Area forecast discussion
National Weather Service Green Bay WI
420 am CDT Thursday Jun 20 2013
Forecast discussion for routine morning forecast issuance
Short term...today...tonight...and Friday
issued at 418 am CDT Thursday Jun 20 2013
Timing of convection the issue through this period. Have followed
the European model (ecmwf) with aid of some mesoscale models for this forecast...though
not any of the models have good handle on ongoing convection over
Minnesota/South Dakota area.
Disturbances moving out of western trough to interact with
increasingly warm...moist and unstable air to bring periods of
convection to the upper Midwest and western lakes region. Two
features keying on this morning are weak front extending south and
west from low vicinity Hudson Bay into Lake Superior/northern Minnesota area
and warm front extending east out of low over western Dakotas into southwest
Minnesota. Convective cells along front over Lake Superior continue to
head east-southeast along front with little movement toward northern WI.
Larger area of convection just north of warm front being supported
by 40kt low level jet. Though weakening...low level jet expected to remain
focused to west of WI today. Mesoscale models/latest hrrr suggests
weakening as this system moves into north central WI this
afternoon...while weakening convection along Lake Superior front.
Main change was to decrease chances southeast half of County Warning Area today.
Ec has been fairly consistent past few runs developing complex
over southern half of Minnesota tonight. Complex to shift southeast
south of County Warning Area...also suggests secondary complex forming further
north aided by rrq of jet over southern Canada. This complex to
shift southeast into County Warning Area during day Friday. Do have some concerns
regarding convective feedback with strength of vorticity ec has moving
through WI during day Friday. Given this scenario...thinking that with
more clouds and precipitation expected Friday will lower temperatures some...keeping
previous temperature forecast for today. Given modest instability am not
expecting widespread severe this period...though a few storms may
produce large hail given lower wet bulb zero heights and some shear.
Due to forecast uncertainty for both days have stayed with chance
to low likely probability of precipitation at this time.
Long term...Friday night through Wednesday
issued at 418 am CDT Thursday Jun 20 2013
Upper ridge across the eastern United States this weekend will
gradually retrograde into The Rockies by the middle of next week.
Position of 500mb ridge will put US in the ring of fire during
the period...at least into early next week. Some questions linger
by next Wednesday if frontal boundary will shift south as upper
trough deepens across the western Great Lakes by the end of next
week.
As always the case with these scenarios...when and where thunderstorm
complexes set up always difficult to pinpoint this far out. To
complicate matters...outflow boundaries can have a big impact on
frontal boundary position and future track of later thunderstorm
complexes. The GFS is scary in the fact it brings a complex across
the western lakes about every 24 hours into early next week.
Also...maximum temperatures each day complicated on timing of these
systems if they should hold off later in the day or even into the
evening hours. Went conservative on maximum temperatures each
day...but latest mex guidance and 925/850mb temperature study
would indicate maximum temperatures could be even several degrees
warmer than what is currently in the forecast. Would rather raise
maximum temperature closer to each day than back track due to more
clouds or rain.
The forecast for Friday night is somewhat muddled tonight due to
the remnants of a convective vorticity maximum from Thursday night complex
which is expected to move across the region Friday afternoon.
There may be some subsidence behind this system early Friday
evening...but low level southerly flow and steep middle level lapse
rates should bring a chance of showers and storms to the region
later Friday evening...especially across the south. The better
chance for showers and storms will be after midnight as a complex
of showers and storms move into the area from the west. This complex
is expected to linger Saturday morning across the Fox Valley and
Lakeshore. There will probably be a lull or no activity late
Saturday morning into the early afternoon hours. Atmosphere should
become unstable again later Saturday afternoon. Modified soundings
indicated 2500 to 3500 j/kg of cape...windex values of 40 to 50
knots and no cin. Models do show 500mb speed maximum moving across the
region Saturday night to help enhance lift. Large hail is a less of
a concern as wet bulb heights rise to around 13 thousand feet.
However...can not rule out large hail due to massive amount of
cape in the forecast soundings. Maximum temperature dependent on
sunshine later Saturday morning and Saturday afternoon...which
some sunshine should be expected this time of year. Another
complex of thunderstorms is expected Saturday night and will
linger Sunday morning across the east. Would suspect based on the
models and model soundings...more storms will reform Sunday
afternoon. With the heat and humidity around...damaging winds and
locally heavy rain are the primary threat with these storms.
Boundary will be lurking around Monday and Tuesday...along with
the heat and humidity. Some of the storms could become strong or
severe...but it is too early to determine if or when organized
severe weather would occur. Some indications that front will
begin to sink southward Tuesday night as upper heights begin
to fall. Wednesday could end up being dry...however did not make
any changes to that period for now.
See hydrology section for more details on expected rainfall into
early next week.
&&
Aviation...for 06z taf issuance
issued at 1236 am CDT Thursday Jun 20 2013
A line of showers ahead of an approaching cold front extended
from Central Lake Superior into northern Minnesota. Another area
of showers and isolated thunder was over the Dakotas and western
Minnesota. This was in association with an approaching upper level
short wave. When to put showers and thunder in the tafs and how
long to keep them there was problematic. Some of the models were
fairly dry in far eastern Wisconsin...so have opted to leave out
mention of precipitation at grb and atw. Models spread quantitative precipitation forecast across
most of the area during the day as the front and short wave move
into the state. Have showers and thunderstorms mainly during the
afternoon...followed by a chance for showers after about 00z.
Conditions should be mainly VFR other than during precipitation
when MVFR visibilities and ceilings will be common...with isolated IFR
conditions. Confidence in forecast timing is not particularly high.
&&
Marine...
issued at 418 am CDT Thursday Jun 20 2013
A increasingly warm and humid air mass will overspread the cooler
waters of the Bay of Green Bay and Lake Michigan Thursday night and
Friday... leading to the formation of locally dense marine fog.
The locally dense marine fog will continue at times into early
next week as the warm and humid air mass remains firmly entrenched
across the region. Several complexes of thunderstorms are expected
from time to time Friday into early next week. These storms will
pose a significant risk for mariners with strong winds...locally
higher waves...large hail and torrential rains. &&
Hydrology...
issued at 418 am CDT Thursday Jun 20 2013
Several complexes of thunderstorms are expected from Friday into
early next week. A very warm and humid air mass will become entrenched
across the region late this week and persist into early next week.
Precipitable water values between 150 to 200 percent of normal are
expected...leading to the likelihood of torrential rains and
potential for urban flooding due to the intense rainfall rates.
Any training or slow moving thunderstorms will increase the risk
of flooding. Rainfall amounts over the next 5 days should range
from 1 to 3 inches with locally higher amounts. Would not be
surprised if there were a few amounts close to 5 inches if storms
move over the same region on several days into early next week.
&&
Grb watches/warnings/advisories...
none.
&&
$$
Short term.....Te
long term......eckberg
aviation.......mg
marine.........Eckberg
hydrology......eckberg
