Scientific Forecaster Discussion
fxus63 kmpx 240558 aac
Area forecast discussion...updated
National Weather Service Twin Cities/Chanhassen Minnesota
1158 PM CST Thu Feb 23 2017
issued at 815 PM CST Thu Feb 23 2017
Will downgrade northern portions of the headlines shortly. Now
that the event is underway and hi-res models are getting a
relatively good handle on the dry air and where the snow shield
will advance, feel confident on tightening the pop, qpf, and snow
amount gradients. Global model resolution is too coarse to resolve
the ridiculously tight gradient expected which results in bleeding
the snow too far northward. This means locations like Redwood
Falls and the Twin Cities proper may not see a flake. The 00z NAM
has shifted south slightly again which confirms this idea.
As for the area that will see snow, we are still not seeing
widespread steady snow, and it appears the models are too eager to
break things out this evening, particularly the NAM. However, it
is still expected to become intense tonight near the Iowa border.
Visibilities are 1/4 mile in several locations of northwest Iowa
and thunderstorms are breaking out just south of there so a
maturing snow shield is under development. Expecting this snow
shield to shift mostly east northeastward tonight, then pivot more
northeast Friday. Amounts on the northern periphery of the
Blizzard Warning may be a little high if the hrrr was to verify,
but can't get too cute.
This southward shift in the past couple days has been a little
surprising to say the least considering the monster totals
advertised on all the models and their ensembles at one point.
Suspect this is due to the very large upper low over Florida which
deamplified the eastern ridge and resulted in a flatter pattern,
sending the low eastward and taking a more southerly track. The
mid level features aren't as strong and compact either and
are instead more sheared which will prevent the monster totals
(15-18"+) from falling where the heavy band does set up.
The west-southwest and the rest of the product suite will be issued shortly.
Short term...(this evening through Friday night)
issued at 342 PM CST Thu Feb 23 2017
Surface analysis this afternoon shows development of a classic
mid-latitude cyclone over the Texas/Kansas panhandles with an extended
surface front from the center of this system east-northeast into lower Michigan and
a trailing cold front over nm/AZ. Aloft, a large longwave trough
which moved onshore the lower California coast has shifts atop the
central-southern rockies. The upper trough is expected to deepen
over the central-northern plains during the day tomorrow through
tomorrow night, helping add plenty of energy to the developing
surface low. The surface low will shift across Kansas tonight and over
northern MO tomorrow morning, on its way to Lower Lake Michigan by
tomorrow evening then Lake Huron by Saturday morning. Deep Pacific
moisture will accompany this system, so much so that quantitative precipitation forecast amounts
for southern Minnesota look to range 1.00-1.50" by the conclusion of this
system. Though there continues to be good model agreement (outside
of the NAM which has and remains a significant outlier) that southeast Minnesota
will be in the crosshairs for the highest snowfall totals, the
consensus track took another slight shift to the se, thus lowering
the snow amounts and hence the impacts mainly north and west of the Twin
Cities Metro. As such, some minor reconfiguring of the headline
products were made but the point must still be made that there
will be a very tight northwest-southeast gradient of snow amounts just to the
south of the Twin Cities Metro. As an example, counties like Le
Sueur or Nicollet or Dakota or Brown could see as much as a 6"
difference in snow totals from northwest to southeast across each respective
Snow has commenced in far SW Minnesota this afternoon and snow will
gradually spread mainly east and slightly north as the overnight
progresses. There may even be a slight mixture with rain at the
immediate onset but any mixture of precipitation will have a short
duration before become all snow. As the surface low shifts east-northeast
and the upper level trough pivots to help enhance the surface
feature, snow coverage and intensity will increase during the
early morning hours, likely with rates increasing to 1-2"/hr for
several hours in far southern Minnesota and into SW WI. The morning
commute for southern/eastern Minnesota into western WI will likely have
ongoing snow then the snowfall will steadily diminish as the day
progresses. However, strong winds for much of the region will
contribute to the blowing snow issues, making for hazardous
travel conditions for much of southern Minnesota. Another concern is
isolated thunderstorms could develop over far southern Minnesota into
southwestern WI. Instability levels are not entirely zero given
the antecedent moisture levels plus the incoming surge and that
additional lift will be created by the pivoting upper level
trough. Confidence not high enough to include mention at this
point but some thunder cannot be ruled out.
Precipitation will end from west to east late tomorrow afternoon
through tomorrow night and the wind speeds will decrease as well,
resulting in partly to mostly cloudy skies for the start of the
weekend. As for temperatures, once temperatures drop below
freezing later this evening, they will remain below freezing
throughout the duration of this event. Lows tonight to highs
tomorrow will have a very small diurnal range, potentially 5
degrees or less from tonight through tomorrow. As colder air wraps
around this system as it departs, lows will drop into the teens
tomorrow night, temperatures this area has not seen in some time
but will be actually right on par with climatological norms.
Long term...(saturday through thursday)
issued at 342 PM CST Thu Feb 23 2017
Temperatures will warm through early next week. A fairly large
storm will develop over the upper Midwest, but thermal profiles
should be warm enough to have mostly rain, especially if the
system translates westward with time.
A weak clipper system will sweep through the region on Sunday and
could bring some snow across MN/WI. On Monday the southern stream
will develop a storm to the southeast over the Ohio River valley.
This will be followed by a third system Tuesday into Wednesday.
This mid-week storm will have a favorable temperature gradient
to work with and should develop strong cyclone. However, there is
a lot of warm air with this storm, so the primary p-type will be
rain, with perhaps some snow mixed in. There is still uncertainty
with this storm, but as of now it appears that the chance is quite
low for this system to develop into a winter storm for the upper
Aviation...(for the 06z tafs through 06z Friday night)
issued at 1158 PM CST Thu Feb 23 2017
The snow now appears to likely remain south of kmsp, krnh, and
krwf. However, some MVFR cigs are likely at times tonight and
Friday. Keau will be under IFR or lower snow for the next 18 hours
or so with several inches expected. Thunderstorms will be
confined to far southern Minnesota. No concerns for kstc or kaxn.
Kmsp...the snow should remain south/southeast of the terminal by
10 to 20 miles. Main concern is the gusty north northeast winds
and occasional MVFR cigs for the next 18 hours.
/Outlook for kmsp/
Sat...VFR. Wind northwest 10-15 kts becoming west.
Sun...VFR. Chc MVFR/-sn. Wind SW-west 5-10 kts.
Mon...VFR becoming MVFR. Chance afternoon -rasn. Wind S 5-10 kts.
WI...Winter Weather Advisory until 6 PM CST Friday for wiz015-023.
Winter Storm Warning until 6 am CST Saturday for wiz016-024>028.
Minnesota...Winter Weather Advisory until 6 PM CST Friday for mnz063-069-070.
Winter Weather Advisory until noon CST Friday for mnz067-073.
Blizzard Warning until 6 am CST Saturday for mnz074>078-082>085-