Area forecast discussion
National Weather Service Great Falls Montana
525 am MDT Sat may 18 2013
Aviation section updated
Discussion...
upper level trough extending from the Pacific northwest through the Great
Basin will continue a slow progression east into the rocky mtn
region today, keeping the forecast area under a moist and somewhat
unstable SW flow aloft. Main focus for precipitation this morning
will be along an old/weakening deformation zone along the hi-line
and over far SW Montana ahead of the next incoming shortwave embedded
within the broader upper trough. Elsewhere, precipitation will be
limited to scattered light showers with patchy fog/drizzle through
early this morning. Daytime heating and the approaching shortwave
from the south should bring an increase in the coverage of showers
this afternoon and evening to most of the forecast area and an
isolated weak/brief thunderstorm or two cannot be ruled out,
however instability parameters are rather marginal. Sunday and
Monday, the upper trough emerges east of The Rockies with the
upper level circulation closing off as the trough undercuts an
upper ridge over central Canada. This results in a shift to more
northerly flow across the forecast area as the upper low center
tracks from Nebraska slowly northeast into the upper Midwest
through early next week. This should shift the focus for
widespread precipitation to north central Mt, however all areas
will have some chance for showers through the period, especially
during the afternoon/evening time-frames. Temperatures will remain
close to seasonal averages today with slightly cooler conditions
on Sunday as the flow aloft turns to the north. Snow levels at or
above 8000 ft will limit any snow accumulation to the higher
peaks. Hoenisch
Monday night through Saturday...extended forecast period begins with
mostly quiet conditions as the upper level system that will cross
Montana this weekend will be moving into the upper Midwest states
and a well-amplified high pressure ridge begins to settle in over
ern Montana and the Dakotas. Meanwhile, a second upper level trof
is forecast to swing through western Canada and form a closed low
pressure center over the pacnw coast by tues eve. With the large
ridge remaining anchored thru the middle of the country, the Pacific
low essentially becomes stationary, drifting along the coast and
occasionally moving inland during the latter half of next week.
Minor shortwaves and accompanying moisture rotating around the
central low will bring intermittent periods of showers and isolated
thunderstorms each day from Wed Onward. Forecast models are in
decent agreement that most parts of central/southwest Montana will see
rain during this time, but precip amounts still look to be generally
light (0.10-0.15 inch or less) over the plains and valleys.
Rainfall totals are slightly higher (0.20-0.50 inch) over the
melting snowfields of the rocky mtn front and little belt mtns,
which gives concern for increased streamflows in creeks/rivers in
those areas. Persistent southerly flow aloft will allow
temperatures to stay close to typical seasonal values with highs in
the mid-60s to the low 70s each day, though that will be modified
quite a bit by a fair amount of cloud cover and the timing of the
shortwave passages.
Waranauskas
&&
Aviation...
updated 1125z.
An upper level trough extending from the Great Basin area into the
northern rockies will make only slow eastward progress today...
keeping central and southwest Montana under broken-overcast skies with
intermittent periods of light rainshowers. There is also a slight
chance for isolated thunderstorms this aftn, but given their low
probability and spotty coverage did not include mention of any ts at
primary taf sites today. Ceilings and visibilities should remain
VFR but brief MVFR/IFR conditions are possible near any
precipitation areas. Also, winds will be light across the region
today.
Waranauskas
&&
Hydrology...
two upper level trofs, one moving through our region this weekend
and the other becoming stationary over the pacnw coast for much of
next week, will bring increased chances for rainfall over the next
several days. Though cooler temperatures will reduce snowmelt
slightly, the primary concern is that rain on the melting snowpack
will cause enhanced rises in stream/river levels. Most area rivers
are currently well below flood stage but a handful of sites are near
or approaching bankfull, so rainfall totals and river stage
forecasts will be closely monitored for significant changes and
impacts. Waranauskas
&&
Preliminary point temps/pops...
gtf 64 45 59 45 / 80 60 80 80
ctb 62 44 60 43 / 80 50 80 80
hln 64 45 60 45 / 70 60 60 70
bzn 64 43 59 42 / 80 70 60 60
wey 55 36 53 34 / 80 70 60 40
dln 61 40 58 40 / 80 60 50 50
hvr 69 49 66 47 / 90 80 90 80
lwt 61 45 57 43 / 80 80 80 80
&&
Tfx watches/warnings/advisories...
none.
&&
$$
Short term...hoenisch
long term...waranauskas
aviation...waranauskas
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