Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 240951
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
351 AM MDT Thu Apr 24 2014

.DISCUSSION...

Today through Saturday...An active westerly flow aloft continues
over the region with water vapor satellite imagery this morning
showing a shortwave trough moving into the Pacific NW coast with
additional shortwave and upper level jet energy upstream of this
trough over the Eastern Pacific. Trough along the west coast this
morning will weaken as it moves across a weak ridge in place over
the Northern Rockies this afternoon, but will bring increasing
cloud-cover to the region this morning with good chance for
showers this afternoon as the upper level energy and Pacific
moisture move across the area. A drier period is expected tonight
before the previously mentioned upstream trough digs into the
Western US and amplifies Friday and Saturday. Flow aloft turns to
the SW ahead of the trough on Friday with moisture and instability
increasing by Friday afternoon. The first piece of energy rotates
north across across MT late Friday/Friday night with another
lifting north through the area on Saturday. Showers and possibly a
few isolated thunderstorms develop over SW MT Friday afternoon
which evolves into an area of widespread precipitation that lifts
north across central and north central MT Friday night. A brief
period of little activity is expected Saturday morning before the
second wave moves through the region later on Saturday, taking a
slightly more eastern trajectory through Central MT. Hoenisch

Saturday night through Thursday...Medium range models are in good
agreement for the early portion of the forecast period. Southern
portion of an upper level trough over the western USA is expected to
move slowly east-northeast Saturday night through Monday night and
gradually forms a cutoff low over the Great Plains. However, the
northern portion of this trough remains anchored along the British
Columbia coast forming a negative tilt trough that keeps the
Northern Rockies in an unsettled northwesterly flow aloft. This
pattern is expected to keep temperatures generally near seasonable
averages with showers lingering over the forecast area. Have updated
the forecast to increase pops for Saturday night and Sunday then
kept isolated to scattered pops in place through Tuesday. By Tuesday
night models begin to inch the closed low further east and build a
positive tilt ridge into the Northern Rockies. This is similar to
what was shown by the ECMWF for early to mid week last night but
models are now showing more of a dry northeast flow transitioning
into a better likelihood for western Montana to come under the
influence of the ridge axis. Have gone with a somewhat drier
forecast for Tuesday night through Thursday with model guidance in
better agreement in a slow warming trend. mpj

&&

.AVIATION...
UPDATED 0415Z.
Weak high pressure will build over central Montana tonight, isolated
evening showers will end, and the air mass will dry. A low pressure
shortwave will move into the Pacific Northwest Thursday morning and
moisture ahead of this will cross the Rockies and increase cloud
cover through the afternoon. Scattered showers will develop over
Southwest Montana during the afternoon as the shortwave approaches
but most locations to the north will remain dry. VFR conditions will
prevail through the next 24 hours.

&&

.HYDROLOGY...
light/showery precipitation is expected this afternoon with a larger
Pacific storm system bringing another round of widespread moderate
precipitation late Friday through Saturday. Total precipitation amounts
from the Fri/Sat system look similar to the last precipitation event
with most areas expected to receive around one-quarter inch of liquid
equivalent precip, with one-half inch or better amounts most
likely across southern Gallatin/Madison counties and arcing from
the Little Belts north and west into the Glacier Park area. Snow
levels will range from around 5000 feet near Glacier Park to
around 7000 feet over SW MT. This will likely mean that the bulk
of precipitation that occurs over existing mtn snowpack will be in
the form of snow and not immediately contribute to runoff. However
many rivers and creeks originating in the mtns are already running
at high levels and this will be closely monitored through the weekend.
Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  57  35  58  39 /  20  10  50  80
CTB  56  31  56  35 /  20  10  10  80
HLN  58  37  58  39 /  30  20  60  80
BZN  59  35  59  37 /  30  20  50  60
WEY  48  32  49  32 /  50  50  60  60
DLN  55  35  55  37 /  30  30  50  60
HVR  62  34  63  39 /  10  20  10  70
LWT  57  32  56  37 /  20  20  40  80

&&

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

$$


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