Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
349 am CDT Sat Aug 27 2016
Short term...(today and tonight)
issued at 349 am CDT Sat Aug 27 2016
There are three main concerns during the short term; shower coverage
through the morning, the potential for severe storms later today
into the evening for central Minnesota, and the threat for low level jet driven
nocturnal convection over southwest Minnesota late tonight.
Aloft, the dominate feature across central noam is a trough axis
centered from the central Canadian prairies down to eastern Colorado.
Numerous weak waves exist within the southwest flow ahead of this
trough and the one of interest locally came up out of central neb
last night and is moving into SW Minnesota this morning. Beside this short
wave, there is a split jet structure across the upper MS valley,
with one 80-90kt jet streak up over northwest Minnesota and a 100kt streak over northwest
WI. This eastern jet streak is associated with the tropical moisture
plume and will be sending the most widespread and heaviest precip
and thunderstorm activity into WI this morning. However, the
Nebraska wave with the northwest Minnesota jet streak along with weak and broad
low level warm air advection will be enough to generate a good area of light rain
west of I-35 through the morning. The hrrr has been the best at
picking up on this, so followed it's idea for working showers
through western Minnesota this morning. We don't bring this activity into
eastern Minnesota and western WI as these areas will be under the influence
of a dry surface ridge axis extending down from the u.P. Of Michigan
coming down on the west side of the precip that will be going into
For this afternoon, attention turns to the wave moving across the
western Dakotas this morning. By the afternoon, models are in good
agreement on a seasonably strong pv boot working into northwest Minnesota, with
strong upper forcing expected with it. There will be very little in
the way of a surface inflection with the wave though, with little/no
surface convergence expected, so afternoon acitivity will be driven
by the pv feature. There looks to be a narrow corridor where
forcing, instability (1000-1500 j/kg of mlcape), and shear (50-60
kts of 0-6km shear) come together to create an elevated severe
weather threat from southeast ND across central Minnesota. The hrrr has been
fairly persistent with creating a small bowing line in this region,
which seems very plausible given the upper forcing along with
instability and shear present where the hrrr has this line.
For tonight, we added a chance of thunderstorms after 6z for SW Minnesota.
Here, the NAM/GFS/ecwmf all show a 925-850 mb low level jet developing, with a
good deal of moisture transport and Theta-E advection noted as the
front currently down along the I-80 corridor lifts north as a warm
front. The NAM/GFS/Canadian all show some quantitative precipitation forecast developing in this
region late tonight and as a result, added in the pops out that
direction. Aloft, there are no mid-level waves noted and this area
is in the left entrance region of the upper jet, but it's hard to
believe we can't get something to develop within this low level
moisture feed tonight.
Long term...(sunday through friday)
issued at 349 am CDT Sat Aug 27 2016
A marginal risk for severe weather on Sunday remains the main
concern in the long term. Strong low level Theta-E advection will
be ongoing Sunday morning across south central Minnesota. This coupled
with a surface boundary lifting north through the area may be
enough for some showers and thunderstorms to persist across the
south in the morning. The NSSL and ncar ensemble forecasts have
only a few members showing some scattered activity. Hence
confidence is not high with only 15-20 percent pops indicated.
Later in the day on Sunday, the boundary will have progressed
northward into central Minnesota and adjoining areas of west central WI.
MLCAPE of 2000-2500 j/kg is forecast along the boundary along with
deep layer shear in the 30-35 knot range. However, the region is
on the north side of the upper level jet. The mid/upper level
flow becomes more anticyclonic through the afternoon along with
height rises. Only three members of the 10 member ncar ensemble
WRF indicated activity near the boundary. So confidence is not
high on afternoon/evening thunderstorm development with low chance
pops remaining in place. If storms can develop, then some isolated
severe weather is certainly possible.
A cool front will work eastward from the Dakotas Sunday night and
slowly pass across the forecast area Monday and Monday night. Better chances
for showers and thunderstorms exist late Monday and Monday night
along the front. The concern is that there may be a dry period
Monday morning across the eastern and southern forecast area which is
currently not reflected in the forecast due to the spread in model
solutions. Hopefully we can refine this in the next few forecast
Quite a bit of collaboration was done with our neighbors
overnight to eliminate the pesky 15-20 pops that littered the
forecast in the wake of the front from Wednesday through Friday.
The ec seems to have the best idea on being dry with surface high
pressure parked over the western Great Lakes as an upper level
ridge expands across the central third of the Continental U.S.. at this
point, we eliminated pops from eastern/southern Minnesota and western
WI. Later forecasts may be able to dry out western Minnesota.
Precipitation chances return at the end of the long term heading
into Labor Day weekend. Both the GFS and European model (ecmwf) are in agreement on
a wet weekend.
Nothing too dramatic in the temperature department through the
period. Monday should be the warmest day in the long term with
highs in the middle to upper 80s. Things will settle back to near
seasonal levels by mid week in the wake of the front with another
warming trend to above normal temperatures heading into next
weekend. Afternoon humidity levels will be a little uncomfortable
from Sunday into Tuesday with dew points in the 65 to 70 degree
range. Values will subside some behind the front for Wednesday and
Thursday with a return of mid to upper 60s dew points for the end
of the week.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1022 PM CDT Fri Aug 26 2016
High clouds are expected tonight, but we think it will remain dry
until early tomorrow morning when isolated to scattered showers
develop in western Minnesota and move east. There is very little
instability in the atmosphere, therefore the threat for thunder is
very low until tomorrow afternoon. The best chance for thunder
will be at kstc and kaxn.
No aviation weather concerns tonight with the high clouds
overhead. There is a chance for thunder tomorrow afternoon, but
still not a sure thing at this point. It might remain well north
and west of the Airport.
/Outlook for kmsp/
sun...VFR. Slight chance -tsra. Wind S-SW 5-10 kts.
Mon...VFR. Wind S 5-10 kts.
Tuesday...VFR. Wind light and variable.