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Area forecast discussion
National Weather Service Green Bay WI
345 am CST Wed Nov 22 2017

Forecast discussion for routine morning forecast issuance

Synopsis...
issued at 341 am CST Wed Nov 22 2017

Still rather chilly today, then warmer for Thanksgiving day and
Friday. Scattered light wintry precipitation could result in
slippery travel conditions tonight.

The primary large scale features are a trough off the West Coast,
downstream ridging just inland from the coast, and a trough over
eastern North America. The features will deamplify some the next
couple days, then reamplify again during the weekend. Slow
progression is expected thereafter.

The modest amplitude and eventual approach of the upper ridge will
result in temperatures varying between near and modestly above
normal after today. Moisture will be limited, so despite some
fairly strong cyclones tracking to our north, only scattered/
light precipitation is expected. Amounts are likely to end up
below normal for the period.
&&

Short term...today...tonight...and Thursday
issued at 341 am CST Wed Nov 22 2017

The cold anticyclone that dropped southeast from Canada will shift
through to the south of the area early today, allowing southwest
winds and a return flow on the back side of the high to set up
by afternoon. The backing low-level flow will take trajectories
away from Lake Superior, allowing the low clouds and flurries to
finally dissipate. Anticipate at least a period of sun before high
clouds stream back in from the west this afternoon.

Isentropic lift will shift southeast through the area tonight,
mainly between 01z and 10z. Forcing may initially be enhanced a
little by the lfq of an upper speed Max, though that aspect of the
forcing won't last as long as the isentropic lift. In general,
the models responded by showing an area of light precipitation
breaking out to the northwest of the area this afternoon. Most of
the models brought the precipitation southeast into the area
during the evening, before wiping it out later in the night. That
seemed reasonable. Precipitaiton amounts will be light, with less
than an inch of snow expected over central Wisconsin, and
probably just a trace or dusting elsewhere. The complicating
factor is that model forecast soundings off both the GFS and NAM
suggest the temperatures in the saturated layer may not always be
cold enough to result in ice crystals, especially toward the
latter part of the event. There may be lingering dry air at low-
levels, so these aren't classic fzdz soundings. If ice crystals
aren't present, it's most likely the precip will fall as very
light freezing rain. Considering the potential impact from that would be
much higher than from just a dusting of snow, manipulated the
grids to get at least a mention of some freezing rain into the forecast.
Will also continue to mention in the severe weather potential statement.

Any lingering precipitation should exit the area by 12z
Thanksgiving morning. So quiet weather with temperatures returning
to a couple degrees above normal is anticipated for the Holiday.

Used a blend of top performing guidance products for highs today,
then used hourly temperatures to generate an atypical diurnal
temperature trend tonight. Stayed close to the broad-based blend
of guidance products for highs on Thursday.

Long term...Thursday night through Tuesday
issued at 341 am CST Wed Nov 22 2017

The medium range ensemble means continue to indicate that
northwest flow will be present in the upper levels over the
western Great Lakes for the next week. Some veering is anticipated
by the middle of next week though. In the meantime, forecast
concerns mainly revolve around the impacts from the passage of a
strong cold front during the Friday-Saturday time period. Will use
a blend of the ECMWF/GFS for this system.

Thursday night through Friday night...as winds shift to the south,
could see some warm advection clouds develop late Thursday night
after a mostly clear start. Dont think that saturation depth will
become sufficient for a chance of any precip. This will likely
change on Friday afternoon, when a strong shortwave pushes a cold
front across the region. Given the magnitude of the forcing, should
see at least scattered rain showers accompany the front. Most of
the shower activity should exit with the front by early Friday
evening. Cold advection will then take place on Friday night driven
by 50 kts of 850mb flow. Mid-levels look pretty dry behind the
front, while winds look too backed for much of a lake effect threat
to the Vilas County snow belts. But suppose could see a few snow
showers move into far northern WI, but without much accumulations.
Should be a windy night otherwise, with some gusts greater than 30
mph. No significant changes to temps.

Rest of the forecast...the windy conditions will persist into
Saturday, with gusts greater than 30 mph possible. Trajectories
become more favorable for lake effect, which could bring a few more
snow showers into north-central WI. Regardless, should see a partly
to mostly cloudy day with highs returning into the 20s and 30s. High
pressure then builds in for Saturday night and Sunday, which should
result in a slow clearing trend. On the back side of the surface
high, strong warm advection is expected to occur early next week.
Some indications of a potent low pressure system will cross the area
by next Tue, though confidence in the details is low this far out.
&&

Aviation...for 12z taf issuance
issued at 341 am CST Wed Nov 22 2017

Ceilings may edge up slowly as the remaining stratocumulus shifts
off to the east this morning. Expect VFR and good flying
conditions until some lower ceilings (probably MVFR or low-end
vfr) and wintry precipitation spread in from the west this
evening.
&&

Grb watches/warnings/advisories...
none.
&&

$$

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