Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion...updated 
National Weather Service Green Bay WI
1020 PM CST Monday Dec 22 2014

Updated aviation portion for 06z taf issuance
and new information added to update section

Update...
issued at 1006 PM CST Monday Dec 22 2014

Going to make one last adjustment to the advisory...and that will
be to add southern Marinette County. There may be a considerable
difference in snow amnts across that forecast zone...but totals in
the northwest part of the area seem high enough to warrant adding them to
the advisory.

Otherwise...forecast now playing out as expected. Initial band of precipitation
has slowed it/S northward progress...and should linger in place until it
weakens. Earlier in the eveing...there was a fairly large precipitation-free
area between this band of precipitation and the next sig shortwave coming out of
the plains...but that has begun to fill in. So will have periods
of precipitation continuing overnight...with another band of heavier precipitation
arriving toward morning.

Also aware that there is keen interest in the eventual track/
impact of secondary cyclone riding up the front on xmas evening. The
overall trend on the 00z guidance products that have arrived thus
far has been to jog the track back to the west. Hard to really
read much into that as guidance has literally been all over the
place with this system. One thing that may be Worth noting is that
the Canadian regional has settled in on the more westerly track for at
least it/S last 3 runs. It now seems likely that precipitation from that
system will at least clip far eastern WI Wednesday aftn/evening. Even
so...precipitation type could be an issue as models suggest temperatures aloft will
be cold enough for snow...but boundary layer temperatures look marginally
too warm. Still lots of forecast details to work out.

Updated product suite out as soon as possible.

Update
issued at 857 PM CST Monday Dec 22 2014

Have decided to add wood and Portage to the advisory. Snow is
falling there now. There will be a lull in the precipitation in that area as
the precipitation switches over to dz for a time overnight...but another
band of heavier snow should arrive in time for the morning
commute. The addition will give the advisory more of a SW-NE
orientation...which should line up pretty well with the southern edge
of the upper deformation zone expected to be across the area tomorrow.

Updated product suite will be out as soon as possible.

Update
issued at 747 PM CST Monday Dec 22 2014

Leading band of precipitation still liftg northward across the forecast area. Northward
progress is likely to slow...but should get snow pretty much across
all but maybe northwest Vilas County by midnight. Roads reportedly
starting to get slick...so will just start the advisory now.

Warmer air just above the surface making northward progress into east-c WI from the
S. So snow should change back to mainly rain over much of east-c WI
by midnight. Expect the changeover line to work about as far north as
a pcz-cli-ocq-sue line by late evening...then pretty much hold in
place the rest of the night. But the band of precipitation should gradually
weaken overnight...and that will hold amnts in check. Alignment of
advisory looks pretty good...mainly need to watch areas just to
the southeast to see if we need to expand it a bit on the southern edge.

Another band of precipitation will surge northward into the forecast area from the S
starting around 4 or 5 am. That will be associated with strong shortwave now
rounding the base of developing upper low...and will set up strong upper
shear zone across the forecast area. Snows in the advisory area will pick
up early tomorrow morning when the precipitation arrives...and snow could
be falling quite heavily in a band from auw-imt for the morning
commute. Were it not for the warm boundary and wet nature of the
snow that will keep snow ratios down...this might warrant an upgrade
to a warning.

Concern even exists for areas to the southeast of the advisory for the
morning commute. Several of the hi-res models indicate sig cooling
of low-levels early tomorrow morning as the strongest lift moves
through. Model forecast soundings cool to the point where they would
support a brief period of moderate or even heavy snow. This would
be occurring near the edge of the dry slot...where precipitation could be
convective. This seems most likely to occur in a band from
Wautoma to atw-grb to sue...as areas to the southeast may just have too
much warmth to overcome. Confidence in this occurring is not high
enough to warrant a change in the forecast at this point...but will try and
highlight the possibility in upcoming Special Weather Statement and updated severe weather potential statement.

Even if a temporary change to snow occurs...precipitation across the southeastern part of the
forecast area would likely flop back over to dz as middle-level dry slot
drives NE into the forecast area late tomorrow morning.

Updated grids just sent. Will be updating the text products as soon as possible.

Update
issued at 551 PM CST Monday Dec 22 2014

Lead band of precipitation surging northward across the southern part of the forecast area
early this evening. Precipitation generally liquid or a mix at the
onset...then changing to mainly snow as evap cooling down to the
wet-bulb temperature knocks low-level temperatures back a couple degrees. Warm air advection in
the 925-850 mb layer should eventually lead to a change back to
liquid over east-c WI later tonight...though timing of that is hard to
pin down. With ground temperatures generally at or above freezing...its hard to
see getting more than a slushy accumulation on grassy areas this
evening.

Slightly colder temperatures back to the northwest in c-NE will allow for more
accumulation in those areas...and some travel problems will likely
develop there as the evening progresses.

Despite now being within 48 hours of the possible event...there is
still a high degree of uncertainty on the possibility of having
secondary cyclone running up the front impact the area on xmas
evening.
&&

Short term...tonight and Tuesday
issued at 249 PM CST Monday Dec 22 2014

Plenty of potential pitfalls in this forecast over the next 24 hours
as a large dynamic system gradually moves into the region. The
main forecast dilema to be precipitation type over most of the forecast area as
warmer air at the surface tries to lift north and will be countered by
dynamic cooling trying to keep the atmospheric column cooler.

The 20z msas surface analysis indicated a pair of low pressure systems
over the northern and Central Plains. A warm front extended from the Central
Plains eastward into the Ohio River valley. The radar mosaic showed a
large band of precipitation lifting north from the Midwest into the Great
Lakes/upper MS valley. This precipitation was mainly in the form of rain...
although some light snow or sleet were mixing in at times.

A closed upper low...located within the broad central Continental U.S. Longwave
trough...is forecast to move from the Central Plains to the Midwest tonight.
As this takes place...the pair of surface lows will consolidate and lift
northeastward into NE Iowa by daybreak with the warm front pulling north to near
the WI/Illinois border. The large slug of Gulf moisture...overrunning
the warm front and ahead of a northern moving shortwave...will continue to
overspread the rest of WI this evening with precipitation continuing through
the night. Precipitation type to be the biggest forecast issue as surface temperatures are
forecast to be around freezing north-central WI to the middle 30s east-central WI
and the 800 mb 0c isotherm to be lifting northwest overnight. Current thinking
is for an inch or two of snow to fall northwest of a line from isw-mnm
with lesser amounts across east-central WI where more rain is
anticipated.

Precipitation will pick up in intensity late tonight through Tuesday morning as this
dynamic system heads into the western Great Lakes region. Precipitation type will
continue to be a major forecast headache...especially over the southeast half of
NE WI where the strong dynamics may provide enough cooling to allow
the mixed precipitation or all rain to briefly change to all snow...despite
surface temperatures in the upper 30s. This could provide for a quick inch or
two of snow from roughly ste-grb-imt back to ste during the
morning. This area is also to reside on the nose of an incoming
dry slot where lift will be maximized. For north-central WI where the precipitation
to remain all snow...look for an additional 2 to 4 inches of
accumulation to occur.

Long term...Tuesday night through Monday
issued at 249 PM CST Monday Dec 22 2014

Slow moving system will continue to drift over the Great Lakes
region Tuesday night into Wednesday night. Light wrap around precipitation
will gradually depart the northwest half of the state Tuesday
evening while a dry slot drives northeastward into much of eastern
Wisconsin.

Of notice for Wednesday into Wednesday night is the subtle shift
westward with the new precipitation shield due to phasing of the Great
Lakes system with southern stream energy. Will confine higher probability of precipitation
along the Lakeshore and later runs can continue to up probability of precipitation and
increase coverage if this trend continues. Otherwise quiet
weather for xmas day at this time.

Forecasts are in good agreement with a surface low with middle level
trough sliding over the area toward Friday night. Temperatures may
be warm enough for a rain snow mix over east central Wisconsin
but will keep it snow due to overnight timing. Another possible
frontal system may slide over the area early next week. This
early next week system may be followed with some colder air mass
but the duration looks minimal on the recent model runs.
&&

Aviation...for 06z taf issuance
issued at 1006 PM CST Monday Dec 22 2014

IFR or poorer flight conditions expected the next 24 hours. Precipitation will
continue but should diminish some overnight...with a new band of
heavier precipitation arriving around daybreak. Middle-level dry slot driving into
the area from the S will likely shut precipitation down to dz/fzdz over at
least the southeast 1/2 of the area tomorrow...but ceilings and visibilities will
remain low.
&&

Hydrology...
issued at 249 PM CST Monday Dec 22 2014

Mild conditions are expected over east central Wisconsin through
Wednesday along with the potential of up to 0.25 to 0.40 inches of
rain tonight into Tuesday. While the rain will produce some
runoff...primary issue is the affects on ice conditions on area
rivers and lakes.
&&

Grb watches/warnings/advisories...
Winter Weather Advisory until 6 PM CST Tuesday for wiz005-
010>013-018>021-030-031-035-036-073.
&&

$$

Update.........Skowronski
short term.....AK
long term......tdh
aviation.......skowronski
hydrology......tdh

National Weather Service Glossary of Abbreviations