Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 270507
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1105 PM CST Sat Nov 26 2016

.UPDATED...
Issued at 1105 PM CST Sat Nov 26 2016

Dense fog has developed across the snow pack of central/northern MN
and northern WI late this evening. The fog was expanding across
north-central WI...and have issued a Dense Fog Advisory to cover the
1/4 mile visibility hazards.

Meanwhile, the HRRR and RAP suggest that fog could expand and become
dense across southeast MN. Without a snow pack to aid and keep the
fog development, any thick fog could be more spotty. We`ll monitor
and expand the Dense Fog Advisory if needed.

.SHORT TERM...(This evening through Sunday)
Issued at 235 PM CST Sat Nov 26 2016

Latest 20z surface analysis shows ridge over the Ohio River Valley.
Southerly winds on the backside of the surface ridge has ushered
in warmer temperatures into the forecast area today. Temperatures
have climbed into the middle 30s to lower 50s per latest metars.

For tonight, the NAM and RAP soundings continue to show a very
strong inversion will develop overnight.  As a result, they are
developing fog and stratus across the area.  The NAM is likely doing
this because it is assuming that our area currently has snow on the
ground. Meanwhile, the GFS soundings show that the boundary layer
will remain well mixed; thus, no fog or stratus development.  For
the time being, kept the mention of patchy fog in western Wisconsin.
This will cover the possibility of fog developing in the sheltered
areas overnight and Sunday morning.

Another potential concern tonight is the low temperatures. The model
consensus has low temperatures ranging from the mid-20s to lower
30s. These temperatures seem just too cold considering the well-
mixed boundary layer and that the dew points are currently in the
mid-and upper 30s.  Due to this, raised the low temperatures up into
the lower 30s.  However I am a bit concerned that these low
temperatures may still be too cold.

With upper level trough digging over the western United States and
upper level ridge amplifying over the Great Lakes Region...southerly
winds aloft will continue to advect a warmer airmass into the
forecast area Sunday. The 26.12z GFS/NAM/ECMWF suggest 925mb
temperatures warming to plus 2 to plus 7 degrees celsius by 00z
Monday. Once again...temperatures will be well above normal...with
highs in the 40s and possibly the lower 50s.

.LONG TERM...(late Sunday afternoon through Saturday)
Issued at 235 PM CST Sat Nov 26 2016

The 26.12z GFS/NAM/ECMWF are in agreement in lifting upper level
closed low/shortwave trough into the Northern Plains States Sunday
night into Monday. The deterministic models indicate strong moisture
transport/converge and forcing in association with the upper level
closed low/shortwave trough. Increasing moisture/forcing will
produce a band of precipitation and move northward across the
forecast area late Sunday afternoon(after 21z Sunday) through Monday
morning. Rainfall amounts of a quarter to possibly three quarters of
an inch is expected Sunday night into Monday across the forecast
area. Next concern are thunderstorm chances...the 26.12z GFS/NAM
continue to show minimal elevated CAPE with the system.
However...with a dynamic system...cannot rule out an isolated
thunderstorm across the forecast area. Will continue mention of
isolated thunder.

Upper level low remains nearly stationary across northern Minnesota
Tuesday and main shortwave trough pushes into southern Ontario
Canada. This will move heavier precipitation over Lake Superior and
southern Ontario Canada. Forecast area may see some light
precipitation...as weak shortwave trough wraps around on the
backside of upper level low into western Minnesota. Precipitation
chances will remain small with light amounts.

Main forecast concerns from Tuesday night into Saturday are light
wintry precipitation chances through the period. The 26.12z
GFS/ECMWF/GEM are in good agreement in weakening or slowly moving
upper level closed low/trough over the northeastern United States
during the period. Main difference between the deterministic 26.12z
models are timing the weak shortwave troughs rotating around the
upper level low/trough over the Great Lakes Region through the
period. Overall...the 26.12z GFS/ECMWF/GEM are in decent agreement
in wrapping weak shortwave troughs into the region and will result
in on and off small precipitation chances through the period.
Temperatures will be cooler and near normal...as the 26.12z
deterministic models advect a cooler airmass into the forecast area
on the backside of the upper level low/trough. Highs are expected to
be mainly in the 30s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1105 PM CST Sat Nov 26 2016

Aviation concerns are two-fold...

1) Fog tonight

Thick sub 1sm fog has developed across the snow pack over northern-
central MN into northern WI. HRRR/RAP models favor sinking it a bit
south in northern WI, but suggest IFR fog could also spread across
much of southeast MN - impacting KRST. Without the aid of a
snowpack, the thicker fog could be more `ebb and flow` rather than
socking the region in. Confidence not high so will just go with MVFR
for KRST - but monitor closely. There certainly is a threat
visibilities could go lower.

For KLSE, confidence even lower that there will be fog impacts. T/Td
spread at 6 F at 04z with bkn high-thin clouds overnight. Light sfc
winds though, along with light winds in the near sfc. No evidence of
valley fog at this time. Again - will keep a close eye and update if
needed.

2) Rain later Sunday

Expansive area of rain/low clouds slated to move northeast into the
area for late Sunday afternoon, holding on into Monday morning.
Expect a drop into ifr/mvfr cigs and vsbys. Winds are also going to
ramp up as the pressure gradient tightens ahead of this rain maker,
staying fairly breezy through the overnight hours Sunday.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Dense Fog Advisory until 9 AM CST Sunday for WIZ017-029.

MN...None.
IA...None.

&&

$$

UPDATED...Rieck
SHORT TERM...DTJ/Boyne
LONG TERM...DTJ
AVIATION...Rieck



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