Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
1026 PM CST Thursday Dec 18 2014
Short term...(tonight through Friday night)
issued at 245 PM CST Thursday Dec 18 2014
Main forecast concerns this period...clouds/temperatures.
18z data analysis had broad area of high pressure centered north of
Lake Superior with ridging south into Illinois/west Tennessee. A weak trough was
located northwest to southeast across WI...with low stratus clouds
remaining across much of west and northeast 1/4 of Minnesota near this trough.
Some partly sunny skies from northwest Minnesota to northeast Iowa but this
area being squeezed by clouds advecting from both the west and east.
Temperatures under the areas of thicker low clouds slow to warm again today.
No big problems noted with 28.12z model initializations. Solutions
quite similar as one shortwave passes mainly north of the area
tonight then the next shortwave drops into Iowa Friday night. Trend
generally favors a compromise of the earlier runs with these
features. Short term forecast confidence is good this cycle.
For the short term...approaching/passing shortwaves slowly push the
surface/low level ridge axis east of the area tonight through Friday night.
Low level flow becomes south to southwest and slowly increases through
the period. Good model consensus for the broad low stratus shield
from ND to MO and eastern Kansas/OK to advect into the area tonight then
spread over much of Minnesota/IA/WI for Friday/Friday night. Much of this
moisture trapped in/under a strong inversion between 950-850mb...
with cloud top temperatures mostly in the -5c to -8c range. Ice
is not likely in these clouds. With some weak warm advection/
isentropic lift in the top of the moisture layer...may yet need a
patchy -fzdz mention Friday night. With some signal for drier air in
the surface-950mb layer left this out for now. Otherwise...more overcast
stratus skies will be the rule tonight through Friday night. These
clouds to reduce the diurnal temperature range with lows trending warmer
and highs cooler. Favored warmer of guidance lows and cooler of
guidance highs...especially Friday/Friday night.
Long term...(saturday through thursday)
issued at 245 PM CST Thursday Dec 18 2014
Main forecast concerns for Sat through Sun night...precipitation chances/types Sat
night through Sun night...temperatures.
Model runs of 18.12z in good agreement quickly sliding the Iowa
shortwave east to northern in by 00z sun. Improving agreement with
the next shortwave trough moving east across the northern plains Sat
night...then pushed northeast Sunday as a stronger wave comes through
the western ridging and digs into the central/northern plains
sun/Sun night. Trend...except for can-Gem...is slowing/stronger with
the central/northern plains troughing by Sun night. Given the
reasonable and improving consensus...forecast confidence in the Sat through
Sun night period is average to good this cycle.
Shortwave moving across Iowa/northern Illinois Sat looks to do little more
than continue pushing the low level ridge axis to the east and
further envelop the area in warming southwest low level flow. Surface-
700mb warm advection continues Sat through Sun night...with 925mb temperatures
a couple degrees either side of 0c by Sun night. Moisture continues
to gradually increase in this flow. Surface-800mb layer quite saturated
by Sat night...with depth of saturation and forcing/lift increasing
sun/Sun night as the surface-middle level trough/low approach. Top of the
moisture column around -5c Sat night...too warm for ice in the
clouds. Any precipitation Sat night to be -fzdz with the boundary layer
temperatures/surfaces expected to be below freezing. Given some lift through
at least the top of the saturated layer...continued the patchy -fzdz
mention across the forecast area Sat night. Increasing depth of
moisture/saturation sun...along with a broad increase of lifting...
precipitation chance transitions to -ra. By later the depth of the moisture
increase enough or distance between increasing middle level moisture
and the lower moisture decreases enough for seeder-feeder to
introduce ice into the clouds. Given this...and the near/above 0c
surface-925mb layer across much of the forecast area by Sun night...left
precipitation chances as -ra/-sn for now. Suspect with the low level warm
advection/thickening clouds and precipitation...temperatures Sun night will not
fall much and stay nearly steady near/above freezing. May yet need a
-fzra mention for Sun night but plenty of time to zero in on those
details. As the surface-925mb layer saturates...fog looks to once again
develop/spread across the landscape. Introduced a patchy fog mention
from later Sat evening into Monday morning for now. With the general
lack of snow cover...at this time not sure how widespread or dense
any fog might become. Trended toward cooler of guidance highs Sat/
sun and warmer of lows Sat/sun nights.
For Monday through Christmas day...days 4 to 7...
Main concerns this period...-ra/-sn chances through the period...
potential snow accumulations...temperatures.
Medium range models of 18.00z/18/.12z continue to indicate strong
longwave troughing over central noam Monday...with the trough deepening
over the lower through upper MS valley Monday night/Tue. Trend is a little
slower Monday then favors a compromise of earlier runs Tuesday. Good
consensus for a 500mb low over the western Great Lakes Wednesday with
southern portion of the trough/energy rotating northeast through the
eastern Continental U.S.. middle level low to migrate east with this energy
Thursday...with weak shortwave ridging to build over the central Continental U.S.
As the troughing in the west reloads. Even with a reasonably good
consensus among the models on the large scale features...plenty
of lower level/smaller scale detail differences continue with
potential impacts on the sensible weather in the day 4-7 period.
Day 4-7 forecast confidence is average this cycle.
Precipitable water values prognosticated to increase into the 1/2 inch range for Monday into
Tuesday. This ahead of the slow moving surface-middle level low/trough and
increasing forcing/lift with it. Precipitation chances in the 40-60 percent
range for Monday into Tuesday reasonable. Precipitation type is the big question
as surface-925mb temperatures will be a few degrees either side of 0c much of
this period. For now will continue with a -ra/-sn mix across much
of the forecast area early next week...until lower level thermal
details become clearer. Tightening signal for the surface through middle
level trough axis to pass later Tuesday/Tuesday night...with lower level
cooling and any precipitation Tuesday night into Wednesday night mainly as snow.
Improving model consensus for a 48hr quantitative precipitation forecast total of as much as 1/2
inch from 12z Monday to 12z Wednesday...but how much of this falls as rain
or falls as snow is the question. Time will tell as the low level
details sort themselves out. Present trends would spread surface through
middle level high pressure ridging across the upper Midwest Thursday...for
a generally dry...possibly partly cloudy...Christmas day. Plenty
of clouds expected Monday into Wednesday...with small diurnal temperature changes.
Blend of the guidance highs/lows for Monday-Thursday looks to have them
well trended for now.
Aviation...(for the 06z tafs through 06z Friday night)
issued at 1026 PM CST Thursday Dec 18 2014
Some changes have been made to the previous aviation forecast...
which begun on a 0130z amendment to rst. MVFR stratus still
persists over the taf sites. However...a wedge of dry air below an
altostratus deck has made its way from I-35 towards rst. Rst
should go VFR for the start of the taf period as a southwest wind
at the height of the clouds pushes the dry air eastward. In
fact...this drying could make it all the way to lse later in the
morning...between 15-19z to allow for temporary scattering out
there. Late in the afternoon...around 20z...MVFR stratus over
western Iowa and Minnesota looks to move into rst. Meanwhile...light flow
should allow MVFR stratus to reassert itself at lse. Anticipating
then conditions to stay MVFR into the evening as the two stratus
decks merge up.