Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
503 PM CDT Monday Apr 27 2015
250 PM CDT
A quiet forecast cycle continues with a slow moving upper pattern
and thus surface reflection across the area. This is preventing
temperatures from climbing back to normal at a more speedy rate
across most of the forecast area with northerly low-level winds
persisting and thus lake influence continuing modify the air mass
some. A 16 degree range is seen across the forecast area as of 230
PM this afternoon from Lakeside of 47 degrees to 63 out in parts
of north central Illinois. This similarly is expected to result a
good part of the workweek.
Given the anticipated surface ridge placement later tonight from the
u.P. Of Michigan through eastern Iowa...less wind is forecast than
last...although still a slight component is expected this evening.
Conditions warrant a slightly better frost potential than last
night in exposed areas...but considering that temperatures did not
drop as far last night in many places as anticipated...still
marginal confidence for an advisory. Patchy to some areas of frost
are mentioned in the forecast along with our other messaging which
Guidance has trended stronger with a buckle in the northern flow of
the jet stream on Wednesday. The forcing for any rain remains
weak...but the low-level reflection of cooler air advecting in
looks more pronounced Wednesday night into Thursday as a closed
upper low is forecast now over Lake Michigan dropping southward.
This pattern should result in more clouds during that
period...especially as low-level lapse rates steepen during the
afternoon. Have highs on Thursday similar to what they were today
with synoptic northeast winds.
300 PM CDT
Thursday night through Monday...
As we flip the calendar from April to may...the dry benign
conditions at the end of April will transition to a warmer...more
humid...and potentially active pattern. Middle and upper pattern will
become generally zonal. While stronger flow aloft will remain near
or north of the Canadian border...midlevel shortwave energy should
still propagate across the region in warm advection regime also
accompanied by low pressure systems developing over plains and
working their way east-northeast. Better moisture return will also
occur around periphery of high pressure expected to setup near or
off southeast coast over the weekend into early next week.
End result in sensible weather is generally dry day on Friday
with onshore flow...but start of warm-up with highs of about 65 to
70 inland. Model thermal forecasts and local climatology supports highs
into 70s Saturday through Monday...and possibly well into 70s at
that. Given prognosticated moisture return/50+ dewpoints in this
period...should be enough instability to support occasional
thunder chances in disturbed flow over the weekend into next
week...but probability of precipitation will undoubtedly be refined in later forecasts.
Uncertainty with how warm temperatures may get over the weekend
into early next week will likely be tied to impact of clouds and
precipitation...and any days that have onshore flow.
//Ord and mdw concerns...updated 22z...
* northeast winds 030-050-ish near 15 knots late this afternoon.
* Winds diminish around sunset...backing more north less than 10
High pressure centered northwest of the region will maintain a
northeast wind direction this afternoon through Tuesday...with occasional gusts
until sunset tonight up to 22kt. Visible imagery this afternoon shows a
channel of scattered/broken deck at 4kft above ground level...however this cloud deck was
thinning as the afternoon progresses. Expect only a few clouds later
this evening except for a few cirrus overnight. Winds will remain
northeasterly for Tuesday...however the gradient looks to be less with
high pressure drifting closer to the region.
//Ord and mdw confidence...updated 22z...
* high for all elements.
//outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z...
Tuesday night through Thursday. Primarily VFR. North-northeast
Friday...VFR. Chance of showers Friday night. Southwest winds.
Saturday...VFR. Southwest winds becoming east-southeast in the
Sunday...chance of thunderstorms and rain. South winds.
212 PM CDT
High pressure will continue to expand over the northern United
States in the coming days. Winds will lighten up some as this
occurs...which should allow waves to subside along the southern
tip of the lake. Waves have been holding around 4-5 feet along the
nearshore...so will let the advisory ride through the evening. The
core of high pressure will remain north of the area through
midweek. Meanwhile...low pressure will move east along the Gulf
Coast through Thursday. Then northeast along the Atlantic coast
Thursday into Friday. This will maintain north-northeast winds
across the lake most of the week...with an uptick expected on
Thursday as the southern low deepens slightly. The high will move
overhead Friday for a lake breeze circulation dominated wind
Lm...Small Craft Advisory...nearshore waters
until 10 PM Monday.
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