Area forecast discussion
National Weather Service Glasgow Montana
216 PM MDT Tuesday may 21 2013
Short term...for northeast Montana...tonight through Thursday...
an upper level low over eastern South Dakota will continue to
drive wraparound light shower activity over the southeast this
evening. Shower activity will quickly taper off after midnight as
the upper low pushes to the east.
Dry and warm conditions will rule on Wednesday as upper ridging
dominates ahead of an approaching upper trough over the Pacific
northwest. Gusty southeast winds will also pick up during the late
morning in response to the approaching low...necessitating a lake
Wind Advisory that will last into late Thursday evening.
Southerly flow aloft associated with the approaching upper low
looks to build into the western zones by Wednesday evening.
Disturbances in this flow will create decent chances for showers
and thunderstorms across the west into Thursday. Gilchrist
Long term...Thursday night through Tuesday...
the upper level ridge gets pushed aside Thursday night and Friday
by a lobe off the strong Pacific-northwest closed low swinging north through
Montana. Weak instability develops to the southwest late Thursday
so the disturbance could generate scattered rain showers and
isolated thunder mainly in the western zones. A more distinct cold
front will push through the area on Friday...but again...the best
instability and moisture remain outside northeast Montana...
according to the GFS. The timing is different in the ec...which could
bring the front into the area Friday afternoon with instability.
With model differences on Friday...will leave probability of precipitation as is. Model
differences continue into Saturday. There could be showers and
thunder or not...depending on the model. So again will leave
along. There is at least the potential with the GFS model to match
up a temperature boundary with some very unstable conditions...
which could lead to some energetic thunderstorms. Sunday looks
like the best day for timing and location to bring about possibly
very energetic storms. The grids handle this uncertainty well...so
few change past day four. Scattered
Previous discussion... the extended period begins Thursday night
with a closed upper trough across the west and an upper level
ridge across the southern U.S. That extends through the plains.
This places northeast Montana in diffluent southwest flow aloft.
The 00z model suite is pointing to a robust shortwave pushing
through the flow which could lead to enhanced dynamic lift across
parts of the County Warning Area. The 850mb level easterly/southeasterly winds
will help to pump in low level moisture...and also suggest the
presence of a low level jet which could help to force convection
if the other dynamic ingredients discussed come together
coincident ly. Decent equivalent potential temperature ridge looks
to be in place across the County Warning Area by Friday afternoon with warming
temperatures out ahead of the trough. The middle levels also look
pretty dry. This leads to the concern that some storms that
develop may produce hail. The 00z European model (ecmwf) forecast soundings show Li
between -1 and 1 across the County Warning Area and total totals in the upper 40s
and not much cape. Notice a nice veering profile in the European model (ecmwf)
forecast soundings with about 40 kts of 0-6 km speed shear. The
GFS shows actually higher lifted index values and is less
suggestive of strong convection in short...the ingredients are
there for good thunderstorm development but some parameters appear
only marginal and so confidence on strong storms is not yet there.
Will need to look at future model runs for consistency. Timing of
the shortwave as it intersects the Theta-E ridge will also be
key...right now it appears the northern zones and southern
Saskatchewan may be in the best location for Friday afternoon for
convection initiation if following the ec guidance. Any convection
should slowly dwindle Friday night with the loss of daytime
heating.
On Saturday...upper flow pattern maintains southwest flow aloft
over the County Warning Area with ridge axis running from Texas...northwestward
through the plains and into NE Montana. This should actually lead to
warmer temperatures which could lend to enhancing the potential
for convection initiation. Nice upper level diffluence is noted
and some of the forecast soundings are certainly indicative of
convective potential. There is also some subtle embedded shortwave
energy that pushes through shortwave flow aloft...but nothing as
impressive as the wave on Friday.
Sunday into Sunday night presently looks to be the best potential
for thunderstorms to develop which could become strong in nature.
There is much better agreement in both the European model (ecmwf) and GFS with both
forecast soundings showing lifted index values between -5 and -8
across the County Warning Area. The GFS BUFKIT sounding shows around 1500 j/kg of
cape and total totals approaching 60. Though a nice veering wind
profile exists in both European model (ecmwf)/GFS soundings... though would like
to see more speed shear in the soundings. Should also point out
that forecast soundings from both models are showing precipitable waters of over
an inch. With again dry air present in the middle levels...would be
concerned about hail formation with any storms. There is again a
low level 850mb easterly jet that forms...which should help to
pump in low level moisture. Nice Theta-E ridge axis over the County Warning Area
along with a couple of shortwaves that pass off to the west could
also help provide decent lift if the timing is right...and that
will be the key for determining the overall extent of the
thunderstorm potential. Of all of the days in the extended
period...this looks to be the one with the best model agreement
and the best in terms of various ingredients coming together both
dynamically and thermodynamically that are coincident with each
other. Will of course need to watch for continuity in the model
solutions to determine if this continues. Will also need to hone
in on mesoscale details which are difficult to determine this far
in advance. Where these ingredients come together...the precise
timing of the shortwave features...and how everything interacts
with mesoscale processes will determine the precise extent of
convective potential.
Monday and beyond...NE Montana is expected to remain in southwest flow
aloft with the European model (ecmwf) guidance suggesting perhaps the upper
pattern could become more zonal late next week. The GFS on the
other hand keeps the overall pattern aloft from the southwest. This
overall pattern would be considered favorable for a continued
potential for convective showers and thunderstorms to develop
depending on timing/track of shortwave features in the flow. Maliawco.
&&
Aviation...
mostly VFR through Wednesday...although brief MVFR ceilings are
possible today at ksdy and kgdv.
The closed low pressure system is now centered over eastern South
Dakota...with the back edge of the low level stratus cloud deck
covering eastern counties in northeast Montana this morning. So
far...seeing ceilings of around 3.5kft-4kft over ksdy and kgdv.
Otherwise satellite imagery reveals an overcast of thin high
stratus over kggw and kolf. By this time on Wednesday the low
stratus should have moved well to the east
Expect wind to become easterly today at 12kt to 20kt. On Wednesday
a developing surface trough in the Lee of the northern rockies
will set up stronger easterly wind across the area. Expect wind to
reach 15kt to 25 knots by afternoon. Scattered
&&
Glasgow watches/warnings/advisories...
lake Wind Advisory from 10 am Wednesday to midnight MDT Thursday
night for Fort Peck Lake for central and southeast Phillips...
central and southern valley...Garfield...McCone...Petroleum.
&&
$$
Weather.Gov/Glasgow
