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000
FXUS65 KTFX 202055
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
255 PM MDT Mon May 20 2013

.DISCUSSION...
Tonight through Wednesday...Scattered showers and isolated
thunderstorms continue to affect portions of Central MT this
afternoon...as upper level energy continues to rotate around the
main storm system that is currently over the upper Midwest. As the
airmass continues to cool this evening...expect the airmass to
become more stable. The end result...the showers/thunderstorms
should diminish this evening. Generally quiet weather is expected
Tuesday morning until early early afternoon. Isolated
thunderstorms will once again be possible over mostly the higher
terrain of the Rocky Mountain Front and Southwest MT on Tuesday
afternoon/evening...as slightly unstable air moves back into the
region. Much warmer air will also move back into the entire region
by the afternoon hours. Isolated thunderstorms continue Tuesday
night...mainly over the western portions. On Wednesday...a
stronger upper level low will approach from the West...resulting
in thunderstorms over the Southern and Central sections of the
CWA. A few storms could be on the strong side...mainly in the
Bozeman and White Sulphur Springs area. Expect more widespread
precipitation to develop by the late afternoon hours over the
Rocky Mountain Front...with snow levels starting to lower quickly
by Wednesday evening. Brusda

Wednesday Night through Monday...Latest forecast model solutions
maintaining good continuity and agreement with runs from past
several days in showing large, closed upper-level low pressure
system remaining mostly stationary over WA/OR/BC from Wed eve
through early Sat. Persistent south-to-southeast flow in the low-
and mid-levels on the front side of the closed low will bring in a
steady stream of moisture for widespread clouds and precipitation on
Thurs and Fri. Thurs aftn/eve is looking particularly interesting as
the closed low drifts into western MT, allowing the jet stream to
become positioned along/slightly east of the Continental Divide.
This will create favorable wind shear and some upper-level
divergence to support scattered thunderstorms, some of which may be
moderate to strong in intensity. With model-projected precipitable
water values increasing to 0.8 to 1.0 inch from Great Falls north to
the US/Can border, locations across north central MT may see
significant rainfall amounts, especially over/near the higher
terrain of the Rocky Mtn Front and Glacier NP.  This could also have
subsequent impacts on river/stream levels in the western third of
the forecast area heading into the Memorial Day weekend. We will
continue to monitor coming model runs and assess possible impacts,
including the need for any hydrological headlines. As for the rest
of the extended forecast period, the closed low is still projected
to gradually fill/weaken and exit across southwest Canada beginning
Sat eve.  Currently, the GFS model moves the system along quicker
than the ECMWF model so forecast grids keep 20-45% chance of precip
over most of our region through the weekend. A second, faster-moving
shortwave trof is projected to arrive late Sun into Mon morning,
bringing another round of light rain.  With all the expected cloud
cover and precip, models have decreased forecast high temperatures a
few degrees, but still expect low-mid 60s each day over the plains
and valleys, with low 50s in the mountains.
Waranauskas

&&

.AVIATION...
UPDATED 1747Z.
Central Montana is between a pair of low pressure systems and the
airmass will be slowly drying. Scattered showers are possible to the
north with numerous showers across Southwest Montana. A slight
chance also exists for thunderstorms. VFR conditions and light winds
are expected to prevail through Tuesday.

&&

.HYDROLOGY...
First of two rainfall-producing systems will gradually exit the
region today, with precipitation expected to taper off this
evening.  Focus now shifts to the second system, currently swinging
south along the Canadian coast.  It is expected to become stationary
over the Pacific Northwest area and persist thru this week.  Small
shortwave troughs embedded in the system circulation will bring
periods of light to occasionally moderate rain during the latter
half of the week, with possibly some significant rain amounts along
the Rocky Mtn Front late Wed through Thurs night. Most area rivers
are still well below flood stage, but we could see some marked rises
by week`s end, especially in smaller tributaries and streams west of
the Interstate 15 corridor.  At this point, cannot rule out that
some hydrological headlines may be needed, but will have to wait for
several more forecast model runs to assess the expected
precipitation amounts.
Waranauskas

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  43  72  42  69 /  20   0  10  30
CTB  42  71  41  64 /  30   0  10  10
HLN  44  74  45  68 /  20  10  10  50
BZN  38  72  39  71 /  30  10  10  50
WEY  33  64  35  65 /  30  20  10  40
DLN  40  72  41  68 /  20  10  20  40
HVR  44  72  44  72 /  20   0  10  10
LWT  40  65  39  63 /  20   0  10  20

&&

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

&&

$$

weather.gov/greatfalls