Scientific Forecaster Discussion
fxus63 kmpx 281155
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
555 am CST Tue Feb 28 2017
Short term...(today and tonight)
issued at 320 am CST Tue Feb 28 2017
Complex short term as we will see multiple pieces of energy
emanating from a mean trough over The Rockies. The models have shown
little run-to-run consistency during what is now the today/tonight
period and there's only decent agreement on what's going to happen
today, with significant differences already showing up tonight.
At the surface this morning, there are two frontal features of
interest. There is what is left of the warm front that lifted across
the area yesterday that can still be found in the wind field from
west central into northeast Minnesota. However, this front is becoming less
and less defined as a surface low over southeast Nebraska
strengthens, with its associated warm front near the I-80 corridor
taking over as the predominate frontal feature in the area. These
two features will drive the two primary areas of precip expected
through the day. The remnant boundary over central Minnesota will drive a
snow chance up there through the day. However, this frontal band has
struggled to produce snow overnight thanks to dry low levels it is
encountering we have seen a downward trend in its potential in
guidance. Still the hrrr and hopwrf show light snow working across
west central/central Minnesota through the day, so continued high
chance/likely pops in these areas, with maybe an inch of snow
possible from Alexandria over to Little Falls. Strong isentropic
lift ahead of the warm front will drive multiple rounds of showers
and thunderstorms into the afternoon from southern Iowa toward
central Minnesota. However, this forcing looks to remain largely southeast
of the mpx, so significantly reduced pops southeast of the remnant
boundary, with a primarily dry day expected from southwest Minnesota back
across the Twin Cities into western WI as these areas remain between
the two areas of forcing. Not expecting any drizzle either, as one
look at the 00z sounding from mpx last night shows we are just
starting off too far on the dry end of things in the low levels with
For tonight, things are beginning to look a bit more intriguing,
with the European model (ecmwf)/NAM/nsslwrf showing snow expanding across southern
Minnesota. This is in response to the wave currently near Seattle that is
beginning to dig into The Rockies trough. European model (ecmwf)/NAM show this wave
taking on a negative tilt and strengthening as a trowal develops
from central Iowa northeast into central WI tonight. The enhancement
of baroclinic forcing on the poleward side of the trowal results in
these models expanding the snow across southern Minnesota through the
night. These models are pretty aggressive with the snow that
develops in this region, with a couple of inches of snow possible by
the end of Tuesday night. However, the GFS and Canadian are more
progressive with this wave and show just light precip moving across
southeast Minnesota overnight. Sref members show a pretty even split
between these two solutions. The European model (ecmwf) solution would represent a
pretty significant change in continuity in terms of our forecast, so
trended that way, but held pops at 50% and quantitative precipitation forecast under a tenth of an
inch, though both of these may need to be doubled if what we are
seeing from the European model (ecmwf)/NAM/nsslwrf is correct.
Long term...(wednesday through monday)
issued at 320 am CST Tue Feb 28 2017
The longer term concern remains the snow potential Wednesday and
again Thursday. After this will be timing and strength of warming
into the weekend ahead of the next trough.
Models continue to have problems handling the western Continental U.S.
Trough and timing of impulses/short waves ahead of the main wave.
It now has slowed overall movement of the upper trough some 6 to
12 hours. This lingers the precipitation chance well into
Wednesday. The deterministic European model (ecmwf) and the 03z sref continue to
produce around 0.10 to 0.15 inches of quantitative precipitation forecast with the passage of the
trough. Lift becomes more enhanced as the surface low lifts off
toward the Great Lakes Wednesday morning. The European model (ecmwf) is slower with
the main upper trough and leaves southeast Minnesota in more of
trowal at least through midday. We will follow this model a little
closer...but not so robust with quantitative precipitation forecast amounts at this time. It does
look like it will be cold enough just snow Wednesday so this will
leave an inch or two of snow across about the southern third of
the cwa through Wednesday evening.
The models diverge on the potential strength of the northwest
flow clipper-type storm moving in for Thursday following this
initial wave. The GFS is a little stronger with the wave and drops
southeast affecting the southern County Warning Area late Wednesday night and
Thursday. The European model (ecmwf)...because it was slower with the initial
wave...is not as strong with this second wave. This has been the
trend of this model while the GFS has continued to show a
potential for a band of snow along the path of the wave. We will
continue the chance pop trend for this system for now. If the GFS
trend verifies...we will need to increase pops and possible snow
accumulations over about the southwest half of the cwa. Taking a
looks at the 00z gefs probability plots > 1 inch in 24 hours...
the potential snow band aligns along a 70 mile wide swath from
kaxn-kmsp...a little northeast of the current forecast.
Following this system...cooler temperatures are expected through
about Friday. A more zonal flow pattern develops with ridging
aloft in response to a strong trough forecast to move ashore over
the Pacific northwest later in the weekend. This should lift
warmer air into the area for the weekend. Sunday and perhaps next
Monday have the potential of being some 15 to 20 degrees above
normal...if the model trend continues. We raised temepratures
about 2 to 3 degrees above superblend guidance which did yield
lower and mid 50s Saturday and especially Sunday for the area.
Both deterministic runs indicate the strong northwest Continental U.S.
Trough to move east and drag a cold front through the region
Monday/Monday night time frame...with cooler temperatures
following for a day or two early next week.
Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 555 am CST Tue Feb 28 2017
Gfslamp is finally catching on to the idea of the dry air we have
had overnight and is delaying arrival of MVFR ceilings at all
terminals. However, as a cold front continues to slip southeast
today and north winds bring cooler air south, we will slowly see
the low levels moisten as the temperature drops, which will
eventually allow for widespread MVFR cigs to develop by the
afternoon. Model that was followed to time in the lower ceilings was
the NAM. Snow out in western/central Minnesota will impact stc this
morning, but most guidance shows this band dissipating before it
reaches msp/rnh. Best chance for snow is actually looking to come
tonight as snow is expected to expand across southern Minnesota.
NAM/nsslwrf reflectivity forecasts reflect what the European model (ecmwf) is
showing for precip placement, so followed the NAM/nsslwrf for this
snow, which looks to primarily impact msp on south.
Kmsp...dry air has really slowed the development of low cigs.
Updated tafs follow the hrrr/NAM in being considerably more
optimistic than the previous tafs for today, though even 18z may
be early for seeing MVFR conditions. Confidence is increasing in
light snow impacting msp late tonight, likely between 8z and 15z
/Outlook for kmsp/
Wednesday...MVFR ceilings/-sn in morning. Winds northwest 5-10 kts.
Thursday...chc MVFR cigs. Winds northwest 5-10 kts.
Friday...VFR. Winds S 5-15 kts.