Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Twin Cities/Chanhassen Minnesota
425 am CDT Friday Aug 22 2014
Short term...(today and tonight)
issued at 420 am CDT Friday Aug 22 2014
Quite the battle going on across the mpx area this morning between
stratus to the north...cirrus in the south...and dense fog which can
be found in between. Surface analysis at 3 am showed the somewhat
diffuse surface low that tracked across the Twin Cities Thursday evening
was centered near Menomonie...WI. Stratus that remained in place
north of the warm front yesterday is still there this morning and
has been dropping southeast behind the surface low into the Twin Cities.
Around the periphery of the stratus...dense fog is the issue until
you hit a cirrus shield in southern Minnesota originating from Iowa
convection. Just expanded the initial advisory east toward Owatonna
and Faribault...and thinking that should be all the more expansion
needed as the cirrus will continue to keep visibilities up along the I-90
corridor...while low stratus will continue to dominate the scene
northeast of the dense fog advisory.
For the rest of today...the surface low will drift toward
Milwaukee...with the mpx area getting into some surface ridging and
divergent low level flow. Beside the divergent low level flow...we
will have slowly building 500 mb heights throughout the day. So although
we will not be kicking the moisture out of here...with dewps still
staying in the upper 60/lower 70s...the combination of the diffluent
low level flow with subsidence induced by the rising heights aloft
will contribute to keeping today a dry...though muggy day.
For tonight...the front currently working into Iowa will make it
down into southern Iowa this afternoon...then begin surging north as a
warm front. With that said...it does not look like it will be
surging north fast enough to impact the mpx County Warning Area overnight...so cut
back probability of precipitation considerably for tonight...leaving some small chance probability of precipitation
in for after 9z SW of the Minnesota river. The majority of models show the
925-850 moisture transport coming north into western Iowa...then backing
hard to the west...going toward eastern Montana in response to a
deepening surface low in the vicinity of the Colorado/NE/KS border. The one
model that sticks out as an outlier at this point is the GFS...which
has the surface low by 12z Sat way out near Yankton as opposed to the
Colorado/NE/KS border region...so this forecast leaned much more toward a
non-GFS consensus for probability of precipitation.
Long term...(saturday through thursday)
issued at 420 am CDT Friday Aug 22 2014
An active period is still forecast for much of the long term as
energy ejects periodically from the base of a trough over The
Rockies. Concern with these systems will be heavy rain due to a
strong connection to the eastern Pacific and its several tropical
The most potent system of the long term will take shape over the
Central Plains Friday night and track north across the eastern Dakotas
through Saturday night. This system will evolve much like that of
an autumn-like cyclone with a well defined warm sector and very
widespread stratiform precipitation to the north and west of the
low track. The warm front extending to the east...aligning with
the Minnesota river Saturday...will be the focus for a secondary
batch of showers and thunderstorms. Strong east to southeast
boundary layer flow north of the front...veering southwest in the
middle levels will provide adequate shear for rotating storms.
However...instability is not forecast to be all that impressive
with residual cloud cover from Friday night convection. This will
be the main limiting factor for severe. Storm coverage will
decrease with eastward extent as forcing becomes less and less
influential. Decreased probability of precipitation to the chance category for these areas.
As the low pushes north and a sserly low level jet strengthens
Saturday evening...expecting to see showers and thunderstorms
become fairly widespread along the cold front heading into western Minnesota.
Heavy rainfall will again become a concern with any training
cells. Rainfall likelihood diminishes again with eastward extent
with capping and dry middle level air. Reduced probability of precipitation again over the
eastern half of the County Warning Area.
The warm sector will be firmly in place on Sunday east of the
thunderstorm activity from the night before. GFS seems too
progressive with a positively tilted trough and thus the warm
sector and thermal ridging are shunted much further east than all
the other models. The NAM...Gem...and European model (ecmwf) all have the thermal
ridge over eastern Minnesota and western Minnesota Sunday afternoon but differ on
intensity. Tossed out the much too hot NAM solution which would
indicate highs near 100f across the eastern half of the County Warning Area. Seems a
bit unlikely with a decent possibility of cloud cover lingering
through the day. Despite its recent performance...the European model (ecmwf) is the
preferred solution with the Gem close behind. Should be a very
warm and humid day with heat indices flirting near heat advisory
The front will swing through Sunday night as the cyclone finally
makes a more meaningful push east. Thunderstorm chances along the
front do not look particularly good with the parent system well to
the north...but there should be sufficient shear and instability to
pop isolated to scattered cells in the evening.
The forecast for the rest of next week will depend how far south
this front sags. Additional disturbances will develop shower and
thunderstorm clusters along and north of the SW-NE oriented front
through Wednesday. Continued to mention medium range probability of precipitation...
highest south during this period. Temperatures may actually be too
warm if we do see an all day rain scenario set up...in which case we
may only be able to muster highs in the 60s.
Aviation...(for the 06z tafs through 06z Friday night)
issued at 1123 PM CDT Thursday Aug 21 2014
No major changes for the 06z taf...still bring in light fog/mist
and low stratus. We trend a little more toward low ceilings rather
than reduced visibility from fog. The stratus currently in central
Minnesota and northern WI should continue to spread south tonight and
lower with time.
High confidence in ceilings below 1500ft before the morning
commute...ceilings could be as low as 300ft. We think there will
be some fog...but the ceilings will be the biggest concern. Little
wind is expected this morning and the ceilings will be somewhat
slow to clear given the weak flow in the wind fields today.
/Outlook for kmsp/
Sat...MVFR with IFR/+tsra possible. Winds east at 10g15 kts.
Sun...VFR with MVFR/thunderstorms and rain possible. Winds northwest 10 kts.
Monday...VFR. Winds variable 5 kts.
Minnesota...dense fog advisory until 10 am CDT this morning for mnz041-047-