Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
558 am CST sun Dec 21 2014
Updated aviation portion for 12z taf issuance
Short term...today...tonight...and Monday
issued at 401 am CST sun Dec 21 2014
Precipitation trends/type and snowfall amounts are the main forecast concerns.
Southerly return flow was increasing across the region early this
morning...but moisture was still too shallow for any significant
precipitation. A few flurries were reported in NC WI...and also suspect
that some patchy freezing drizzle was occurring. Widespread
overcast conditions covered the forecast area...and temperatures
were in the 25 to 30 range.
A low-level jet is expected to gradually strengthen to 30-40 kts
today...resulting in increasing warm air advection/isent lift during the late
morning and afternoon. Patchy flurries/fzdz will start to mix with
light snow as the morning progresses...then change over to all
snow as deeper saturation and stronger lift occurs this afternoon.
Have increased probability of precipitation to categorical over NC/far NE WI late this
afternoon...with snow accums around an inch anticipated there. Quantitative precipitation forecast
and snowfall amounts will taper off quickly farther southeast. High temperatures
will be in the lower to middle 30s.
Steady light snow (and categorical pops) will continue through
the evg across NC/far NE WI...with just patchy light snow over the
southeast part of the County Warning Area. Another inch or two of snow accumulation is
expected over the Northwoods. A dry slot will work its way into
the region late tonight into early Monday...which will cause snow
to decrease and mix with freezing drizzle...as ice crystals get
stripped out of the mid-levels. Precipitation will increase again by Monday
afternoon...as an upper low moves into northwest Iowa and a short wave trough
lifts north into WI. Precipitation should be mainly snow across NC/far
NE WI...with a rain/snow mix changing to rain farther southeast. Low
temperatures tonight are expected to be around 30 degrees...with highs
on Monday in the 30s.
Long term...Monday night through Saturday
issued at 401 am CST sun Dec 21 2014
The latest GFS and European model (ecmwf) ensemble means have become more consistent
with the track of a deepening cyclone during the middle of the
upcoming week. This track would take intensifying low from
southwest lower Michigan to roughly Sault Ste Marie. However...the
00z GFS and European model (ecmwf) now shows flatter upstream flow...and consequently
takes the low on a weaker and more east heading...thereby missing
the region completely. This is a new development...but until the
ensemble means show this shift...plan on keeping continuity as much
as possible. Will therefore rely on the ensemble means for much of
this forecast after 12z Wednesday.
Monday night through Tuesday night...low pressure will be moving
east into southwest Wisconsin Monday night ahead of shortwave energy
rotating northeast into the region. With the approach of better
forcing...should see precipitation surge northward through the
night. Still looking at mainly snow over north-c Wisconsin...a mix
north and west of the Fox Valley...and mainly rain over the rest of
the forecast area. The low will move northeast on Tuesday which
will transport warmer air into north-c Wisconsin where snow could change
to a mix...while a dry slot moves overhead over eastern Wisconsin.
This may lead to break in the precipitation across the east...or a
transition to light drizzle. Accumulating snow will likely occur
over north-central Wisconsin before the change over occurs...that could
potentially prompt an advisory. Temperatures cool aloft a bit
across central and north-central Wisconsin Tuesday night...but the
dry slot will remain an issue...which means precipitation will remain
rather light. Not expecting much accumulations if temperatures cool
enough for snow over the i39/Route 51 corridor. Late in the
night...will be watching an intensifying cyclone move north into
Southern Lower Michigan. The eastern edge of the precipitation shield from
this low may be felt along the Lakeshore...but temperatures look warm
enough for a rain/snow mix. Forecast temperatures look good Monday
night and Tuesday...but raised low temperatures Tuesday night.
Rest of the forecast...the low pressure will continue to strengthen
as it heads towards the Soo on Wednesday. This track would bring a
glancing blow to northeast Wisconsin...with sub-advisory type snows. Any
deviations though could lead to much higher totals...or none at
all...so uncertainty remains high in any case. Behind the
low...expect west winds to strengthen and become gusty Wednesday
afternoon through Thursday...which would blow/drift any new snow.
Christmas day continues to look quiet besides the gusty winds. Then
will be watching the next chance of snow arrive on Friday. Next
weekend looks quiet.
Aviation...for 12z taf issuance
issued at 539 am CST sun Dec 21 2014
A surge of warmer air will generate some fzdz and flurries this
morning...which will change to light snow as deeper saturation
arrives this afternoon. The most significant snow (and associated
IFR conditions) should occur over north central and far NE WI later
this afternoon and evening. A dry slot will arrive after midnight...
which will likely cause some fzdz to redevelop overnight. Widespread
MVFR/IFR conditions are expected through the taf period.
Some marginal low level wind shear is possible this evening. For now...just added
a mention at the rhi taf site.