Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
346 am CST Monday Nov 30 2015
Short term...(today and tonight)
issued at 346 am CST Monday Nov 30 2015
Main forecast concerns this period...wintry mix of precipitation and snow/
sleet/ice amounts today/tonight as the system lifts into the upper
Data analysis at 06z had high pressure retreating out of the upper
Midwest. Pressures were falling across the region and Central Plains
as shortwave energy rotates around the east side and out of The
Rockies trough. A lead wave and increasing lower level Thermo-
dynamic forcing producing a broad area of mainly rain with a wintry
mix on the north end...lifting north through eastern Iowa...southeast Minnesota
and into western WI. Per observation...2m temperatures...temperatures aloft and ground
temperatures a couple degrees either side of 32f...creating potential for
the mixed precipitation and icing on cold Road surfaces anywhere in the forecast
area early this morning.
No major issues noted with 30.00z model initializations. Models
continue to offer a tightening consensus as The Rockies middle level
low moves into eastern Nebraska by 00z Tuesday...then to far northwest Iowa by
12z Tuesday. Trend favors more westerly of earlier solutions with the
500mb low position...and surface low position at 12z Tuesday...with the forecast
area now well on the warm-sector side of the system as it lifts into
the upper Midwest. Given the tighter model consensus...forecast
confidence would generally be good. However...subtle differences
with the surface-800mb thermal profiles...with values straddling 0c...
and their impacts on precipitation type leaves forecast confidence at least on
precipitation types in the average range.
In the short term...some of the bigger precipitation type impacts may be
through middle morning this morning...with Road temperatures hovering around 32f.
This with boundary layer air temperatures in the lower 30s and model
soundings generally showing a nearly isothermal layer in the 925-
800mb layer...again about 1c either side of 0c. Thus the headache
with precipitation types and icing threat this morning. Already moved
Winter Weather Advisory to start at 06z across the west end of the
forecast area. With reports of icing of roads north of kdbq...short term
forecaster has expanded the Winter Weather Advisory across the
southeast half of the forecast area through 15z to account for the cold
roads and additional icing potential as the band of mixed precipitation
lifts north across the forecast area.
The lead wave of precipitation weakens/lifts north across the forecast
area this morning as the shortwave aiding it moves into northern WI.
Signal among hi-res/meso-scale models that much of the forecast area
would get a break from the precipitation through the middle morning into early
afternoon hours. Based on radar trends early this morning this
appears quite reasonable and will adjust precipitation chances downward
through much of the middle-day period. As the stronger shortwave energy
coalesces on the south side of The Rockies trough this morning...
then rotates around the east side of the trough this afternoon/
evening and across the forecast area overnight tonight. Strongest of the
500-300mb pv advection moves across the area later this afternoon/
evening. This along with the stronger of the 925-700mb warm
advection/lower level Thermo-dynamic lift. Tightening model
signal that a broad band of precipitation to rotate northeast into/across
the forecast area from roughly 30.21z to 01.09z. Spread nearly 100
percent precipitation chances northeast across the forecast area this period.
With the trend toward a more westerly surface through 500mb low
track...area ends up more deeply on the warmer side of the system.
925-850mb forecasts continue to show temperatures at/above 0c intruding
further and further northwest across the forecast area this
afternoon/ evening. This with surface temperatures climbing into the
middle/upper 30s. With surface observation already indicating mixed precipitation as
Dodge Center Minnesota...appears bulk of precipitation this afternoon/evening
will be rain or a rain/sleet mix...with any snow limited to the
far west or far north parts of the forecast area. Will leave headlines
for today/tonight for a the wintry mix of precipitation... but did lower
most snow amounts across the west/north ends of the forecast area
these periods. Fear may not have lower snow amounts enough but
time will tell. By the time the colder air arrive/column starts to
cool to support more snow later tonight...deeper moisture is
pushed North/East of the forecast area and precipitation may be more dz/fzdz
vs. Snow with the shorter moisture column and lack of ice in the
cloud layer. Tried to leave precipitation as the wintry mix for much of
the through the late night hours tonight. Used a blend of the
guidance highs/lows for today/tonight.
Long term...(tuesday through sunday)
issued at 346 am CST Monday Nov 30 2015
For Tuesday through Wednesday night...
Main forecast concerns this period...wintry mix yet Tuesday...then
mainly snow chances Tuesday night...temperatures.
30.00z models showing a tighter consensus as the middle level low
tracks east Tuesday/Tuesday night. Models still showing some speed
differences on this at 12z Wednesday...with European model (ecmwf) slower and NAM/GFS
faster. Trend does favor slower of the earlier model runs with
the eastward moving middle level low Tuesday/Tuesday night. Signal remains
good for rising heights/shortwave ridging to build in behind the
departing low/trough Wednesday/Wednesday night. Forecast confidence is average to
good this period.
With the deeper moisture lifting north of the forecast area Wednesday...precipitation
type remains a problem...even as the column cools. Weaker forcing/
lift across the area Tuesday...with 925-850mb cold advection and the
main shortwave moving east/north of the area. Precipitation
Tuesday...whatever type...looking to be on the light side. With
shallower moisture over the area at least Tuesday morning and
potential for no ice in the clouds...left precipitation as a wintry mix
for most of the area Tuesday morning. With continued cooling as the
middle level low moves over the area...top of the moisture column
looks to intersect some -10c to -15c air at the top Tuesday
afternoon...with precipitation trending toward more snow by later Tuesday
afternoon/Tuesday night. However...by Tuesday night the 850-500mb low is
moving east of the area and forcing/lift is becoming rather
limited. Carried highest precipitation chances across the north end of
the forecast area Tuesday morning...with some weak deformation band lift
and deeper moisture spreading over the northwest half of the forecast
area Tuesday afternoon. 30 percent precipitation chances southeast to 60-70
percent northwest Tuesday afternoon reasonable. Mainly 30-40 percent
-sn chances Tuesday night as the 850-500mb low drift across the area
reasonable as well. With the cold 850-500mb airmass sliding across
the area Wednesday morning and lingering deeper cyclonic flow...added
some flurries mainly along/east of the MS river. As system departs
and heights start to rise...trend is for clouds to decrease for Wednesday
afternoon/night as surface-500mb ridging builds into the region. Used
a blend of the guidance highs/lows for Tuesday through Wednesday night.
For Thursday through sun...days 4 to 7...
Main forecast concerns this period...above normal temperatures for early
Medium range models in rather good agreement for longwave ridging to
build east across central noam Thursday into Sat...ahead of the next
troughing to dig into The Rockies. Reasonable agreement among models
for a northern-southern stream split of the western noam troughing
late Sat into sun..with the southern portion slowing and the
northern stream portion remaining more progressive across southern
can. This ridging/splitting pattern leaves the upper Midwest
generally high and dry in the day 4-7 period with forecast confidence
Lower level warm advection returns for Thursday into Sat. This flow is
rather dry with a strong westerly component to the surface-700mb flow.
Thursday through Sat shaping up to be a dry and seasonable mild period...
with highs trending at least 5f to 10f above normal. Depending on
how much snow cover we end up with from today through Tuesday night...
model/ensemble consensus highs by Friday/Sat may be even warmer as the
naefs ensemble mean has 850mb temperatures 1.5 to 2 Standard deviations
above normal Fri/Sat. Time will tell...but highs approaching 50f in
the MS/WI river valleys Friday/Sat not out of the question. Stayed with
the ensemble blend for now as spent most of the time focusing on the
short term. Some cooling Sat night/sun as the passing northern
stream shortwave trough drags a weak...and dry...cold front through the
region. Even so...temperatures Sunday looking to remain above normal.
Aviation...(for the 06z tafs through 06z Monday night)
issued at 1158 PM CST sun Nov 29 2015
VFR conditions late this evening will gradually give way to lower
MVFR ceilings/visibility as an area of wintry mixed precipitation
moves across the taf airfields. Tried to keep weather type simple
between rain/snow...but could see periods of sleet and perhaps even
some freezing rain. Multiple updates likely as this complex storm
system moves across the region over the next 24 to 48 hours.
WI...Winter Weather Advisory until midnight CST Tuesday night for
Winter Weather Advisory until 9 am CST this morning for
Winter Weather Advisory until midnight CST Tuesday night for
Minnesota...Winter Weather Advisory until midnight CST Tuesday night for
Winter Weather Advisory until 9 am CST this morning for mnz096.
Iowa...Winter Weather Advisory until midnight CST Tuesday night for
Winter Weather Advisory until 9 am CST this morning for iaz010-