Area Forecast Discussion
Issued by NWS Twin Cities, MN

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302
FXUS63 KMPX 202317
AFDMPX

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

.UPDATE...
Issued at 604 PM CDT Fri Oct 20 2017

Updated to include 00z aviation discussion below.

&&

.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
blend.

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 precipitation.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 604 PM CDT Fri Oct 20 2017

VFR conditions will generally prevail into the early morning hours
then ceilings will steadily lower through daybreak in advance of a
cold front approaching the area. Precipitation looks to hold off
until close to daybreak, then move through in a hit-and-miss
variety, and in this batch of precip there will be a combination
of lower-end VFR and upper-end MVFR ceilings. After a break in the
precip, a more organized line of -RA/TSRA will push through in the
afternoon which will likely keep ceilings in MVFR range.
Visibilities will be more tricky due to rainfall rate so have run
with an optimistic 6SM but lower ceilings would not be surprising.
Breezy conditions are again expected through much of the period,
with directions mainly southerly through tomorrow afternoon then
winds will shift to WNW tomorrow evening behind the frontal
passage.

KMSP...Have made very few changes from the 20/18z TAF as timing
among the models looks similar to what the TAF had going. Have
maintained ceilings above 1700ft but visibilities could easily
drop into MVFR or IFR range.

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

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
&&

$$

UPDATE...JPC
SHORT TERM...MPG
LONG TERM...
AVIATION...JPC



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