Area Forecast Discussion
Issued by NWS Duluth, MN

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471
FXUS63 KDLH 220525
AFDDLH

Area Forecast Discussion
National Weather Service Duluth MN
1225 AM CDT Fri Sep 22 2017

.SHORT TERM...(This evening through Friday)
Issued at 313 PM CDT Thu Sep 21 2017

An active pattern will set up over the Northland tonight through
Friday time frame. On the large scale, a longwave trough,
characterized by a 110 to 120 knot upper-level jet over the
Intermountain West states northeast into Manitoba and northwest
Ontario, will build across the western United States. This trough
will set the stage for this active weather pattern as strong
southwest flow will develop over the region, enhancing isentropic
upglide. At the sfc, an area of high pressure that dominated our
weather today will continue to move off to the east, maintaining
southwesterly winds and increasing 850-700 mb layer warm and
moist air advection. Moisture transport and instability will
increase overnight, leading to MUCAPE values between 1500 and
3000+ J/kg, as well as sfc dew points into the middle to upper
60s F. Showers and thunderstorms will result, with some possibly
strong to severe given the instability and a strengthening low-
level jet. For tonight, the main threats will be damaging winds
and some large hail. There is some uncertainty regarding the areal
coverage of these storms, as there is some timing and areal
differences between the convective-allowing models. The Storm
Prediction Center has a Marginal Risk of severe weather for nearly
all of the forecast area for tonight.

For Friday, there should be a brief respite from showers and storms
during the day, according to the high-res guidance. However, due to
the strong southerly flow, very warm and humid air will become
entrenched over the region. Temperatures will approach record
values Friday, with high temperatures in the middle to upper 70s
along the International Border into the middle to upper 80s over
our southern counties, especially over northwest Wisconsin -
values well above climatology. Due to the unseasonable heat and
humidity, heat indices could reach into the upper 80s to lower 90s
over our southern counties.

Another round of showers and thunderstorms will be possible late
Friday afternoon as a sfc low pressure system that develops due to
lee cyclogenesis. This area of low pressure will be accompanied by
an inverted sfc trough and stationary boundary trailing the low
pressure center. These features will act as the mechanism for this
next round of showers and storms. Instability will linger over
the region, and even increase some, with MLCAPE values between
2000 to 4000 J/kg between the GFS and NAM models. Deep layer shear
will be enhanced late in the afternoon, with values between 30 to
40 kts. Due to the moderate to high instability, SPC has a Slight
Risk of severe weather north of a Brainerd to Hibbing to Ely
line. The timing of this activity appears to be more in the Friday
night and Saturday morning time frame.

.LONG TERM...(Friday night through Thursday)
Issued at 313 PM CDT Thu Sep 21 2017

The Northland will have an extended period of wet and stormy weather
through the weekend and early next week. An area of high pressure
over the eastern US and eastern Canada will meet resistance from the
tropical storms in the Atlantic and have a difficult time going
anywhere fast. This will in turn result in a cool front to slowly
work its way eastward across the Northern Plains and and western
Great Lakes region over the weekend through early next week. The
Northland will find itself in days of warm and humid southwest flow
aloft (with precipitable water values of 1.5 to 1.75 inches, which
is very deep moisture for this time of the year). Passing
shortwaves, the front, and nocturnal low-level jets will provide
forcing for multiple rounds of showers and storms during this
period. Days of showers and storms, tapping into this deep moisture,
could result in very saturated soils and some flooding. It is too
soon to narrow down the flooding risk to any particular area, but we
will be keeping on eye on the flooding potential. Much of it will
depend on where and how much rain falls from thunderstorms with
reach round.

There is risk of strong to severe storms Friday night. A strong low-
level jet will develop, about 35 to 50 knots, as the front begins to
enter the western areas of the forecast area. Lowering heights,
because of a deep upper-trough digging through the western US, and
passing shortwaves will provide moderate synoptic forcing for
ascent. This forcing will help trigger storms across the region,
with better chances of storms in the western and northern forecast
area. Widespread most-unstable CAPE of 1500 to 3000 J/kg and 0-6 km
deep layer wind shear of 25 to 40 knots will provide a favorable
environment for organized updrafts/storms. Hail, damaging winds,
and heavy rain will be the main threats with these nocturnal storms.

Temperatures will begin the weekend on the warm side, with parts of
the southeast forecast area about 15 to 20 degrees above seasonal
normal. Highs on Saturday are forecast for the low 80s in northwest
Wisconsin to as low as the middle 60s over north-central Minnesota.
Temperatures will gradually cool with each successive day as that
front works its way through the region. Temperatures should return
to near seasonal normal by Tuesday or Wednesday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1225 AM CDT Fri Sep 22 2017

High pressure will continue to lose its grip on the Northland
weather overnight and into Friday, as an area of low pressure
approaches our region from the west. A warm front developing just
south of the Northland will continue to lift slowly northward
overnight. The warm front will be the focus for showers and
thunderstorms for the area TAF sites overnight. The focus will
then shift to the cold front that will develop to the west of our
area on Friday and Friday evening. This will bring the next chance
of showers and thunderstorms. While we should see largely VFR
conditions, there will undoubtedly be some MVFR to IFR CIG`s and
VSBY`s.

&&

.PRELIMINARY POINT TEMPS/POPS...
DLH  57  79  67  78 /  50  40  40  50
INL  59  73  60  62 /  50  70  70  40
BRD  66  85  65  73 /  50  30  40  70
HYR  68  88  68  85 /  50  30  30  30
ASX  57  88  69  85 /  50  30  30  30

&&

.DLH WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
LS...Small Craft Advisory until 5 AM CDT early this morning for
     LSZ140>145.

&&

$$

SHORT TERM...JTS
LONG TERM...Grochocinski
AVIATION...DAP



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