Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1150 PM CST Sat Jan 31 2015
Short term...(this evening through sunday)
issued at 256 PM CST Sat Jan 31 2015
Confidence is rather high for tonight and Sunday. Global models
remain tightly clustered near the consensus through Sunday...
including strong support from the higher res models of the rap
and hopwrf. There will be a sharp cut off to the snow as dry air
erodes the shield north of about Madison to Red Wing. South of
that line...there will be several hours of light to moderate
snowfall gradually picking up in intensity with southward extent.
Temperatures will cool into the teens tonight which will set up a
very deep dgz extending from the surface to about 600mb. While
forcing is forecast to be weak...Omega will be negative throughout
the entire dgz. Banding won't be too much of an issue with the
absence of frontogenesis...and therefore not expecting isolated
extreme amounts. Should be more of a uniform snowfall. Looking at
accumulations of 3 inches along Highway 14 from Mankato to
Owatonna...and 6 or 7 inches along and south of Interstate 90.
Issued a Winter Weather Advisory for these areas through Sunday
morning. Amounts will drop off quickly north of Highway 14 and
am not expecting anything measurable north of Interstate 94.
Northeast winds will increase overnight...with some gusts to 30
miles per hour possible across southern Minnesota. The snow will be prone to blow around
given its light nature...so included patchy blowing snow as well.
High pressure will build south Sunday afternoon with clearing
skies expected by evening.
Long term...(sunday night through saturday)
issued at 256 PM CST Sat Jan 31 2015
Large scale pattern evolution...
the Hudson Bay upper low will remain in place through the
period... with the overall pattern continuing to favor a trough
over the eastern part of the continent and ridging to our west. At
the start of the period we will be in northwest upper flow... and
will be subject to the western periphery of the Arctic air. But...
as we head into next weekend the Hudson Bay low looks to retreat a
bit northward and the western ridge Delaware-amplifies some... allowing
for more in the way of westerly/warmer flow into our area. In
terms of large scale signals... it still looks to remain
predominantly dry across our area with better moisture remaining
well to our south/east.
Forecast item/S/ of concern...
other than the cold air... the only items of concern are a chance
for some light snow on Tuesday... and perhaps a better shot for
some light snow by the Friday-Saturday time frame... although
nothing at this point looks particularly significant.
the GFS... European model (ecmwf)... Canadian... and gefs are in fairly good
agreement on the overall large scale pattern evolution. There are
differences in the timing and position of shortwave features
expected to bring the few chances for light precipitation... and at this
point there isn/T much evidence to favor any given solution over
another. A consensus approach was used... together with the
Sensible weather expectations...
we/ll see things dry out tomorrow night with temperatures tumbling as
high pressure works into the area. Some light snow is possible
Tuesday... although guidance has shifted farther south over the
past several runs... so probability of precipitation have been lowered and could
eventually be eliminated if the trend continues. Cold and dry
conditions will prevail through midweek... with chance for snow
returning by Friday/Saturday as we see a few shortwaves ripple
through along the baroclinic zone. Moisture return ahead of each
feature would appear to be limited... so nothing too significant
would appear likely at this point.
Detailed discussion for item/S/ of concern...
as mentioned... guidance has shifted south with the weak shortwave
moving through on Tuesday. The GFS has been farther south for a
couple of runs... and the latest European model (ecmwf) has also shifted a bit
south. Regardless... forcing appears quite weak and there isn/T
much in the way of deep moisture... so any snow that can be wrung
out would likely be in the inch or less department. There is a
better window of return flow later in the week along with a modest
increase in precipitable water values... so the shortwaves and
associated surface features prognosticated to impact the region Friday
and late Saturday could produce a bit more in the way of
snowfall... although it still appears precipitation amounts would struggle
to exceed a quarter of an inch liquid. In addition... model
guidance is quite variable in the details after Thursday... so
confidence in position/timing of the better precipitation/snowfall is low
at this point.
confidence is high in the occurrence of somewhat below normal
temperatures for much of the week. Confidence is moderate to high
that overall precipitation amounts will be light through the
period. There is greater confidence in the chance for light
accumulating snow toward the end of the week and weekend than
there is for the Tuesday system... although confidence is low to
moderate in the occurrence of either.
Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1150 PM CST Sat Jan 31 2015
No really big change for the 06z taf from the ooz taf. It does
look the snow may creep a little farther north than originally
thought...so there could be a brief window of IFR visibility and a
dusting to 1/2" snow at kmsp and Keau near or just after sunrise.
Kstc/krnh/kaxn...are still too far north for a snow concern. We
also slowed the snow timing a bit with this taf.
We decided to include a tempo group for a period of IFR
visibility and light snow near or just after sunrise and push the
snow timing back a bit. We really don't think the Airport will get
much more than 1/2"...but the visible would still likely drop to near
or below 2 miles for a time if we end up with 0.25-0.5" snowfall
/Outlook for kmsp/
Monday...VFR. Wind SW 5-10kt.
Tuesday...chance MVFR/-sn. Wind west 5 knots.
Wednesday...VFR. Wind northwest 10-15 knots.
Minnesota...Winter Weather Advisory until noon CST Sunday for mnz082>085-