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Area forecast discussion 
National Weather Service Great Falls Montana
1030 PM MST Sat Nov 28 2015


Tonight...the upper level high pressure ridge remains in place and
temperatures have already dropped to near forecast lows in some locations.
Updated to lower temperatures mainly across the central zones. Also
freshened winds. Zelzer


Aviation...updated 0530z.

High pressure aloft will generally keep the airmass dry with clear
skies persisting through 18z. At lower levels, moisture is
available...and areas of fog with IFR/LIFR conditions in some
valleys and near rivers, including vicinity khln, may develop during
the night. Otherwise...VFR conditions will prevail through the next
24 hours. Middle to high level clouds expected to develop after 18z
mainly...across southern weak energy and some moisture
wrap around closed low pressure centered over northern Wyoming.


Previous discussion...
/issued 412 PM MST Sat Nov 28 2015/

Tonight through Monday...this period will remain dry with
generally clear skies for most of the area. However, temperatures
will be very slow to warm through the period. The upper level
ridge currently over the Pacific northwest/southwest Canada will
gradually shift to over the northern rockies by Monday as the
upper low currently over the central rockies moves out onto the
Continental U.S. Plains. This will result in the easterly flow aloft shifting
more northerly. Have also generally undercut model consensus
temperatures for this period, as it has not handled the snow
cover well. The mostly clear skies will aid in slowly melting the
snowpack, which will allow high temperatures to warm closer to
normal (30s) over the plains of north central Montana by Monday.
Winds will also remain fairly light there, allowing overnight lows
to fall mostly into the single digits. Temperatures along the
Rocky Mountain front will continue to warm into the lower 40s due
to less snow cover there, as lows fall into the teens. The cold
air will likely remain trapped in the southwest valleys through
the period with very slow erosion of the airmass through the
period. This will allow lows to fall into the single digits and
teens below zero, as highs only warm into the teens and lower 20s
above zero. The fog in the Helena valley only dissipated by the
middle afternoon, so am expecting it to redevelop this evening and
continue through at least Sunday morning. Am thinking that the
significant cooling in the river valleys and some higher mountain
valleys in the southwest may also allow patchy fog to redevelop,
so have included it there as well. The moisture wrapping around
the upper low will likely only reach as far northwest as the West
Yellowstone area, so have kept a slight chance of flurries there
for much of the period. Coulston

Monday night through Saturday...upper level closed low pressure
system expected to be over Nebraska at start of period with
another large low pressure trough over the eastern Pacific.
Montana left under the narrow high pressure ridge in between
these systems. With an anticipated slow eastward progression and
weakening of closed low initially...ridge expected to be the
dominant weather feature for the area through Thursday. Effect
should be mostly sunny skies and seasonably mild temperatures.
Pattern becomes more progressive by Friday. The closed low
finally weakens and moves out...allowing the low pressure trough
that had been building over the eastern Pacific to begin pushing
inland. As leading edge approaches Montana...expect winds to
increase...especially along the Rocky Mountain front. May also
see some precipitation...however at this time appears any snow or
rain would be limited. The trough begins to move off to the east
as quickly as Saturday...with high pressure again rebuilding over
the area.

In terms of temperatures in the extended...looks like the cold
air will remain trapped in the valleys of southwest Montana through
Tuesday. Winds are expected to increase Wednesday into
Thursday...this should help mix things out a bit. Thus through
Wednesday morning...went a bit below guidance...but would not be
surprised if temperatures are a bit colder than currently
forecasted through Wednesday morning in the southwest. For
Wednesday through Saturday...both the ec/GFS have some fairly
high 500mb thickness over the region...resulting in the potential
for temperatures near 50 degrees at lower elevations by late in
the week. Brusda


Preliminary point temps/pops...
gtf 3 28 12 31 / 0 0 0 0
ctb 6 41 14 41 / 0 0 0 0
hln 1 16 4 23 / 0 0 0 0
bzn -8 19 -2 22 / 0 0 10 10
wey -16 28 -12 26 / 0 10 20 20
dln -7 16 -1 20 / 0 0 0 0
hvr -2 25 6 28 / 0 0 0 0
lwt 5 32 13 33 / 0 0 0 0


Tfx watches/warnings/advisories...



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