137 
FXUS65 KTFX 211743
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1142 AM MDT Tue May 21 2013

.UPDATE...

Forecast is on track with clear skies across central MT this morning.
Early morning patchy valley fog dissipated quickly as expected. Made
only cosmetic changes to the POP and hourly temperature grids
following an earlier adjustment to min temperatures. Forecast
still indicates increasing clouds with isolated thundershowers
over the higher terrain of SW MT. Expect increasing east-southeast
winds up to 30 mph this afternoon over the north central plains as
low pressure deepens to our west. Nutter

&&

.AVIATION...
UPDATED 1742Z. 
Central and Southwest Montana are sitting underneath weak high 
pressure which is generally keeping the airmass dry. Weak 
instability will develop over Southwest Montana this afternoon and 
isolated thunderstorms are possible. A low pressure system to the 
west is approaching the region. Southwest winds will increase and 
become gusty, and the airmass will moisten as this system continues 
to approach. Low level moisture could cause some MVFR cigs beginning 
tonight. Areas affected include KCTB, KHVR, and KLWT. 

&&

.PREV DISCUSSION...
/ISSUED 535 AM MDT Tue May 21 2013/

Today through Thursday...Weak upper level ridging between the
large upper low centered over the northern plains states and an
incoming upper low off the Pacific NW coast will drift east across
the Northern Rockies and MT today for generally dry and seasonably
warm conditions. Relatively weak flow aloft under the ridge axis
will begin to shift more southerly late this afternoon and evening
as the ridge axis shifts east and this combined with surface
heating may allow for the buildup of convective clouds over the
higher terrain of SW MT with an isolated shower or thunderstorm
possible. Southerly flow strengthens on Wednesday as the Pacific
NW upper low moves inland and the first in a series of shortwaves
and associated upper level jet energy rotates up through the
Northern Rockies late Wednesday through Wednesday night with
another wave expected to rotate around the nearly stationary upper
low across the region late Thursday. Showers and thunderstorms
will develop Wednesday afternoon over SW MT and lift north across
the remainder of the forecast area Wednesday evening with steadier
precipitation developing along the Rocky Mtn Front/Glacier Park area
Wednesday night with a short break early Thursday morning before
another surge of precipitation lifts through the area Thursday
afternoon. Models continue to focus greatest precipitation amounts
across the Rocky Mtn Front/Glacier Park region where total amounts
late Wednesday through Thursday may exceed an inch. Snow levels
will also lower to around 6500 ft in this area by Thursday with
several inches of wet snow likely over the highest elevations near
Glacier National Park and the Bob Marshall Wilderness areas.
Thunderstorms on Wednesday and Thursday will be most likely over
portions of SW MT where instability will be greatest with lesser
chances further north where easterly low level flow and its
associated stable layer will make it difficult for surface based 
convection, although some elevated thunderstorms will be possible. 
Hoenisch

Thursday Night through Tuesday...Large closed low pressure system 
discussed above expected to remain over PacNW through Fri eve then 
begin to fill/weaken and lift into southwest Canada during the 
weekend.  Forecast models still indicating broad area of light to 
moderate rain over much of central and southwest MT Thurs night into 
Fri morning, with precipitation amounts over the Rocky Mtn Front and 
adjacent slopes possibly exceeding 0.5 inch during that period.  
There will also be rain/snow mix near mountain pass levels and a few 
inches of accumulating snow above pass level along the Rocky Mtn 
Front.  The rain/snow activity will gradually taper off Fri eve as 
much of the energy and moisture of the PacNW system swings northward 
into BC/Alb, bringing drier conditions across our region for the 
first half of Memorial Day weekend.  A secondary shortwave trof 
rotating through the remnants of the weakened closed low circulation 
is forecast to arrive Sun eve and should generate scattered light 
rain showers that continue through most of Mon. Kept at least a 
chance of showers across our region on Tues, mostly as a compromise 
between the wetter GFS and drier ECMWF solutions. Daily high 
temperatures still expected to remain in the low to mid 60s, which 
slightly warmer temps possible on Sunday thanks to decreased cloud 
cover and good southerly flow aloft.
Waranauskas

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  72  43  68  45 /   0  10  20  60 
CTB  71  43  63  42 /   0  10  30  70 
HLN  74  45  67  44 /  10  10  50  70 
BZN  71  41  70  43 /  10  10  50  60 
WEY  62  37  63  37 /  10  10  40  40 
DLN  70  42  67  40 /  10  20  30  30 
HVR  72  45  71  51 /   0   0  10  40 
LWT  65  41  65  46 /   0  10  20  60 

&&

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

$$

weather.gov/greatfalls