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Area forecast discussion 
National Weather Service La Crosse WI
451 am CST Sat Jan 31 2015

Short term...(today and tonight)
issued at 345 am CST Sat Jan 31 2015

Challenges abound tonight through Sunday with impending winter
storm...especially as it relates to snow amounts and the expected
sharp gradient from north to south. Overall...31.00z model suite is
in good agreement except for the amount of available moisture and
will address that below. Otherwise...will use a blended model

Cold front across ND into northwest Minnesota early this morning will drop
south across the upper MS River Valley later today. Temperatures to
rise into the middle 20s to middle 30s before beginning to drop middle-
afternoon with increasing north-northeast wind. In addition...expect
increasing clouds through the day.

For tonight...850 hpa low ejects northeastward from Four Corners
trough and into southeast NE/northeast Kansas by 01.06z tonight. Strong
925 to 850 hpa frontogenesis develops on north side of this cyclone
mainly across northern Iowa. Upper level (500 to 300 hpa) q-vector
convergence indicative of large scale lift a bit delayed with slower
wave dropping south from Canada...although right entrance region
from departing 300 hpa jet streak will still provide some additional
lift. Expect area of light snow to lift from southwest to northeast
this evening across northeast Iowa and southwest WI as moisture
increases with precipitable waters from 0.3 to 0.4 inches. As west to east
oriented low-level frontogenesis slides eastward into Illinois...expect
prolonged period of light snow to continue overnight and into
Sunday. Snow amounts overnight along and south of I-90 from 1 to 5
inches possible with a very tight gradient likely...especially
along northern flank of precipitation shield given weaker overall
forcing and drier air pushing south with a 1035 hpa surface high.

Long term...(sunday through friday)
issued at 345 am CST Sat Jan 31 2015

Light snow to continue through much of Sunday...especially across
northeast Iowa into southwest WI. Fairly deep dendritic growth layer
develops by Sunday morning but by this time...greatest lift which is
only 2 to 4 ubar/ generally above it in the 600 to 300
hpa layer. Surface gradient will also be quite strong Sunday morning
with roughly a six hour period with 30 to 35 kts at the top of the
boundary layer. This should translate to north-northeasterly winds
with occasional gusts up to 35 miles per hour creating areas of blowing and
drifting snow...especially across the wind prone areas of southeast
Minnesota into northeast Iowa. An additional 1 to 3 inches of snow along
and south of I-90 is possible on Sunday.

Overall...still somewhat uncertain with snow totals...especially
north of the Minnesota/Iowa border. Operational models generally paint a
broad swath of 0.25 to 0.3 liquid inches as far north as the I-90
corridor and as much as 0.6 liquid inches across the southernmost
forecast area. With the strongest low-level moisture transport
expected across southern MO into central Illinois and models over
prediction of low-level moisture this winter season...knocked off a
solid tenth of an inch from these values. With snow ratios
increasing through the event from near 10:1 to 20:1 with cooling
column...this generally translates to snow amounts from 1 to 4
inches from the Minnesota/Iowa border north to the I-90 corridor...3 to 6
inches across extreme northeast Iowa into adjacent portions of
southwest WI and 5 to 8 inches across the southernmost counties.

Normally...6 plus inches in 12 hours is considered warning
criteria...but considering it will take 24 to perhaps even 36
hours for snow totals to reach these higher end amounts...will
cover northeast Iowa into southwest WI with a Winter Weather
Advisory from 01z tonight through 03z Sunday night. As mentioned
above...snow totals north of the Minnesota/Iowa border not as
held off on an advisory for now. All said...the possibility exits
for a warning across the far south and northern extension of the
advisory into the I-90 corridor. Will need to watch this carefully
and adjust as necessary. Untreated roads across northwest Iowa into
southwest WI for sure to become slick...creating difficult travel
at times through Sunday.

Lingering snow to move out of the region Sunday night. Additional
light snow chances possible Monday night through middle-week with a
series of weak short-waves in northwest flow aloft. The latter half
of the week looks dry. Bigger story will be much cooler temperatures
with daily highs from the middle teens to middle 20s and nighttime lows by
Thursday/Friday mornings dropping below zer0.


Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 451 am CST Sat Jan 31 2015

An MVFR stratus deck will gradually drift south into the taf sites
this morning and looks to remain over the area through much of the
day. Plan on cloud bases in the 1500 to 2500 feet range. Will have
to keep a close eye on ceilings could drop to IFR this
afternoon. Snow will spread into the taf sites this evening with
conditions falling into the IFR to LIFR categories. Plan on
visibilities falling into the 1/2 to 1 mile range. The snow looks
to continue through the overnight hours.


Arx watches/warnings/advisories...
WI...Winter Weather Advisory from 7 PM this evening to 9 PM CST
Sunday for wiz054-055-061.

Iowa...Winter Weather Advisory from 7 PM this evening to 9 PM CST
Sunday for iaz008>011-018-019-029-030.



Short term...Rogers
long term...Rogers

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