Scientific Forecaster Discussion
fxus63 kmpx 211719
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1219 PM CDT Fri Jul 21 2017
Short term...(today and tonight)
issued at 343 am CDT Fri Jul 21 2017
The next 24 hours will continue to be active as thunderstorms with
heavy rain and severe weather are expected to move across Minnesota
and Wisconsin again later today and tonight. The Storm Prediction
Center has an enhanced risk of severe weather for most of the area,
and the weather prediction center has a moderate risk of heavy rain
across southeast Minnesota and southwest Wisconsin. Lastly, heat
indices could approach 100f across southern Minnesota if skies
manage to clear behind this morning's convection and high
temperatures reach the lower 90s with dewpoints in the mid to upper
70s. Looking ahead, storms should exit the region tonight setting
the stage for a drier couple of days to start the week.
As for timing, the 21.03 hrrr experimental and most members of the
21.00 ncar ensemble bring showers and thunderstorms across the
southern part of Minnesota into western WI this morning, and then develop
the main line of severe convection across eastern North Dakota and
drive the storm southeast during the evening hours. This solution
seems very plausible, and trended toward that timing with the
hourly pops. The precipitation should be over behind this line of
The magnitude of the severe weather threat will likely to be dependent
on the morning convection and its impact on the instability ahead
of the aforementioned trailing line of thunderstorms. As was the
case Wednesday, morning convection could limit the severe
potential for the rest of the day if the atmosphere isn't able to
recover. The current thinking is this morning's convection will
not significantly inhibit the northward advancement of the warm
front and we should manage to develop a few thousand j/kg of
MUCAPE by late afternoon given the mid/upper 70s dewpoints that
are forecast to be in place across the region along and south of
the warm front. Deep layer shear should be around 40 to 50 kts
given the veered wind profile associated with the warm air
advection. Low level helicity values will be sufficient for quick
spinup tornadoes if a qlcs does manage to develop as some of the
In summary, no single solution is capturing this event. Feel that
the warm air advection precip is underdone by some of the models,
and the timing may be too slow on the other models. Overall expect
morning precip currently over the NE/South Dakota border to move into Minnesota/WI,
and then have some recovery behind it in time for a forward
propagating line of storms to move across Minnesota this evening and
into western Wisconsin. Should see some elevated storms develop
ahead of this line, and these would pose the flood threat if the
manage to train over the same area for a couple of hours.
Long term...(saturday through thursday)
issued at 343 am CDT Fri Jul 21 2017
An upper low will descend southeastward from Manitoba to Lake
Superior Saturday and Saturday night. A belt of 40-50 kt mid level
westerlies will overspread the region during the afternoon, but
poor mid level lapse rates will limit the degree of instability
despite lingering 70 degree surface dew points. Despite these
factors, a few thunderstorms could develop over western Wisconsin
where the deeper moisture will reside. The meager instability will
limit the severe threat overall, but the degree of shear could
organize a storm or two enough to produce a large hail threat
given the dry air aloft.
Cooler and drier air will follow for Saturday night into Monday.
Could be a bit chilly Sunday night with lows possibly in the 40s
in some areas of central Minnesota and WI.
The warm, humid, and very wet pattern is looking to make a
comeback mid to late week. A potent system tracking east across
central Canada will send a front southward into the upper
Mississippi Valley Tuesday, which will eventually stall as forcing
shifts away from the region. Meanwhile, the ridge will make an
attempt to build back east into the Missouri Valley, with an
active mesoscale convective system zone setting up nearby. Best severe threat appears
Tuesday and Tuesday night while the strongest mid level flow is
overhead, due to the short wave trough passing to the north.
Biggest threat next week is again heavy rainfall totals with
repeated rounds of convection into Thursday.
Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1220 PM CDT Fri Jul 21 2017
Rain showers/thunderstorms and rain that affected a large area of southern Minnesota since
earlier this morning has modified the forecast greatly this
afternoon, as instability, due to the clouds/rainfall, has lessen
the overall chance of precipitation in the next 6 hours.
Therefore, VFR and little chance of redevelopment through 00z,
with even a low chance of thunderstorms in the vicinity until 6z looks reasonable. I will
continue the thunderstorms in the vicinity after 6z for most of the tafs but timing and
overall coverage remains low. Winds will gradually shift from the
south/southeast this afternoon, to a more west/north/northwest
As previously stated, kept nearly no chances of thunderstorms in the vicinity in the
forecast until after 6z. Only concern this taf period is the
development of MVFR/IFR cigs in the eastern/northern forecast area
due to light and variable winds in the boundary layer and nothing
to move the moisture out of the area until after 18z Saturday.
Started to introduce sct015 bkn040 after 12z which could be too
late and/or not low enough.
/Outlook for kmsp/
Sun...VFR. Wind northwest 10 kts.
Mon...VFR. Wind east-northeast - east-southeast 5 kts.
Tue...VFR. Chc thunderstorms and rain late. Wind S/south-southeast 5-10 kts.