Area forecast discussion
National Weather Service Green Bay WI
1048 PM CDT Monday may 20 2013
Updated aviation portion for 06z taf issuance
Update...
issued at 839 PM CDT Monday may 20 2013
A large cluster of thunderstorms extending from Missouri into wstrn Illinois will
likely stay mainly to the southeast of grb County Warning Area tonight. However...
several smaller lines or clusters of storms over southeast Minnesota and Iowa are
expected to consolidate into a large area and lift north through
the forecast area during the very late evening and overnight. The hrrr and
rap models seem to have a decent handle on this situation...and
show the convection overspreading c/ec WI between 04z-06z...and
northern WI between 06z-08z. Instability is not expected to be
great...with MUCAPE in the 500-1000 j/kg range...but deep layer
shear should be fairly strong...in the 40-50 knots range. Dynamic
forcing from a short wave trough and 100 knots jet streak...plus moisture
convergence on the nose of 35 knots low level jet will provide the trigger.
Suspect that isolated severe thunderstorms will be possible in c/ec WI
toward the beginning of the event...before the storms form into a
larger cluster. Wet bulb zero heights around 9k feet are supportive of
large hail...and dcape values of 800-900 j/kg are marginally
supportive of strong wind gusts.
Update
issued at 631 PM CDT Monday may 20 2013
Updated probability of precipitation for this evening and overnight. Scattered showers and isolated
thunderstorms were occurring over NC WI due to a short wave trough and lfq of an
upper level jet. This activity should lift north of the region by
01z-02z.
Latest hrrr/rap suggest that a more significant round of thunderstorms
will arrive in our southern counties late this evg...and quickly
spread NE through the County Warning Area during the overnight hours. This convection
will be driven by another short wave trough and jet streak...as well as moisture
convergence on the nose of a 30 knots low level jet. Although surface based instability
will be waning by that time...some elevated instability will persist
and shear profiles will be supportive of organized storms. The area
remains in a slight risk for severe thunderstorms...so will watch closely
to see how this develops as the evg progresses.
&&
Short term...tonight and Tuesday
issued at 306 PM CDT Monday may 20 2013
The threat for severe storms continues to be the primary focus of the
forecast for both tonight and on Tuesday.
The 19z msas surface analysis showed a strong area of low pressure over
the NE corner of South Dakota with a frontal boundary extended eastward through
central WI. A cold front stretched southward from the low through the plains. A
look at the water vapor imagery indicated an middle-level area of low
pressure over the northern plains. Thunderstorms have fired over in lower Michigan where
better instability resides and showers continued to spin northward over
western WI.
An initial surge of drier air aloft was not handled well by the
models earlier today...however prevailing SW winds to pump more
moisture into WI later this afternoon/early evening and help to
lift convective available potential energy into the 1000-1500 j/kg range. Moderate to strong 0-6km
shear remains over the region...thus as moisture increases and a
trigger arrives (mid-level speed max)...expect to see convection
Blossom to our south and overspread NE WI later this evening. The
thunderstorm activity should be line segments which could bring some of
this stronger wind aloft down to the surface...thus the greatest severe
threat would be damaging wind gusts. Storms should weaken a bit
during the overnight hours as instability weakens...but have kept
shower/isolated thunderstorm wording overnight as shortwave energy to still be
moving through the region.
The upper trough to our west is forecast to slowly edge east across the
plains and weaken a bit. Meanwhile...the surface low tracks across the
upper MS valley and also weakens somewhat on Tuesday. The main question
for Tuesday for NE WI is how many clouds will still be around during the
morning from overnight convection? The atmosphere should have
stabilized by this time and will have to wait for the clouds to
break up to permit sufficient heating to allow destabilization to
take place. Another concern would be the anticipated mesoscale convective system over the
Southern Plains/mid-MS valley which could rob some of the incoming Gulf
moisture. Once instability occurs...models show enough shear to help
organize the next round of strong to severe thunderstorms from the Great
Lakes SW to the Ozarks. Since there is some doubt as to when the
new storms would fire and exactly where they would fire...prefer
to focus higher probability of precipitation in the afternoon.
Long term...Tuesday night through Monday
issued at 306 PM CDT Monday may 20 2013
The latest GFS ensemble mean indicates that the pesky upper low
thats currently over the Central Plains will shift east across the
area Wednesday night before departing. Thereafter...northwest flow will
prevail for several days...which will lead to quieter and cooler
conditions. The next chance of precipitation following the upper low will
occur due to return flow next weekend...which models offer different
solutions. As is usual...the GFS is more progressive in advancing
a warm front into the region Saturday night into Sunday...which the
European model (ecmwf) portrays a strong Hudson Bay high keeping the warm front to
our SW. The European model (ecmwf) has support from the Canadian so think a
compromise between the two solutions is in order.
Tuesday night through Thursday...upper low over the Dakotas will
finally shift east Tuesday night and Wednesday. Broad surface low
pressure will move east across Wisconsin Tuesday night before a
secondary surface low organizes over northern Illinois. Should have
enough surface instability and forcing for the threat of storms to
continue through Tuesday evening. Storms should dissipate overnight
on Tuesday...but some showers may continue over north-c Wisconsin closer
a stalled out front. Showers look to be a decent bet on Wednesday as the
surface low moves from northern Illinois into lower Michigan and the
upper low moves overhead. Out of the warm sector...temperatures will be
cooling off into the 60s with plenty of clouds around. Will lower highs a
tad. Precipitation will be diminishing from northwest to southeast Wednesday night as
the low heads towards the eastern Great Lakes...and a dry Hudson Bay
high builds over the northern Mississippi Valley. This high will
then reside across the area on Thursday with highs in the low to middle
60s.
Rest of the forecast...Hudson Bay high will continue to be present
across the area on Friday with highs not that different than on
Thursday. By Friday night and Saturday...a building ridge over the
central part of the continent will push a warm front north across
the Great Plains. Should see a band of precipitation develop north of the
boundary from the northern plains to the southwest Great Lakes. The
Hudson Bay high pressure will continue to funnel in very dry air
across Wisconsin during this time...and tend to think that the dry
air will win out for much of the forecast area. Think precipitation
chances will improve over the second half of the weekend as the
middle-level ridge moves east and the warm front lifts north.
&&
Aviation...for 06z taf issuance
issued at 1043 PM CDT Monday may 20 2013
Rain showers/thunderstorms and rain will become more numerous as they spread north into the
forecast area overnight. MVFR/local IFR conditions should accompany
the thunderstorms...along with a threat of heavy rainfall...small hail and
brief gusty winds. The storms should taper off from south to north
late tonight into Tuesday morning...although showers may linger
over far northern WI. Some lingering low clouds and fog may also
persist across the forecast area through mid-morning. Low pressure
will move into the region on Tuesday afternoon...and should
generate another round of thunderstorms. The storms should taper off
from west to east during the late afternoon and early evening.
Partial clearing may cause some fog and low clouds to develop
Tuesday evening.
&&
Marine...
issued at 306 PM CDT Monday may 20 2013
Next round of showers and storms should occur early this
evening and linger into the early morning hours. These storms could
produce strong winds. Moist air flowing over the colder waters of
the Bay and lake will result in fog at times through Tuesday
night. The fog will be locally dense with visibilities below one
mile at times.
&&
Grb watches/warnings/advisories...
none.
&&
$$
Update.........Kieckbusch
short term.....AK
long term......mpc
aviation.......kieckbusch
marine.........AK
