Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 291728

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1228 PM CDT MON AUG 29 2016

.UPDATE...For 18z aviation discussion below


.SHORT TERM...(Today and tonight)
Issued at 409 AM CDT Mon Aug 29 2016

Interesting two segment LEWP stretching from the Arrowhead to just
west of Alexandria early this morning. The storms are being fed
through strong low level moisture transport heading northeast from
SD with considerable upglide evident on the 310k theta surface. The
HRRR over the past several hours is struggling badly with the
southwest portion of the storms while the 00z ARW, NMM and NSSL WRFs
have a good handle on this activity. As the morning progresses, the
strong low level southwest flow will weaken considerably as a low
pressure system in the low levels near the International border
moves northeast into Ontario. In addition to the flow weakening, it
will become more westerly. Hence, thunderstorms will pass across
central MN early this morning with significant weakening expected by
mid morning. An isolated severe storm cannot be ruled out with heavy
rainfall the main concern.

For this afternoon and tonight, a cold front in the eastern Dakotas
will reach central MN by early this afternoon and then head slowly
southeast to the far eastern and southern reaches of the FA by
daybreak Tuesday. The low level wind field is quite weak this
afternoon across central MN with essentially a cul area stretching
from south of Duluth to Aberdeen. This opens the door to several
problems for new convective development during the afternoon and
evening. The PWAT values are forecast to be AOA 1.5 inches along the
frontal boundary. This is well above the 90th percentile for the end
of August. In addition, with the weak wind field, RAP BUFKIT
profiles at points across central MN show MBE velocities less
than 10 knots. Hence, storms that develop will likely be slow
movers and contain torrential rainfall. One such complex of storms
developed Sunday evening near Fertile in northwest MN. The storms
produced between 5 and 7 inches of rain based on gauge data and
MRMS. Another concern with the storms will be damaging winds (wet
microburst) with DCAPE values progged to be in the 1000-1500 J/KG
range. Based on the aforementioned 00z WRFs, thunderstorm
initiation is between 20 and 22z with the activity sinking slowly
southward through the FA during the late afternoon and early
evening with a significant weakening trend over the eastern and
southern FA late in the night.

Highs today a few degrees either side of 85 still look good. The
warmest readings of 85 to 87 degrees stretch from west central MN
through the Twin Cities to Eau Claire. The warm temperatures
combined with dewpoints around 70 yield heat indices in the lower 90s
with 93 progged for MSP. Lows tonight will remain warm from the Twin
Cities on east and south with middle to upper 60s forecast. However,
cooler air filtering in behind the front in central MN will drop
lows there to the middle 50s.

.LONG TERM...(Tuesday through Sunday)
Issued at 409 AM CDT Mon Aug 29 2016

For the extended, the first week of school for many will bring
gorgeous weather, while the upcoming holiday weekend is
looking unsettled, with multiple chances for thunderstorms
looking possible.

For Tuesday through Friday, there`s not much to write about. An
upper ridge will amplify across the nations mid-section, with deep
troughs building down the west coast and into eastern NOAM. We will
remain to the east of the ridge trough Friday, with high pressure
over the Great Lakes keeping temperatures near normal with dewpoints
comfortably in the 50s, the kind of weather pattern tailor made for
blended/model averaged forecasts. The only period where we may need
to do some work in the future is with winds on Friday. GFS bufr
soundings are showing wind gusts to 30 mph being possible in western
MN Friday afternoon as our gradient tightens between a 1025mb high
over the Great Lakes and a sub-1000 mb low over southern
Saskatchewan.  Current forecast from SuperBlend has everything about
5-10 mph lower.

For Labor Day weekend, this whole trough/ridge pattern shifts east,
placing the upper MS Valley into southwest flow aloft. We still have
disagreements on when the initial moisture surge and precip chances
get here, but at this point we have narrowed it down to Saturday on
the fast end with the GFS and Saturday night/Sunday on the slower
end with the ECMWF. Unfortunately, we look to remain in this active
southwest flow pattern through the duration of the holiday weekend
with multiple shortwaves looking to move through, giving is several
opportunities for showers and thunderstorms. The most significant of
the waves looks to come Sunday night into Monday. Both the GFS/ECMWF
show a surface low working across the nations mid-section then,
though the ECWMF brings it up across MN while the GFS slides it
south through Iowa. Where this low goes will be important as along
its track we should see swatch of fairly healthy rain with the ECMWF
taking it up across here while the GFS would keep it down in Iowa.
The ECMWF solution would also bring with a risk for severe weather on
Labor Day as well, but lots of time for those sorts of details to
get worked out before you start getting too worried about weather
impacting your Labor Day plans.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1230 PM CDT Mon Aug 29 2016

The primary TAF concern is timing of shower and thunderstorm
activity this afternoon and tonight. The cold front of interest
was stretching from eastern South Dakota across northwestern
Minnesota at TAF issuance, while an outflow boundary (generated
from earlier thunderstorm activity) was found across south
central MN. Spotty shra/ts development is possible this afternoon
in the vicinity of the old outflow boundary, although that should
stay south of the TAF sites and therefore the main show will be
tied to the approaching front. Have delayed the onset of activity
a bit due to a slightly slower frontal timing. Expect shra/ts to
start to develop in the northwest (KAXN) around 21z, then drift
east-southeast toward KSTC/KRWF around 23z, and reach KMSP after
00z (most likely closer to 02z). MVFR vsbys/cigs will be possible
for a several hour window after onset, and then some additional
showers are possible early Tuesday morning with the frontal
passage. Outside of the precipitation time frames, VFR conditions
will prevail. South winds ahead of the front will gradually shift
to the northwest then north-northeast behind the front.

SHRA/TS activity is expected to impact KMSP a bit later, and it is
now looking more like 02z and after. The window for thunder looks
to be 02z-06z with scatter showers possible with the frontal
passage around/after daybreak Tuesday.

TUE NIGHT...VFR. Wind N 5 kts.
WED...VFR. Wind NNE 5 kts.
THU...VFR. Wind E 5 kts.
FRI...VFR. Wind SSE 7-10 kts.


.MPX Watches/Warnings/Advisories...


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