Scientific Forecaster Discussion
fxus63 kmpx 271004
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
504 am CDT Tue Jun 27 2017
Update...for 12z aviation discussion below
Short term...(today and tonight)
issued at 416 am CDT Tue Jun 27 2017
The main forecast concern in the short term period is timing for
precipitation onset with the next system.
Thunderstorms are expected to develop across the Dakotas this
evening as a shortwave trough approaches and moisture advection
ensues ahead of the related surface low over the central/northern
plains. A capping inversion will likely delay convective initiation
to the west until early evening, with convection not looking to
reach west central Minnesota until around midnight. Have tweaked pop/weather
grids to reflect a delayed onset, with showers and thunderstorms
becoming widespread across Minnesota counties between 05z and 11z as the
low level jet increases to 50+ knots. The activity looks to spread
across west central WI around/after daybreak Wednesday.
With precipitable water values nearing 2 inches during this time
frame, heavy downpours and street flooding in urban areas will be a
Temperatures will generally moderate 5-7 degrees from monday's
readings given the increasing heights and mixing supplied by breezy
southerly winds. Highs will range from 75 to 80 degrees.
Long term...(wednesday through monday)
issued at 416 am CDT Tue Jun 27 2017
Widespread showers and thunderstorms will be ongoing early
Wednesday morning across much of the upper Mississippi Valley
region. A seasonally strong 50-60 kt low level jet will veer easterly
Wednesday morning, taking the impressive moisture transport and
the bulk of the convection with it. It is unclear how the rest of
the day will unfold convectionwise, but the current thinking is
that showers and storms will clear out for a time late morning
into early afternoon before another mid level wave rotates
through. NAM shows little or no atmospheric recovery behind the
first round, while the GFS and Euro show quick destabilization
during the afternoon. The WRF-nmm and to a slightly lesser degree
the WRF-arw also show this, with thunderstorms redeveloping around
mid afternoon across eastern Minnesota and working east into WI by early
evening. Shear isn't too favorable for severe thunderstorms with
mid level winds easing as the base of the trough pushes through,
but a few severe storms are possible across southeastern Minnesota near
the belt of stronger mid level westerlies across Iowa into
northern Illinois and southern WI.
With the surface front settling down to the I-80 corridor
Thursday, we should have a break in the action before another
trough dives southeast across the northern plains Thursday night.
An inverted trough will settle in Friday. Several waves rotating
around an upper low with weak surface based instability should
generate numerous showers and thunderstorms with daytime heating
Friday. Activity will wane Friday evening and then regenerate
again Saturday. Increased pops through this period to high chance
or low end likelies.
The trough will begin to lift out Sunday, and ridging building
eastward across the Southern Plains will allow a system to
propagate across the Central Plains to the Midwest Sunday night
and Monday. There is a lot of variability with this system, but it
could bring another round of soaking rain.
The pattern is showing more signs of changing drastically later
next week with the ridge shifting from the Desert Southwest to the
central U.S. Heights building to 595 dm centered over the Central
Plains will send a hot and potentially very humid airmass
northward back to the upper Midwest. Tough to say so far out if
the pattern will favor mesoscale convective system development in our neck of the Woods,
or if it will be dry and capped. Nonetheless, a big difference
from the last half of June.
Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 505 am CDT Tue Jun 27 2017
VFR conditions are expected through most of this evening. High
clouds will be on the increase this afternoon/eve ahead of an
approaching low pressure system. Associated showers and
thunderstorms will spread eastward across the area after 05z
Wednesday, reaching the Minnesota/WI border around 10z. South/southwest
winds will increase for this afternoon ahead of the system,
generally reaching 15g25kt speeds between 17-18z (with the
exception of Keau where gusts are not expected). Many sites will
remain gusty this evening.
expect showers and thunderstorms to arrive around 9 or 10z
Wednesday...tapering off after 14z. Additional redevelopment could
Wednesday afternoon. MVFR ceilings will be attendant with the
showers/ts, and also in the wake of the showers.
/Outlook for kmsp/
Thu...VFR. Winds northwest 5-10 kts.
Fri...VFR. Chance MVFR with thunderstorms and rain. Winds variable 5 kts.
Sat...VFR. Chance MVFR with thunderstorms and rain. Winds west-northwest 5-10 kts.