Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
400 PM CST sun Dec 21 2014
Short term...(this evening through monday)
issued at 400 PM CST sun Dec 21 2014
A short wave...that moved into southwest Minnesota this morning...was
over the Twin Cities late this afternoon. This feature will move
across west central WI this evening...bringing the snow or a mix
of rain and snow to an end. Accumulations today have been an inch
or two...mainly to the north and east of New Richmond. Up to an
inch of additional snow is forecast this evening around Ladysmith.
In the wake of this feature will be a stronger wave that will
begin to influence our weather during the overnight hours.
Fortunately...temperatures are forecast to stay above freezing
overnight...with a 1 or 2 degree rise in the temperature from
their current readings. Hence...no ptype issues are expected with
this second round of precipitation pushing into western and
southern Minnesota overnight.
One concern overnight is the threat for dense fog due to the
highly anomalous dew points for late December (middle-upper 30s)
pushing across what is left of our snow cover. Webcams through
the day have been showing a steady improvement...especially those
in our western County Warning Area near the South Dakota border. Winds will be on the
increase tonight with speeds from 10 to 20 miles per hour as the pressure
gradient increases ahead of a low pressure system. There will
likely be a downward trend on the visibility overnight...but the
thinking is that widespread dense fog will not occur.
The 12z model trends today for the forecast on Monday are still
in line with what was shown with last nights 00z runs...and that
is for the wave approaching from the Dakotas to pass south of US
Monday night and become a closed low. With low level southerly flow
tomorrow...we are still able to maintain a mix layer (500m) wet
bulb temperature from 1 to 2 degrees c across much of central and
southern Minnesota through the day. This is based on the sref as an
ensemble approach seemed the way to handle the variations seen in
the deterministic models. The exceptions are across our far
northern counties in central Minnesota as well as west central WI where
the ml wet bulb temperature struggles slightly above 0.5 degrees.
The powt grids handled the situation well with a mix of rain and
snow across central Minnesota and west central WI along with patchy areas
of just plain snow. Mainly rain is seen across far southern Minnesota.
Accumulations for those areas of central Minnesota and west central WI
that have some snow tomorrow should be minor with temperatures in
the middle 30s. Conditions will likely change rather quickly
Monday evening (rain changing to all snow) from south to north as
strong forcing arrives from Iowa which the long term will discuss below.
Long term...(monday night through sunday)
issued at 400 PM CST sun Dec 21 2014
Picking up the forecast Monday evening...we'll be looking at a pv
boot pushing north through southern Minnesota...which will be coupled with
strong f-general and hence quickly increasing Omega with these features.
Precipitation type remains a huge concern in forecasting amounts
obviously...but with the consensus being a southern shift...we are
now looking at mostly snow for Monday night. With the enhanced
forcing during the evening...we could see some decent snow rates for
Monday night. Southeast of the metropolitan however...went with lower snow
ratios in the forecast due to maximum wet bulb temperatures closer to +1
or warmer for much of the night. There will be periods of rain
mixing in with snow there during the night. Through the night..the
low will work its way across Iowa and start moving northeast
Tuesday. Quite a bit of spread remains in the models in terms of
quantitative precipitation forecast. On Tuesday...the European model (ecmwf) solution is concerning. The sref/NAM
solutions offer about a tenth of an inch of liquid through much of
the area as the deformation band weakens and Omega decreases. The
European model (ecmwf) on the other hand has about 4 times that amount of quantitative precipitation forecast and
keeps the deformation zone going through the day on Tuesday.
Meanwhile...the GFS holds a deformation band further northwest
through Tuesday with a dry slot overtaking much of southern Minnesota. I
find this dry slot difficult to believe due to the placement of the
low at that time. Also...the GFS has been rather inconsistent as of
late...so not as confident in this dry slot working out on Tuesday.
For now...elected to go with the ensemble approach...leaning on the
sref for probability of precipitation and quantitative precipitation forecast. Another notable difference between the ec
versus some of the other guidance...is the fact that it is slower
with the movement of the low through Tuesday...and hence the
evolving deformation band. There is potential for more snow than
currently in the forecast if the European model (ecmwf) nails it...but again its best
to handle it with an ensemble approach until more consistency and
agreement can be reached.
The next precipitation chance comes on Thursday night when another low
pressure area moves through. At this time...the ec keeps the low
south of our area and spreads snow through southern Minnesota. The
GFS meanwhile...brings the directly through central Minnesota...with
no accumulating snow in southern Minnesota. The parallel GFS
though...looks closer the ec. Kept the blended probability of precipitation in the forecast
for now for this period.
Its becoming more and more likely we'll see an Arctic air instruction
next weekend...so confidence is high we'll have colder than normal
temperatures by that time.
Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1157 am CST sun Dec 21 2014
IFR ceilings forecast through the period with less than MVFR
visibilities. A moist southerly flow will persist through the taf
period as low pressure approaches from SD/NE. Extensive br/fog
through the period. Some improvement in the visibilities will
occur this afternoon...but visibilities drop back tonight to
between a 1/2 and 1 1/2sm at most locations. Complicating matters
is a weak area of low pressure aloft passing by this afternoon and
evening. This will aid in the generation of mainly light rain
across the Twin Cities and points southwest with snow mixing with
and changing to rain across west central WI by evening. Model
trends today continue what began last night and that is for the
low pressure system approaching from the west to pass south of US
on Monday. This means a colder profile with a rain and snow
mixture during the day. Just another degree or two colder will
mean most taf sites would be all snow by Monday afternoon along
with accumulation. Right...we are going with a changeover to all
snow by Monday evening with several inches of accumulation likely
across the area Monday night.
Kmsp...bouts of light rain/snow this afternoon. No accumulate with
temperatures holding in the middle 30s. In fact...slowly rising temperatures
tonight with upper 30s by daybreak Monday. Although a rain/snow
mix is forecast on Monday...it could become all snow in the
afternoon. Definitely all snow by evening.
/Outlook for kmsp/
Monday night...IFR. Snow. Several inches of accumulate. Winds southeast 5-10 kts.
Tuesday...MVFR/IFR. -Sn or -ra/-sn mix. Winds northwest 5-10 kts.
Wednesday...MVFR. Slight chance -sn. Winds northwest 10-15 kts.