Area Forecast Discussion
Issued by NWS La Crosse, WI

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971
FXUS63 KARX 152015
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
315 PM CDT Sun Oct 15 2017

.SHORT TERM...(Tonight through Monday)
Issued at 315 PM CDT Sun Oct 15 2017

Forecast focus for tonight is on the frost potential.

First though, upper level shortwave dropping across the northern
plains this afternoon, sliding it across the region tonight. Mostly
mid level saturation with the feature, but it has resulted in a
smattering of light pcpn - per sfc obs. HRRR/Hopwrf runs suggesting
this trend would continue into northern WI later this evening, and
looks reasonable. Not expecting measurable, and will hold north of I-
94.

The bigger, potentially widespread impact for the local area is
frost.

PRO`s:
* colder air in place: cold highs (mostly near 50) won`t have far to
  go to drop into the low to mid 30s.
* light sfc winds: bufkit soundings point to shallow decoupling
  (under 600ft)

CON`s:
* clouds: shortwave is bringing down a mid level deck that will
  cover at least the northern 2/3rds of the forecast area for a few
  hours this evening/overnight. Thus, cutting into radiational
  cooling time.
* near sfc winds: while soundings and guidance suggest decoupling
  and relatively light sfc winds, just off the deck (by 600 ft) its
  blowing +10 kts. How much decoupling really occurs and/or how much
  stirring/turning results isn`t certain.

Not a perfect scenario or "slam dunk" for widespread frost tonight.
If extent, time of coverage in the clouds...and winds do decouple
with little influence from a loft...areas of frost a lot more
likely. Conversely, if winds a bit higher and/or clouds linger...not
so much.

All that said, per coordination with surrounding offices will hoist
a Frost Advisory for a central/northern WI. Again, some
contingencies with that, but certainly low lying/shelter areas face
a pretty good risk for frost later tonight. Might need to expand the
adv into parts of southeast MN and northeast IA.

.LONG TERM...(Tuesday through Sunday)
Issued at 315 PM CDT Sun Oct 15 2017

After a spate of wet days, a stretch of dry and mild conditions is
on tap for next week - a nice break and a great opportunity to catch
some of those fall colors.

After the passage of the upper level trough east this evening, both
the GFS and EC have been insistent of moving to a zonal/ridging
upper level flow pattern for the better part of the new week. A
couple ripples in the flow around mid week, but hold mostly across
southern Canada and not much moisture to work with. Should stay dry
through Friday. Come Saturday, both the EC and GFS take an upper
level trough/sfc front combo east across the plains and through the
Upper Mississippi River Valley. Would see at least some scattered
showers (isolated storms?) in this scenario.

For temperatures - a return to above normal readings as the upper
level ridging becomes more pronounced. NAEFS 850 mb temp anomalies
generally +1/+2, with Friday looking like it would be the mildest
day. Highs in the mid 60s to low 70s looking likely for Tue into
Sat.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1135 AM CDT Sun Oct 15 2017

Plan on MVFR stratocumulus clouds to erode from west to east through
this afternoon. A mid-level trough of low pressure will then bring
some altocumulus in later this afternoon into this evening with
bases generally above 10kft. The bulk of this cloud is expected to
pass north of the KRST/KLSE airports but will likely see a couple
brief periods of broken ceilings, then clearing again/VFR conditions
after midnight as a ridge of high pressure builds in from the
Plains. Otherwise, plan on northwest winds to continue on a downward
trend through this afternoon with light westerly winds, generally
less than 10kt, tonight into Monday.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Frost Advisory from 2 AM to 9 AM CDT Monday for WIZ017-029-034-
    042>044.

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

&&

$$

SHORT TERM...Rieck
LONG TERM....Rieck
AVIATION...DAS



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