dissipate by 10pm or so.

The weather system driving the activity is a large low pressure
system currently just off the WA/OR coast.  Forecast models still in
good agreement that the system`s central low pressure area will lift
northeast toward southern British Columbia tonight then track east
just north of the US/Can border on Thursday.  Forecast pressure
gradients remain strong both aloft and with the surface cold front
that should move across the region tomorrow morning, so widespread
gusty winds are still likely over the northern half of the forecast
area.  As such, the high wind warning for areas from Toole County
west to the Northern Rocky Mtn Front remains in effect starting late
tonight.  As the system heads into Saskatchewan on Fri, the
gradients should lessen, along with the winds. High pressure then
steadily builds into the region from the west, bringing dry
conditions and temperatures slightly below normal (mid-upper 70s)
for Friday.
Waranauskas


Friday night through Tuesday...Models are still on the same page in
developing a strong upper ridge over the western U.S. over the
weekend which will continue into early next week. Expect dry
conditions through the weekend. By Monday models forecast some
monsoonal moisture over the area as a weak weather disturbance moves
through the strong upper ridge. Models disagree on details with the
the weather disturbance and associated spotty precipitation. Will go
with isolated showers and thunderstorms over southwest Montana
Monday through Tuesday. Models have spotty precipitation farther
north but due to differences with the timing and placement of the
precipitation will hold for now in mentioning any isolated showers
or thunderstorms over central and northern Montana. Blank

&&

.AVIATION...
UPDATED 1755Z.
VFR conditions are mostly expected through the next 24 hours, unless
otherwise noted. Weak thunderstorms in Hill County to move northeast
into Canada over the next hour or so. Otherwise, under an unstable
southwesterly flow aloft, scattered showers and thunderstorms will
develop over the higher terrain of western Montana after 20Z and
spread out onto the plains of north central Montana after 22Z. Some
of these storms may become strong or even severe (mainly north of a
KBTM-KBZN line) with hail over 1 inch in diameter and wind gusts in
excess of 60 mph possible. Brief heavy rain is also likely with many
of these storms, which may cause conditions to briefly lower to
MVFR/IFR. After 06Z, the upper low pressure area responsible for the
flow aloft will move across the Pacific Northwest, forcing
thunderstorm activity to the east. However, precipitation will
likely move into northwest Montana and obscure mountain tops. A
Pacific cold front ahead of the upper low will also move east across
the area between 03Z and 12Z, shifting winds more westerly and
increasing them significantly. Gusts will mostly be 20 kt or more at
that time, but gusts along the Rocky Mountain Front will likely
exceed 40 kt.
Coulston

&&

.FIRE WEATHER...
Fire Weather Watch for fire zone 115 (Chouteau/Fergus counties)
and the southern third of Blaine County remains in effect with no
changes at this time. Gusty winds, sustained at 20 to 30 mph with
gusts around 40 mph, are likely on Thurs with afternoon humidities
dropping down to 20 to 25 percent range. There is also concern
that lightning starts from thunderstorms this evening may hold
over and then spread as the strong winds develop tomorrow.
Waranauskas

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  61  75  52  76 /  70  10  10  10
CTB  55  69  50  72 /  80  10  10  10
HLN  59  78  53  80 /  40  10  10   0
BZN  54  81  43  80 /  20  10  10   0
WEY  44  74  33  72 /  10  10   0   0
DLN  53  76  43  77 /  30  10  10   0
HVR  60  80  52  77 /  70  20  10  10
LWT  58  81  49  77 /  40  20  10  10

&&

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

HIGH WIND WARNING from Midnight tonight to Midnight MDT Thursday
Night Northern Rocky Mountain Front.

HIGH WIND WARNING from 6 AM Thursday to Midnight MDT Thursday
Night Eastern Glacier...Toole.

&&

$$

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