Area Forecast Discussion
Issued by NWS Great Falls, MT
FXUS65 KTFX 291732
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1132 AM MDT Fri Aug 29 2014
.UPDATE...Updated early this morning to add areas of fog until 10
AM. With some areas of fog persisting I have updated again to
extend the fog until noon. Have also made tweaks to sky condition
in areas where fog has lifted into a low stratus deck, and have
updated winds to be more in keeping with observed and expected
conditions for the remainder of today. mpj
An upper level low pressure system will move into the Pacific
Northwest tonight and an associated cold front will cross the
Canadian border early Saturday and move into Southwest Montana by
noon. Scattered showers will develop near frontal passage although
widespread precipitation is not expected. Local MVFR CIGS are
possible near rains showers although VFR conditions will generally
prevail through the next 24 hours.
/ISSUED 540 AM MDT Fri Aug 29 2014/
Satellite imagery early this morning over the Gulf of Alaska and E
Pacific Ocean displayed a large wrapped-up cyclone and expansive
trough, with an impressive cold frontal cloud band, making steady
progress to the ESE. The low was positioned near 55 N 140 W. The
mid-level cold front will reach Montana tonight, signaling a
change to below average temps that will last into next week.
Today...Quiet weather is forecast with the exception of a few
isolated showers/storms over the far Southwest. Highs will still be
near 80 F.
Tonight and Saturday...Models are in agreement with mid and upper
dynamics moving across the N Rockies early Saturday morning into the
aftn. 300 mb divergence will overspread the region as a strong jet
exit region dives through Idaho. A defined area of 700 mb
frontogenesis will also pass through during the morning...and linger
into the evening over the SW. PWAT values will be near 0.75
inches, close to 2 standard deviations above the mean, 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 0.20 to 0.40 inch
amounts south of a HLN-to-LWT line. Areas that receive heavier
showers or embedded thunderstorms may accumulate over 0.50 inches.
The NAM and GFS both predict a heavier band with amounts near 1
inch over central and southern Madison and Gallatin Counties. This
area is superimposed with the low/mid-level frontogenesis that
remains in the region into the evening.
Sunday...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 PWATS falling to
around 0.40 or 0.50 inches. Highs will be in the 60s. Uttech
Sunday Night through Friday...Relatively quiet weather conditions
expected for first half of next week. A weak, quick-moving
shortwave trof will sweep across the state on Mon. GFS and GEM
forecast models indicate little, if any, precipitation with the trof
while the ECMWF continues to indicate broad coverage of showers
mainly over the southwest counties on Mon aftn/eve. Since a
northwest flow pattern aloft (like we`ll have on Mon) usually will
generate isolated showers with even minimal moisture and
instability, have kept in a slight chance of rain showers over the
astern third of the forecast area on Mon. Weak, transient high
pressure ridging will make for dry conditions from Tues through Wed,
with slightly warmer high temperatures (low to mid 70s) as winds
aloft transition to westerly. Stronger shortwave trof is forecast
to arrive on Thurs, though model solutions still showing some marked
run-to-run differences on the shortwave`s timing and precipitation
coverage for Thurs and Fri. General model consensus is that most of
the precipitation will occur Thur eve through midday Fri, behind a
surface cold front that tracks across the region on Thurs morning.
Some locations, especially along/south of a Helena-Lewistown line
may see rainfall totals of 0.25-0.50 inch, with lighter amounts
further north. The front will also usher in a much-cooler Canadian
airmass, that will keep high temps on Fri only in the mid to upper
50s and bring snow levels down to the 6500-7500 ft range over the
central mountains and the Rocky Mtn Front.
.PRELIMINARY POINT TEMPS/POPS...
GTF 79 54 72 48 / 0 20 60 10
CTB 77 53 70 46 / 0 0 20 10
HLN 83 54 73 46 / 0 20 50 20
BZN 80 49 69 41 / 10 20 60 30
WEY 70 42 63 33 / 10 30 60 40
DLN 80 51 68 44 / 20 30 60 30
HVR 76 54 76 49 / 0 10 60 10
LWT 74 53 69 47 / 10 20 60 20