Area Forecast Discussion
Issued by NWS Twin Cities, MN

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000
FXUS63 KMPX 182341
AFDMPX

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

.UPDATE...For 00z Aviation discussion below

&&

.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
thunderstorms.

.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 00Z TAFS through 00Z Tuesday evening)
Issued at 642 PM CDT Mon Sep 18 2017

Expect MVFR ceilings to continue to degrade to IFR this evening,
and stay reduced through Tuesday morning. Models indicate gradual
improvement to VFR between 15z and 18z. Stratus will prevent
dense fog from forming overnight, but could see some 4-5SM
reductions. Southeast winds prevail through the period, and become
gusty during the late morning and afternoon hours on Tuesday.

KMSP...
Ceilings now look to lower to IFR and remain there overnight.
Expect to stay at least MVFR through most of Tuesday morning, with
improvement to VFR Tuesday afternoon.

/OUTLOOK FOR KMSP/

Wed...Mainly VFR. Chance TSRA/MVFR early. Winds W 5-10 kts.
Thu...VFR. Winds SE 10 kts.
Fri...VFR. Chc MVFR/TSRA late. Winds S 10-15 kts.

&&

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

$$

SHORT TERM...ETA
LONG TERM...JLT
AVIATION...LS



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