Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 202013

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
313 PM CDT Sun Aug 20 2017

.SHORT TERM...(This evening through Monday)
Issued at 312 PM CDT Sun Aug 20 2017

Uncertainties are abound in this forecast, with how far north we
will see convection tonight along with coverage of any storms Monday
afternoon. Then you throw in the eclipse and the added pressure on
getting a good handle on sky cover (which we don`t) and you get a
forecast where there isn`t as much confidence in as one one like
over the first 24 hours.

Frontal boundary at 3pm stretched from near Fairmont, MN to the Rice
Lake, WI area. Temps are in the low-mid 80s with dewpoints around 70
ahead of the boundary. This is resulting in a rather unstable
environment, with the SPC meso analysis showing around 3k j/kg of
sbCAPE along the boundary. But the 12z MPX soundings shows why we
just have not been able to sustain any surface based convection
today and its is the relatively stable layer of air between h85 and
h7. Convective trigger temp on the MPX sounding was 88 and we are
just not going to quite get there. Still, given the degree of
instability, can not rule out something like what the 12z NSSLwrf
and HRRR show happening around 00z, with a line of storms developing
along the front out toward Rochester over toward EAU, so have pops
for this evening limited to this area.

For tonight, there will be a focus for convection to the northeast
of the surface low developing along the Neb/KS border. Lots of
spread with where storms develop tonight as there are a couple of
regions of focused moist ascent. One along the 85-h7 theta=e
gradient from about Redwood Falls to the Twin Cities, and to the
north of the surface warm front, which will be down in IA. Favor the
idea of convection north of the warm front being the main show
tonight, with an MCS tracking across IA. This should really limit
the amount of moisture return we see into MN, so not really buying
into the convection the HRRR shows for south central MN, with the
HopWRFs and NMM/ARW looking a bit more realistic.

What happens with this convection will be key to what we see for
potential storms and severe weather Monday. The front working across
the area now will be stalling out from south central MN up into
central WI. We`ll likely again see dewpoints pool up to around 70,
with continued steep mid level lapse rates, so we will again see a
large degree of instability. Forcing looks a bit better Monday, with
the upper jet strengthening to our north, with right entrance region
setting up over southern MN. In addition, we`ll have a strong trough
dropping down from Canada, with mid-level flow picking up in
response. Favored pops tomorrow along where the NAM and GFS show the
CAPE gradient setting up, which is from about St. James, MN to
Ladysmith, WI. If we get storms to go, CAPE/shear parameters
certainly pose a severe threat and agree with the northward shift to
severe probabilities made to the updated Day2 convective outlook.

For clouds and the eclipse, with incoming trough and jet, along with
potential for overnight convection nearby, think it is fair to say
that we will be hoping from some breaks in the cloud cover as
opposed to worrying about the passing of the occasional cloud.

.LONG TERM...(Monday night through Sunday)
Issued at 300 PM CDT Sun Aug 20 2017

Showers and thunderstorms may be ongoing near the front across
southern MN into western WI at the beginning of the long term
period early Monday evening. The strong/severe threat will likely have
shifted southeast already with an abundance of development ahead
of the front. Any lingering activity will be pushed well to the
southeast by the early overnight hours as much drier air arrives.

A deep trough across the eastern U.S. and surface high pressure
over the Upper Midwest will bring another round of below normal
temperatures for much of the week. Highs in the upper 60s to mid
70s and lows in the 40s and 50s are expected.

The next system approaching from the west will spread warmer and
more humid air northward across the Plains late week, arriving
here next weekend. High chance PoPs continue for this feature due
to relatively good consistency between the models.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 102 PM CDT Sun Aug 20 2017

Difficult forecast ahead, mainly with uncertainties with how
convection will evolve tonight. Cold front is working through the
Twin Cities/Mankato at 18z and will be over to EAU by 00z.
Atmosphere along/ahead of the front looks to remain capped much
of the day, but the cap may break down around 00z out by EAU and
this would be the only site that has a risk of TSRA this
afternoon. Tonight, confidence is highest in a large MCS
developing to the north of the warm front over IA. How far north
this convection ends up is uncertain, but not buying how far
north the HRRR develops sct storms this evening (RWF to MSP).
Think once a mature cluster gets going in IA, that should greatly
limit amount of moisture making it into MN, keeping the greatest
TSRA threat tonight to MKT and points south.

KMSP...An elevated instability gradient will be laid out across
the Twin Cities tonight, and this is what the HRRR and RAP are
picking up on for the scattered storms they have around MSP
between 7z and 12z. As mentioned above, threat for storms looks
much better south of MSP, so kept tonight dry for now. Will see
the IA warm front push back into MN again Monday afternoon and
will likely serve as the focus for storms. Once again though, tsra
chances look much better south and southeast for MSP Monday
afternoon, so kept the TAF dry through 30 hours.

TUE...VFR. Wind NW 10 kts.
WED...VFR. Wind N 5 kts.
THU...VFR. Wind SE 5 kts.




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