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Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
1048 PM CDT Thursday Sep 18 2014

Short term...(this evening through friday)
issued at 400 PM CDT Thursday Sep 18 2014

There is finally the potential more active weather in the short term
period....besides frost...fog and stratus clouds.

Trough-ridge-trough pattern exists across the Continental U.S. Today with long
wave ridging building into the northern High Plains and a healthy
trough now entering the West Coast with a 70-90kt jet streaming
north out of Southern California and into the northern rockies.
This is triggering convection in Idaho/WY/Mt this afternoon.
Meanwhile the low level jet has been shifting east across the Dakotas today
and we've managed to advect in a little more boundary layer
moisture across the forecast area. There are a lot more middle 50
dewpoints this afternoon...when compared to the past couple days.
We expect a leading shortwave moving from the Pacific northwest
into the Dakotas early tomorrow morning. A surface trough will
move from the eastern Dakotas around 00z into the upper MS valley by
12z Sat as a ribbon of 60s dewpoints streams northward into southern Minnesota
Friday evening. Tonight...with the low level flow turning more
southwest and increasing...we could also see some of the cloud
cover observed in NE/KS/MO today be advected into western Minnesota
before sunrise. Mixing off the forecast soundings suggest a warm
day with highs in the 70s and perhaps even a few 80s out west. As
mentioned in previous discussions...there is strong capping
overhead tomorrow due to strong warm advection over the next 18
hours. This should limit the convective potential locally and
focus more of the early Friday activity farther north in Minnesota where
there is expected to be cooler middle level temperatures. Shower coverage
locally will be very sparse during the day tomorrow across most of
the forecast area. We have seen a continual lowering in both probability of precipitation
and quantitative precipitation forecast over the past several model cycles across nearly all the
modelings systems available.

Long term...(friday night through thursday)
issued at 400 PM CDT Thursday Sep 18 2014

Long range models continue to show upper ridging becoming the
dominate player next week east of The Rockies. Hence a mainly dry
and mild week is expected. 12z models have also continued the trend
of the front coming through Friday evening doing so mainly dry for
the mpx County Warning Area...so have continued to reduce probability of precipitation/quantitative precipitation forecast then.

Friday night through Saturday looks to be our main period for precipitation
chances as first a cold front works across the area Friday
night...with a strong trailing upper wave leading to renewed shower
chances Saturday afternoon NE of I-94. To start the long term...the
problem with the front in the short term will still be in place
Friday evening...a strong capping inversion with a warm nose
centered around 850 mb. However...this warm nose looks to start
breaking down shortly after 00z...with the Prospect for thunderstorm
initiation along the front looking better by 3z. Coverage looks to
be scattered along the front...add into that questions about
when/where storms will be as well...decided to cut probability of precipitation back to no
higher than the chance category Friday evening...as veering winds
and weak convergence along the front...along with neutral height
falls and the lack of any significant short waves look to make it
unlikely we will see widespread precipitation with this front. At this
point...best chances for storms looks to be east of a Mankato to
Rice Lake line...so removed quite a bit of probability of precipitation west of this
line...while reducing likelies east of it down to no higher than
50%. For the rest of the night...this front looks to shift east
fairly quick...so reduced probability of precipitation quite a bit after 6z Friday
night...with a dry forecast now in place Saturday morning. There
still looks to be a conditional threat for severe weather with any
activity Friday evening given steep middle-level lapse rates and
40-50kts of bulk shear. However...the probability for surface based
convection looks relatively low...since as the elevated capping
inversion breaks down...the nocturnal surface inversion will be
developing...hence why all the day2 Storm Prediction Center convective outlook mentions
is there being a small threat for large hail with elevated
convection.

Saturday afternoon...guidance is in good agreement with a strong
wave rotating around the mean eastern Canadian trough into northern Minnesota.
Beside a strong pv signature...this wave will be coming down with a
120+ knots nwrly jet streak. The pv advection and left exit region of
the jet streak will be coming down north of I-94 Saturday
afternoon/evening. Being in the wake of the cold front...the forcing
looks to have some low level dry air to battle...but given decent
model agreement...stuck close to a model blend for probability of precipitation/quantitative precipitation forecast from 18z
Sat to 6z sun...with chance probability of precipitation working across areas north of I-94.

Sunday...we will see a fairly Stout Ridge build up The
Rockies...with an upper low getting cut-off over California. This
ridge and upper low will shift east during the first half of next
week. As the upper ridge moves into the Great Lakes...it will be
accompanied by a Dry Ridge of high pressure at the surface moving
into the lakes as well. The cut-off/closed low will stay on the west
side of the ridge as it moves east...with precipitation looking to slowly
move east across the plains next week. Still questions as to whether
or not this wave will make it to the upper MS valley...with the
European model (ecmwf) keeping the wave mainly to our west as it strengthens the
upper ridge through the Great Lakes. The GFS does not show it
getting here either...though that is because it shears it out
overhead as it attempts to crest the ridge. 850 mb moisture transport
from both the GFS/European model (ecmwf) starts moving into western Minnesota ahead of the wave
Wednesday night...with limited quantitative precipitation forecast continuing into Thursday across the
area...so allowed for the chance probability of precipitation Wednesday night/Thursday from the
blending procedure to remain...but given strength of the upper ridge
along with dry surface high remaining centered over the Great
Lakes...confidence is pretty low that we will be able to get any
measurable precipitation this direction.

Looking beyond...this bit of a pattern shift looks to remain in
place...with a western trough joining in with the eastern ridge by the last
weekend of September...so maybe by the end of September/beginning of
October we may slip back into a more active pattern. At the very
least...we look to have shaken northwest flow for a little bit after this
weekend.

&&

Aviation...(for the 06z tafs through 06z Friday night)
issued at 1048 PM CDT Thursday Sep 18 2014

Only small changes made to the 06z taf package. MVFR/IFR ceilings
are still expected to advect northward across western Minnesota during
the late night hours and persist through Friday morning before
lifting to low VFR for the afternoon/evening. Light showers are
becoming more likely during the morning as well at kaxn...krwf and
kstc with prevailing rain showers indicated. Backed off a little bit on
the extent of the morning stratus and fog for Keau and krnh as
middle/high level clouds are really starting to thicken over the
area. Still think there will be a few hours near sunrise that both
krnh and Keau experience MVFR br and LIFR/IFR ceilings. A few
thunderstorms are likely Friday evening from south central Minnesota
through west central WI. The one taf site in line for this is Keau
with thunderstorms in the vicinity indicated after 00z. The winds will be a hassle factor
late tonight and Friday as the pressure gradient tightens across
the area. South winds with speeds of 15 to 20 knots sustained are
likely with gusts 23-27 knots. The highest winds (20g27kts) will
be from krwf through kaxn.

Kmsp...still forecasting VFR conditions through 15z with MVFR
ceilings and some light showers in the 16z-20z time frame. South
winds by daybreak reaching 15 knots with gusts 23 knots. The
sustained winds and gusts increasing 3 knots or so for the late
morning and afternoon hours. Highest confidence for scattered thunderstorms and rain
Friday evening (00z-06z) is from the south metropolitan to the Iowa
border.

/Outlook for kmsp/
Sat...VFR. Morning MVFR ceilings possible. Winds northwest 10-15 kts.
Sun...VFR. Winds northwest 10-15 kts.
Monday...VFR. Winds northwest 5-10 kts.

&&

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

$$

Short term...clf
long term...mpg
aviation...rah

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