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000
FXUS65 KTFX 230000
AFDTFX

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

UPDATED AVIATION SECTION

.DISCUSSION...

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.

&&

.AVIATION...
UPDATED 2355Z.
Showers will turn to widespread rain this evening except at KHVR and
KLWT. The rain will turn back to showers late tonight into Thursday
morning. The main forecast problem will be how low conditions will
go. Like yesterday some guidance forecasts VFR conditions while
other guidance forecasts MVFR/IFR conditions. Tended to take a
middle-of-the-road approach so over southwest Montana went with
primarily VFR conditions with occasional MVFR conditions. At KGTF
and KCTB on the other hand went with MVFR/IFR conditions later
tonight and continuing into Thursday morning. Do expect generally
VFR conditions Thursday afternoon over the entire forecast area with
MVFR/IFR conditions in local heavy precipitation. As for
thunderstorms the only threat for thunderstorms early this evening
will be over southwest Montana. For Thursday afternoon there will be
a few thunderstorms except at KCTB and the best threat for
thunderstorms at the taf sites will be at KLWT. 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.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  41  55  40  64 /  90  70  70  40
CTB  40  54  38  62 / 100 100  70  50
HLN  40  58  39  63 /  80  70  70  40
BZN  42  63  37  66 /  70  60  60  30
WEY  38  61  34  59 /  50  30  40  10
DLN  38  59  35  62 /  70  50  60  30
HVR  49  68  45  69 /  20  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.

&&

$$

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