Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
914 PM CDT sun Jul 5 2015
Updated aviation portion for 06z taf issuance
Short term...tonight and Monday
issued at 242 PM CDT sun Jul 5 2015
Main forecast focus to be on the severe potential Monday afternoon as a
cold front moves into the region. In addition to the severe possibility...
this system may produce heavy rainfall with over an inch of rain
accumulation appearing more likely.
The 19z msas surface analysis showed a retreating area of hi pressure over
the eastern Great Lakes. An area of low pressure was situated over the
central hi plains with a cold front extended northeastward through central South Dakota...eastern ND/
western Minnesota border into western Ontario. Radar mosaic picked up a band of
showers and thunderstorms in the vicinity of the cold front. Finally...a broad
southerly wind was continuing to pull Gulf moisture northward through the
central Continental U.S. Toward WI with dew points pushing 70 degrees as close as
Relatively quiet conditions expected for tonight as the prevailing
south wind continue to Pump Gulf moisture into WI. Clouds will be on
the increase over central WI after midngt as the cold front approaches
from the west. There is a small chance that the leading edge of
showers/thunderstorms could reach north-central WI by daybreak. For the eastern
sections of the forecast area...mostly clear to partly cloudy skies
with patchy fog to hold through the night. A mild night in store with min
temperatures ranging from the lower 60s near Lake Michigan...generally middle 60s
elsewhere with a few upper 60s possible.
The cold front is forecast to move through northwest WI Monday morning and essentially
bisect the state roughly from imt-lse by 00z Tuesday. A plethora of
moisture will be waiting for the fnt as dew points climb into the
middle to upper 60s and precipitable water values rise to around 2.00". In the middle-
levels of the atmosphere...a shortwave trough is expected to sweep
eastward along the U.S.-Canadian border and reach the upper MS valley Monday
afternoon. This feature would provide stronger forcing across northern
WI...along with higher shear values. Storm Prediction Center has raised most of the
forecast area into a slight risk of severe thunderstorms Monday afternoon into Monday
evening. Would like to see better instability values than the
1500-2000 j/kg convective available potential energy and the 5.5 c/km middle-level lapse rates. The
NAM is too aggressive with the moisture return sending dew points
into the lower to middle 70s...thus overdoing the instability.
Overall...this looks primarily like a heavy rain event with small
hail/wind gusts in that 30-40 miles per hour range. Have added heavy rain to
the forecast and severe weather potential statement as most locations could easily see an inch of
rain with isolated heavier amounts under a persistent thunderstorm. Much
of the severe potential will come down to how fast clouds overspread
the area...especially over eastern WI. Maximum temperatures will range from the lower
to middle 70s over north-central WI where the rain starts first...to the
lower to middle 80s east-central WI (cooler near the lake).
Long term...Monday night through Sunday
issued at 242 PM CDT sun Jul 5 2015
Timing of the cold front is still the main concern during the
early part of the extended Monday night. The NAM and European model (ecmwf) have
come in a bit slower this run than the GFS and Canadian
models...lingering the heavy rain across much of the County Warning Area during
the evening hours as the front tracks through the western Great
Lakes. Given the general trend of a slower frontal passage will adjust probability of precipitation
to reflect a longer residence time in the heavy rain and add heavy
rain to the evening periods.
Severe weather will still be a concern during the early evening
hours as MLCAPES ahead of the front are still 500 to 1000 j/kg
with 0 to 6 km bulk shear values of 25 to 30 knots. Soundings
indicate the main threat will be heavy rain given precipitable water values of 2
to 2... damaging winds possible with any severe storms.
Given the cape profiles and wet bulb zero heights around 14k feet
large hail does not appear to be as much of a threat.
High pressure will build in behind the departing cold
front...which will bring in a much drier and cooler airmass to the
region Tuesday and Wednesday. This will bring high temperatures
down into the 60s and 70s...with overnight lows down into the 40s
across central and north central Wisconsin Tuesday night.
The rest of the forecast is a bit murky as several shortwaves
track through the Great Lakes towards the end of the week and into
the weekend. Given the weakness of these systems the models are
not in much agreement regarding the exact timing of these systems.
Therefore the typical slight chance and low chance for
precipitation will suffice with this forecast package.
Temperatures in the late part of the forecast will moderate back
into the 70s by the end of the week...with low 80s back in the
forecast by next weekend.
Aviation...for 06z taf issuance
issued at 914 PM CDT sun Jul 5 2015
VFR expected through most or all of the night. An
approcahing cold front will bring showers and thunderstorms as it
moves slowly across the area Monday with MVFR or IFR conditions.
The activity may arrive in Rhinelander and Wausau by noon but will
probably not reach Lake Michigan until late in the afternoon.