Area Forecast Discussion
Issued by NWS Twin Cities, MN

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434
FXUS63 KMPX 220008
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
708 PM CDT Thu Sep 21 2017

.SHORT TERM...(This evening through Friday)
Issued at 355 PM CDT Thu Sep 21 2017

A surface warm front will continue to lift north across the area tonight.
Surface dewpoints have already moderated into the upper 60s over
southern MN this afternoon. As low level moisture advection
increases this afternoon, hi-res models continue to indicate the
potential for isolated showers to pop up this eve. The more
concentrated activity looks to develop around/after 05z over
central/northern MN/WI as the low level jet strengthens.
Anticipate most of this activity to stay north of the Interstate
94 corridor.

Tomorrow expect the nocturnal activity to wane as the CAP builds
in the warm sector. The primary forecast concentration
transitions to how warm we will get, with near-record temperatures
in the forecast. Progged Bufkit soundings indicate we will mix to
between 875-900mb, which would yield highs mostly in the upper 80s
to lower 90s. Given the elevated dewpoints in the lower 70s, heat
indices along and south of I-94 moderate into the 95 to 100 degree
range. Given the breezy southerly winds, the true impact may be
abated slightly, but Wet Bulb Globe temperatures still look to
rise into the lower 80s across the metro area. While this is
somewhat borderline for a typical heat advisory day, we do meet
advisory criteria for much of the metro, and given heat
acclimatization has been non-existent given the cool August we
had, issued an advisory for the TC metro for tomorrow afternoon.

.LONG TERM...(Friday night through Thursday)
Issued at 300 PM CDT Thu Sep 21 2017

The longer term concerns remain timing/evolution of the western
conus trough and movement of the surface front.

The front will slowly move into western Minnesota overnight. Day2
outlook continue to paint slight risk of severe thunderstorms in the
vicinity of the front. We should be capped until the front moves
into the area. Models trend a weak surface low traveling across
eastern South Dakota into northwest Minnesota during the night and
this should help move the front east into Minnesota.
Severe weather parameters continue to favor large hail and damaging
winds, with a small tornado threat along and north of the path the
surface low.

We trimmed the eastern extent of the PoP forecast with some leeway
given the small eastward progression of the front on the 12z ECMWF.
Still looks like it should mainly affect the western and central
portion of the area through the day Sunday. This will leave the
eastern cwa in the heat and humidity, but shouldn`t be as extreme as
Friday afternoon. The thunder threat will continue with the frontal
boundary through at least Monday with the slow passage of the upper
trough and surface front into Tuesday. Heavy rainfall remains a
threat as well, with the NAEFS anomalous PW`s showing around 3
standard deviations through the weekend.

The front exits the east Tuesday followed by cooler and drier
conditions through at least midweek.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 634 PM CDT Thu Sep 21 2017

There remains a chance for thunderstorm activity along a warm
front overnight, primarily near AXN. Otherwise, the other TAF
locations should remain dry. The next concern for the overnight
hours is low level wind shear becoming greater than 30kt for
KSTC/KRWF/KMSP/KMKT with increasing wind speeds aloft. The height
of the wind shear is up to 2,000ft. Next, there is a chance for
patchy fog in Western WI with increasing moisture overnight, so
visibilities of 4-5SM have been mentioned in the KRNH/KEAU TAFs.
There is a possibility that cloud cover and wind may prohibit fog
from developing, so that will continue to be monitored.

Otherwise, winds are expected to veer slightly from southwest to
south-southwest during this time period. Winds will gust near
20-25kt during the late morning and through the afternoon on
Friday, especially in MN. Chances for thunderstorm activity
return late Friday with best chances being after 00Z Saturday.


KMSP...
Dry conditions are anticipated overnight with any thunderstorm
activity that occurs likely staying to the west. The main concern
is low level wind shear overnight. The shear will decrease by
late morning as surface winds become breezy again through the
afternoon. Wind gusts of 20kt have been mentioned, but that may
need to be increased should wind speeds aloft that mix down to the
surface supersede what is predicted.

/OUTLOOK FOR KMSP/
Sat...Mainly VFR. Slight chance MVFR/TSRA. Winds SSW 5-10 kts.
Sun...Mainly VFR. Slight chance MVFR/TSRA. Winds SW 5-10 kts.
Mon...MVFR/TSRA. Winds S 5-10kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...Heat Advisory from 1 PM to 7 PM CDT Friday for MNZ060>063-
     068>070.

&&

$$

SHORT TERM...LS
LONG TERM...DWE
AVIATION...AMK



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