Area forecast discussion
National Weather Service Great Falls Montana
1020 PM MDT Mon may 20 2013
Updated aviation section
Update...
have begun to decrease pops across north central and central
Montana as a stabilizing airmass develops from the east. Some
showers and an isolated lightning striker or two is still
possible near the Front Range of the Rocky Mountains where some
late shower activity is still on going. Some patchy fog remains
possible as some clearing is expected overnight. However not
confident that temperatures will sufficiently cool enough to yield
widespread fog at this point so have decided against putting it in
the weather grids for now. Suk
&&
Aviation...
updated 0420z.
Expect VFR conditions. Skies will clear the rest of the night.
Isolated showers and thunderstorms will develop over southwest
Montana near and after mid-afternoon on Tuesday. Think the activity
will be isolated enough that its mention was not included in the
khln and kbzn tafs. This activity should diminish by 06z Wednesday.
Blank
&&
Previous discussion...
/issued 520 PM MDT Mon may 20 2013/
tonight through Wednesday...scattered showers and isolated
thunderstorms continue to affect portions of central Montana 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 Montana 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
County Warning Area. 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 Washington/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 Montana 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 European model (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
&&
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 / 20 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.
&&
$$
Short term...suk
long term...waranauskas
aviation...blank
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