Scientific Forecaster Discussion
fxus63 karx 191739
Area forecast discussion
National Weather Service La Crosse WI
1139 am CST Fri Jan 19 2018
Short term...(today and tonight)
issued at 316 am CST Fri Jan 19 2018
Main short-term fcst concern is the mild temperatures.
Data analysis at 06z had low pressure over south-central can and
high pressure centered over the lower MS valley. Broad gradient flow
between the 2 was producing MDT SW to west lower level flow and
warm advection across the upper Midwest early this morning. This
flow combined with plentiful high level moisture/clouds spilling
across the region was responsible for seasonably mild temps, with
temps across Minnesota/IA/WI early this morning mainly in the mid 20s to
lower 30s, some 10-20 degrees above the normals.
Model runs of 19.00z offering good agreement as ridging aloft builds
across the north-central Continental U.S. For today/tonight, ahead of strong
troughing moving into the western Continental U.S.. short-term fcst confidence
is good this cycle.
For the short-term, another mild day in store for the area with the
SW to west lower level flow sourced from central/northern plains.
925mb temps progged to warm into the 0c to +6c range this afternoon.
Mixing these temps to the sfc would result in highs today in the mid
40s to mid 50s. However, there is the snow cover and the question of
how deeply will the column mix as plenty of high level moisture/
clouds ahead of the western trough to continue streaming over the
upper Midwest. Given the high level clouds, the snow cover and a
slacking pressure gradient, appears column not likely to mix to
925mb. Even so, with the warm start the blend of the guidance highs
in the upper 30s to lower 40s looks quite reasonable. The low moving
across southern can drags a weak cold front across much of the
region tonight. This results in little more than a wind shift as
little in the way of actual colder air behind the front. The feed of
high level moisture/clouds continues over the region tonight, to
limit diurnal cooling. The blend of guidance lows in the 20s tonight
appears well trended.
Long term...(saturday through thursday)
issued at 316 am CST Fri Jan 19 2018
Saturday looks to be the calm before the storm with the forecast
area between weather systems and a rather weak surface pressure
pattern. Expect increasing clouds through the day, but winds will
be light and temperatures will rise into the upper 30s to lower
40s by mid-afternoon.
Focus quickly turns to Sunday into Monday as a Four Corners
trough closes off and ejects northeastward into the upper Midwest.
For Sunday, watching an increase of low-level moisture and warm
air advection across the region. Modest isentropic lift on the
280-290 k surfaces suggest there could be periods of light drizzle
and/or freezing drizzle depending on surface temperatures. Best
chance for freezing precip looks to be Sunday morning for areas
mainly south of I-90. Temperatures should warm enough into the
afternoon for any additional precip to transition to drizzle or
even some light rain.
Current model consensus is to bring the surface low into southwest
Iowa by 12z Monday. This results in a large swath of warm advective
precip across the forecast area Sunday night into Monday morning.
The GFS is quickest in lifting this precip northward, whereas the
European model (ecmwf) is slowest with the NAM somewhere in-between. In addition,
the thermal and ice present profiles for each model differ. The
NAM is very warm aloft with little to no ice, but also colder at
the surface resulting in substantial freezing rain. The GFS is not
as warm aloft, but does include ice with warmer surface temperatures,
so it favors either rain or snow. The European model (ecmwf) is closer to the GFS,
but because it's slower, it brings a slug of higher quantitative precipitation forecast across
the forecast area later Monday afternoon whereas the American
models are much lighter as the system quickly lifts northeastward.
Despite all these differences, a general consensus would suggest
rain or drizzle Sunday afternoon will gradually give way to a
wintry mix from west to east Sunday night through the day on
Monday, eventually changing over to all snow for some areas. Best
chance for snow accumulations looks to be across the northwest
third of the forecast area. If NAM thermal profiles verify, much
more concerned about ice accumulations and this is something that
bears watching over the next couple days. At this time, it's safe
to say anyone with travel plans Sunday into Monday should watch
the latest forecasts and statements as travel is likely to be
impacted in some way.
Tuesday through Thursday look dry with seasonable temperatures as
a broad area of high pressure works its way across the nation's
Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1139 am CST Fri Jan 19 2018
Low level wind shear is expected this afternoon as southwest winds
at 2 kft increase to 40 kts. The low level wind shear will mainly be a concern at
klse as surface winds look to stay around 10 kts through the
afternoon. MVFR to IFR conditions are possible late tonight into
Saturday morning with visibilities falling into the 2 to 4 sm
range in br, mainly at krst.