Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
655 PM CDT Friday Aug 28 2015
Short term...(this evening through saturday)
issued at 244 PM CDT Friday Aug 28 2015
The mpx area this afternoon sits between an area of low pressure
slowly moving east across Iowa and a low pressure trough that
sits from nodak back into northwest Ontario. We have seen about a 40 knots
low level jet build north to the eastern Iowa/Minnesota border...which has allowed light
rain to build about as far north as Eau Claire...but no further
northward expansion is expected. Cool cloud tops associated with
the precipitation shield on the nose of the low level jet and within a zone of
850 mb-h6 fgen is moving east-northeast at about 12 miles per hour. Based on current
speed...the rain should clear the Eau Claire shortly after 21z and
looks to completely be out of the mpx area after 03z. Only real
changes needed to the pop/weather forecast were to one...speed up the
departure of the rain this evening and two...remove thunder...as
instability has remained confined closer to the surface low down near
Other area to watch for precipitation is the trough to our north. 20z
Storm Prediction Center mesoscale analysis shows MLCAPES between 2000 and 3000 j/kg have
developed ahead of the boundary from eastern nodak across the
international border. The cams have been fairly consistent with
developing a scattered line of thunderstorms along this boundary late
this afternoon from Fargo to east of inl up in The Arrowhead.
The question with this activity is how far south does it make it
tonight. For now...have kept the forecast dry across central Minnesota
through tonight...but will have to see what happens with this
convection to the north this evening...as some decaying showers
moving into central Minnesota between 6z and 9z can not be completely
The bigger issue for tonight though will be the threat for
widespread dense fog. Tonight...the mpx area will find itself in a
cull between the low moving across Iowa the trough to the north.
Clouds are also slowly clearing out from west to east and combined
with light winds...we will see excellent radiational cooling
conditions setup. Guidance is in good agreement with lows dipping
down into the middle 50s...which is 5 to 10 degrees lower than our
dewpoints this afternoon...so we will be primed for dense
fog/stratus. There will be a Sweet spot though for getting fog
tonight...to the east of west central Minnesota...where south winds look
to remain elevated enough to keep the atmos mixed and fog at
Bay...and to the west of where the upper cloud shield associated
with the wave down in Iowa right now ends up. Right now...the
cloud shield looks to get about as far east as an Albert
Lea/Hastings/Bloomer line. Between the cloud line east and the
mixing west we did beef up the fog mention.
After the fog Burns off...the rest of Saturday will be perfect
with mostly sunny skies and light south winds. 850 mb temperatures around
24c...which would support highs around 90 will remain in the
central Dakotas...with 850 mb temperatures here between 16c and 20c...which
corresponds to highs in the upper 70s/lower 80s.
Long term...(saturday night through friday)
issued at 244 PM CDT Friday Aug 28 2015
Generally warm and uneventful extended portion of the forecast
for this issuance.
Upper level ridging currently over the western Continental U.S. Will steadily slide
eastward over the weekend and settle over the eastern Continental U.S. For much of
the upcoming week. Increased 500 mb heights will allow for a slow warming
of temperatures to unseasonably warm levels for the end of August
into early September. Highs will commonly reside in the middle-upper
80s with lows in the middle 60s to around 70. There may be a high-level
thin layer of smoke over the weekend incoming from the western Continental U.S.
Fires but nothing that should impact air quality.
As for sensible weather features...the deep ridging aloft originating
from the southwestern Continental U.S. Will also allow warmer high pressure to take up
residence at the surface. This will prevent an approaching cold front
from the Canadian rockies through the northern plains to exit the Dakotas.
As such...any synoptic scale systems will not make it to this
region...thus mitigating anything other than airmass pop-up showers
thunderstorms should any weak midlvl shortwave disturbances make it this
far east for much of the week. Any probability of precipitation have been capped at no
higher than 20 percent.
Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 648 PM CDT Friday Aug 28 2015
The only concern with the aviation forecast overnight is the
potential for IFR visibilities with fog late tonight. We didnt
change much from the 18z taf that had fog developing late tonight.
High clouds are the wild card tonight and if they become
widespread enough...fog formation will be limited. Otherwise
tonight is a decent set up for fog development.
The Airport is usually fog resistent...but the fog late tonight
could be a little more widespread that what we typically see. We
will monitor the trends late tonight. As mentioned
above...advancing middle/high clouds and even some of that advancing
smoke evident on satellite could keep US a few degrees warmer
tonight and limit the fog formation.
/Outlook for kmsp/
Sun-Tue...VFR. Winds S 5-15 kts.