Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 302337
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
540 PM MDT Sat Aug 30 2014

UPDATED AVIATION SECTION

.DISCUSSION...

Tonight through Monday...Quasi-zonal mid/upper flow pattern
remains in place across north-central and southwest MT this
afternoon. Next in a series of embedded shortwave troughs is
currently sparking convection over southwest MT with a few showers
over north-central MT. With weak to moderate instability and
moderate effective shear...can`t completely rule out an isolated
strong wind gust over SW MT this evening but threat is minimal.
Trough axis sweeps eastward across the forecast area overnight
resulting in slightly cooler temps for most locations on Sunday.
Could see an isolated shower/thunderstorm across primarily SW MT
and along the Rocky Mountain Front again tomorrow afternoon. Weak
northwest flow will set up across the area on Monday as a flatten
ridge begins to translate into the region. This will aid in
keeping afternoon temps around 5 to 10 degrees below normal for
the 1st of September. Foltz

Monday Night through Saturday...Quiet weather conditions expected at
the start of this period, as transient high pressure ridging will
result in mostly fair skies, no precipitation, and comfortable early
autumn temperatures on Tues and Wed. By Wed night, a shortwave trof
is expected to move through southwest Canada and the Northern
Rockies and push a cold front across our region, but frontal
passage looks to be mostly dry with just a minimal chance of a
shower or two near Glacier NP and portions of the Hiline counties.

From Thurs onward, latest forecast model solutions made significant
changes from solutions seen the previous couple of days.  The most
notable changes are seen in the ECMWF and GEM models which give a
new prediction of much drier conditions over central and southwest
MT. Meanwhile, the GFS kept closer to its earlier runs in showing
periods of widespread showers and thunderstorms from late Thurs
night through Sat evening.  The disparate forecasts hinge on the
placement and evolution of an upper-level trof that will extend from
Alberta/Saskatchewan to central California beginning Thurs.  The
drier ECMWF/GEM models bring the trof axis into western MT which
keeps a re-developing monsoonal moisture flow well southeast of our
forecast area.  The GFS, on the other hand, holds the trof axis much
further west over WA/OR, allowing a steady flow of combined Pacific
and monsoonal moisture to generate the aforementioned
precipitation.  However, there are some inconsistencies in the GFS
solution, mainly that is doesn`t develop any precipitation upstream
in eastern Oregon or southern Idaho as we often see with a Pacific
moisture stream in this type of weather pattern.  Given all these
differences, confidence is low at this point that any of the models
has an accurate read on conditions for the latter half of next
week.  The current extended period precipitation grids are a very
rough compromise between the solutions, and rely heavily on
climatological probabilities of precipitation (about 20-25% for the
region during early Sept). Further, and possibly significant,
changes in the forecast are likely, but those will have to wait for
signs of increased model run consistency/agreement in the days ahead.
Waranauskas

&&

.AVIATION...
UPDATED 2340Z.
Showers will continue over far southwest Montana including KBZN
through the night and into Sunday morning. Showers could diminish
Sunday afternoon but do not have high confidence on this happening.
Expect local MVFR conditions and mountain obscurement in the
showers. Also expect isolated thunderstorms this evening. Farther
north a few showers could affect KHVR and KLWT early this evening
otherwise expect it to be dry with VFR conditions. Sunday afternoon
there should be scattered showers and isolated thunderstorms over
the mountains. Although not indicated in the tafs there could be a
few showers or possibly thunderstorms over the plains as well. Blank

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  49  68  46  68 /  20  10  10  20
CTB  47  67  46  66 /  10  10  10  20
HLN  48  69  46  69 /  20  20  10  20
BZN  45  65  41  67 /  50  20  20  20
WEY  35  56  31  60 /  50  20  20  20
DLN  45  63  43  66 /  40  20  20  20
HVR  50  73  47  72 /  20  10  10  20
LWT  49  67  46  66 /  30  10  10  20

&&

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

$$

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