Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 kmpx 210946 
afdmpx

Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
446 am CDT Sat Sep 21 2019

Short term...(today through sunday)
issued at 446 am CDT Sat Sep 21 2019

Multiple rounds of showers and thunderstorms are expected today, as
an upper level trough and surface cold front slowly work through the
region. Already seeing a band of showers and a few isolated
thunderstorms early this morning, and expect these to continue to
track north and east through daybreak on the leading edge of the
strongest low-level warm advection. Instability will remain low this
morning, but can't rule out a few isolated instances of thunder
embedded within this line of showers.

After the precipitation moves through this morning, it looks like
most of the area will see a lull in precipitation through at least
early afternoon as the dry slot of the surface cyclone moves over
the region. It'll still remain plenty moist near the surface
however, and widespread stratus is expected to develop below a
shallow inversion, along with some patchy drizzle/light rain. The
main question this afternoon will be whether any breaks are able to
form in this stratus later this afternoon, and provide enough
surface instability for storm to initiate along the surface cold
front as it crosses through the County Warning Area this afternoon. Convection-
allowing models have largely backed off on the coverage of these
thunderstorms across the area this afternoon, but a few (namely the
hrrr) have been consistent in developing a broken line of
thunderstorms by early afternoon across west-central Minnesota and
tracking these east across central Minnesota. Should skies clear out
some and these storms are able to initiate, a conditional threat for
severe weather will exist with 2000-3000 j/kg of SBCAPE and 50+ kts
of shear in place ahead of the cold front. A few supercells capable
of large hail and damaging wind would be possible given these
parameters, and can't rule out a tornado or two as well with 150-200
m2s2 of 0-1 km storm-relatively helicity in place in addition to
very low local heights. The best overlap of parameters looks to set up
mainly north of the I-94 corridor across central Minnesota and west-
central Wisconsin, so will have to monitor this area for areas of
clearing and convective trends later this afternoon. Isolated heavy
rainfall is possible as well with any storms that develop this
afternoon, given precipitable water values approaching 1.75". Expect the heaviest
rainfall and main flooding risk to remain northwest of the area
across ND and northwest Minnesota, but some localized amounts of around an inch
are possible across central MN and west-central WI.

The front moves east into central Wisconsin Sunday night, with
cooler and drier air filtering into the region through Sunday as an
area of high pressure slides into the region. One more muggy day is
expected today with highs in the 70s to low 80s depending on
clearing this afternoon, and dewpoints again around 70s. Fall
weather returns on Sunday with lows dropping into the 50s, and highs
in the mid to upper 60s.

Long term...(sunday night through friday)
issued at 446 am CDT Sat Sep 21 2019

The parade of upper lows and short waves continues this coming week,
with the result being southwest upper flow, then northwest, then
back to southwest by late in the week. Many gefs and European model (ecmwf) ensemble
members are in decent agreement with this overall picture, with the
deterministic European model (ecmwf) being a bit slower.

Sunday night starts out with the upper low still affecting the area,
then exiting to the east overnight. The GFS is faster by a few
hours, but the only impact of this will be the timing of some higher
level clouds.

Ridging takes place Monday with the next short wave beginning to
move in from the west-northwest. Winds at 250mb strengthen to 130-
140 knots. The European model (ecmwf) is somewhat more amplified with this wave,
hence its tendency to be slower. Will need to continue precip
chances Tuesday afternoon through Wednesday owing to the slight
timing differences of the models. Temperatures will also be quite
warm Tuesday prior to the arrival of the short wave, with 70s
common, and even some lower 80s in our western counties. Will not be
surprised if it is even a little warmer.

After that GOES by, the area will be in northwest flow and cool
temps for a short time before upper pattern takes on a southwest
flow once again. This transpires as a strong upper low is projected
to drop southward along the West Coast. An upper high in the
southeast will also shift westward late in the week, and so Minnesota/WI
will be in a prime area for very moist air to be brought into the
area from the southwest. There are even hints of a couple
tropical plumes merging over the Central Plains and upper Midwest
- one from the western Gulf and the other coming across Mexico. If
that happens, there could be rather heavy rains late next week
for a day or two. We will see, but many ensemble members have been
consistent in showing this for at least a couple days now.

&&

Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1037 PM CDT Fri Sep 20 2019

All sites to start out as VFR this set but the complications begin
after midnight with the timing of potentially multiple rounds of
precip from around daybreak then thru the day Saturday. As a cold
front slowly pushes east over the Dakotas tonight, rain showers/thunderstorms and rain will
develop over western Minnesota overnight then steadily push east,
reaching eastern Minnesota around daybreak then continuing into western
WI during the late morning hours. Conditions will likely drop to
MVFR levels with the morning round of precip, and this round will
have the highest potential for cb/ts. Once this round moves thru,
then there may well be a break in the action and not just lesser
coverage. In addition, IFR ceilings look likely to develop during
the day tomorrow in close vicinity of the cold front. Additional
showers may reach the terminals tomorrow afternoon but intensity
is expected to be less and cumulonimbus/ts chances are much lower.
Conditions look to improve around tomorrow evening and beyond.

Kmsp...two main rounds look to push thru msp, with best timing
around and shortly after 12z then a second round late tomorrow
afternoon. Chances for ts/cumulonimbus higher for the morning precip, so
have omitted mention for the afternoon tempo at this point. Breezy
conditions also likely with slowly veering wind direction.

/Outlook for kmsp/
sun...VFR. Wind west 5-10 kts.
Mon...VFR. Wind light and variable.
Tue...VFR. Wind SW 5-10 kts.



&&

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

National Weather Service Glossary of Abbreviations