Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
659 PM CDT Sat Oct 25 2014
Short term...(this evening through sunday)
issued at 409 PM CDT Sat Oct 25 2014
Pretty quiet short term is expected as high pressure currently
over the Dakotas will make its way over to the Ohio Valley by the
end of this period. For winds...we will see our breezy northwest
winds die off pretty quickly at sunset...go light and variable
overnight as the high moves overhead and setup out of the
southeast on Sunday. Will see winds get pretty breezy out in western
Minnesota as well as gradient tightens up and 850 mb winds increase to
between 20 and 30 kts.
We have seen dewpoints mix out into the 20s this afternoon...so we
should see a pretty rapid drop in temperatures tonight given the dry
airmass and light winds as the high moves overhead...with lows
mostly in the 30s being common tonight. We will see good warm air advection at 850 mb
during the day on Sunday...but we will not realize those
temperatures with southeast boundary layer winds...still 925 mb temperatures look
more than warm enough to support highs making a run back into the
70s out in the upper Minnesota River Valley.
Have also maintained the dry forecast...despite the healthy 850 mb
Theta-E advection we will have moving in. Isentropic sfcs between
300k and 310k do not moisten up until after 00z Monday. Beside
isentropic moistening...both the GFS/NAM do not show modest elevated
instability developing until after 00z Monday...further supporting any
precipitation Sunday holding off until the evening. Still given the degree
of Theta-E advection...would not rule out seeing some light showers
over eastern Minnesota after 21z. The 12z European model (ecmwf) hints at this possibility with
the quantitative precipitation forecast it generates over La Crosse between 18z sun and 00z Monday.
However...with the sref precipitation probs non existent and the nmm/arw
reflectivity forecasts coming in dry as well...felt comfortable
keeping the dry forecast going.
Long term...(sunday night through saturday)
issued at 409 PM CDT Sat Oct 25 2014
The coming week will feature a rather active pattern for the
northern states... with upper troughs moving through every couple of
days. Other than Sunday night...there will normally not be enough
significant low level moisture moving northward into our area.
Models are in decent agreement in terms of overall trends...and
details are also in good agreement...at least through Wednesday night.
Main difference late next week seems to be the 12z European model (ecmwf) is a
little faster...but this is relatively minor difference with other
First system to affect the area will arrive Sunday with broad area
of lift. First weak short wave arrives Sunday afternoon but a more
substantial one arrives later Sunday evening. An unusual amount of
moisture for late October reaches our area... with precipitable
water values generally 1.O to 1.3 inches from SC Minnesota into wc wisc.
Low level jet increases to about 35 knots...lifted indices go
negative aloft...and cape spikes above 1000 j/kg. Thunderstorms are
anticipated to develop Sunday night in eastern Minnesota and western wisc.
Might even be some small hail here and there.
Pacific front moves through late Sunday night and Monday
morning...with front arcing from low in southern Manitoba...into
Wisconsin and then looping back to the SW into Kansas. Next short
wave on southern periphery of trough will help bring precipitation from the
Central Plains into Minnesota/WI Monday late morning into the evening.
Strong middle level winds with 500 mb maximum on the order of 60-70 knots will
move into western Minnesota midday Monday and help set off the next batch
of showers...especially from SW/SC Minnesota into wisc where the best lift
will match up with more significant low level convergence.
Wrap around showers are possible Tuesday as upper trough tracks
east...especially for central Minnesota into wisc. Weak ridge moves across
on Wednesday before next trough drops out of Canada Thursday into
Friday. Showers will once again be possible...especially from
central Minnesota into wisc. Probability of precipitation are meager for now since it looks like
best lift will be just to the north and east. But if trough encroaches
a little more into our area...there may be a shot at some wet snow
mixing in with any light rain showers Thursday evening. Looks
like decent ridge arrives after that... with a brief warmup during
the latter part of next weekend. Active progression of troughs
looks to persist beyond the seven day time frame...not an unusual
pattern for the autumn.
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 615 PM CDT Sat Oct 25 2014
High confidence in VFR tonight and tomorrow. Winds will ramp down
this evening and could even be calm for a time tonight before
increasing out of the southeast by late tomorrow morning. By the
end of the taf period...00z-Monday...light showers are possible in
eastern Minnesota and western WI. It is also entirely possible most of
the light rain will pass to the east.
No weather impacts through the taf period. Wind direction will be
tough to pin down late tonight with high pressure moving right
overhead. We do think the winds will finally establish a southeast
direction by 14z. Rain is not impossible by the end of the taf
period...but the chances don't look great either - maybe some
sprinkles out of a middle deck of clouds.
/Outlook for kmsp/
Monday...MVFR ceilings likely early. Chance rain showers. Winds becoming west-northwest 5-10 kts.
Tuesday...chance MVFR/shra. Winds west-northwest 15g25 kts.
Wednesday...VFR. Winds SW 5 kts.