Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 182050

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
350 PM CDT Mon Sep 18 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 350 PM CDT Mon Sep 18 2017

Showers are diminishing from west to east as forcing aloft from
an upper-level wave & the ascent region of a 100 kt jet move off
to our east. Should see the last of the precip move off into
central WI before midnight tonight so we`re looking at dry
conditions overnight across the area. Continued SE winds at the
surface & rather impressive moisture return at 850 mb has provided
plenty of isentropic ascent even behind the showers where a
widespread area of stratus has developed. This stratus will keep
our lows from temperatures from dropping much at all overnight &
should prevent any dense fog from forming despite the wet ground &
light winds.

Low clouds will stick around for much of the morning before
dissipating as we get into the early afternoon. Southerly flow
will increase during the day in response to a developing surface
low over the Dakotas which will allow temperatures to rise into
the upper 70s & dewpoints into the mid-60s. This along with the
clearing skies will allow instability to build during the day,
especially across western MN. MUCAPE will increase to 1000-2000
J/kg by Tuesday evening but capping aloft should limit any storms
developing during the afternoon. The cold front from the
aforementioned low will move into western MN at the beginning of
the long-term period along with the potential for strong/severe

.LONG TERM...(Tuesday night through Monday)
Issued at 350 PM CDT Mon Sep 18 2017

A few concerns/questions for this afternoon discussion.

1. Severity Tuesday night along a sharp short wave moving northeast.
2. Anomalous 85H temperatures and associated high temperatures.
3. Anomalous PWATs this weekend/early next week.
4. Upper Ridge strength and/or movement this weekend.

The combination of increasing wind shear values /both directional &
speed shear/ develop early Tuesday evening in western Minnesota, in
advance of a sharp/fast moving shortwave, will likely generate a few
strong to severe storms. Location, and the amount of instability
remains questionable Tuesday evening, but the best chance of MLCAPES
of 1500-2000 J/KG occur in west central Minnesota before sunset.
Other areas of central and southern Minnesota will likely have lower
MLCAPES, along with weaker wind shear. Please monitor the latest SPC
information for more updates.

As with yesterday discussion, the upper ridge strength and location
across the Great Lakes is very important in terms of temperatures
and precipitation chances late this week and into the weekend. Even
if the placement of the upper ridge is either further to the west or
east, our region will likely see a strong contrast between the
thermal boundary, and hence surface temperatures.  The most likely
scenario based on model biases of lifting systems /or ejecting
systems/ too fast across the plains, is to increase temperatures in
the southeast one-third of MPX forecast area. This is roughly south
central Minnesota, and portions of east central Minnesota, as well
as west central Wisconsin. A scenario of highs in the 90s in south
central/southeast Minnesota, with only 50s in west central Minnesota
is a very good possibility for Friday. The only concern is morning
convection /Friday morning/ that may linger longer and keep full
isolation from developing. The morning convection is based on an
increasing low level jet Thursday night across southern Minnesota in
advance of the next system moving out of the Northern Plains.

Finally, even if this system slows down, the orientation of the
mid/upper level flow, combined with anomalous PWAT values 2-4
standard deviations above normal, the chances of very heavy rainfall
exists. Plus, if these anomalous PWAT values materialize, training
of storms, due to the directional flow, will cause copious amounts
of rainfall. I woudn`t be surprised to see broad amounts of 2 to 5
inches of rainfall across portions of MPX forecast area.

Once this system moves off to the east/northeast, does our weather
become cooler and less active.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 106 PM CDT Mon Sep 18 2017

Showers have ended at AXN, RWF, & STC and will continue taper off
from west to east this afternoon. Little in the way of visibility
or ceiling restrictions has been seen this morning & expect this
to continue in the SHRA. Low stratus has already begun to move in
behind the rain in western MN where borderline IFR/MVFR ceilings
are present. Expect this stratus to spread across all terminals
into this evening but the question will be how far east the IFR
ceilings get. Ensemble guidance shows it advancing as far east as
the Twin Cities, so most confident in IFR ceilings tonight at AXN,
RWF, & STC and will have to monitor MKT & MSP in future
forecasts. SE winds throughout the period around 10 kts with some
gusts developing tomorrow afternoon.

Expect shower activity to clear the terminal by 22Z. Ceilings will
lower behind the showers this evening, but expecting ceilings to
remain MVFR overnight. However MSP will be right on the eastern
edge of the more widespread IFR ceilings so can`t totally rule
out a drop to IFR later tonight.

Tue...Mainly VFR. Chance MVFR ceilings. Winds SE 10-15 kts.
Wed...Mainly VFR. Chance TSRA/MVFR early. Winds W 5-10 kts.




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