Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
643 PM CDT Monday Jun 1 2015
259 PM CDT
Expansive high pressure over the northern Great Lakes will settle
southward...eventually bringing clear skies even to areas south of
I-80 where pesky stratus has hung on today. Light to calm winds
and clear skies overnight will yield lows by early Tuesday
primarily in the 40s...but expecting some rare June upper 30s in
far northeast Illinois. Also it still appears Rockford could come close
to its record low of 41 for June 2nd. Have some concern that there
could be patchy fog development in southeast portions of County Warning Area still
holding onto cloud cover as of middle afternoon. Dewpoints remain in
the 50s in these areas that also observed the most significant
rain amounts over the weekend. None of the MOS and hi-res guidance
is hitting fog hard...but will pass on concerns to evening shift
Other than maybe a few puffy fair weather cumulus at top of mixed layer
nearing 850 mb on Tuesday...expecting sunny skies. Position of
surface high over central Great Lakes with ridging back across
Lake Michigan will result in synoptic east-northeast to east-
southeast flow...and slightly enhanced lake breeze push easily
able to spread inland during the afternoon. Climatology of prognosticated
850/925 mb temperatures supports Lower-Middle 70s inland...so a nice
recovery from today. Also aiding in warming could be some mixing
out of dewpoints during the afternoon. Unfortunately the Lakeshore
areas will again be kept cooler by onshore flow...generally in the
upper 50s to lower/perhaps middle 60s.
317 PM CDT
Tuesday night through Monday...
While a surface ridge will be slow to depart the area during
midweek...its weakening along with veering flow aloft will allow for
gradual moderation in temperatures. Readings at normal are expected
by Wednesday. The weakening upper low across the Tennessee Valley /
southeast U.S. During Wednesday into Thursday will limit any
appreciable return flow into the area. Low pressure across the
Central Plains is forecast by most guidance to weaken as it moves
eastward...though the 12z Gem does actually strengthen this but is
an outlying solution. Lift and moisture are limited on Thursday ahead
of this feature...with slightly greater moisture convergence and
support for showers and thunderstorms Thursday night.
A cold front is forecast to develop ahead of an impinging high from
the north on Friday and drop south through the area. Confidence is
low on the precise timing of this and likewise is low on
temperatures into this weekend. The magnitude of the cool air is
certainly not to what we just experienced...though Lakeside areas
could be only around 60 for highs by Friday afternoon and again on
Saturday. While there is a lot of spread in solutions...there is
consensus on return flow by late in the weekend which could
provide for better chances for thunderstorms at the start of next
Rockford is presently forecast to be close to a record low
Tuesday morning June 2nd. The record low is 41 set in 2003.
//Ord and mdw concerns...updated 00z...
* northeast winds 10-12 knots early this evening. Winds more
easterly and likely just under 10 knots Tuesday.
High pressure ridge settling across the area tonight with
northeast winds diminishing below 10 knots early. Ridge drifts off to
the east Tuesday...with a more easterly component aided by weaker
lake push. Forecast soundings indicate not much wind 10 kts or
higher through or above the mixed layer during the day...so while
a brief 10-12 knots wind not out of the question the prevailing speed
is expected to be just under 10 kts.
Other than a few flat VFR cumulus Tuesday...and some patchy thin cirrus
aloft...not much in the way of cloud cover expected. Areas south
of the terminals...particularly south of gyy...could see some fog
development tonight where dew point temperatures linger in the 50s after
heavy weekend rains. Will watch observation at gyy into the night for some
ground fog potential...though have greater confidence that any
significant fog will mainly be south of gyy and well south of
//Ord and mdw confidence...updated 00z...
* high all elements.
//outlook for Ord/mdw for 12z Wednesday-12z Monday...updated 00z...
Wednesday and Thursday...dry/VFR. Slight chance thunderstorms and rain Thursday
night. East-southeast winds.
Friday...chance of thunderstorms and rain. MVFR likely. Northeast winds.
Saturday...dry/VFR. Northeast winds.
Sunday...slight chance shra/tsra. Southeast winds.
254 PM CDT
A high pressure ridge extending southeastward over the northern and
central part of Lake Michigan is preserving onshore flow into the
Illinois and Indiana nearshore into this evening. Wind speeds will
diminish though and waves which have been steady today are expected
to slowly drop after dark. The ridge will ever so slowly inch south
with diurnal onshore components in Illinois and Indiana from lake
breezes. A cold front is expected to develop over the lake ahead of
another high approaching from the northwest on Thursday night...and
move southward over the lake sometime on Friday. High pressure then
looks to briefly move over the lake on Saturday. There do not appear
to be any regimes for gales through Saturday nor Small Craft
Advisory conditions along the Illinois and Indiana nearshores
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