Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
849 PM CDT Monday may 20 2013
Update...
Latest RUC shows a 105 knot 250 mb jet maximum over eastern Iowa pushing
north and increasing to 110 knots as it reaches central Wisconsin
by midnight. This increases the upper divergence. The 50 knot 850
mb wind maximum also lifts northeast mainly toward Chicago. This
increases the low level moisture convergence across much of
southeast and south central Wisconsin. As a result expect
thunderstorm chances to increase considerably. The limiting factor
is the instability. Zero to 1 km cap is modest at best...but with
the increasing low level jet...instability may be more than models
indicate...since the rap is saturating the soundings to moist adiabatic.
Given the initial middle/low level drier air downdraft cape is near
1000 j/kg. Therefore damaging wind potential will exist into the
early morning hours.
&&
Aviation/06z tafs/...
Expect a band of thunderstorms to affect south central and
southeast Wisconsin overnight. Some IFR in cigs/vsbys...along with
strong wind potential. Some brief MVFR around sunrise...then
mainly VFR Tuesday with a chance of afternoon thunderstorms.
&&
Previous discussion... /issued 331 PM CDT Monday may 20 2013/
Very short term...tonight and Tuesday. Forecast confidence...low
to medium.
Biggest concern into tonight is timing and strength of expected
thunderstorms. Storm Prediction Center is maintaining a slight risk for severe storms
across the forecast area. Latest NAM and rap soundings agree that
the environment still looks favorable for strong to severe storms.
With the continued sunshine...should see surface based cape values get
in the 1000-1500 j/kg range by late afternoon or early evening.
Additionally...plenty of 0-6 km bulk shear...generally 35-45 knots.
Plenty of moisture as well...with precipitable water values
increasing to around 1.25 inches this evening...above climatological
normals.
With a decent environmental setup...will just be waiting for the
trigger. Timing of upstream shortwave along with latest hrrr and
local wrf4l model indicate that convection may hold off across
southern Wisconsin until the middle-evening hours. One thing to note is
that the last few runs of mesoscale models have been all over the
place with convective solutions for this evening...so not a ton of
certainty at this point.
Low pressure remaining to the west will keep southern Wisconsin in a
warm an unstable environment through Tuesday...thus chance for
thunderstorms will continue. Similar to today...not a lot of
certainty in timing and placement of development. Any storms that do
develop tomorrow will have the chance to become strong to
severe...with Storm Prediction Center maintaining a slight risk for severe storms.
Short term...Tuesday night through Thursday. Forecast confidence
medium.
Closed low finally begins to move as it opens and becomes phased
with trough dropping toward the northern Great Lakes as it rotates
around polar vortex just east of Hudson Bay.
One more round of convection expected with 500 mb short wave Tuesday
night/Wednesday am with layer q-vector maxima and frontogenetic forcing
indicating best chances mainly after 06z Wednesday...with maximum
frontogenetic forcing over the County Warning Area at 12z Wednesday. Forcing lifts
through...stalls and then drops back across region as system opens and
moves right across the state...with middle-level trough axis south of County Warning Area
around 18z Thursday. The slightly differing solutions to timing and
structure among the models leads to various amounts of quantitative precipitation forecast. Went
with blended quantitative precipitation forecast amounts with a lean toward the higher numbers...but
if convergence band sets up right over the region...then amounts
could go even higher and areal/river flooding could come into play.
Not enough confidence to re-issue an efs at this time but will need
to be watched.
Temperatures do not fall much Tuesday night with clouds and the area
staying roughly to the warm side of the low as it moves through so
followed lows closer to the warmer guidance numbers. Highs Wednesday
will be around middle day...then cool with winds turning northerly as
surface low/trough slides by...though lingering troughiness will
prevent much of a gradient until closer to Thursday morning. Highs
Thursday will only get to the middle 60s well inland...with 50s along
the Lake.
Long term...Thursday night through Monday. Forecast confidence
Amount of cooling varies with depth of evolving 500 mb trough over
eastern Canada. Omega block sets up with an amplifying ridge over
the plains between eastern Canada vortex and closed low over the
Pacific northwest...with the forecast area remaining under the northwest flow of
the eastern trough through Saturday. Less amplified European model (ecmwf) allows
warm air advection precipitation ahead of short wave undercutting
the ridge and associated inverted surface trough crossing the region
Friday night and Saturday. More dominant surface high pressure with
more amplified GFS/Gem holds precipitation chances off until Saturday
afternoon/Saturday night on the Gem...and not until Sunday with the
GFS. While the consensus blend has precipitation chances through the
Holiday weekend due to the timing spread in the models...there will
be dry periods. Temperatures will be below normal...with highs
trying to get back into the lower 70s Monday away from the lake.
Aviation/00z tafs/...
VFR conditions are expected to continue through Tuesday...though
lower ceilings/visibilities possible in any thunderstorms. Big issue through
tomorrow remains timing rounds of thunderstorms. Looks quiet through
late afternoon...with latest mesoscale models suggesting development
possible this evening and into the night. Took a Stab at best chance
for storms in the tafs...though still had to keep it fairly general
due to a good amount of uncertainty. Chance for storms will continue
Tuesday.
Marine...
Will likely see gusts to 25 knots from North Point light southward
into early evening...until daytime heating/mixing decreases. Will
thus keep the Small Craft Advisory going until 01z. Farther
north...could see a few gusts to 25 knots...but overall gradient is
a bit weaker. Plus...winds continue to have an onshore component at
times across the north...which should limit gust potential due to
the stable airmass over the lake.
Winds will mainly be south to southwest through Tuesday...so overall
fog potential will be limited. Could see some fog for a time if
winds are southeast longer than expected.
&&
Mkx watches/warnings/advisories...
WI...none.
Lm...none.
&&
$$
Update...hentz
tonight/Tuesday and aviation/marine...ddv
Tuesday night through Monday...rem
