Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 171135

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

Issued at 624 AM CDT Mon Jul 17 2017

Updated to include 12z aviation discussion below.


.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 12Z TAFS through 12Z Tuesday morning)
Issued at 624 AM CDT Mon Jul 17 2017

Cloud cover will slowly increase in the upper levels and lower in
height through the day today in advance of a cold front over the
Dakotas. Greater instability in western MN today will spark
scattered showers and thunderstorms which may impact KAXN-KSTC
this afternoon and evening. The front will remain far enough north
and have slow enough forward progress such that precipitation will
likely hold off until during the day tomorrow. Winds will
generally come from the south in the 8-15 knot range.

KMSP...Precipitation looks to hold off until after 12z tomorrow
then chances increase fairly quickly tomorrow morning into
tomorrow afternoon for showers/t-storms. Have advertised a start
time of 14z at this point but that may change in later TAF
issuances. Have also kept conditions at VFR but conditions may
easily drop to MVFR-IFR tomorrow afternoon when chances are much
greater for having showers/t-storms near and potentially at MSP.

Tue night...MVFR possible with chc SHRA/TSRA. Wind NE 5 kt.
Wed...Mainly VFR. MVFR possible with chc SHRA/TSRA late. Wind NE
5-10 kt becoming SE.
Wed night...MVFR possible with chc SHRA/TSRA. Wind SE 5 kt
becoming NE.
Thu...VFR. Wind light and variable.
Fri...MVFR possible with chc SHRA/TSRA. Wind SE 5-10 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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