Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 201802

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
102 PM CDT Sun Aug 20 2017

.SHORT TERM...(Today and tonight)
Issued at 335 AM CDT Sun Aug 20 2017

The short term concerns are convective potential early and again
later this afternoon along frontal boundary.

Initially, the Iowa/Nebraska complex appears to impeding flow with
the LLJ and as of 08z nary a shower has developed into south central
MN. The 06Z HRRR did try and develop some showers after 10z so we
will continue to mention small chance into the southern metro south
through much of the morning. Another forecast challenge is the
convective band across South Dakota. The FGEN induced convection is
forecast to move into central MN late morning/afternoon and
gradually weaken as the trough translates east and forcing is
redirected along frontal boundary further southeast. The GFS
indicated fairly strong mid level FGEN but produced little in the
way of QPF with this feature. The HRRR moves the band east into
central MN late morning/afternoon and does produce some light QPF.
We will have to hold onto small chance PoP into west central MN for
this potential.

Instability increases with MUCAPE to 2500 J/kg, best LI`s to minus 8
over the southeast area and deep layer shear around 40kts along the
surface front. The FGEN may be enough to generate at least some
widely scattered convection along the boundary as it drops southeast
during the evening. The exiting MCV associated with this mornings
complex may retard development some. If anything does develop it
could be severe. Due to the somewhat conditional nature of the
threat, the DAY1 outlook of marginal risk over the far south appears
reasonable at this time.

The front sags south this evening and we will continue to hold PoPs
along and south of the boundary. Some of the HIRES models were
indicating a possible increase in convection late tonight mainly
along and south of I90.

.LONG TERM...(Monday through Saturday)
Issued at 335 AM CDT Sun Aug 20 2017

The long term forecast is bookended with active weather periods,
with thunderstorm chances on Monday/Monday night and again
Friday night/Saturday.

The period starts off with the cold front of recent interest
having stalled from northern Iowa across central Wisconsin Monday
morning. The front should be the continued focus for showers and
thunderstorms, especially during Monday afternoon when surface
heating is forecast to allow for decent destabilization along the
Iowa border and south. The shortwave trough sinking south out of
Canada will also supply sufficient ascent for storm development,
possibly severe along/south of I-90.

Tuesday through Thursday surface high pressure broadens across
the Upper Midwest. This will bring dry weather and temperatures
generally in the 70 to 75 degree range for highs.

Medium range models are in pretty decent agreement that the next
trough will move in on Friday evening, then gradually work across
the area through the weekend. It looks to be a fairly stormy
weekend if these progs hold.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 102 PM CDT Sun Aug 20 2017

Difficult forecast ahead, mainly with uncertainties with how
convection will evolve tonight. Cold front is working through the
Twin Cities/Mankato at 18z and will be over to EAU by 00z.
Atmosphere along/ahead of the front looks to remain capped much
of the day, but the cap may break down around 00z out by EAU and
this would be the only site that has a risk of TSRA this
afternoon. Tonight, confidence is highest in a large MCS
developing to the north of the warm front over IA. How far north
this convection ends up is uncertain, but not buying how far
north the HRRR develops sct storms this evening (RWF to MSP).
Think once a mature cluster gets going in IA, that should greatly
limit amount of moisture making it into MN, keeping the greatest
TSRA threat tonight to MKT and points south.

KMSP...An elevated instability gradient will be laid out across
the Twin Cities tonight, and this is what the HRRR and RAP are
picking up on for the scattered storms they have around MSP
between 7z and 12z. As mentioned above, threat for storms looks
much better south of MSP, so kept tonight dry for now. Will see
the IA warm front push back into MN again Monday afternoon and
will likely serve as the focus for storms. Once again though, tsra
chances look much better south and southeast for MSP Monday
afternoon, so kept the TAF dry through 30 hours.

TUE...VFR. Wind NW 10 kts.
WED...VFR. Wind N 5 kts.
THU...VFR. Wind SE 5 kts.




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