Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 151142

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
542 AM CST Sun Jan 15 2017

.UPDATE...For 12z Aviation discussion below


.SHORT TERM...(Today and tonight)
Issued at 414 AM CST Sun Jan 15 2017

The warming trend continues today ahead of the highly advertised
system set to impact our area Monday into Tuesday. Expect readings
to top out in the lower 20s to lower 30s with light winds and mostly
sunny skies making it another classic January day. The system for
tonight has slowed down 6 hours with the onset of precipitation,
which now looks to hold off until between 09z-12z Monday. Per
preferred Nam/SREF guidance/BUFKIT, precip looks to be a freezing
rain-snow mix overnight.

.LONG TERM...(Monday through Saturday)
Issued at 414 AM CST Sun Jan 15 2017

Main concern in the extended continues to revolve around the messy
system expected for Monday/Monday night. After that, the big
question is how warm do you go as we look to get into a legit
January thaw from Tuesday into the weekend.

For the Monday system, the biggest change noted in the 00z models
was a significant slowing, with precip onset delayed by about 6
hours across the board. There has also been a southeastward shift in
the precip as well, with quarter inch or more type QPFs looking to
stay south and east of the Twin Cities. What has not changed is
spread in P-types. The reason for all the varying ideas for P-types
stems from the fact that our forecast soundings show we will have a
more or less isothermal profile through the lowest 5k ft of the
atmosphere. Of course the isotherm we will be following is 0C.
Because of this we are getting differing answers from the two
primary p-type techniques we use. In the top-down method, the max
temperature in the profile never warms enough to fully melt snow, so
it produces a snow and sleet scenario for us. With the Bourgouin
technique, though the max temps in the layer may never get much past
1C, there`s enough depth to the above freezing layers with a
corresponding lack of deep sub-freezing layers that this technique
gives a snow/freezing rain scenario. P-type probabilities from both
the SREF and GEFS are either snow or freezing rain as well. At this
point, the NAMnest/ARW-WRF/NMM-WRF/HopWRF-TS get out long enough in
time to capture at least some of this event. As CAMs, algorithms are
not used to forecast p-types. Instead, they explicitly attempt to
forecast hydrometeor types within their model physics. These CAMs
say we are heading for an either snow or freezing rain setup, with
little in the way of sleet.  In GFE though, the procedure used to
create p-types uses the top-down method, so our grids came out on
the snow and sleet side of things. However, we did replace the top-
down probability of freezing rain grids with the SREF probability
for freezing rain to at least get freezing rain into the forecast.

So what does all of this mean for the watch?  Well first of all, the
6 hour delay in precip onset bought us more time before needing to
go into warning decision mode, so no part of the watch was upgraded
to a warning or advisory, with the start times for our two watch
segments pushed back 6 hours. The lower QPF means we will likely not
be reaching warning criteria for snowfall (6" in 12 hours). However,
what are seeing for p-type from the Bourgouin technique and CAMS,
there is enough QPF that the possibility for seeing ice
accumulations at or above 0.25" is still very much plausible, so we
felt it best to maintain a watch, as an upgrade to a winter or even
ice storm warning may be needed if we get enough ice accumulation.
There was discussion of expanding the watch to include the rest of
our CWA to the east of the watch, but collaboration with neighbors
to our east kept the watch location unchanged.

Behind this system, models continue to show a very blocky pattern
developing, with a significant blocking ridge developing over Hudson
Bay this weekend, with deep troughing over Alaska. This is a very
mild pattern for us and as a result we have highs getting into the
40s by Thursday. By Friday/Saturday, our temperatures are on the
conservative side of the model envelop as it looks like our warmth
will come with ample stratus. However, we should have no problem
running average temperature departures up to around 20 degrees above
normal by the end of the week as our highs get up to around 15
degrees above normal and our lows go 25 to even 30 degrees above
normal. For the weekend, the GFS/ECMWF continue to show a system
working across the upper midwest, though the Canadian is sending
it across the Ohio Valley. Though there`s a good deal of spread in
location/amounts of precip, where there isn`t any disagreement is
that this would be a warm system with the p-type being all rain.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 542 AM CST Sun Jan 15 2017

High confidence in VFR through this evening with mostly sunny
skies and light southerly winds. There could be some patchy BR
with MVFR vsbys Monday morning, and then mid/high clouds increase
early in the next period as the storm system lifts north toward
the area.

High confidence in VFR with mostly sunny skies and light winds.
Deteriorating conditions Monday pm.

Mon...MVFR/IFR. -SN likely with chance of -fzra/-sn. Winds SE 4
to 7kt.
Tue...MVFR possible early then clearing. Winds NW bcmg W 5-10 kt.
Wed...VFR. Winds SW 5-10kt.


MN...Winter Storm Watch from Monday afternoon through Tuesday morning
     for MNZ051>053-058>063-065>070-075>077.

     Winter Storm Watch from Monday morning through late Monday night
     for MNZ073-074-082>085-091>093.



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