Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 170812

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
312 AM CDT Mon Jul 17 2017

.SHORT TERM...(Today and tonight)
Issued at 315 AM CDT Mon Jul 17 2017

Two main issues during this short-term period: elevated heat index
values, especially in western MN, and potential for strong/severe
storms tonight in central MN.

Surface analysis shows high pressure centered over the western Great
Lakes with a low pressure center developing over central Manitoba
province and its associated cold front extending southwest through
Montana and Idaho. A leading warm front is pushing east through the
Dakotas. Aloft, an upper level dome of high pressure is centered
over the central Plains with a ridge axis extending northeast into
the Great Lakes. In addition, an upper level low is rotating slowly
eastward through central Alberta province with a primary upper level
trough on its southern periphery. The warm front will punch through
the western portions of the WFO MPX coverage area this morning and
continue shifting east into WI by this evening. The associated cold
front will be slow to follow, only making it to northwest MN by this
evening then take up a position from the Arrowhead to west-central
MN by daybreak Tuesday morning. At the same time, the upper level
flow will become primarily zonal across the area while the upper
level low in central Canada broadens, making its trough feature slow
to shift east. The warm sectoring will not only result in a
significant rise in temperatures but also dewpoints as Pacific
moisture rides in with the front and a plume of moisture from around
the upper level ride is shunted northward from the Deep South. These
features will allow dewpoints to climb to around 70 degrees while
max temps out west surge to near 100 degrees (with highs in the 80s
in western WI through eastern MN. This combination will nudge heat
index values into the low 100s in a handful of counties in far
western MN so have opted to issue a Heat Advisory. As for the
precipitation aspect of the incoming surface fronts, the combination
of heat/humidity will undoubtedly surge instability levels across
the area, particularly in western MN where convective temperatures
will be reached and CINH is eliminated. Strong heating will likely
result in isolated to scattered thunderstorms in western and
northern portions of the coverage area, eventually spreading east
but generally remaining north of the I-94 corridor. With bulk shear
in the 30-40 kt range and modest veering going upwards, some storms
may become strong/severe with the main hazards being large hail and
damaging winds. Not looking for a lot of coverage in the coverage
area until nearly daybreak tomorrow morning but the potential for
severe weather will remain due to the highly unstable airmass and
available frontal lift.

.LONG TERM...(Tuesday through Sunday)
Issued at 315 AM CDT Mon Jul 17 2017

The forecast concerns in the longer term remain severe weather and
heavy rain threat through Tuesday through Thursday as the region
remains on the favored location of quasi stationary boundary.

For Tuesday, frontal boundary sags southeast during the day and
should remain the focus of thunderstorms development much of the
day as it moves through. The 00Z NAM was rather impressive with
developing a MCS over the central Dakotas and racing it east over
the cwa Tuesday afternoon and Tuesday evening. The GFS and ECMWF
are not so aggressive with that type of development. Will likely
see broken convection develop along the front and move southeast
with possible bowing segments generating damaging winds. With the
degree of instability and shear and the possibility of a short
wave moving with this system, the slight risk DAY2 for severe
storms looks good at this time. There will be some hydro concern
with any real organized acitivity with PW`s over 2 inches and
development of low level jet into Tuesday night. It appears the
surface boundary will drop into far southern Minnesota and will
be on the edge of the mid level cap and very unstable airmass.
Development of a MCS is possible riding across southern MN along
the instability gradient. At the moment we have some 1 inch plus
totals from Tuesday afternoon/Tuesday night over south central
MN. If an organized system does develop, we could certainly see
some 2-3 inch totals during the period.

Wednesday morning should be a break in overall activity with
chances into again to the west ahead of the surface boundary,
which should lie close to the MN/IA border by late in the day.
Very unstable airmass in place with development of 50kt low level
jet by early evening, we should see another complex develop
generating damaging wind, hail and torrential/possible flash
flooding into Wednesday day. This would focus once again across
about the southern third of the area. The threat will also be
driven by the previous nights convection.

There could be another round of convection Thursday night into
Friday as well. The GFS keeps the area near the boundary through
this week. Finally, models have been trending the Canadian trough
to drop southeast over the western Great Lakes and this should
finally give the southern push to the boundary by late in the
weekend and early next week. THis should provide somewhat cooler
and drier air.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 937 PM| CDT Sun Jul 16 2017

Made no significant changes from earlier forecast. We`ll see winds
veer around to the south by mid-morning as high pressure continues
to work eastward. It still looks like any precipitation associated
with return flow should stay north/west of the TAF sites until
after the forecast period.

KMSP...No specific concerns at this time.

Monday overnight...VFR. South wind 10 to 15 kt becoming southwest.
Tuesday...MVFR possible with a chance of showers and
thunderstorms, especially in the afternoon. Mainly southwest wind
around 10 kt.
Tuesday night...MVFR possible with a chance of showers and
thunderstorms. South wind 10 kt or less.
Wednesday...VFR, but brief MVFR possible with slight chance of
showers and thunderstorms. South wind less than 10 kt becoming
Wednesday night...VFR, but brief MVFR possible with a slight
chance of showers and thunderstorms. Mainly northeast wind 5 kt or
Thursday...VFR. Northeast wind 5 to 10 kt.


MN...Heat Advisory from 1 PM this afternoon to 7 PM CDT this evening
     for MNZ047-048-054>057-064-065-073.



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