Scientific Forecaster Discussion
fxus63 kmpx 300929
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
429 am CDT sun Apr 30 2017
Short term...(today and tonight)
issued at 429 am CDT sun Apr 30 2017
A powerful Spring cyclone is developing across the central US,
and this storm will bring widespread precipitation over all of
Minnesota and Wisconsin over the next 36 hours. Areas on the cool
side of this system will wake up to white lawns Monday morning,
while the rest of is see just a cool rain. The clouds and precip
will keep temperatures well below the seasonal average for late
There are two schools of thought in terms of how much snow will
fall. On the one hand this storm is already producing snow as far
south as the panhandles of Oklahoma and Texas, so one can assume
that snow should fall as far north as Minnesota. However, forecast
highs today are expected to be in the upper 40s and it as almost
may, so snow seems unlikely as far north as Minnesota. In the end,
decided the snowfall potential is limited to only a few inches to
converted the Winter Storm Watch to a Winter Weather Advisory.
Confidence is still high that snow will develop across
western/central Minnesota, but confidence is low that amounts will
exceed 5 or 6 inches.
First things first, the Laws of physics that govern the atmosphere
do not adhere to a calendar. With that said, it is very challenging
to get snow with this high of a sun angle, but that should not be
a problem since the heaviest snow is expected overnight. One
thing that is of concern is the lack of cold air with this system.
This is a mature cyclone, but there is not persistent cold air
advection in the boundary layer. If anything, there is warm air
advection and the "cooling" that will cause the change over from
rain to snow is due primarily latent cooling, with some component
contributed to upward vertical motion as well. Along those lines,
the highest Omega is at or above 600mb across western Minnesota where the
positive vorticity advection and upper level divergence are
maximized. For that reason precipitation will be light to
moderate, but over a larger area than if there was a tight low-
level fgen band driving the precip as was the case in may 2013.
Speaking to that may storm, that system has sub-freezing temps in
the low levels, an incredible low level temperature gradient, and
strong isotherm-normal winds leading to continued forcing along a
narrow line which kept the atmosphere cool and allowed heavy snow
to pile up. Since the forecast snow with this system for Sunday
night is located in the deformation band, decided to go on the
warmer side of model guidance both in terms of surface temps, but
also in terms of the thermal profile. This leads to only a few
inches of snow across western Minnesota, which matches up well with the
wpc snowfall forecast.
Long term...(monday through saturday)
issued at 429 am CDT sun Apr 30 2017
By Monday, wet bulb temperatures aloft will be below freezing
west of I-35 and surface temperatures will dictate p-type. If
temperatures are in the low to mid 30s, snow will be the main
p-type until some modest warming occurs by late morning. Not
expecting much in the way of accumulations during the day, perhaps
an inch around Alexandria with a half inch or less elsewhere. Will
let the advisory expire at noon, but it could be cancelled early
if roads are plain wet. Deep cyclonic flow will continue through
Monday night, and even into Tuesday for the WI counties.
Widespread cloud cover and a few passing showers are possible
until surface ridging arrives late Tuesday.
The pause in precip chances will be brief as a weak disturbance/
inverted trough from the stronger system across the mid south
pushes in late Tuesday night into Thursday. Pops and cloud cover
may need to be increased going forward with some solutions
hinting at fairly widespread, albeit light measurable precip.
This week will feature a gradual trend toward normal temperatures
with a decent chance of above normal temperatures by the weekend.
The blocky pattern that develops should send heights up into the
570s well into central and western Canada, but the troughs on
both coasts will ultimately dictate where and how the ridge
develops. The 00z GFS and some of the gefs members have shifted
the surface ridge eastward from the MS valley, allowing warmer air
to build further east across the County Warning Area. The Gem and European model (ecmwf) keep it
overhead, however, and chillier temps into the weekend. There
will likely be a rather impressive temperature gradient across
this ridge with 50s and 60s east to 80s and maybe 90s across the
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 1243 am CDT sun Apr 30 2017
Abundant mid and high level clouds continue to spread northward
tonight. MVFR ceilings as close as central Iowa late this evening
will take their time edging northward, as Southern Plains low
takes its time moving this way. Have maintained the south to
north arrival of MVFR, reaching krwf/kmsp around 17z-18z. A slight
delay by an hour or two is possible.
Other main issue is the persistent northeast wind in southern Minnesota
into WI, with sustained winds of 12-15 knots as of 05z. Gradient
does remain overnight, so it may well hang around 10 knots and
then strengthen Sunday morning. Winds in central Minnesota should remain
below 10 knots, since gradient is not as strong there.
persistent northeast wind will remain sustained over 10 knots the
rest of the night and early Sunday morning as gradient is firmly
in place. Even stronger northeast winds are expected by late
Sunday morning and through the afternoon and evening as low
pressure center makes it move toward the upper Midwest. MVFR
conditions will also develop Sunday afternoon as low pressure
approaches, with a gradual descent to IFR Sunday evening.
/Outlook for kmsp/
Mon...IFR/ra mixing with snow aftn/eve. Winds north 15-20g30kts.
Tue...MVFR ceilings possible. Northwest 10-15 kts.
Wed...VFR ceilings. S 10 kts.
Minnesota...Winter Weather Advisory from 9 PM this evening to noon CDT
Monday for mnz041>045-047>050-054>057-064.