Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service La Crosse WI
510 PM CST Friday Dec 19 2014

Short term...(tonight through Saturday night)
issued at 300 PM CST Friday Dec 19 2014

Main forecast concerns this period...low level moisture/clouds and
any -fzdz chances...temperatures.

18z data analysis had a broad area of high pressure from eastern ont
southwest to MO. 19.12z radiosonde observations showed a rather strong inversion near
900mb across the region...with plenty of moisture trapped in/under
it producing rather widespread lower stratus clouds across the
region. Some breaks in the clouds over parts of Illinois/eastern WI.
Otherwise developing southeast to south flow on the west side of the
ridge axis was advecting more moisture northward into IA/MN. With
lack of snow cover...temperatures actually warming a bit today under the
clouds in the developing southerly warm advection flow.

19.12z models look to have initialized well...though all were about
1mb too high with surface pressures in the broad high from ont to the
middle MS valley. Solutions similar for tonight through Sat night and
converging toward a tighter compromise of their earlier runs as
one shortwave passes south of the area tonight and another moves
into Minnesota Sat night. Short-term forecast confidence is generally good
this cycle.

In the short term...the main concern tonight through Sat night is the
depth of the lower level moisture/stratus and extent of any lift
in/just above the moisture layer. This moisture remains trapped
under an inversion around 850mb tonight and around 900mb Sat night.
Deepest moisture is over the forecast area late this afternoon/evening
but most cloud bases in the 1500-2500 feet range with surface temperature-dew
point spreads of 5f to 10f. This not overly conducive to to -dz/
-fzdz and removed any -fzdz mention through this evening. Weak/MDT
925mb warm advection and 280-285k persist across the area tonight
through Sat night. Some of the stronger of this Sat night as the 925mb
flow increase to around 20kts. Surface-925mb saturation increases through
tonight...with lowering ceilings and temperature-dew point spreads. Will
continue the patchy -fzdz spreading across much of the forecast area
during the late evening/overnight hours into early Sat morning.
Added a patchy fog mention overnight into Sat morning as well. Left
the bulk of Sat/Sat evening dry but cannot rule out isolated -dz/
-fzdz and patchy fog. Continued the patchy -fzdz mention later Sat
night with the increase of lift late but did remove any -sn mention
as top of the moisture column later Sat night would only be about
-5c at the coldest with no ice in the clouds. Added some fog for
this period as an increasingly moist surface-925mb airmass is pushed
northward over the cold landscape. Favored warmer of guidance lows
tonight/Sat night with the thickening low clouds and a blend of
guidance highs for Sat.

Long term...(sunday through friday)
issued at 300 PM CST Friday Dec 19 2014

Main forecast concerns for sun through Monday night...precipitation types/chances as
the trough/low deepen and approach...temperatures.

19.12z models show a tightening consensus as strong shortwave energy
tops the western noam ridging sun then drops into the central/
northern plains Sun night/Mon. This consensus favors slower/
stronger of the earlier runs at 00z Tuesday. Good agreement among GFS/
European model (ecmwf)/NAM for a 500mb low near kfsd by 00z Tuesday...migrating toward
kdsm by 12z Tuesday. Can-Gem more progressive/further north and
appears as an outlier by Monday night. Given good/tight consensus
among NAM/ECMWF/ GFS through 12z Tuesday...forecast confidence for sun through
Monday night remains generally good.

Falling heights and strengthening low pressure in the northern plains
continues the south/southwest low level flow into the area sun/Sun
night. Surface through 850mb low remains northwest of the forecast area through
00z Tuesday. This persistent south/southwest warm advection flow ahead
of the low warms the surface-925mb layer to near/above 0c by Monday/Monday
night. Depth of moisture the area starts to increase sun along with
the strength/depth of the forcing/lift. Moisture/lift increase even
more for Sun night through Monday night with precipitation becoming likely for
much of the area Monday/Monday night. Transitioned any precipitation from -dz
to -ra Sun morning with deeper moisture and one stronger surge of
850- 700mb warm advection lifting across the area. Bit of a lull
in the lift Sun night with the increase of dynamic lift/pv
advection ahead of the trough/shortwave moving across Monday/Monday
night. Raised precipitation chances Monday into Monday night. Added patchy fog
for Sun night/Monday morning as well with increasing dew points over
the cold ground. Trended precipitation Sun night through Monday night toward
more -ra as model soundings and x-sections for this period showing
at least the surface- 900mb layer warming into the 0c to +3c range...
especially over the central/south parts of the forecast area. Lingered
a -sn chance mainly during the nighttime/early morning hours with
a cooler boundary layer and any heavier precipitation rates if they occur
capable of cooling the column enough for snowflakes to reach the
ground. Will also have to watch the late night/early morning hours
of sun/Monday nights for -fzra on cold ground surfaces. Used a blend
of the guidance highs/ lows for sun through Monday night...raising lows
a bit Sun night with the southerly boundary layer flow/thick low
clouds over the area.

For Tuesday through Friday...days 4 to 7...

Main forecast concerns this period...precipitation chances/types Tuesday/Wednesday
and again Friday...temperatures.

Medium range model runs of 19.00z/19.12z in quite good agreement Tuesday
with the trough deepening over the central Continental U.S.. trend favors
slower/stronger of the earlier runs. Consistency remains good for
the trough to be progressive into the eastern Continental U.S. Wednesday/Thursday as
strong energy rotates up the east side of the trough. Improving
signal for shortwave ridging to build over the middle/upper MS valley
for Christmas one trough lifts into the can Maritimes and
more energy digs into the central/southern rockies. Potential for
some portion of the energy to lift toward/into the middle/upper MS
valley Friday. Plenty of spread among the models and ensemble
members with the details by day 7. Confidence in the day 4-7
period is average to good Tuesday-Thursday then average to below for Friday.

Improving model consensus for the surface-700mb low to be occluding/
as it moves across the upper Midwest Tuesday. Forcing/lift over the
forecast area weakens through Tuesday/Tuesday night with the moisture conveyor
being pushed further from the area by the occlusion. Column does
cool Tuesday as the occluding low/middle level trough axis pass. Higher
chances of precipitation Tuesday/Tuesday night as a deformation band would
rotate across the area. Mix of -ra/-sn Tuesday morning giving way to
-sn for late Tuesday/Tuesday night. Strong energy in the southern portion
of the longwave trough rotating up the eastern Seaboard Wednesday drags
the middle level low into the eastern Great Lakes by Wednesday night...
spreading drying northwest 850-500mb flow over the region. Any
lingering -sn looking to taper off from southwest to northeast
Wednesday/Wednesday evening. Surface through middle level ridging prognosticated to slide
across the upper Midwest Thursday...with our best shot at some sunshine
next week on Christmas day. Confidence average at best Friday but
latest deterministic model runs would lift a portion of The
Rockies energy into the upper Midwest at the end of the week.
Plenty of lower level detail differences by Friday with any
potential surface low track/strength and how much warm air is drawn
north ahead of the system for precipitation type issues. Will stay with
the model/ensemble consensus on Friday for now. Model/ensemble
consensus of highs/lows for Tuesday-Friday seems to have them well
trended as well.


Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 510 PM CST Friday Dec 19 2014

A 5-10kt southerly flow is forecast to persist at the taf sites
through Saturday...which ever so slowly will bring in more
moisture. Expect this moisture to manifest as a persistent stratus
deck...with ceilings mainly MVFR at lse and MVFR/IFR at rst. The
best chance for IFR at rst as well as MVFR br is from late this
evening into Saturday morning...coinciding with the diurnal
minimum. Additionally...IFR ceilings and MVFR visibilities are not
far away from rst...near I-35.

Cannot rule out some patchy freezing drizzle at rst...but
confidence is too low to include at this time.


Arx watches/warnings/advisories...


Short term...rrs
long term....rrs

National Weather Service Glossary of Abbreviations