Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 211638
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1138 AM CDT Fri Jul 21 2017

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Saturday
Issued at 308 AM CDT Fri Jul 21 2017

Main forecast concerns center around potential for both severe
thunderstorms and locally heavy rainfall tonight.

The 07z MSAS surface analysis showed weak high pressure stretched
from northern WI into northern Lower MI. A quasi-stationary front
was draped from northern IL west-northwest into northern Nebraska
where an area of low pressure was located. A cold front extended
south from the low pressure into the central High Plains. Regional
radar mosaic indicated two convective systems. One along the
quasi-stationary front from IA east-southeast into Indiana and the
second system over the northern Plains.

The quasi-stationary front is forecast to slowly lift north as a
warm front today, reaching roughly from southern MN southeastward
into southern WI by 00z Saturday. A continuation of warm, moist
air flowing over this frontal boundary, coupled with strong shear
and the right entrance region of the upper jet, all point to the
development of another thunderstorm complex today. Latest timing
shows this complex rolling into central WI mid-afternoon and
eastern WI late afternoon. Even though instability parameters are
marginal over northeast WI, the ample shear alone could produce
damaging winds along with torrential downpours due to PW values
approaching 2.0". Large hail may be difficult to come by due to
high wet bulb zero heights > 13K ft. Highest pops placed over
central WI (closest to the warm front), while far northeast/
eastern WI may not see any precipitation until around 00z. Max
temperatures could be a little tricky depending on the arrival of
the showers/thunderstorms. Look for readings in the upper 70s to
around 80 degrees central WI, lower to middle 80s eastern WI.

Potential for strong to severe storms and torrential rainfall
will continue over the region tonight as a weak area of low
pressure moves into the region, the warm front to be situated over
south- central WI, a 40kt southwest low-level jet aimed at WI and
the cold front accompanying the surface low. Sufficient shear
remains in place, thus there is concern for a MCS to run along the
instability gradient/north of the warm front. If this scenario
does develop, a damaging wind threat would exist with torrential
downpours that could lead to minor flooding, focused mainly on
central/east-central WI. Will issue a ESF for this heavy
rain/flooding threat. Min temperatures to range from the lower 60s
north, around 70 degrees south.

Expect to see some showers/few thunderstorms carry over into
Saturday morning as the surface low and cold front to still be in
the process of exiting the forecast area. There are questions as
to the extent of anymore precipitation for the rest of Saturday as
the next system of interest to still be over southwest Ontario/
northern MN in the form of a shortwave trough. Central/north-
central WI may start to destabilize in the afternoon if we can
shake the clouds. Main forcing remains to our north and west, so
find it difficult to see much in the way of showers/thunderstorms
until Saturday evening. May keep a token chance pop in the
forecast mainly across north-central WI where forcing will be on
the increase. Temperatures are forecast to cool a bit, along with
slightly less humid conditions. Look for max temperatures to be in
the middle to upper 70s north/near Lake MI, lower to middle 80s
south.

.LONG TERM...Saturday Night Through Thursday
Issued at 308 AM CDT Fri Jul 21 2017

Long term forecast highlights include lingering shower/storm
chances Saturday night through Sunday, a couple of DRY days early
next week, then a return to active weather mid-week. Temperatures
look to remain near or slightly below normal through most of the
period.

Severe weather chances will be quickly diminishing Saturday
evening as instability wanes. However, upper level trough and
mid-level shortwave will dive over Lake Superior through midnight
then continue east/southeast into Sunday. This will keep the
chance for showers and storms through the weekend, with the best
chances residing over the northeast toward Upper Michigan. With
upper trough directly overhead on Sunday and lower freezing/wet
bulb levels, if we can get any instability to build, there will
be at least the small chance of strong/severe pulse-type storms
with hail/wind the main threats. Precipitable waters remain over
an inch, so brief heavy rain will be possible as well.

Things finally dry out across the entire area Sunday night into
Monday, as mid-upper level ridging builds and surface high
pressure builds over the western Great Lakes. Look for mostly
sunny skies and comfortable humidity levels. Sunday night and
Monday night will be cooler than normal with some 40s possible
across the typical cool spots across northern WI.

The dry/quiet weather comes to an end mid-week as return flow sets
up on the back side of the departing high pressure. Dewpoints will
be on a slow climb Wednesday ahead of an approaching frontal
boundary from the northwest. A little early to pin down the
severe weather threat, but early indications point toward at
least a marginal/slight severe weather risk, especially if the
front can come through during peak heating. Some questions on how
far south this boundary will get Wednesday night and Thursday (no
surprise there!), so will keep only low POPs on Thursday. A heavy
rain set up could develop along this boundary (precipitable water
over 1.5"), but latest models keep the better chance of seeing any
flooding issues south of the area.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1138 AM CDT Fri Jul 21 2017

For the afternoon hours, Thickening high level clouds
will lower to mid level during the afternoon as convection over
southeast Minnesota, Iowa and southwest Wisconsin late morning,
gradually shift eastward. Convection has been weakening while
approaching Wood county late this morning, but still anticipate
some debris type precipitation to drift across parts of central
and east central wisconsin along with patchy mvfr cigs. Trend of
most model runs this evening into Saturday morning is to shift
the heavy rain potential southward. Still anticipate cigs lowering
to mvfr and possibly ifr for a period later tonight as the
convection passes through. These conditions may linger Saturday
morning before improving late Saturday morning into the afternoon.
&&

.HYDROLOGY...
Issued at 308 AM CDT Fri Jul 21 2017

Showers and thunderstorms will be on the increase this afternoon
from west to east. Given the high moisture content (PW values
approaching 2.0") and the threat for additional storms tonight,
rainfall totals across parts of central and east-central WI could
easily range from 1 to 3 inches by Saturday morning. Even locally
higher amounts are possible if the axis of heavy rain shifts
north even a little bit. Rainfall of this intensity could lead to
minor flooding or even isolated flash flooding tonight. People
living in central and east-central WI will want to keep abreast
of the latest forecasts.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM.....Kallas
LONG TERM......Bersch
AVIATION.......TDH
HYDROLOGY......Kallas



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