Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 132107

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
407 PM CDT Sun Aug 13 2017

.SHORT TERM...(This evening through Monday)
Issued at 407 PM CDT Sun Aug 13 2017

Surface low in eastern South Dakota with inverted trough extending
into eastern North Dakota. Upper low in North Dakota has been
weakening and by tomorrow afternoon, it is expected to be drawn
into a trough extending from the large eastern Canadian low now
in northern Quebec. As this happens, upper confluent flow will
develop over MN and any ongoing precip will come to an end
from west to east tomorrow.

Until that happens, trough extending southward from the North
Dakota upper low has been the locus of thunderstorms that have
been in eastern South Dakota and now far western MN. There have
been a couple breaks in the cloud cover in the form of narrow
lobes spiraling around upper low, but in general there has not
been enough instability. Most of the region has been overcast, and
lapse rates are rather meager. Decent 30 knot low level jet has
been helping some storms, but system in general is not as strong
as advertised yesterday. Upper jet is farther south and weaker. So
all this means that the thunderstorms should mostly be in our
western counties, with a weakening trend as precip moves east.

.LONG TERM...(Monday night through Sunday)
Issued at 407 PM CDT Sun Aug 13 2017

Raised pops above guidance for Tuesday night into Wednesday. Could
not find any guidance that had a dry forecast. The only uncertainty
would be if convection developed across the southwest, and another
area of precip develop more in the deformation zone to the northwest
of the CWA essentially splitting central MN and western WI. This
scenario has probably a 10 to 20 percent chance of happening, so
feel that the 80 to 90 pops are warrantied.

There is still a small risk for severe weather, but the main threat
with this system is heavy rain. Forecast soundings continue to show
very high tropopause heights with PWAT values above 2 inches. The
potential for training thunderstorms also exists.

Looking ahead, high pressure and dry weather will follow for
Thursday and Friday, but the parent upper level waive remains across
central Canada and another shortwave is forecast to move across the
region and bring an MCS along the Missouri River valley into
southern MN. The ECMWF is weaker and farther south with this system
which would keep the region dry through Saturday before developing
and area of storm over the eastern Dakotas Saturday night. The
considerable disagreement between the ECMWF and GFS led to a blended
solution for the extended forecast.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 407 PM CDT Sun Aug 13 2017

Scattered storms in western MN may make it into eastern MN, but
most should weaken and mainly be showers instead by the time
precip reaches eastern or WI. However, confidence in this
scenario is only average at best, since the short term models have
had difficulty today.

Other concern is development of mvfr/ifr later tonight in the wake
of precip. As surface low moves in, gradient will weaken. Models
are in very good agreement with spreading low clouds across the
area late tonight and keeping them through Monday morning.
Confidence is above average on this.



Tue...VFR. MVFR/IFR possible late with SHRA/TSRA. Wind E-SE at 5-10 kts.
Wed...Likely MVFR/TSRA. Wind S-SE at 5-10 kts.
Thu...VFR. Wind N-NW 5-10 kts.




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