Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 142103

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
403 PM CDT Fri Jul 14 2017

.SHORT TERM...(This evening through Saturday)
Issued at 402 PM CDT Fri Jul 14 2017

Main concern in the short term is the increased heat and
humidity, as well as the chance for severe thunderstorms late
Saturday. At this point it appears that the heat indices will remain
in the mid 90s, with wet-bulb globe temperatures reaching the lower
80s. This is below the heat advisory criteria, but still could pose
a threat to people outdoors if proper precautions are not taken
like hydration and period breaks from strenuous activity.

As for the severe weather threat, there will be a weakening upper
level wave moving across the forecast area and along with a diffuse
frontal boundary. These two features will be the focus for
convective initiation, but there is still doubt as to how widespread
the storms will be, especially as you head west.  Feel the 12Z WRF
NMM/ARW have a good handle on the timing and placement of the
convection, so tried to mimic that with the hourly pops tomorrow
afternoon. As has been the case for pretty much all of July, deep
layer shear is around 40 to 50 kts, and should see a few thousand
joules of CAPE along and ahead of the boundary. If storms do
develop, they should have no problem acquiring a rotating updraft,
and could produce large hail, with a secondary threat of wind, and
tertiary threat of tornadoes. Precip chances will fall off to zero
overnight as the boundary moves further south.

In the extended there appears to be a favorable set up for heavy
rain starting late Monday night and lasting through the middle of
the week as a frontal boundary stalls out across the region. This
would set the stage for repeated chances for showers and
thunderstorms. Severe weather is also possible during this time
frame. Did not stray from the blended guidance.

.LONG TERM...(Saturday night through Friday)
Issued at 402 PM CDT Fri Jul 14 2017

Early next week the weather pattern will remain fairly consistent
with an upper level ridge to the west, but more zonal flow will set
up along the International Border. Another surge of warm moist air
will return on Monday, with highs in the mid 90s out west and mid
80s elsewhere. Expect dewpoints to climb to near 70, which will lead
to large amounts of instability over the Upper Midwest.

Forecast models continue to show an weak PV anomaly rotate up from
the high plains of NE/KS and enhance the convergence along the warm,
moist boundary as it leans across MN/WI. This should lead to quick
development of an MCS late Monday night, which should propagate
southeast across Minnesota and Wisconsin and lead to an east/west
oriented boundary over southern MN and western WI.

This boundary will remain fairly stationary as zonal flow leads to
lee side troughing over the Dakotas, and maintains the low level
convergence along the boundary east over MN/WI. For that reason
expect multiple rounds of thunderstorms, but the exact details are
unpredictable at this time since they will likely be influenced by
the convective outflow of the storms themselves.

The main take away is that early next week looks particularly active.
There will be a few thousand J/kg of MUCAPE, with a surface boundary
nearby, and favorable deep layer shear. GFS shows PWAT values will
be near 2 inches, and higher in the areas that have convection. It
is unclear if the storms will backbuild and set up a training
thunderstorm scenario with heavy rain and flash flooding, or whether
or not they will become forward propagating and pose a severe/wind
threat. CIPS analogs show more of a heavy rain threat. Realistically
there will probably be some combination of the two, so this remains
the period of interest in the extended forecast.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 125 PM CDT Fri Jul 14 2017

VFR conditions expected this afternoon through tomorrow. The
nagging stratus will slowly lift and dissipate this afternoon with
clear skies expected by the evening. Light and variable winds
should eventually become southerly. Overnight a few showers will
move across northern Minnesota and Wisconsin, but these are
expected to stay north of the TAF sites and should produce nothing
more than mid/high clouds.

VFR conditions throughout. Clouds should break up this afternoon,
and clear out this evening. Light winds will become southerly
overnight and southwesterly on Saturday. A few thunderstorms are
possible late Saturday afternoon, but it looks like they will be
east of MSP so did not include mention in the TAF.

Sunday...VFR. Northeast wind 5 to 10 kt becoming east.
Monday...MVFR possible early, otherwise VFR. Southeast wind 5 to
15 kt becoming south 10 to 20 kt.
Tuesday...MVFR with TSRA possible early. Wind S at 10kt




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