Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
614 am CDT Friday Aug 29 2014

Short term...(today and tonight)
issued at 338 am CDT Friday Aug 29 2014

Rather ugly morning out there in terms of aviation...with plentiful
amounts of low ceilings and fog out ahead of a warm front that was
starting to make progress north into Minnesota and at 3 am was near the
I-90 corridor. Beside the low clouds...isentropic lift and elevated
instability advection associated with a modest 20-30 knots southwesterly low level jet
has allowed for an area showers/thunderstorms to move north-northeast across the
area. The hrrr has had a good Handel on this activity
overnight...and followed it fairly closely to time this mornings
batch of activity through the area. This batch of rain looks to be
mostly north of the mpx area by 15z...with a brief break in precipitation
expected until thunderstorms get going again later in the afternoon.

For the afternoon...we will be dealing with a rather messy surface
pattern as the mesoscale low currently near Sioux Falls washes out over
Minnesota as it gets absorbed by the main cold front...which was still back
across western nodak. Biggest change seen from the 28.12 to the 29.00
model runs was a wholesale slowing of the front/surface trough across
the area. This has resulted in higher probability of precipitation being pushed back west
this afternoon...as what was looking like was mainly going to be a
show for areas along/east of I-35 is now more of an along/east of a
St. Cloud to Fairmont line. The hi-res models that look to adhere
to what is seen forcing wise with the deterministic models the best
are the 29.00 nmm and mpx wrfs...which both show storms forming west
of the Twin Cities between 19-21z and moving east from there as a
broken line of storms. One thing that has not changed...is the most
widespread rain/highest amounts look to fall across southeast Minnesota/western WI as
the afternoon activity to the west of I-35 congeals into a broader
mass east of I-35.

Still not anticipating much in the way of severe weather today
either...as cloud cover looks to be extensive...which will limit the
degree of destabilization we see by the afternoon. In addition...middle
level lapse rates will remain rather weak...further pointing to
instability somewhat lacking when it comes to the severe weather
potential. Beside the instability...the surface pattern looks to be
rather diffuse...with no clear focus existing for thunderstorm
activity /see front Sunday for a clear focus for thunderstorm
development/. With that said...bulk shear will be up in the 30-40 knots
range ahead of the surface trough...sufficient enough to allow for a
multi-cell cluster or two to pulse up enough to pose an isolated
wind/hail threat today.

For highs today...there is admittedly some bust potential with the
current forecast of highs in the 70s as that is predicated on cloud
cover being dominate today. If we instead get some decent sun before
storms develop this afternoon...then highs in the Lower/Middle 80s will
be more common.

For tonight...shower and thunderstorm activity looks slowly clear
out to the east...with to much left outside of some drizzle in western
WI by 12z Saturday. However...the surface through/front will be very
slow in working across the area...with plentiful low level moisture
looking to hang around most of the night. This should allow for
another round of low stratus and fog to expand across the area
tonight...which in turn will help keep lows up mainly in the
middle/upper 60s.

Long term...(saturday through thursday)
issued at 338 am CDT Friday Aug 29 2014

The main story in the extended is the system expected Sunday
afternoon and Sunday night with the high likelihood of widespread
thunderstorms and the potential for severe weather. The rain
should clear early on Labor Day...leaving mainly dry weather
across the forecast area through at least middle week.

The trough expected to move across the northern Continental U.S. Sun-Monday and
bring widespread thunderstorms to the nation's middle section is
already an established circulation with a well-defined pv feature
in the Gulf of Alaska this morning. The trough will dig southeast
over the next 48 hours and develop more of a negative tilt before
lifting north toward Hudson Bay Monday night. The jet structure
supports widespread upper divergence and 500mb 12-hour height falls
between 80-100m slip across northern Minnesota Sunday night. Lower in the
atmosphere...850-700mb fgen increases in western Minnesota near or just
after 18z on Sunday. The boundary makes gradual progress to the
east from 18z Sunday until 12z Monday. A slowly deepening and
eastward-moving surface cyclone produces 4-6mb 6-hour mslp pressure
falls across the forecast area on Sunday. Convective initiation
should not be a problem...in fact...storms should already be
ongoing in the eastern Dakotas Saturday morning. Elevated
convection should transition to more surface based convection with
heating and the likely establishment of large line segments or
clusters of thunderstorms due to the large-scale forcing. The deep
layer shear should be good enough to organize storms into multi-cell
with embedded supercells /even if they exist in lines/. The
instability should be sufficient to support strong updrafts. It
looks primarily like a wind and hail threat in the afternoon and
evening hours across Minnesota. Both the NAM and GFS have 0-3km shear on
the order of 25-40kts from 18z-06z across the forecast area. The
severe threat diminishes in WI given the loss of instability
Sunday night and quickly northward lifting surface low. Beyond the
weekend...the trough flattens out and we're left with a relatively
dry and cool westerly flow pattern through at least middle week.

&&

Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 557 am CDT Friday Aug 29 2014

The rap caught on to the VFR conditions moving north out of
southern Minnesota pretty quick this morning...and followed it/S idea for
bringing VFR conditions up into eastern Minnesota/western WI. With the surface low
moving into SW Minnesota...will take a bit longer for axn to see
improvements. For thunderstorms and rain this afternoon...followed the trends of
hrrr/mpxwrf/nmmwrf...which are all pretty similar with their
handling of thunderstorms and rain this afternoon/evening. Storms look to develop
between 18z and 20z near/east of a rwf to stc line and move east
from there. Behind the afternoon storms...the pressure gradient
looks weak...with the main cold front and its dry air still hung
up northwest of the mpx County Warning Area. This means we will have another night
with vlifr/LIFR ceilings. Given low level winds...stratus is most
likely in Minnesota...but winds will be light enough in western WI to lead to
heightened threat for fog at rnh/eau.

Kmsp...VFR conditions moving up from the south look to be here to
stay for the rest of the morning given the fact we have not seen
ceilings fill back in across southern Minnesota/northern Iowa...so will keep ceilings VFR
until tonight. Frontal boundary for the afternoon is looking
slower in getting here...allowing confidence to be quite high in
msp seeing another round of thunderstorms and rain this afternoon. Tonight...again
there is high confidence in another round of IFR stratus...just a
question of how quickly does it come in and how low does it go.

/Outlook for kmsp/
Sat...VFR. Wind north-northeast at 5-10 kts.
Sun...VFR with MVFR/thunderstorms and rain late. Winds south-southeast at 10-20 kts.
Monday...MVFR/tsra ending early. VFR by aftnoon winds west 10-15kts.

&&

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

$$

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

National Weather Service Glossary of Abbreviations