Area Forecast Discussion
Issued by NWS La Crosse, WI
FXUS63 KARX 162002
Area Forecast Discussion
National Weather Service La Crosse WI
302 PM CDT Thu Mar 16 2017
.SHORT TERM...(This evening through Friday night)
Issued at 302 PM CDT Thu Mar 16 2017
The forecast for tonight remains tricky with regard to precip type.
Primarily mid/high clouds and a narrow band of lower clouds were
on the increase this afternoon as 850-700 mb warm advection
increased ahead of a shortwave currently out over the northern
Rockies. A dry air low-level air mass has remained in place over
much of the area, but the depth of saturation will increase late
this afternoon and this evening as warm advection/isentropic
upglide increase. Generally, the main message of the forecast
remains unchanged, with the highest precip chances (and potential
wintry mix) along and north of I-94. Precip amounts and chances
will decrease westward, especially west of the Mississippi. As
with many of our mixed precip events this season, several factors
likely will influence precip type. Warm advection will continue
tonight and with clouds lowering, temps should not drop off much
and remain in the 30s. One thing to watch though will be how
quickly surface moisture increases. If dew points are slower to
rise, wet bulb effects could impact surface temps a bit.
Similarly, did slightly slow the arrival time of precip as it may
take slightly longer to saturate the low-levels, as the RAP/HRRR
indicate. The NAM/GFS/RAP suggest that the area with the strongest
forcing north of I-94 will maintain deep saturation through much
of the night, which may allow for a longer period of light snow,
possibly an inch or two north of I-94, if temps remain cool enough.
A glazing of ice is also possible in this area if cloud ice is
lost before precip exits and temps remain below freezing. In
collaboration with surrounding offices will not issue any
headlines at this time, but will continue to monitor trends this
evening, given potential for some slick roads for a time overnight
especially north of I-94.
Most of the precip should exit the area by early to mid morning
Friday with the passage of the cold front. Cold 500 mb temps,
steepening low-level lapse rates and the passage of an upper level
trough during the day may be enough to produce a few rain/snow
showers in north central WI during the afternoon/evening. There will
be increasing 1000-850 mb cold advection especially during the
afternoon, but with a mild start to the day and favorable westerly
winds for mixing, seasonably mild temps in the 40s to low 50s are
expected, along with gusty westerly winds.
.LONG TERM...(Saturday through Thursday)
Issued at 302 PM CDT Thu Mar 16 2017
This weekend still is shaping up to be a more spring-like one across
the area. An upper level ridge will progress into the Upper Midwest
later Saturday and Sunday. There will be a slightly cooler air mass
that sinks into the area behind Friday`s frontal system and some
lingering cloud cover ahead of the upper ridge, but highs should
still reach near average in the upper 30s and 40s.
Return flow will get going on Sunday ahead of a shortwave trough
tracking across southern Canada. With the thermal ridge axis
approaching and 850 mb temps climbing to 6-11C, highs in the 40s and
50s are expected with breezy south winds. Model RH fields do show
hints at some low-level moisture working northward ahead of the
upper wave, so will have to watch for any stratus that may try to
advect northward with the moisture return.
The 13.12Z models are still mixed with respect to precip chances
with the system on Sunday night. The shortwave will pass well north
of the region, but warm advection/frontogenesis along the associated
cold front may be enough to generate some rain. The ECMWF remains
on the drier side across the area. The GFS/ECMWF actually
indicate the potential for elevated convection developing along
the front Sunday night within a pool of higher precipitable water
values, steep mid-level lapse rates and modest MUCAPE ahead of the
front, but current model runs keep this mainly to the south and
east of the area. Even behind the cold front on Monday, temps
should remain fairly mild in the 40s and 50s.
Heading into mid-week, surface high pressure builds back across the
Midwest bringing cooler, but seasonable temps to the area. By late
Wednesday and especially Thursday there is loose model agreement in
a stronger upper level trough ejecting across the plains with
increasing precip chances as a surface low lifts northeastward from
the plains. It`s too early at this point to have much confidence
in timing, amounts, or precip type yet.
.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 250 PM CDT Thu Mar 16 2017
Cigs: a persistent, small band of mostly mvfr clouds advanced more
north than east (vs originally thinking), and have resulted in a
few hours of low cigs at KLSE. Bufkit hrrr/rap soundings insistent
that these will start to break up by 22z, although satellite
trends would hold then an hour or two later. Have updated KLSE and
sided with the soundings depiction for now.
High/mid level clouds the on the increase ahead of an upper level
shortwave/sfc warm front. Increasing low level moisture should
result in a lowering cloud deck this evening, ahead of an
approaching cold front. IFR/MVFR looks likely for the better part
of tonight into Fri morning. Then see an opportunity to break out
of the low cigs post a cold front Fri morning, but lower cigs
quickly return Fri afternoon as low pressure sweeps southeast
across the eastern Great Lakes.
WX/vsby: warm air advection and some north-south running
frontogenetic lift should result in areas of light showers (mainly
rain) later this evening through the overnight. Higher threat across
western WI, and will add some mention to KLSE as a result. Not
expecting much accumulation. Models favoring some mvfr vsbys and
seems reasonable with melting snowpack, shower threat.
Winds: southeast-south then going west with the passage of a cold
front Friday morning. Looks fairly breezy later in the day Friday as
the pressure gradient tightens up.