Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 220521
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1121 PM CST Mon Jan 21 2019

.SHORT TERM...(This evening through Tuesday night)
Issued at 251 PM CST Mon Jan 21 2019

Water vapor satellite early this afternoon shows troughing over the
Rockies with a northern stream short wave trough over Montana and a
stronger southern stream short wave trough over Nevada and Colorado.
The 21.12Z models continue to remain consistent within their own
solutions which is helping to maintain a fair dose of uncertainty
with the snowfall amounts and locations.

All the models are in agreement that the southern stream short wave
trough will swing northeast across the area Tuesday bringing a round
of moderate pv advection in the 300-500 mb layer over the region. It
also still looks like there will be some jet dynamics that come into
play from the left exit region of the 300 mb jet. Where the
differences in the models lie are with the strength of the
northern stream short wave trough and how much of an influence it
has on the overall system. The 21.12Z NAM and several runs of the
RAP today have this northern stream wave maintaining its strength
as is come across the Upper Midwest which results in these two
models placing the mid level frontogenesis farther to the
northwest from southern Minnesota into northern Wisconsin. The
21.12Z GFS does not maintain the strength of the norhtern short
wave trough and allows the strongest frontogenesis to form farther
southeast from northern Iowa into southern Wisconsin. The 21.12Z
GFSFV3 does not provide and useful insight as it does not show any
organized mid level frontogenesis occurring. These differences
have made it difficult to determine if there is going to be one
band of heavier snow across the south or possibly two bands with
one across the south and one across the northwest sections.
Incorporating some of the short range meso-scale guidance into the
forecast maintains the higher qpf amounts across the south but
does increase them over the previous forecast across the
northwest. This leads to a general 4 to 6 inches across the entire
area with some pockets of northeast Iowa exceeding 6 inches by a
few tenths of an inch.

There also looks to still be the potential for some light icing with
this event as well. As the precipitation starts to move into
tonight, the saturated column initially does not look deep enough to
support ice nucleation leading the possibility of some freezing
drizzle. This potential lack of ice nucleation looks to continue as
the far south on the north side of the surface low through Tuesday.

With the precipitation starting overnight in the areas west of the
Mississippi River, not comfortable continuing the watch. With the
snow amounts currently in the forecast, we are on the dividing line
between a high end advisory or a low end warning. After coordination
with surrounding offices, will go with a low end warning south of
Interstate 90 and advisory for the rest of the area.

.LONG TERM...(Wednesday through Monday)
Issued at 251 PM CST Mon Jan 21 2019

A cold, relatively active northwest flow pattern will set up
through the middle of the week into early next week. There remains
good agreement that the flow pattern will amplify late this week
with an Arctic cold front sweeping through Wednesday night into Thursday.
Gusty winds and possibly some flurries or snow showers will
accompany the front given the strong cold advection, with 850 mb
temps falling towards -25C Thursday night. With the ridge axis
passing to the west Thursday night, winds won`t completely
decouple, with wind chills likely eclipsing 30 below across parts
of the area. Highs on Friday may struggle to top 0. Temps will
remain well below average through the weekend.

21.12Z model guidance shows several upper shortwave troughs rotating
southeastward through the broad longwave trough Friday into early
next week. Although timing/placement is uncertain, any of these
systems may lay down a swath of light snow. Recent runs of the GFS
and ECMWF have suggested a stronger wave dropping south later in the
weekend with chances for accumulating snow, but confidence isn`t
very high given the low predictability nature of embedded shortwaves
at this time range.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1121 PM CST Mon Jan 21 2019

Lower clouds continue to very slowly develop eastward, and that
trend will continue, with MVFR conditions eventually enveloping
RST and then LSE prior to sunrise, though this is much slower than
initially thought. Eventually, some light snow should also develop
for both locations prior to sunrise, with visibility mainly MVFR
but eventually becoming IFR through the day as light snow
continues. Still many questions about where any heavier snow bands
may set up through the coming day and into this evening as well,
and unfortunately confidence is VERY low regarding the outcome for
any given hour at RST or LSE. Suffice it to say that changes to
the TAF forecasts are expected as we start to get a sense of where
any heavier snow bands may set up.

For now, will carry a general 1-2 miles in light snow, but under
any stronger bands, visibility could be reduced to 1/2 mile at
times. Conversely, it is possible that conditions will improve
back to MVFR levels outside of any snow bands. Simply put, this is
one tough forecast, but it does still appear that the most
widespread heavier snow may hold off until Tuesday evening and
even night, perhaps near or even after the end of the current TAF
period. Look for winds from the southeast diminishing the next few
hours before eventually shifting northeast ahead of approaching
low pressure later Tuesday afternoon and evening.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Warning from 6 AM Tuesday to midnight CST Tuesday
     night for WIZ053>055-061.

     Winter Weather Advisory from 6 AM Tuesday to midnight CST
     Tuesday night for WIZ017-029-033-034-041>044.

     Winter Weather Advisory until midnight CST Tuesday night for
     WIZ032.

MN...Winter Storm Warning until midnight CST Tuesday night for MNZ095.

     Winter Storm Warning from 6 AM Tuesday to midnight CST Tuesday
     night for MNZ096.

     Winter Weather Advisory until midnight CST Tuesday night for
     MNZ079-086>088-094.

IA...Winter Storm Warning until midnight CST Tuesday night for
     IAZ008>010-018-019-029.

     Winter Storm Warning from 6 AM Tuesday to midnight CST Tuesday
     night for IAZ011-030.

&&

$$

SHORT TERM...04
LONG TERM...JM
AVIATION...Lawrence


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