Scientific Forecaster Discussion
fxus63 kfgf 121830
Area forecast discussion
National Weather Service Grand Forks ND
1230 PM CST Tue Dec 12 2017
issued at 1230 PM CST Tue Dec 12 2017
Still seeing echoes on radar east of a line from Roseau to Wadena,
with corresponding light snow in the observations in that area.
Will therefore maintain some light snow there throughout the
afternoon. Otherwise, continuing to watch the temp rise. Temps in
the 30s plus have moved into the western Red River valley, but not
so much yet within the valley. Hallock and St. Vincent are still
in the low 20s. So a big west to east gradient will still occur.
Finally, decreasing clouds are occurring across the far western
forecast area. This may reach portions of the Red River valley by early
evening, before stopping.
Short term...(today through wednesday)
issued at 432 am CST Tue Dec 12 2017
Main concerns will revolve around the next clipper system tonight
into Wednesday. The potential for freezing rain and then strong
winds will be the main impacts.
Water vapor imagery shows a strong upper wave crashing onshore
across western Canada. The track and strength of this feature is
very similar to the system that affected the region Sunday night
into Monday. The main difference is that the initial band of warm
air advection snowfall is a bit further east, which means the low
level airmass will be warmer across the region. Upper level
forcing (associated with the upper level circulation/jet streak)
will lead to precipitation later this evening and overnight.
Temperatures aloft support liquid across the majority of the
region. Surface temperatures should rise above freezing along and
west of the valley, but remain below freezing east of the valley.
Road temperature forecast supports Road surface temperatures
remaining below freezing most areas. Thus, freezing rain potential
exists across the region tonight. Impacts will be dependent on
actual surface temperatures, how quickly Road temperatures rise,
and precipitation amounts/coverage. Still some questions, but
will be ramping up the messaging for icing impacts. The next
concern will be strong wind potential following the cold frontal
passage. Strong wind ingredients (mixing, winds aloft, pressure
rise, etc) support Wind Advisory criteria Wednesday morning into the
early afternoon hours. 06z NAM suggests mixing to 850mb with around
60knts at this layer. Other guidance is a bit weaker, but this
suggests at least a low potential for wind warning criteria. This
potential not enough for a high wind watch at this point, but
something to monitor. Any existing snowpack will not be blowable,
but there is the potential for Post frontal snow showers with
another upper wave/jet streak quickly following the initial system
(which would occasionally limit visibility).
Long term...(thursday through monday)
issued at 418 am CST Tue Dec 12 2017
Busy times ahead as there will not be much reprieve in precipitation
and wind mid to late week. A couple of compact, quick clippers move
through the region Wednesday overnight and Thursday, with the latter
affecting more of Minnesota whereas the former has more impacts in
North Dakota. Behind the North Dakota clipper Wednesday overnight,
cold air advection behind its associated front could continue snow
showers through the day Thursday within North Dakota. The Minnesota
clipper late Thursday is deeper into colder temperatures aloft which
should increase snow ratios within Minnesota and enhance the
realization of snow with little moisture to work with.
Behinds this clipper is, you guessed it, another clipper, or at
least a cluster of mid level pulses, some time around Friday. This
one is different than the clippers we have been recently seeing in
that it will originate more from the Pacific northwest as the
western ridge flattens. This should increase its moisture content
compared to those coming out of the Canadian interior. Ahead of this
clipper will be strong warm air advection from the west increasing a
thermal gradient oriented more zonal than meridional. This could set
up the possibility of snowfall moving across the region more
west/northwest to east/southeast along the colder side of its
baroclinic zone. Gefs plumes for northern counties show a spike in
snowfall, some appreciable, Friday overnight into Saturday. Signals
within ensemble guidance has been consistent for the past few days
of this clipper moving through the region late Friday into Saturday.
After the Friday, deterministic guidance starts to vary however it
does seem possible the heightened precipitation pattern continues
over the weekend. Closer to early next week, ensemble and
deterministic guidance is suggesting a potent system to develop
within Canada. There are still considerable questions on the track
and development of this system however so will limit mentioning of
impacts at this time.
Temperatures continue to be above normal during the forecast period,
especially within the low temperature forecast. Normal high
temperatures this time of year are in the mid 20s with normal low
temperatures in the single digits above zero.
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1230 PM CST Tue Dec 12 2017
MVFR clouds and some flurries or light snow will continue to
affect kbji and possibly ktvf this afternoon. By evening, expect
conditions to hold about where they are at. Models have fairly
high confidence in a freezing pcpn potential developing by late
evening/very early Wednesday morning. There could be a period of
several hours before this would mix with or change to light snow.
About this same time, gusty north winds will also kick in. Expect
these gusty north winds to combine with periodic snow showers
Wednesday morning and afternoon, to reduce vsbys. These vsbys may
not constantly be low, but may be up and down. However, any
falling snow with 45 to 55 mph winds will cause low vsbys.