Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
254 PM CDT Friday Jul 25 2014
254 PM...forecast concerns include thunderstorm chances and
timing through Sunday evening...high temperatures this weekend and then
much cooler temperatures early next week.
Confidence regarding convective trends over the next 12 to 18
hours is fairly low. A weak upper wave is moving east/southeast
across southern Wisconsin which may allow isolated activity to
develop over the next few hours. Isolated activity over western Illinois
appears to be on the trailing edge of the wave pushing across
southern Illinois but also along an instability axis extending into
eastern Iowa. Short term/hi res guidance suggests just isolated
coverage through early this evening then dry through late evening
and possibly into the overnight hours.
As low level moisture returns tonight another wave currently over
eastern South Dakota may aid thunderstorm development overnight
toward sunrise and perhaps over the eastern County Warning Area depending on
when...and if...anything does develop...then exit to the southeast
quickly Saturday morning. Opted to maintain likely probability of precipitation for this
time period but there is little continuity in the models and its
possible much of the overnight could end up dry.
Trends become a bit more confident on Saturday as the northern County Warning Area
remains capped but instability across the southern County Warning Area/central Illinois
increases by late afternoon for expected thunderstorm development.
The severe threat would be primarily damaging winds but with
dewpoints surging back into the 70s...heavy rain will be possible
and with training of storms over the same areas...some potential
for flooding will also be possible. Bulk of this activity appears
to remain south of the I-80 corridor with the activity moving
southeast of the County Warning Area Saturday night.
High temperatures on Saturday will be dependent on thunderstorms...
and how soon they form as well as cloud cover. Low level thermal
profiles would suggest upper 80s to around 90 is doable but made
no changes to current forecast of middle/upper 80s. With little
change to airmass Saturday night...and likely wet ground across
the southern County Warning Area...combined with light winds...fog development
seems likely late Saturday night into Sunday morning...primarily
across the southern County Warning Area.
Low pressure will move across the western lakes Sunday with a
trailing cold front moving across the area Sunday morning. Winds
will shift west/northwest behind the front and forecast soundings
suggest it would become quite breezy/windy Sunday afternoon. Any
thunderstorm potential would likely be short-lived and along/ahead
of the front.
A second cold front/surge of colder air will then move south
across the western lakes Sunday night. This could trigger a few
showers overnight but bulk of this activity should remain
northeast of the County Warning Area. Winds will shift northerly behind this front
with highs on Monday only in the Lower/Middle 70s. Temperatures moderate
back into the upper 70s/near 80 by the middle of next week with
periodic chances for showers/thunderstorms...mainly in the
afternoon/early evening. Cms
//Ord and mdw concerns...updated 20z...
* south-southwest winds gusting around 20 knots this afternoon.
* Period of MVFR ceilings 2500-3000 feet for a time this afternoon/early
* Chance of thunderstorms and rain increases late tonight.
* Period of MVFR ceiling/fog possible early Saturday.
* Wind direction somewhat variable Sat morning.
Remnant boundary from Iowa convection has pushed across northern
Illinois with an area of MVFR stratocu spreading in from the northwest
behind it. Have indicated a few hour period of ceilings for remainder
of afternoon hours. Rain showers/thunderstorms and rain potential continues to appear fairly
low until later tonight.
Previous discussion from 18z...
Low pressure was over eastern Nebraska at midday...with a warm
front extending southeast across the lower Missouri River valley.
Circulation associated with a middle-level disturbance/remnant mesoscale convective vortex
over southern WI was producing some rain across WI...but only
spotty rain showers into northern for the most part...while a complex of
thunderstorms continues to move southeast along the Mississippi
River into western/southern Illinois. Terminals will remain in an area
of broad moist ascent into tonight...with nocturnal increase in
low level jet and an approaching middle-level wave expected to
eventually shift the elevated warm frontal zone eastward and
produce scattered thunderstorm threat over the terminals later
tonight. Pattern is relatively messy however...with fairly low
confidence in timing details beyond the overall trend at this
time. At this time however...it appears that the greatest
thunderstorm threat will be toward/after midnight. Low level jet
then to become more west/northwest by Saturday
morning...decreasing thunderstorm potential over terminals after
sunrise. Some potential will exist for brief period of MVFR
ceiling and some ground fog will exist early Saturday morning
especially where organized rains fall.
Southerly surface winds gusting around 20 knots this afternoon in
response to pressure falls across WI/IA...apparently enhanced over
western Illinois by wake low behind Mississippi Valley thunderstorm
complex. Winds diminish tonight after sunset as gradient weakens
with approach of surface trough. Winds should shift more
west/northwest Saturday morning as surface trough/cold front sags
southeast across the area...though guidance indicates some
variability in wind direction in weak gradient near the trough.
//Ord and mdw confidence...updated 20z...
* medium confidence in period of MVFR ceilings at Ord/mdw this afternoon.
* Low confidence in rain showers this aftn/evening. Medium confidence in
thunderstorms and rain late tonight.
* Medium confidence in wind trends this afternoon/evening. Low
confidence in wind direction details Saturday morning.
* Low confidence in MVFR ceiling/visible development late tonight/early
//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...
Saturday night...small chance of thunderstorms and rain.
Sunday...chance of thunderstorms and rain.
Monday...VFR likely. Strong north-northeast winds.
Wednesday-Thursday...slight chance of thunderstorms and rain.
205 PM CDT
Southerly winds will continue to strengthen over the lake this
afternoon and tonight as high pressure moves off the middle Atlantic
coast while low pressure moves across the middle and upper
Mississippi Valley. The surface trough leading to the increase in
winds will weaken and dissipate Saturday. Low pressure dropping
out of the Canadian rockies and tracking across the southern
Canadian prairies will become the more dominant feature to affect
Lake Michigan as it tracks across Lake Superior and Lake Michigan
Saturday night. Winds will become weaker and more variable with the
low weakening as it passes across the western Great Lakes region
Saturday night. A surge of much cooler air with push south down
the length of the lake Sunday night and Monday with winds becoming
northerly and increasing to around 30kt...with a few gale force
gusts to 35 knots possible. The most likely period to see the gale
force gusts should be Saturday night into Sunday morning when the
cold advection following the passage of the low and an associated
cold front will be the strongest. High pressure will quickly build
across the plains Monday and spread across the upper Mississippi
Valley and western Great Lakes Monday night and then settle into
the Ohio Valley by Tuesday. Another weak low tracking
southeastward across Ontario will keep generally northwesterly to
westerly winds over the lake into Tuesday.
While a gale headline will not be issued with the strong north
winds Sunday night and Monday...winds and waves will likely be
sufficient to necessitate a Small Craft Advisory.
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