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000
FXUS65 KTFX 230432
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1030 PM MDT Wed May 22 2013

UPDATED AVIATION SECTION

.UPDATE...
Numerous light rain showers continue to push northward over the
Treasure state...as this area of precipitation continues to rotate
around the main area of upper low pressure over the Pacific
Northwest. Expect this steadier shower activity to continue
through tonight over the western half of the CWA...while areas
around Lewistown...Harlem and Havre only seeing a passing shower
at best. Thus pops have been raised in the West and remain low in
the East for tonight. Also...the threat for thunderstorms has
greatly diminished...thus those have been dropped for tonight.
Forecast models indicate that most of the precipitation will come
to an end early on Thursday morning...and only a passing light
shower is expected in most areas during the day. The next main
area of precipitation should affect the region Thursday
night...especially over much of North Central MT.

The winter weather advisory continues for the Rocky Mountain
Front. A few inches will be possible on Marias Pass...but the most
significant snowfall should occur near ridge tops. Brusda

&&

.AVIATION...
UPDATED 0430Z.
It now appears the only widespread rain the rest of tonight will be
near the Continental Divide with KBZN/KHLN/KCTB/KGTF seeing only
occasional showers. It also now appears that farther east (KHVR and
KLWT) should see mainly dry conditions the rest of tonight. There
could still be scattered areas of MVFR ceilings Thursday morning
but conditions should become VFR by early afternoon. There also
should not be much precipitation during the morning and early
afternoon with the exception of near the Rocky Mountain Front. Then
during the afternoon scattered showers and thunderstorms will
develop over southwest Montana. Thunderstorms will diminish by dark
as showers turn to widespread rain and spread north. With the
precipitation expect at least scattered MVFR ceilings and
visibilities with these conditions possibly becoming widespread over
the plains during the evening. Also with the precipitation expect
local IFR ceilings in heavier precipitation. Mountains will be
generally obscured over the Rocky Mountain Front with areas of
mountain obscurement elsewhere particularly late in the afternoon
and evening. Winds are another forecast problem as guidance differs
quite a bit on both wind speeds and directions. Hence confidence in
forecast winds in the tafs is lower than average. Blank

&&

.HYDROLOGY...Widespread rain and high elevation snow will
develop tonight along the Rocky Mtn Front and continue through
Thursday morning with additional precipitation expected Thursday
and Thursday night. Heaviest precipitation is expected late
tonight through Thursday morning with precipitation amounts locally
exceeding an inch and a half along the continental divide. Snow
levels should lower enough to lessen the impact of moderate
precipitation on the remaining mtn snowpack in these areas, however
local streams and rivers could see delayed rises through the
upcoming weekend. 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.

&&

.PREV DISCUSSION...
/ISSUED 555 PM MDT Wed May 22 2013/

Tonight through Friday...The airmass is slightly unstable and some
good shear is available from Southwest Montana up towards Lewis and
Clark county. Scattered showers and thunderstorms are expected into
early evening. The main threat from these thunderstorms are strong
winds...hail should remain small. A closed upper level low will
rotate over the Pacific Northwest tonight and bring a period of
widespread precipitation centered over the Northern Rockies through
Friday morning. A jet will move through the flow aloft and develop
strong diffluence over central and north central Montana tonight
into Thursday morning. Snow levels show most of the snow to be above
6000 feet and the current watch will be changed to a winter weather
advisory. Elsewhere, all but the northwest zones will be slightly
unstable Thursday and the extreme southwest zones will be the most
unstable. The jet aloft will move across the zones again Thursday
afternoon, and the pattern should once again develop strong
diffluence over the zones. Winds will switch from the southeast to
southwest Thursday night and hasten the end of precip east of the
Rockies Friday...although higher elevations along the Divide will
remain very moist.  Zelzer

Friday night through Wednesday...The period begins with a slowly
filling upper low over British Columbia. Southwest flow aloft
associated with this feature will continue to bring moisture and
instability in the form of scattered thunderstorms to the area
Friday night. Generally drier conditions will return Saturday though
afternoon isolated thunderstorms will remain as the flow aloft
becomes more southerly. By Sunday, southwest flow aloft strengthens
as an upper trof approaches the west coast. The airmass remains
moist and instability increases during the afternoon with scattered
thunderstorms the result. The medium range models differ in their
handling of QPF with the European being the wetter model. Model
confidence is low for Memorial Day. The GFS brings the upper trof to
the Pacific Northwest and develops widespread precipitation. The
European model, on the other hand, develops weak ridging over the
area and dry conditions. Given the progressive nature of the GFS
solution, have leaned toward the wetter outcome for Memorial Day.
Continued southwest flow aloft Tuesday and Wednesday ahead of a west
coast upper trof will keep scattered showers and thunderstorms in
the picture. Temperatures will remain above seasonal averages
through the period.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  41  55  40  64 / 100  70  70  40
CTB  40  54  38  62 / 100 100  70  50
HLN  40  58  39  63 / 100  70  70  40
BZN  42  63  37  66 / 100  60  60  30
WEY  38  61  34  59 /  60  30  40  10
DLN  38  59  35  62 / 100  50  60  30
HVR  49  68  45  69 /  30  60  80  70
LWT  45  63  41  65 /  50  60  90  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

WINTER WEATHER ADVISORY from Midnight tonight to Noon MDT Friday
FOR ELEVATIONS ABOVE 5500 FEET FOR Northern Rocky Mountain
Front...Southern Rocky Mountain Front.

&&

$$

weather.gov/greatfalls