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000
FXUS65 KTFX 220427
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1030 PM MDT Tue May 21 2013

UPDATED AVIATION SECTION

.UPDATE...
No significant changes to the forecast this evening as some
scattered showers continue over Southwest Montana. Will slightly
increase pops for these showers through Midnight...shower activity
is not currently expected to continue into the morning hours. And
has already decreased significantly after sunset. Moisture aloft
continue to flow into the area and the wetter conditions for the
rest of the week currently appear on track. Suk


&&

.AVIATION...
UPDATED 0430Z.
The main forecast problem will be the possibility of MVFR or lower
ceilings near and northeast of a line from KLWT to KCTB late tonight
through Wednesday morning. Latest guidance still has significant
differences with regard to ceilings Wednesday morning. Some guidance
keeps conditions VFR while other guidance has ceilings as low as
LIFR. Do not expect LIFR ceilings. Did go with MVFR ceilings at KCTB
and KLWT with confidence in these lower ceilings at a moderate but
not high level. Even if the MVFR or lower ceilings develop Wednesday
morning expect ceilings will become VFR Wednesday afternoon. Expect
gusty southeast to east winds over the plains. Scattered showers and
possibly isolated thunderstorms will develop over southwest Montana
Wednesday afternoon then will increase in areal coverage Wednesday
evening as they spread northeast. There could be brief MVFR ceilings
and visibilities in the showers. Widespread showers will develop
over the Rocky Mountain Front later Wednesday afternoon and continue
through the evening. Expect areas of mountain obscurement with the
showers. Blank

&&

.HYDROLOGY...
The low pressure weather system expected to remain over the PacNW
this week will bring periods of light to occasionally moderate rain
from Wed evening through Fri afternoon.  Forecast models continue to
show some rainfall amounts in excess of one inch are possible along
the Rocky Mtn Front, including Glacier NP.  Local streams and rivers
could see some quick rises, especially over the foothills and
adjacent plains west of Interstate 15.  Right now, widespread
flooding is not expected, but the situation will be closely
monitored over the coming days in case any hydrologic headlines are
needed. Persons living near or planning activities near the
mentioned areas during the holiday weekend should be alert for
rising river levels and increased stream flows.
Waranauskas

&&

.PREV DISCUSSION...
/ISSUED 540 PM MDT Tue May 21 2013/
Tonight through Thursday...Expect gradually more active weather to
move into the region over the next 24 to 48 hours...as the main
upper level low that is currently just off the West coast starts
to move inland over the Pacific Northwest. A few isolated
thunderstorms are starting to develop over southern Idaho this
afternoon...and these storms should move into Southwest Montana by
this evening and then continue to expand northward along the
divide over the Rocky Mountain Front. For Wednesday...expect
another upper level disturbance to rotate northward through the
region around the upper level low over the Pacific Northwest. Some
thunderstorms will once again be possible over Southwest
Montana...while numerous showers are expected along the front
range. The precipitation becomes a bit heavier Wednesday night and
into Thursday morning along the divide...as snow levels also begin
to lower. The snow level will lower to around 6000 feet by
Thursday morning...with a significant snowfall possible for
elevations above 6000 feet. Thus a winter storm watch has been
issued for the higher elevations of the Rocky Mountain Front. A
steadier rain is expected over much of Central and North Central
Montana on Thursday morning per the NAM model. The NAM model is
also much colder than the GFS model...bringing snow levels down
to around 4000 feet. I have not gone this low with the snow
levels...but will continue to monitor this closely. Afternoon
temperatures will generally be near/slightly below normal for both
Wednesday and Thursday for most locations. Brusda

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  74  41  67  40 /   0  10  10  70
CTB  74  40  63  39 /   0  10  30  80
HLN  76  42  67  42 /  10  10  40  80
BZN  73  38  68  40 /  10  10  40  60
WEY  61  34  64  34 /  10  10  40  50
DLN  74  39  66  38 /  10  30  30  50
HVR  75  43  70  46 /   0   0  10  30
LWT  69  38  63  42 /   0  10  20  60

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH from Wednesday evening through Thursday
morning FOR ELEVATIONS ABOVE 6000 FEET FOR Northern Rocky
Mountain Front...Southern Rocky Mountain Front.

&&

$$

SHORT TERM...Suk
LONG TERM...WARANAUSKAS
AVIATION...BLANK

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