000
FXUS65 KTFX 221804
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1200 PM MDT Wed May 22 2013

.UPDATE...

Scattered showers have developed along the Rocky Mountain Front this
morning as moisture streams in from the southwest. POPS were raised
in this area to better reflect activity shown on radar...and
furthermore blended with 12Z model guidance. Gusty southeast winds
continue over the north central plains...which were backed about 20
degrees closer to east in line with observations. Finally, added 1-3
degrees to high temperatures over the plains after blending with
fresh guidance and noting gaps in clouds over the area. Rest of
forecast package remains on track. Nutter

&&

.AVIATION...
UPDATED 1800Z.
VFR conditions prevail with the exception of MVFR conditions from
KLWT to KHVR that will slowly lift this afternoon. The airmass will
become unstable after 21z with scattered showers and thunderstorms
developing first over Southwest Montana before spreading north and
east during the afternoon and early evening. Best chance for
thunderstorms will be from West Yellowstone to Helena and south.
East to southeast surface winds will remain gusty through this
evening. Expect more widespread steady rain to move into the region
through the night with most impact along the terrain of the
Continental Divide. Widespread terrain obscuration due to low
ceilings and precipitation is expected after 00z and will continue
through the 24 hour TAF period. Widespread MVFR conditions will be
possible after 06z with the increase in shower activity.

&&

.PREV DISCUSSION...
/ISSUED 555 AM MDT Wed May 22 2013/
Today through Friday...A vigorous upper low, currently centered
over the Pacific NW, will remain nearly stationary through the
remainder of this week with several lobes of shortwave energy and
associated upper level jet maximums rotating around the low
through the Northern Rockies and MT. First piece of energy will
lift north through western MT tonight with convection initiating
over SW MT this afternoon, which then looks to lift north across
north central MT tonight. Instability and shear parameters are
marginal for a few stronger storms across SE portions of the
forecast area through early this evening, mainly across Gallatin and
Meagher counties. Expect convection to weaken as is spreads north
into a more stable airmass over north central MT this evening with
an area of widespread precipitation developing over western portions
of north-central MT overnight. At the same time, a frontal boundary
and cold pool aloft associated with the upper low will spread east to
areas along the Continental divide overnight, lowering snow levels
to around 6000 ft by Thursday morning. Heaviest precipitation will
be focused along the Rocky Mtn Front region where favorable jet
dynamics and upslope SE flow will exist. Precipitation amounts
through Thursday morning could exceed 1 inch along the divide in
this area with significant snow accumulations generally confined to
areas above 6500 feet. Some of the cold air aloft spills east into
central MT Thursday, however S to SE flow aloft around the low will
limit its extent with the bulk of the colder air remaining west of
the divide. Additional lobes of shortwave energy lift across the
region Thursday and Friday with periods of more widespread showers
associated with each disturbance along with a chance of
thunderstorms during the afternoon/evening periods. Models are still
having some difficulty with timing and location of these
disturbances, but all focus on western portions of north central MT
for greatest precipitation amounts. Snow levels will fluctuate
around 6500 along the rocky Mtn front with additional snow
accumulation possible across the highest elevations.

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  66  43  58  43 /  20  70  60  60
CTB  63  41  55  41 /  40  80  80  70
HLN  68  44  60  43 /  60  70  60  60
BZN  67  42  66  44 /  60  70  50  50
WEY  64  35  61  36 /  60  50  30  30
DLN  68  39  62  41 /  40  60  40  50
HVR  71  49  68  47 /  10  30  60  70
LWT  63  44  63  44 /  20  50  60  70

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

WINTER STORM WATCH from 6 PM MDT this evening through Thursday
morning FOR ELEVATIONS ABOVE 6000 FEET FOR Northern Rocky
Mountain Front...Southern Rocky Mountain Front.

&&

$$

weather.gov/greatfalls