Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 181717
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1217 PM CDT THU AUG 18 2016

.UPDATE...
Issued at 1218 PM CDT Thu Aug 18 2016

Update to include 18Z Aviation Discussion below.

&&

.SHORT TERM...(Today and tonight)
Issued at 400 AM CDT Thu Aug 18 2016

Early morning radar/satellite imagery showed scattered convection
across mainly northern North Dakota, with some isolated activity
developing in eastern South Dakota. Convection across North Dakota
was being fed with a strong low level jet with 85h winds in excess
of 40 kts. Further to the south, the low level jet is weaker but
the 85/70h theta-e gradient is much more pronounced. In addition,
water vapor and regional radar did indicate a weak circulation in
central South Dakota which is likely aiding some of the convection
in eastern South Dakota.

Although the forecast is nearly straight forward with convection
likely developing/continuing in west-central/southwest Minnesota
this morning /before 9 am/ as both the increasing theta-e
advection and weak circulation moves eastward. However, usually by
mid/late morning the increasing cap will lead to an overall
decrease in the activity. How the North Dakota convection
interacts with the South Dakota activity this morning will
determine the overall coverage in southern Minnesota, especially
along the Minnesota/Iowa border. Further to the north across
central Minnesota, the interaction of the outflow boundaries from
thunderstorms and the strengthening surface boundary as low
pressure begins to intensify in South Dakota, should lead to
widespread showers/thunderstorms this morning.

As PWATs increase to 1.5 to 1.75", the threat of heavy rainfall and
flooding will increase. In addition to the heavy rainfall and
other severe weather parameters, based on the frontal boundary
location, and if supercell can form, tornadoes are a good
possibility along this boundary. See the latest SPC day 1 outlook
for more information. Although flooding is becoming a concern, the
exact location of where the thunderstorms begin to train/evolve
this afternoon/evening, is questionable. Therefore, a flash flood
watch will not be issued early this morning. Please see the latest
WPC information on the flood threat this morning as the system
evolves.

Models have most of the precipitation moving east of MPX forecast
area late tonight, which is questionable due to the frontal boundary
near the area and continued outflow boundary interaction and upper
level support. I would not be surprised to see several waves of
thunderstorms overnight across the area.

Several of the CAMS support the decreasing convection this morning
in southern Minnesota, with more development across western
Minnesota by the late afternoon. This is highly dependent on how the
morning convection evolves and the strengthening cap. Temperatures
reflect a warmer bias in southwest Minnesota due to strong mixing
and 85h temperatures in the low to mid 20s/C.

.LONG TERM...(Friday through Wednesday)
Issued at 400 AM CDT Thu Aug 18 2016

Friday and Saturday...This period will commence with a low
pressure center nearly atop the Twin Cities metro, with a warm
front extending to the NE into N-central WI and a cold front
extending SW through N and W IA. Aloft, one relatively weak
shortwave axis will be helping slowly nudge the front along while
a stronger more stout trough axis will be swinging through eastern
MT and the western Dakotas, associated with a deep upper level low
spinning through central Canada. Deep moisture will be associated
with the front as PWATs increase to around 1.7-1.8 inches.
Southerly flow in advance of and along the front will aid in
bringing MLCAPE to around 3000 j/kg. The main question going into
Friday is if there will be enough energy lagging back with the
front to have severe weather potential because the chances for
precipitation itself are already high, in the likely to
categorical ranges, so it can be said with high certainty that
there will be precipitation ongoing at the start of this period.
Back to the severe potential, since the front will have made good
progress through the coverage area by daybreak, there will be
little time for insolation to make a significant impact while the
front still lingers in the coverage area. The best potential lies
in far southeastern portions of the coverage area where shear will
be greatest (bulk shear of 30-40kt) and instability highest. This
lines up with the Day 2 Outlook from SPC delineating far SE MN in
a Slight Risk. With the strong upper level trough trailing but
steadily moving east, showers and storms will last well into
Friday night and Saturday, tapering off Saturday night. As for
temperatures, the cold frontal passage and the deep trough aloft
will bring in a noticeable airmass change to the area. Highs will
drop to the 70-80 degree range on Saturday then fall further on
Sunday to 65-70 degrees. Humidity levels will also significantly
drop off, making for a cool but comfortable weekend with respect
to temperatures.

Sunday through Wednesday...Cool high pressure will slide across
the region for the start of the week, keeping the area dry.
However, as a more southerly flow resumes on the backside of the
high, temperatures will go on a gradual warming trend through the
first half of next week. Highs will climb back to around 80
degrees by midweek. The next weather-making system looks to come
Tuesday night into Wednesday in the form of a slow- moving frontal
system. Precipitation chances will increase but have capped pops
in the chance category at this point.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1218 PM CDT Thu Aug 18 2016

.AVIATION...

Several round of showers and thunderstorms will move across the
region over the next 24 hours. The main threat is for strong wind,
but will likely see impacts outside of that with reduced visibility
from heavy rain and low clouds. The storms should shift southeast by
morning, but forecast soundings show low level moisture and MVFR/IFR
clouds Friday morning.

KMSP...

There is a long duration of thunder in the Tafs, but that is because
the system responsible for the storms is very slow moving. The best
chance appears to be later this evening, so have a tempo wi reduced
visibility during the period.

/OUTLOOK FOR KMSP/

Sat...MVFR with TSRA/RA possible. Winds NNW 10-15 kts.
Sun...VFR with morning MVFR Cigs possible. Winds NW 5 kts.
Mon...VFR. Winds S 5-10 kts.

&&

.MPX Watches/Warnings/Advisories...
WI...None.
MN...None.
&&

$$

UPDATE...JRB
SHORT TERM...JLT
LONG TERM...JPC
AVIATION...JRB



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