Scientific Forecaster Discussion
fxus63 kgrb 240418
Area forecast discussion
National Weather Service Green Bay WI
1018 PM CST Thu Nov 23 2017
Updated aviation portion for 06z taf issuance
issued at 750 PM CST Thu Nov 23 2017
Strong warm air advection/isentropic lift has resulted in scattered sprinkles
over northwest and north central WI this evening. Do not think
there will be measurable precipitation, as the showers are
falling from a mid-level cloud deck. Temperatures are tending to
warm above freezing as the thicker cloud cover and showers arrive,
but Road temperatures are hovering near the freezing mark, so
will have very light rain and freezing rain mentioned in the
forecast. Suspect that bridges and overpasses will be most
susceptible to a light glaze of ice.
Short term...tonight and Friday
issued at 245 PM CST Thu Nov 23 2017
Temperature trends will be the primary issue through Friday.
Surface data suggests a subtle wind shift working over eastern
Wisconsin this afternoon was associated with an 850 trough sliding
over the area. After a period of weak ridging in its wake,
southwest winds will redevelop and the warming trend will
intensify. First forecast issue tonight will be overnight lows.
Anticipate some evening lows with light winds and clearing skies
before 850 mb temps rise with the increasing winds aloft.
Height 850 temps expected to climb to at least +12 c by noon
Friday, with many locations climbing well into the 50s. Winds will
be gusty and if sufficient boundary layer mixing can occur, a few
wind gusts to at least 30 mph will be possible midday Friday, but
usually this time of the year, the inversion is rather robust.
These warmer temps may need to be expanded further north, but at
this time, clouds and scattered showers will be dropping into the
state during the day. The progs continue to indicate a faster
trend of moving this frontal system and associated precipitation
over the area on Friday. Lapse rates become very steep in the mid
levels Friday but dry lower levels producing saturation issues
with respect to shower coverage. The 850 mb trough is already
departing eastern Wisconsin toward late afternoon.
Long term...Friday night through Thursday
issued at 245 PM CST Thu Nov 23 2017
Expect low amplitude upper level flow to prevail across the region
into early next week.
Some precipitation may linger into Friday night as a surface low
moves away from the state and a mid level short wave passes.
Light rain is possible in eastern Wisconsin Friday evening and
there may be some rain or snow in north central Wisconsin
throughout the night. Colder temperatures return on Saturday
behind the departing low and there is still a slight chance for
snow showers in far north central Wisconsin.
Surface high pressure and a mid level ridge will bring sunny skies
for Sunday, so highs should be at least a few degrees warmer than
Saturday. Warm advection ahead of the next surface system should
allow the entire forecast area to reach the 40s on Monday, and
some lower 50s are possible in central Wisconsin.
The timing of a cold front that is expected to move across
Wisconsin differs among the models. The GFS took it through the
entire forecast area by 12z Tuesday, while the European model (ecmwf) had it about
Half Way through, and the Canadian only had it reaching northwest
Wisconsin by then. Made no changes to the model blend after 00z
Tuesday due to the differences among the models.
Nothing more than slight chance pops until Thursday when another
surface system brings rain and/or snow chances to part of the
area, mainly during the afternoon. Cooler, but still above
normal temperatures can be expected on Tuesday, but highs on
Wednesday and Thursday should be within a few degrees of normal.
Aviation...for 06z taf issuance
issued at 1005 PM CST Thu Nov 23 2017
VFR conditions will prevail overnight into Friday morning, as
mid-level clouds arrive ahead of an approaching frontal system.
Spotty sprinkles or light freezing rain should move out of far
northeast WI in the beginning of the taf period. The main
aviation concern will be low level wind shear through mid-morning Friday, with
west-southwest winds of 40-45 kts just off the surface. Surface
wind gusts should increase due to daytime heating mid to late
Showers are expected to develop over northern WI late Friday
morning, and over the rest of the forecast area as the front
arrives during the afternoon. The showers should end from west to
east in the early evening. MVFR ceilings should develop over
north central WI during the late afternoon and evening, but it
is uncertain how much farther south and east they will progress.