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000
FXUS65 KTFX 201028
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
425 AM MDT Mon May 20 2013

HYDROLOGY SECTION UPDATED

.DISCUSSION...
Today through Wednesday...Upper level low centered over the Dakotas
will slowly track NE today keeping the the forecast area under a
moist northerly flow aloft. Satellite imagery this morning shows a
slot of drier air working south into into central MT from Havre
south to Lewistown while a swath of higher moisture extends south
across the remainder of north central and SW MT. Cloudy and
showery conditions will persists across much of the area today,
however precipitation will be rather light and driven by upslope
northerly flow as stronger forcing associated with the upper low will
remain east of the region. Weak upper level ridging between the
upper low to the east an an incoming upper low over off the
Pacific NW coast will bring drying/clearing to the entire forecast
area tonight through Tuesday with north to northwest winds today
becoming light tonight before shifting to the east on Tuesday in
response to low pressure developing over the interior NW US ahead
of the incoming upper low. By Wednesday, the Pacific NW upper low
moves inland with strengthening S to SE flow aloft ahead of the
system spreading over the Northern Rockies and MT. A frontal
boundary associated with this system will approach the Northern
Rockies late Wednesday with shower and thunderstorm development
ahead of this feature likely by Wednesday afternoon. Hoenisch

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 0455Z.
Weak instability and widespread moisture will continue to keep
scattered showers under broken ceilings for the next 24 hours. A mix
of VFR/MVFR ceilings through the TAF period. Light rain showers can
be expected near all TAF sites through the day with an isolated
thunderstorm possible through the early evening hours...especially
at KLWT where a persistent thunderstorm is moving into the area .
Flight Categories may briefly drop to IFR category while
precipitation is occurring. Winds should decrease slightly overnight
before strengthening again after sunrise Monday. Suk

&&

.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  64  44  70  43 /  60  20  10  10
CTB  64  44  69  43 /  50  20  10  10
HLN  65  44  72  45 /  50  20  10  10
BZN  63  40  71  41 /  60  30  10  10
WEY  56  34  63  37 /  40  30  20  10
DLN  64  40  71  43 /  30  20  10  20
HVR  68  45  70  45 /  30  20   0  10
LWT  60  41  63  41 /  70  20  10  10

&&

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

$$

SHORT TERM...HOENISCH
LONG TERM...WARANAUSKAS
AVIATION...WARANAUSKAS

weather.gov/greatfalls