Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 180501
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1201 AM CDT SUN SEP 18 2016

.SHORT TERM...(This evening through Sunday)
Issued at 301 PM CDT Sat Sep 17 2016

Clouds are slowly clearing from west to east this afternoon but
should be east of the CWA by early evening. Clear skies tonight
and a weak wind field, coupled with crossover temps being bet by
5-10 degrees and an increasing hydrolapse with height, should
bring areas of fog to eastern Minnesota and western Wisconsin
overnight. Winds will be a bit stronger across western and
southern Minnesota and may largely prohibit fog development there.

Ridging will build in behind the current trough and ahead of the
advancing trough from the northern Plains Sunday. A thermal ridge
with 925 mb temps of +21 to +26C will push into the area by
afternoon, warmest across western MN. Mid/high level cloud cover
will increase across MN in the afternoon, however, with the better
heating across WI. Therefore, the high temp gradient probably
won`t be as extreme as mixing purely from 925 mb would indicate.
The official highs are fairly close to MOS guidance.

The trough axis/cold front will be located from north central MN
to eastern Nebraska Sunday evening. Cannot rule out a stray shower
or storm firing along it, but boundary layer moisture is lacking
and should keep the cap intact. PoPs are non-zero, but probabilities
are too low to mention any TSRA explicitly.

.LONG TERM...(Sunday night through Saturday)
Issued at 301 PM CDT Sat Sep 17 2016

The upcoming week will start out quiet, but we`ll see much more
active weather develop by Wednesday and last through the remainder
of the week.

A strong, zonal jet stream will move across the Upper Mississippi
River Valley Sunday night into Monday. Westerly winds throughout the
atmospheric column increases and drive a dry High Plains air mass
into MN/WI late Sunday night. 2-meter dewpoints, which start in the
low to mid 60s Sunday evening across most of the area, will fall
into the mid 40s by Monday morning. The dry air mass and will keep
the region precipitation free through Tuesday morning. Both the NAM
and GFS are forecasting a weak surface low in IA on Monday, but we
think that will do little to change the sensible weather in MN and
western WI - perhaps a few more high clouds on Monday. This low will
be a bigger impact farther east in southern WI and Illinois where
there will be deeper moisture on Monday.

There will be another notable air mass change Tuesday night into
Wednesday when the low level jet increases and advects moisture
from the Southern Plains northward into MN/WI. The GEFS/NAEFS both
indicate well-above normal precipitable water values across the area
by Wednesday - 1.50"-1.75" off the GFS forecast soundings for next
Wednesday. This moist air mass will force the dry air north and
setup a pronounced warm front across the area. The convergence along
this boundary looks really good and both the GFS/ECMWF continue to
indicate the potential for a widespread 1-2" (locally higher, 2"-3")
rainfall amounts mid week - perhaps coming in two rounds of
convection Tuesday night through Thursday morning. 12z numerical
guidance then develops another surface low in the plains that lifts
northeast FRI-SAT, perhaps giving the area another quick shot of
rain before dumping widespread precipitation across northern MN,
northern ND and southern Ontario.

We don`t often get 2"+ rain events in MN/WI after September 20th,
in fact it`s only happened about a dozen times at our main climates
sites over the past 100 years. When it does, it happens in patterns
like what is expected mid week. The CIPS warm season analogs suggest
this. When comparing the 12z GFS run to climate data, roughly 60% (9
of the last 15) of the top pattern analogs have widespread 2-5" rain
events somewhere in MN/WI. The top match is September 23, 2010, an
infamous, more recent heavy rain and flooding event.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1200 AM CDT Sun Sep 18 2016

Main concern this period is the potential for fog overnight.  Much
of the area has calm winds to start the 6z TAFs, though tightening
pressure gradient continues to work in from the west. We still feel
EAU will have the lightest winds and will be more susceptible to
fog, though the longer STC/RNH remain with calm winds, the greater
the potential is they will see some fog as well. Sunday afternoon
into the evening, we will see cold front working across the area,
which explains the increase in mid-level clouds. Models continue to
show enough moisture may make it along the front as its crossing
into WI to kick of some scattered showers there toward the end of
the period, but deep moisture looks like, so kept RNH/EAU dry for
now. Only change made to 00z TAFs was to increase wind speeds and
add gusts based on forecast sounding from the 00z NAM which show
decent winds mixing down Sunday.

KMSP...No additional concerns for MSP.

/OUTLOOK FOR KMSP/
Mon...VFR. W winds 5-10 kt...becoming NW.
Tue...VFR. SW winds 5 kt.
Wed...VFR/chc MVFR. TSRA likely. SE wind 5-15 kt. &&

.MPX Watches/Warnings/Advisories...
WI...None.
MN...None.
&&

$$

SHORT TERM...BORGHOFF
LONG TERM...CLF
AVIATION...MPG



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