Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
527 am CST Friday Mar 7 2014
issued at 515 am CST Friday Mar 7 2014
Updated to include 12z aviation discussion below.
Short term...(today and tonight)
issued at 224 am CST Friday Mar 7 2014
Radar imagery currently shows the main forecast issue for the short
term... that being an area of mainly light precipitation along and
immediately behind the frontal boundary that is dropping through the
area this morning. Temperatures remain below freezing over most of
the area... and with a nice elevated warm wedge in place we/re
seeing some freezing rain. Could see some sleet as we head into the early
morning hours and colder air filters in aloft... with perhaps a
brief transition to snow over the northwest. Things are being forced
by some front relative upglide... along with some upper divergence
in the right entrance region of a jet streak near the Canadian
border. We/ll lose much of that upper support this morning... and
will need to rely on the lower level frontal forcing. Most of the
guidance... including the high resolution output from the hrrr and
hopwrf... weaken the precipitation through the early morning hours... and
Don/T do much more than trace precipitation as things drop through the
central and southern portion of the forecast area later this
morning. If anything remains... it could be pure liquid for the
east/southeast part of the area as temperature are expected to get
up above the freezing mark before the front moves through.
However... with the northwest and north Central Area already seeing
mixed precipitation and with the colder air on their doorstep... put out a
short duration Winter Weather Advisory to cover the potentially
difficult travel conditions which will greet folks up there as they
awaken this morning. We should see clouds linger through the
afternoon even behind the front... with some cold air stratocu in
the brief window of low level cyclonic flow and cold advection.
However... ridging does look to move into the area tonight... which
should help break up most of the cloud cover... with the far
southern County warning forecast area holding onto it longest. Temperatures are likely close
to their maximum now over the northwest... will be at their maximum early
this morning over central parts of the area... and over the
south/east late this morning... with readings falling behind the
Long term...(saturday through thursday)
issued at 329 am CST Friday Mar 7 2014
Long term still looks fairly quite...with milder temperatures prevailing
for once. Outside of a chance for some light precipitation Monday night into
Tuesday...it will be a dry period as well.
The coldest air in the period will come right out The Gate on
Saturday as a 1035-1040mb high works across the area. After
that...the jet stream gets shunted well north into Canada...which
will slam the door to the Arctic shut /for a short time anyways/ and
fling the door to the Pacific wide open for Sunday and Monday. As we
work through next week...both the GFS/European model (ecmwf) show that all to
familiar western ridge/eastern trough pattern returning...but with one
big difference...both features look to be shifted a bit farther east
than what we saw much of the winter...which means the blow of cold
air we get for Tuesday and Wednesday behind a Monday night frontal passage
will be of the glancing variety...with the core of the Arctic air
heading for the Great Lakes and New England. In fact...both the GFS
and European model (ecmwf) show Chinook winds and a Pacific airmass quickly returning
For this forecast package not much was changed...with the proverbial
slight upward nudge to the highs Sunday and Monday completed.
Confidence remains high with most of the area topping 40 Sunday and
everyone joining in on the fun Monday as the mainly west winds will
help keep the fog/stratus in check. Only period that has a higher
degree of uncertainty are the lows Saturday night. For the start of
the evening anyways...the surface ridge will still be across the eastern
County Warning Area...with clear skies and light winds likely allowing temperatures
to quickly fall off after sunset Saturday night. Net result here is
that lows Saturday night over mainly western WI could end up several
degrees colder than we currently have advertised.
As for that precipitation potential Monday night into Tuesday...medium
range guidance is beginning to settle on the idea of a band of light
precipitation moving across southern Minnesota. There will be a pronounced split flow
pattern Monday...with a strong northern stream wave dropping out of the
Canadian prairies toward the Great Lakes...while a closed low begins
to emerge out of the Southern Plains. Eventually...these two waves look
to become one...but that will come late enough in the game to lay
down a swath of heavy precipitation from the Ohio Valley into New
England. For US...that northern stream wave will lead to a strengthening
of the upper jet from Minnesota across the northern Great Lakes...with a decent
right entrance region setting up over southern Minnesota into Iowa. Upper
divergence in this region will work with a middle level frontal
boundary to help kick off an fgen precipitation band Monday night. Thicknesses
from the European model (ecmwf)/GFS show this precipitation maybe starting as rain...but
quickly transitioning to snow as cold air advection brings thicknesses down into
snow territory. The Gem is a warm outlier with the majority if its
precipitation falling as plain rain. With this forecast update...continue
to move probability of precipitation higher in the chance category across the south...while
the p-type forecast follows the idea of the ECMWF/GFS.
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 515 am CST Friday Mar 7 2014
Things will remain challenging with some amendments likely through
the morning hours due to ceiling... visibility... and precipitation-type
issues. Band of precipitation along frontal boundary has held together more
than suggested by most of the numerical guidance... so keeping
more of it in the forecast than originally anticipated doing.
There remain some pockets of freezing rain and fzdz... but colder air aloft
is moving into the area and mitigating the elevated warm layer...
so sleet is becoming more prevalent. Included a mention of both
for most of the taf sites... although the precipitation window looks like
it will mainly just be an hour or two for most locations. Ceilings
and visibilities remain tough to pin down as well... with the bulk
of the IFR and low MVFR conditions generally north of the area.
However... there are some reduced conditions with the frontal
boundary... so continued to keep them as a possibility. Should see
some MVFR for a time after the front given upstream observations
and the lingering boundary layer moisture along with cold
advection and cyclonic flow.
Kmsp...fairly confident in the time frame highlighted for precipitation and
potentially worst conditions... but have fairly low confidence in
how low ceilings and visibilities will go. Would expect some
variation in timing of up to an hour or two... and ceilings could
potentially get below 010 for a period of time as well.
/Outlook for kmsp/
Saturday afternoon...VFR. West wind 5 to 10 knots.
Saturday night...VFR. Southwest wind 5 to 15 knots.
Sunday...VFR. Southwest wind 15 to 25 knots.
Sunday night...VFR. West wind 10 to 20 knots
becoming northwest less than 10 knots.
Monday...VFR. West wind 5 to 15 knots.
Monday night and Tuesday...MVFR ceilings possible. West wind 5 to
15 knots becoming northwest.
Minnesota...Winter Weather Advisory until 7 am CST this morning for
Winter Weather Advisory until 9 am CST this morning for mnz044-
WI...Winter Weather Advisory until 9 am CST this morning for wiz014-