Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
1118 PM CST Thursday Dec 25 2014

Update...
issued at 1109 PM CST Thursday Dec 25 2014

Updated to include 06z aviation discussion below.

&&

Short term...(this evening through friday)
issued at 132 PM CST Thursday Dec 25 2014

A band of persistent light snow is expected to continue through
the evening and tonight. The band should actually lift a bit to
the north this evening and lose some intensity...before dropping
back south across the rest of forecast area late tonight and early
tomorrow morning. Light snow accumulations are expected...with
most locations getting a dusting to 1".

The more enhanced radar returns line up really well with the rap
analysis of 700mb fgen. With this in mind we tried to move the
probability of precipitation/weather around over the next 24 hours following the forcing at
this level. The band will likely weaken this evening /although the
precipitation isn't particularly heavy to begin with this afternoon/.
There is pretty good agreement in the slight weakening trend
amongst the short term guidance...which also shift the band a bit
farther north with time. Airports under this band are reporting
1-2sm visibility...which is typically good for about 1/4" per hour
snow rates - so...just a little more than a dusting every hour.
That should limit the high end of the snow totals through the
evening and tonight at about 2 inches /again...accumulated at a
very slow pace/. When the snow shifts south tonight...the fgen
signal becomes a little more unclear and perhaps even a bit more
diffuse. One thing is for sure...there will be cold advection
tonight...where as the precipitation this afternoon is tied more toward
warm advection processes and also moisture transport at 850-700mb.
Those will be lacking tonight...thus the snowband tonight should
be weaker. There was some thought about whether or not to include
freezing drizzle tonight because we do lose relative humidity aloft and therefore
might not introduce ice into the precipitation process...but it might
also just mean the snow stops. There is shallow moisture on the
forecast soundings...but it doesn't seem nearly as shallow as our
recent fog/drizzle stretch this month. It seems we'll simply keep
the 1000ft ceilings...but drizzle should be patchy at best.

Long term...(friday night through thursday)
issued at 132 PM CST Thursday Dec 25 2014

Large scale pattern evolution...
as has been advertised for quite a while... the pattern will be
undergoing a significant change over the course of the forecast
period as the ridge off the West Coast amplifies well to the north
and colder air over the pole is able to work its way southward
into the eastern North American upper trough. This will bring
things back down below normal temperature-wise by the start of the
week... with things remaining that way through the balance of the
forecast.

Forecast item/S/ of concern...
the main item of concern in the longer term will be right at the
start of the period... with a swath of southwest-northeast
oriented snow expected to impact a portion of the area Friday
night into Saturday morning. After that... cold temperatures and
wind chills will be the primary concern... although the lack of
deep snow cover may help mitigate very cold temperatures to some
extent.

Model discussion/preferences...
the GFS and European model (ecmwf) are in decent agreement on the overall large
scale pattern evolution through the forecast period... even with
some of the trickier features like a cutoff upper low expected
over the southwest by next Thursday. However... there are some
differences in details amongst the various models at the start of
the period which will have an impact on the forecast snow amounts
for Friday night into Saturday. Prefer the slightly slower and
farther west solution advertised by the European model (ecmwf) for that period. The
various cam runs... including the arw and nmm high resolution
nests and the hopwrf-ts... are even farther west than the European model (ecmwf)...
and are some cause for concern since they also produce precipitation
amounts from 0.3-0.6 inches. With the highly amplified upper
trough currently digging over the west... think there is some
merit to slowing things down with a bit of a westward shift...
although the northern stream wave dropping into the region during
the Friday night/Saturday morning time frame should help to create
a well defined northwest edge to precipitation given the confluent upper
flow it will help setup somewhere over the area. For now... stuck
with a blend of the European model (ecmwf)/GFS/NAM solutions... but with a bit more
weight toward the European model (ecmwf). Subsequent shifts will certainly need to
keep an eye on things should the solution offered by the cams pan
out... since it would bring at least advisory level amounts into
much of the area including the Twin Cities metropolitan.

Sensible weather expectations...
snow should spread in from the southwest Friday evening... then
overspread much of the area into Saturday morning before drier air
and surface ridging work in from the west and northwest by
Saturday afternoon. At this point... the southeastern half or so
of the area looks to be the most likely location for some
accumulating snow... with at least 1 to 3 inches looking like a
safe bet along the main precipitation axis. However... as mentioned
above... it is possible the precipitation axis could shift west and if that
occurs it would likely be a bit heavier since it would be due to a
somewhat stronger and more slowly moving surface low and upper
wave.

Generally benign weather will prevail through the remainder of the
forecast period... although there is a chance for some light snow
over the south on Monday as the baroclinic zone tightens a bit...
then again over much of the area on Thursday as another Arctic
front drops south through the area.

Detailed discussion for item/S/ of concern...
the main concern will be the potential for some accumulating snow
over at least the southeast half of the area Friday night into
Saturday morning as a surface low moves northeast along the
baroclinic zone to our southeast. A piece of the southwest upper
trough will be lifting northeast into the region during that time
frame... helping to enhance an area of upper divergence while also
tightening the middle-level baroclinic zone and aiding in
frontogenesis development. However... a northern stream trough
will be swing southeast then east across North Dakota and northern
Minnesota late Friday night and Saturday... which will help to
push the southern feature eastward while also helping to shear out
that upper wave and setup an area of upper level confluence
somewhere over Minnesota into northern Wisconsin. That area should
provide for a tight edge on the northwestern edge of any precipitation. In
terms of pcpn-type... it appears we/ll have sufficient deep layer
moisture and a completely sub-freezing profile... so it looks like
snow should be the only concern. The NAM... GFS... and European model (ecmwf) are
all fairly close in quantitative precipitation forecast with around 0.2 inches... although the
high resolution cams are about double that amount and farther
northwest as discussed earlier. The sref mean is also a bit
northwest of the NAM/GFS... and more similar to the European model (ecmwf). At this
point... went with a swath of 1-3 inches mainly southeast of a
line from Redwood Falls through the Twin Cities metropolitan and west
central Wisconsin.... with the highest amounts from near Mankato
to Red Wing. However... this bears watching since changes in
position and amounts are possible... and it is a possibility that
amounts could wind up needing a headline.

Forecast confidence/uncertainty...
there is high confidence in temperatures become much colder by the
end of the weekend... with below normal temperatures then
remaining through the forecast period. There is moderate-high
confidence in a southwest-northeast oriented band of snow Friday
night into Saturday morning... but low-moderate confidence in its
position and snowfall amounts.

&&

Aviation...(for the 06z tafs through 06z Friday night)
issued at 1109 PM CST Thursday Dec 25 2014

Small area of clearing in central-eastern Minnesota is allowing for patches
of VFR at 26/06z taf initialization time while far western Minnesota and western
WI remain under fairly solid MVFR conds...although varying within
the 1-3 kft range. Will look for more uniform MVFR ceilings overnight
with a good portion of the weather forecast office mpx coverage area having some IFR
stratus late morning into the afternoon. As the next system approaches for
tmrw afternoon and evening...ceilings look to hold in the upper-end IFR to
lower-end MVFR range...ahead of -sn developing over mainly southeastern Minnesota
into west-central WI. The -sn that develops late tmrw night into Sat
morning looks to be enough to reduce visibility into MVFR category...and
possibly IFR if precipitation rates are adequate. However...will limit
visibility to MVFR at this time.

Kmsp...ceilings bouncing between VFR/MVFR at the start of the
26/06z tafs...but these will gradually settle back down to MVFR
range and possibly into IFR range at times overnight tonight and then
again tmrw night. -Sn looks to move in after 02z-03z tmrw evening and
remain in place fairly steadily overnight Friday night into Sat morning.
Still may need to adjust timing of the start time of the -sn so
this may continue to be refined with later model runs. Have
indicated a drop to IFR conds at the tail end of the taf.

/Outlook for kmsp/
Sat...VFR...possibly MVFR ceilings in the morning. Winds northwest 5-10
kts.
Sun...VFR. Winds S 5 kts...becoming east.
Monday...VFR...possibly MVFR ceilings. Winds north 10 kts.

&&

Mpx watches/warnings/advisories...
Minnesota...none.
WI...none.
&&

$$

Update...jpc
short term...clf
long term...
aviation...jpc

National Weather Service Glossary of Abbreviations