Scientific Forecaster Discussion
fxus63 kmkx 202242
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
542 PM CDT sun may 20 2018
drier air is working in from the east this evening. The new 18z
and rapid update guidance suggests we may be able to delay precip
chances a few hours later tonight...vs what we have currently
going. Still taking it in, but will likely tighten things up
through at least 12z Monday.
A period of VFR ceilings is expected this evening as drier air works
in from the east. However, it will be short lived. Moisture will
quickly return as low pressure lifts northeast from the Central
Plains overnight. The taf sites should be back to MVFR levels by
about 09z Monday with IFR conditions expected by 12-14z Monday.
The low ceilings/visibilities will then linger through the remainder of the
taf period. Winds will be fairly steady out of the east northeast.
Previous discussion... (issued 312 PM CDT sun may 20 2018)
Tonight and Monday...forecast confidence - medium.
Clouds are gradually eroding away from the northeast as drier air
works its way in. Plan on quiet weather with mostly cloudy skies
overnight. Temperatures will drop into the mid 40s near the lake and
upper 40s well inland.
The next round of rain will arrive early Monday morning as a
decaying mesoscale convective system (mesoscale convective system) slides northeastward
through northern Illinois and southern WI. The leading edge of the
storm complex should remain over IL, but expect a period of
widespread rain showers with a small chance for thunderstorms across
all of southern WI during the morning hours. Forecast soundings are
not showing much instability, but just enough to support a few
lightning strikes, similar to this morning. Scattered rain showers
are possible Monday afternoon due to an inverted surface trough and
weak mid level vorticity advection. Not expecting thunder in the
afternoon unless we get some breaks in the clouds. Marine fog may
develop in the wake of the showers Mon afternoon, and onshore flow
would advect that inland. There's not a lot of confidence in its
development, but it's a possibility.
Expect a steady easterly breeze and clouds to keep temperatures cool
Monday night through Wednesday...forecast confidence - medium.
Will continue shower threat Monday night across the area as deep
moisture lingers across the area as a mid-level short wave moves
across WI. Synoptic scale lift however will be tempered by
subsidence from left entrance region of upper jet spreading across
the region. Elevated instability is very marginal so wl hold off
from thunder mention. Increasing subsidence from approaching low to
mid-level ridging should help the clouds clear later Tuesday into
Tuesday night. Mid-high clouds will be quick to return Tuesday
night and Wednesday from upstream convection across the northern
plains. Low level jet remains anchored across the northern plains
Tuesday night and Wednesday, there is a small chance some weakening
convection may hang together long enough to affect parts of western
County Warning Area on Wednesday. Otherwise, mild temperatures are expected both
days with a lake breeze developing by late morning.
Wednesday night through Sunday...forecast confidence - medium.
Short wave ridging over the western Great Lakes will prevent deeper
moisture from surging into southern WI, at least initially. Can not
rule out small chances for weakening convection spreading into
southern WI from Minnesota/Iowa later Wednesday night and Thursday. Strong
warm air and moisture advection will be triggering widespread
upstream convection across the northern plains during this period.
Medium range guidance in general agreement on protective short wave
ridge over the western Great Lakes weakening and sliding off to
the east late in the week. Upstream long wave trof over western
Continental U.S. Will weaken and transition to several weaker short waves
moving through the northern plains and western Great Lakes region
on Friday and Saturday. Deeper moisture and better synoptic scale
forcing will result in a period or two of more widespread
convection across southern WI during this period. At this point,
the wettest period of the Holiday weekend is looking like Friday
into Friday night, with the chance for showers and storms
lingering into Saturday. Above seasonal normal temperatures early
in the period will be replaced by slightly cooler and drier
conditions Sunday and Memorial Day.
The IFR cloud bases are gradually diminishing and expect ceilings to
continually improve to VFR through late afternoon. Clouds will
remain intact over much of southern WI this evening and overnight.
Another round of showers and perhaps a few thunderstorms will slide
northward into southern Wisconsin early Monday morning.
These will bring IFR/MVFR ceilings and reduced visibility in rain
once again. The easterly component off the lake and moist conditions
could lead to lake fog pushing inland during the Monday afternoon
Northeast winds are steadily diminishing this afternoon. Waves
should drop below 4 feet in the nearshore waters by 4 PM north of
Milwaukee and 8 PM south of there, so will allow the Small Craft
Advisory to expire as planned.
Plan on steady east winds and resultant waves to remain just below
advisory criteria Monday. Lighter winds will ensue for the middle of
Lm...Small Craft Advisory until 8 PM CDT this evening for lmz645-646.