Area Forecast Discussion
Issued by NWS Duluth, MN

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FXUS63 KDLH 210239

Area Forecast Discussion
National Weather Service Duluth MN
839 PM CST Mon Feb 20 2017

Issued at 839 PM CST Mon Feb 20 2017

The rain showers are spreading east through the Northland as
expected. Some fog has developed across parts of the Northland,
including from the Twin Ports down the Interstate 35 corridor.
However, this probably will not last long since breezy westerly
flow will kick in soon with the coming cool front. The west flow
should help lift it. However, there is increasing confidence
widespread fog will develop late tonight and around dawn when the
west flow weakens with the coming surface ridge. Our abundance of
moisture from today`s rain, and above freezing temperatures to
continue melting the snow, will likely combine with the light wind
flow tonight to develop radiational cooling fog. Increased cloud
cover through tomorrow morning and ramped up the fog in the
forecast. May need to ramp it up even more. Held off on dense fog
wording, but it is possible we may need to issue a dense fog
advisory tomorrow morning.

UPDATE Issued at 536 PM CST Mon Feb 20 2017

Updated for the new 00Z Aviation Discussion below.


.SHORT TERM...(This evening through Tuesday)
Issued at 330 PM CST Mon Feb 20 2017

A low pressure system centered over Manitoba this afternoon has a
cold front extending south along the MN/Dakotas border region,
with a warm front extending from around Grand Forks southeast
across central MN to around the Twin Cities, and on to around
Madison. These features, along with a strong upper level trough
and shortwave have produced a band of rain with embedded thunder
which is in the process of moving across the forecast area as the
low pressure system moves to the east. The back edge of the
precipitation should reach the western edge of the forecast area
around 5 pm, gradually moving east across the area before moving
completely out of the area shortly after midnight tonight.
Precipitation should remain all rain with a few lingering rumbles
of thunder during the late afternoon and early evening. The fog
currently in place ahead of the cold front and behind the back
edge of the precipitation is likely to also spread east, and
expect most areas to have at least a few hours of fog this
evening. Have kept to patchy wording for now as am uncertain on
exact timing, placement and intensity. Tuesday the surface low
will be well east of the forecast area and we will get a small
ridge of high pressure that builds in for the morning hours before
a very weak shortwave and surface trough moves across the
northern portions of the area in the afternoon. The ridge is
fairly strong for it`s size and should produce clearing skies and
temperatures in the upper 40s and 50s. The trough is weaker and is
likely to only produce some clouds and a few scanty showers along
the Canadian border during the afternoon hours.

.LONG TERM...(Tuesday night through Monday)
Issued at 330 PM CST Mon Feb 20 2017

Initially in the extended flow is quasi-zonal at 500 hPa. This
will bring a shortwave through the Northern Plains and into the
Upper Midwest Wednesday into Wednesday Night. This will spread
precipitation into the region from west to east. Precipitation
will begin as a mixture of rain, snow and a wintry mix early on
Wednesday before becoming all rain Wednesday afternoon. The latest
GFS/NAM/ECMWF/GEM show a low level warm nose ranging from 0 to
3.5 degrees with surface temperatures in the low to mid 30s.
During the afternoon temperatures warm up to the 40`s and 50`s
with a similar low level warm nose, which will result in all rain.
There are significant differences in the model guidance in the
strength of surface low and its associated frontogenesis aloft.
The NAM/ECMWF depict a much stronger system, which would bring
more precipitation and snowfall to the region. Whereas the
GFS/GEM depict a much weaker system, bringing lighter
precipitation and lower snowfall amounts. Decided to compromise
between these two camps at this time due to the significant
differences, but if things trend towards the NAM/ECMWF then
amounts will need to increase. Precipitation will gradually change
over to all snow Wednesday evening before precipitation comes to
an end. This is due to northerly flow advecting colder air into
the region at 850 hPa.

On Thursday, a weak ridge will briefly build into the region
bringing dry and quiet conditions with cooler temperatures. High
temperatures will still be above normal for this time of year with
readings ranging from 30 to 40 degrees. Lows heading into Thursday
range from the low 20s to the low 30s.

Focus then turns to the the system for late in the week and heading
into the weekend. A trough will dig into the West Coast Wednesday
evening. The trough will dig and deepen moving into the Southwest US
by late on Thursday. At the surface, an area of low pressure will
develop in the Central Plains. The trough will then lift and become
slightly negatively tilted as it moves into the Mid Mississippi
River Valley on Friday, while the surface low moves northeastward
into the Iowa/northern Illinois/southern Wisconsin area. By Saturday
the low will continue moving northeastward into the Central Great
Lakes. There still are differences in the track and speed of the
system moving through the region. The GFS is currently the slowest
of the latest guidance, while the ECMWF/GEM are more progressive.
Based on the latest thermal profiles precipitation will remain as
all snow through the event. Still highly uncertain on snowfall
amounts due to model differences in the track of the low, however
think that best chance for accumulating snowfall is across northwest
Wisconsin at this point in time. Those with travel interests late
this week should continue monitoring the forecast for updates. High
pressure will build in behind the exiting system. Aloft, flow will
become northwesterly, which will bring lake effect snow chances to
the South Shore of Lake Superior Saturday into Sunday. High
temperatures late this week and this weekend will return to normal
for this time of year with readings in the 20s and low 30s. Low
temperatures range from the single digits above zero to the teens.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 536 PM CST Mon Feb 20 2017

The forecast is challenging, and confidence in flight categories
through early Tuesday is lower than usual.

Rain will continue to spread east through the Northland this
evening in advance of a cool front to move through the region
overnight. The rain will continue to result in widespread IFR
conditions because of low ceilings and reduced visibility from
mist. However, do not have a good feel for what will happen once
the higher clouds pull out of the region overnight and 5 to 10
knot westerly kick in after the cool front passage. There is a
concern that much of the Northland could get widespread fog
overnight, considering the abundant moisture from the recent rain
and cold and melting snowpack. The lack of upper-level cloud cover
will certainly help radiational cooling to help develop a shallow
inversion, but also concerned there may be just enough wind
overnight to help stave off the fog. A surface ridge will be
spreading in from the west, reaching the Northland around dawn, so
winds will likely weaken later tonight into the morning. Placing
bets on the fog, but since there is not super good confidence it
will occur, just trended the forecasts in that direction. Might
trend more to VLIFR or widespread LIFR with the 06Z forecasts if
confidence increases even more.

Light southwest flow will develop Tuesday morning, and expect a
return to VFR conditions by the middle to late morning.


DLH  34  50  34  44 /  90  10  10  50
INL  32  46  30  40 /  80  10  20  30
BRD  34  55  32  49 /  50   0  10  50
HYR  36  54  34  53 /  80  10   0  40
ASX  37  57  37  49 /  90  10  10  50




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