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Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
1248 PM CDT Wednesday Aug 20 2014

Update...for 18z aviation discussion below

&&

Short term...(today and tonight)
issued at 415 am CDT Wednesday Aug 20 2014

Early morning satellite imagery and metars together with the rap13
surface analysis identified an area of low stratus rotating down
across the eastern County Warning Area from Lake Superior. Ahead of this cloud deck
light winds and ample level moisture caused some morning fog to
develop along and north of the I-94 corridor. Visibilities were
generally around 1/2 to 1mi...but should improve after sunrise.
Meanwhile down to the south a northwest/southeast line of convection
developed along the Missouri River valley. These storms were
handled very will by the hires models...and matched up with the
h850 temperature advection off the Storm Prediction Center mesoanalysis Page.

Today will be dry with increasing clouds from the southwest. Later
tonight models show the low level theta_e advection spreading across
the County Warning Area...so have high confidence that the low level moisture
transport together with the upper level forcing for ascent from the
shortwave will produce a widespread band of showers and
thunderstorms across the region. The threat for severe weather still
exists with elevated cape and deep layer shear. The highest threat
for large hail and damaging wind would be across the southwest
before midnight on Wednesday. Heavy rain is also a concern...but the
front is fairly progressive and the mean layer winds should keep the
storms moving. The one caveat is if the storms backbuild against the
low level flow such that the propagation is less than the steering
winds...but for now feel that 1-2in is the most likely scenario with
the heavier thunderstorms. The precipitation will depart to the
east/northeast overnight with the southwestern portion of the County Warning Area
expected to dry out towards sunrise.

Long term...(thursday through tuesday)
issued at 415 am CDT Wednesday Aug 20 2014

Showers and thunderstorms will be ongoing Thursday morning from
portions of central Minnesota southeastward into southeastern Minnesota and western WI. With a
weakening and veering low level jet...the activity will be in a waning phase
but should still be capable of producing heavy rain with precipitable water values
in excess of 2 inches. The showers and thunderstorms will be
located on and just ahead of a surface warm front with a rather
impressive thermal ridge to the south and west. This thermal ridge
should push into much of central/southern Minnesota and far western WI before
its propagation slows by late morning due to prefrontal convective
cold pools. This will allow for continued redevelopment through
the afternoon and early evening across most of WI. High dew
points and a Stout eml advecting northeastward will contribute to a
moderately unstable atmosphere and 500 mb winds of 30 to 35 knots may
be enough for an organized severe threat to persist Thursday. Main
threat will be large hail with the potential for a stable near
surface layer in place due to convective outflows. Meanwhile
further west in the warm sector...skies will begin to clear as 925
mb temperatures of +25 to +27c advect in from the southwest. Surface
winds veering southwest behind the front will bring a unidirectional
flow and ease the development of a well mixed boundary layer
despite winds of only 10 to 15 knots. Forecast soundings show mixing
potentially extending to 850 mb where temperatures will be in the
lower 20s c. Thinking lower 90s should be attainable in such a
situation. The nam12 is several degrees hotter still with a warmer
thermal ridge. Cannot discount this entirely with the better
resolution but not willing to go as warm as the middle to upper 90s
at this time. Mixing to 850 mb won't do much to surface dew points
because of the high precipitable water values in place and upper
60s to lower 70s appear likely. Could be higher from heavy rain
the night before. This will send heat indices well into the 90s
and locally 100 degrees and a heat advisory may be needed.

Weak cool front will sag south Thursday night with the thermal
ridge axis being squashed into Iowa and southern WI. Nevertheless...
boundary layer temperatures will remain quite warm and it should be
another summerlike day Friday with heat indices in the 90s across
southern Minnesota. May see some storms near the front across southern Minnesota as
well...but the forcing will be weak so nothing too widespread is
expected.

Attention then turns to the weekend as the western long wave
trough begins to push east. Surface cyclogenesis over the western
High Plains will occur sometime Friday as a disturbance rotates
and lifts northeast from the base of the trough. Widespread
thunderstorm development should occur along the baroclinic zone
from Nebraska into sodak...northern Iowa...and eventually into Minnesota
and northern WI Saturday. Precipitable water values may be the highest of the Summer if
current model projections are correct. GFS and NAM bring these
numbers into the 2.25 to 2.50 inch range over the MO/MS valley.
Heavy rainfall will again be a concern this weekend as the surface
cyclone tracks northeastward.

Some uncertainty remains for early next week...and sensible
weather will largely depend on the cyclone evolution this
weekend. There are some signs that the front will stall nearby
with additional chances for showers and storms persisting with
warmer temperatures than currently forecast. Models have been
trending in this general direction. If the trend persists...probability of precipitation
and temperatures will probably need to be increased in future forecasts.

&&

Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1249 PM CDT Wednesday Aug 20 2014

Few-scattered cumulus/strato cumulus along with some high clouds this
afternoon...then conditions deteriorate significantly tonight into
Thursday morning as a trough enters the region. Leaned toward the
hi-res model consensus for timing of shra/ts...which indicate the
upper low over the Great Lakes region will provide some residual
blocking and hinder the eastward progression of convection
overnight. Expect activity to affect our western sites /kaxn-krwf/
between 00z and 05z...reaching kstc-kmsp-krnh between 05z and
09z. Once the convection develops ceilings/visibilities in the MVFR and IFR
range will become common. The precipitation shield is expected to lift
north and east of most of the taf sites /except possible kstc/
throughout Thursday morning...with some clearing/destabilization
developing over south central into central Minnesota during the
afternoon. Additional development is then expected...but
confidence on the amount of destabilization that will occur and
where the key boundaries will lie is somewhat low at this point.
Winds will be from the east/southeast through the period generally
below 10kts.

Kmsp...
few-scattered MVFR to low-end VFR clouds this afternoon...then rain showers/ts
arrive around/after 05z. The main time frame for heavy rain showers/ts
appears to lie between 09z and 14z...with lighter/scattered activity
expected for a couple hours on both sides of that time frame. A
break is expected late Thursday morning and early afternoon...with
additional ts development possible around/after 21z.

/Outlook for kmsp/

Friday...VFR with MVFR/thunderstorms and rain late. Winds NE at 10kts.
Sat...MVFR with IFR/tsra+ possible. Winds east at 10g15kts.
Sun...VFR with MVFR/thunderstorms and rain possible. Winds northwest 10kts.

&&

Mpx watches/warnings/advisories...
Minnesota...none.
WI...none.
&&

$$



Short term...jrb
long term...borghoff
aviation...ls

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