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Area forecast discussion 
National Weather Service Great Falls Montana
525 am MDT Sat Aug 30 2014

Aviation section updated

Discussion...

Upper trough and mid-level cold front is moving in as advertised
and below average temps are anticipated into next week. Storm Prediction Center
mesoanalysis shows that a 700 mb thermal ridge is already moving
into eastern Montana with cold air advection occurring to the west. Sct
showers with a few isolated t'storms have developed over western
and southwestern Montana. This region of precip is coincident with
upper-level divergence aloft due to curvature of the trough and a
localized right entrance region...weak 700 mb frontogenesis was
also analyzed.

Today...previously mentioned forcings for ascent will slide from west
to east across the forecast area this morning. Best chance for
measurable rain is southeast of a roughly gtf-to-hvr line. Left exit region
of a stronger jet across Idaho will act to enhance/sustain precip
over the SW into tonight. Additionally, a defined area of 700 mb
frontogenesis will linger into the evening over the SW. 1000-500 mb
column-average relative humidity nicely depicts the cold frontal structure moving
through this morning, the tail end of the boundary stalling out over
the SW late this aftn, and the potential for rejuvenation of the
showers and embedded storms after 00z/Sunday as the strong Idaho jet
influences the area.

Precipitable water values will be near 0.75 inches, close to 2 Standard deviations
above the mean over the SW, which means plenty of moisture for
moderate to heavy rain producing showers. Precip forecast best
follows the sref but with a tenth or two less at lower elevations.
This method produced 0.15 to 0.30 inch amounts in the SW valleys
with up to 0.85 inches in the mountains of Madison and Gallatin
counties. The ec is lower with total quantitative precipitation forecast values, while the GFS is
higher.

Sunday and Monday...mid-level trough axis will move eastward out of
Montana. Heights will rise slightly and breezy west winds will bring
in much drier air with dewpoints falling into the 30s and precipitable waters
falling to around 0.50 inches. Highs will be in the 60s; chances
or showers will be low. Uttech

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 European model (ecmwf) and Gem models which give a
new prediction of much drier conditions over central and southwest
Montana. 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 European model (ecmwf)/Gem models bring the trof axis into western Montana 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 Washington/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 1125z.
Upper-level trof and surface cold front currently working their way
across central and southwest Montana with scattered showers and a few
isolated thunderstorms near/approaching all taf sites except kctb.
This initial round of convective activity should move into eastern
Montana by 17-19z, leaving mid-level (9000-15000 ft) ceilings over the
region before second round of showers and a few thunderstorms
develop this aftn. Second round will occur mainly along/south of
khln-to-klwt line, with slight chance of precipitation possibly
reaching as far north as kgtf. Expect VFR conditions through the
day, even in areas receiving rainfall. Can't rule out brief MVFR
conditions if a thunderstorm passes right over a terminal, but
confidence in that happening currently too low to warrant MVFR
mention in 12z tafs. Winds will generally be less than 15 kts today,
with some gusts of 25-30 kts possible from thunderstorms outflows
this aftn.
Waranauskas

&&

Preliminary point temps/pops...
gtf 74 49 68 46 / 60 20 10 10
ctb 72 47 67 46 / 10 10 10 10
hln 74 48 69 46 / 50 20 20 10
bzn 70 45 65 41 / 70 50 20 20
wey 61 35 56 31 / 60 50 20 20
dln 69 45 63 43 / 60 40 20 20
hvr 77 50 73 47 / 60 20 10 10
lwt 72 49 67 46 / 60 30 10 10

&&

Tfx watches/warnings/advisories...
none.
&&

$$

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