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

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 06z tafs through 06z Wednesday night)
issued at 1107 PM CDT Tuesday Aug 19 2014

Winds have become calm over all of Minnesota except the far western part.
Still some very light north-northeast wind in west central wisc plus some scattered-
bkn080. The scattered-broken clouds will continue rotating through western
wisc and fog should therefore hold off at krnh/Keau for a little
while...then go IFR or LIFR after 08z-09z. Good chance for LIFR
fog in central and east central Minnesota as well where there was spotty
precipitation Tuesday. Will not be surprised if kstc to Keau experiences
some 1/4sm fog for a couple hours around and before sunrise. Fog
should dissipate after 14z. Surface high should be in far western
Wisconsin at 12z. Tightening gradient should keep southeast winds around
10 kts in Minnesota through Wednesday evening. Surge of precipitation should
arrive in western Minnesota Wednesday evening with thunderstorms a decent bet.

Kmsp...
fog is anticipated...and should certainly be around 4-5sm...but
confidence is not great that visibility will drop below 3sm
Wednesday morning. In part because of current temperature/dpt spreads and
light north-northeast wind. Satellite airports will certainly have some dense
fog. Surface high departs Wednesday morning and southeast wind will remain
nearly constant through the afternoon. Some cumulus should develop
around midday...then storms may move in late Wednesday night.

/Outlook for kmsp/
Thursday...VFR. Chance MVFR with thunderstorms and rain...especially in the morning.
Winds southeast 5-10 kts becmg SW.
Friday...VFR. Slight chance MVFR with thunderstorms and rain. Winds NE 5-10 kts.
Sat...VFR. Chance MVFR with thunderstorms and rain. Winds SW 5-10 kts becoming northwest.



&&

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

$$

Short term...jrb
long term...borghoff
aviation...tdk

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