Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 202007

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
307 PM CDT Fri Oct 20 2017

.SHORT TERM...(This evening through Saturday)
Issued at 304 PM CDT Fri Oct 20 2017

It`s another beautiful, though breezy fall day, with widespread
temperatures in the 70s as we continue to reside in a warm southerly
flow to the east of a 988mb over eastern MT that is on its way
toward the Canadian Prairies. Water vapor imagery is showing upper
level moisture streaming north across the central CONUS. Within this
moist southwest flow to the west of a trough digging across the
Rockies, there is a strong short wave moving into southwest KS. The
NAM/ECMWF/HRRR show showers/storms developing after midnight over
central Iowa and moving north into eastern MN/western WI through the
morning Saturday. Trended the forecast this direction with likely
pops coming into south central MN around 9z that work NNE across
western WI through the morning. There will be other weak shortwave
energy working north across the area out ahead of this wave which
may result in a few stray showers developing across eastern
MN/western, but moisture quality looks lacking until this wave in KS
gets here, so slowed down considerably the arrival of mentionable
precip tonight. Beside the late arriving precip, we`ll see
increasing clouds through the night, with no real decrease in wind
speeds expected. Therefore, we warmed lows overnight several degrees
over what we had, close to a blend of the raw deterministic models
and CAMS. This netted lows in the 60s for most of our MN area, which
is in the ballpark of 5 degrees above our average highs.

Behind the morning wave, we will likely get a several hour break in
precipitation as we await the arrival of the cold front and
associated upper trough. That looks to cross into western MN around
18z. Still some uncertainty in the CAMS with when convection
develops along the front and how widespread it will be, but we did
slow down the eastward progression of the categorical PoPs to be
more in line with the NAM/ECMWF, which are a little slower than the
GFS. For the severe threat, there`s certainly nothing about the CAMs
that screams there being much of a severe threat with this and think
the reasoning from the previous discussion still holds up in that
the widespread forcing will result in lots of weak updrafts as
opposed to a few strong ones. Still, 50kt winds will be residing
down to 3k AGL, so it is plausible that we could mix some of that
wind down to the surface with any stronger downdrafts and the
marginal risk from the SPC day 2 outlook looks reasonable.

.LONG TERM...(Saturday night through Friday)
Issued at 304 PM CDT Fri Oct 20 2017

The medium range guidance is in decent agreement with respect to
the big picture through the forecast period. Overall, we`ll see an
upper trough establish itself over central and eastern portion of
the continent with ridging over the west coast. However, it won`t
be a steady state by any means, with the pattern sort of re-
establishing itself a few times as successive shortwave troughs
(and associated frontal boundaries) drop into the central/eastern
CONUS. This results in some marked differences in the details of
the forecast, but an overall trend toward cooler temperatures with
periodic chances for precipitation is the main theme amongst all
of the guidance. A consensus approach looks to be the best coarse
of action at this point given the aforementioned disagreements in
model details, so did not stray too far from an initial consensus

We`ll see some lingering shower/thunderstorm activity over the
eastern portion of the area Saturday evening, with most of it
expected to be out of our area by around midnight as the frontal
boundary continues to push east. We`ll see a brief period of cold
advection before return flow quickly sets up by Sunday evening.
Fairly good warm advection then looks to persist into early Monday
before we see a more potent cold front push through the area. Some
light precipitation will be possible with this boundary, but there
doesn`t look to be much moisture return and the better large scale
forcing will be north of our area. Much cooler temperatures will
then work into the area Monday night into Tuesday. However, this
will once again be short-lived as warm advection ahead of the next
upper wave looks to develop from west to east late Tuesday afternoon
and evening. It is at this point that the differences in guidance
become more apparent. The GFS is much more progressive with a
northern stream shortwave and drags a cold front back across the
area by late Wednesday evening. The ECMWF, on the other hand,
develops a surface low over the northern Plains, and doesn`t push
the cold front through our area until Thursday afternoon and
evening. The Canadian solution is closer to the GFS, but slightly
slower. All of the solutions would cool the profile sufficiently for
some mixed precipitation behind the front, but it`s tough to say if
there will be much if any post-frontal precipitation at this point.
At this point, just have a period of chance PoPs from late Wednesday
night into Thursday night, which are likely to be increased but
shortened in duration as things get into better agreement. Some
mixture with snow toward the tail end of that time window is
mentioned over the eastern portion of the area, but doesn`t appear
as though it would be of any significance at this point. We should
see things push sufficiently east by Friday to end any lingering


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1247 PM CDT Fri Oct 20 2017

Our run of VFR conditions and dry weather will come to an end
during the course of this TAF period. For MVFR cigs, favored the
forecast of their return more in line with the slower GFS since they
are just now approaching the I-20 corridor in northeast TX, so
will take a while for that moisture to get here. Do like the
NAMnest idea of a cluster of shra/tsra coming out of Iowa after 9z
tonight as short wave currently in the CO/NM/TX/OK/KS borders
region lifts northeast toward the Twin Cities. SHRA/TSRA chances
look to increase across central MN after 18z Saturday ahead of a
cold front that will be moving across MN.

KMSP...Was conservative with the arrival of MVFR cigs and it is
possible we could see MVFR conditions as early as 11/12z. Also
kept cigs above 017, though depending on how much rain we see in
the morning (and the moistening of the lower atmosphere seen
because of it), we could certainly see cigs below this level by
the afternoon.


Sun...VFR. Winds WSW 10G20 kts.
Mon...VFR. Winds W 15G25 kts.
Tue...Chc MVFR. Wind NW at 10G20 kts




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