Area Forecast Discussion
Issued by NWS Twin Cities, MN
FXUS63 KMPX 271724
Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1224 PM CDT SAT AUG 27 2016
.updated for 18z taf discussion below...
.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 MN, and the threat for LLJ driven
nocturnal convection over southwest MN late tonight.
Aloft, the dominate feature across central NOAM is a trough axis
centered from the central Canadian prairies down to eastern CO.
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 MN 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 NW MN and a 100kt streak over NW
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 NW MN jet streak along with weak and broad
low level WAA 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 MN this morning. We don`t bring this activity into
eastern MN 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 NW MN, 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 SE ND across central MN. 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 MN.
Here, the NAM/GFS/ECWMF all show a 925-850 mb LLJ 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 QPF 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 MN. 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 MN 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 FA 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 FA 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 CONUS. At this
point, we eliminated pops from eastern/southern MN and western
WI. Later forecasts may be able to dry out western MN.
Precipitation chances return at the end of the long term heading
into Labor Day Weekend. Both the GFS and 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 18Z TAFS through 18Z Sunday afternoon)
Issued at 1225 PM CDT Sat Aug 27 2016
Initial rain shield lifting northeast around 32 kts. Should exit
the taf sites within an hour or so. Then we will likely see a
period of mvfr cigs over the east and south into the afternoon. It
may break out briefly in spots...but believe overall trend will
be for more stratus to lift north/develop into the evening. Went
ifr/lifr developing over eastern areas with light winds and some
breaks in clouds. Moist boundary layer should be enough to
generate the stratus and fog overnight...ahead of developing warm
front. This afternoon...still have possibility of tsra affecting
the northwest area into the afternoon/evening before dissipating.
Then another shot of thunder possible to the southwest as low
level waa develops along the front. This may affect krwf after
06z. After this afternoon...that appears to be the only area to
see rain until 18z Sun. Light winds remain over the eastern areas
with high pressure becoming se-s and increasing into Sunday.
KMSP...Rain exiting area now. Mvfr cigs should remain into the
afternoon before breaking out toward 00z sun. Then expect some
stratus/fog to lift/develop over the region overnight perhaps
going ifr after 10z or so. Looks like thicker clouds to remain
over the southwest well into the night. Some threat of shra/tsra
into the afternoon along surface front. But timing still in
question so will leave out for now.
/OUTLOOK FOR KMSP/
Sun nite...vfr. Slight chance afternoon mvfr -TSRA. Wind S 5-10
Mon...VFR. Chc -tsra. Wind S 5-10 kts.
TUE...VFR. Wind light and variable.