Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 231800
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1155 AM MDT Wed Jul 23 2014

UPDATED AVIATION SECTION

.UPDATE...
Area of showers and a few embedded thunderstorms that moved through
central MT this morning are now over the Hiline counties and pushing
quickly to the northeast.  This activity should be out of forecast
area in the next 60-90 min.  Now we wait while the atmosphere resets
itself for the strong-to-severe thunderstorms for this aftn.  Going
forecast is in good shape, with main storm activity projected to
occur along/north of a Butte-to-Lewistown line.  As a result of some
model solutions showing marked increase in low-level rotation with
some of the storms, we have added mention that an isolated tornado
is possible as the storms evolve, but likelihood and confidence that
a tornado may occur is low. The primary threats for all storms today
remain large hail and strong/damaging outflow winds.
Waranauskas

&&

.DISCUSSION...
Tuesday through Friday...The next two days of the week will have
some very active weather in store for Central Montana especially
the high plains of Central Montana and the Rocky Mountain Front. A
closed low currently off the coast of Washington will quickly move
across the Pacific Northwest over the next 24 hours and be the major
source for weather impact our region. A shortwave trough ahead of
the low is currently moving across Idaho and will bring showers and
thunderstorms to our area starting this morning. Another round of
thunderstorms should be able to develop this afternoon as a second
shortwave moves into the region. Model solutions for the pre-storm
environment are not as aligned as one might hope for. GFS is
currently one of the drier solutions...with only limited convective
potential for storms. However most other solutions including the NAM
and ECMWF show a strong potential for possibly severe storms to
occur from the mid morning hours through to the late evening hours.
Storm shear looks like it should allow for the easy organization of
supercells. A decent cap does exist at the mid levels and could
inhibit some storm growth, if storms are able to punch through or
erode this inversion the amount CAPE that becomes available should
allow for some very powerful storms capable of producing some very
large hail due to the steep lapse rate through the hail growth zone.

The closed low will track into the area Thursday morning and will
create a strong pressure gradient similar to a fall or spring
storm pattern. This gradient will cause winds to become breezy
throughout the area with strong potentially damaging over Glacier
county including Glacier National Park, Cut Bank, and exiting out
towards the Sweet Grass Hills of Toole county. These winds are
expected to last throughout the day Thursday before winding down
Friday as the low pressure system exits the area. A high wind
warning has been issued due to these conditions. Another set of
concerns with the winds will be the dry and warm environment
before the cold frontal passage across Central Montana. Dewpoints
are going to be near critical fire weather levels. A fire weather
watch is in effect from noon Thursday through Friday.

Friday a ridge of high pressure will begin to build into the area
in the wake of the low pressure system and drying the system out.
Temperatures should be above seasonal averages ahead of the low
pressure system. Thursday night temperatures should drop
dramatically behind the cold front, and slowly warming back toward
seasonal averages. Suk

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...
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  89  61  75  51 /  80  70  10  10
CTB  86  54  69  50 /  70  80  20  10
HLN  92  60  78  52 /  80  40  10  10
BZN  91  55  81  43 /  40  20  10  10
WEY  82  45  75  32 /  30  10  10   0
DLN  87  53  76  44 /  50  30  10  10
HVR  94  59  79  51 /  40  70  20  10
LWT  92  60  80  48 /  60  40  20  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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