Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
654 am CDT Friday Aug 22 2014
349 am...forecast concerns include heavy rain with thunderstorms
through Saturday night...in addition to timing and location of
storms. Additional concerns and challenges include temperatures/
dewpoints and associated heat indices through early next week.
One complex of thunderstorms moving across far northeast Illinois and
Northwest Indiana with a second complex moving southeast across
the southern County Warning Area. These are very efficient rain produces with
dewpoints in the 70s and precipitable water values around 2
inches. Coop observer 3sw of Midway Airport measured 3.60 inches
of rain in just 40 minutes with several reports of flooded roads
in that area. While additional activity continues across the
area...the general trend is for all this activity to move
southeast this morning. However...additional activity over
western Illinois and southern Iowa may continue moving east across the
southern County Warning Area later this morning.
Convective trends are still somewhat uncertain over the next 36
hours or so. If current activity across southern Iowa/western Illinois
drifts southeast...much of the daylight hours could end up rather
dry with tonight possibly being more active as hinted at by the
European model (ecmwf). Have trended this direction but confidence is low. With
dewpoints remaining in the Lower/Middle 70s and precipitable water
values remaining around 2 inches...additional torrential rain will
be possible with any storms that develop. If trends become more
clear...its possible a Flash Flood Watch may be needed for part of
the area...but given the uncertainty of when/where additional
heavy rain will occur...no headlines this morning.
Convection then appears to shift northeast during the day on
Saturday as the upper ridge builds into the area. Thus a dry
period is expected from later Saturday night through Sunday night
and possibly into much of the day on Monday. Chance probability of precipitation then
enter back into the forecast through Thursday. Medium range
models are in good agreement with a cold front moving across the
area midweek...though timing differences remain. Additional heavy
rain will be possible during this time as well.
While Sunday may end up being completely dry as the ridge caps the
area...it does appear there could be a lake breeze along the Illinois
shore Sunday afternoon. How much convergence occurs is uncertain
but given how warm/humid conditions will be Sunday afternoon...if
this is enough for thunderstorms to develop...there could be some
locally heavy rain across far northeast Illinois. Added slight chance
probability of precipitation during this time to account for this potential.
Dense fog has formed across far northeast Illinois and may spread
southwest into northwest Illinois early this morning as rain ends.
Issued a dense fog advisory for lake/McHenry counties earlier this
morning and may need to expand that as trends emerge. Depending on
precipitation coverage tonight...areas of fog possibly dense may develop
again into Saturday morning.
Temperatures today and Saturday will be dependent on cloud cover
and precipitation coverage. Made only small tweaks...basically no
changes. Did bump temperatures up a few degrees on Sunday and Monday with
expected less or no precipitation and more sunshine. Overnight lows will
likely remain in the lower 70s most areas...possibly middle or upper
70s downtown Chicago Sunday night/Monday morning. Forecast heat
indices will be near 100 today...retreat just a little Saturday
and then again near 100 Sunday and Monday. Across the southern
County Warning Area...heat indices approach the 105 mark. If current trends
continue...its possible portions of the southern County Warning Area may need a
heat advisory Sunday/Monday. Cms
//Ord and mdw concerns...updated 12z...
* brief IFR ceiling possible next 1-2 hours.
* Variable SW winds turning northeast early this afternoon. Speeds
expected to be under 10 knots.
* Isolated-scattered rain showers/thunderstorms and rain possible middle/late afternoon. May develop
west of terminals and drift back to the east.
* Chances for rain showers/thunderstorms and rain tonight and Saturday morning...timing
Weak low pressure extends from southern Minnesota/northern Iowa
southwest into the Southern Plains. A frontal boundary extends
from eastern Iowa across northern Illinois to northern Indiana
this morning with strong low level winds helping drive
thunderstorm activity to the south of the terminals. Some spotty
IFR is in proximity to the terminals...with the bulk near rfd but
this should be improving...with Ord/mdw/dpa/gyy likely seeing
little if any IFR. Light and somewhat variable south to southwest
winds are in place but are expected to turn east and northeast
this afternoon as the front re-establishes itself perhaps with
the help of Lake Michigan. This boundary coupled with the very
humid airmass may trigger isolated-scattered rain showers/thunderstorms and rain this afternoon
inland of the lake which would then drift back towards the lake
after development. Have included thunderstorms in the vicinity mention at Ord/mdw/rfd/dpa
with decent confidence that activity will occur but with question
Marks surrounding the extent.
Winds will diminish this evening/tonight which may allow some
shallow dense fog or more widespread MVFR type fog. Thunderstorms and rain chances
remain a challenge but have pretty low confidence in when the best
opportunity will be this evening through Saturday. With the
frontal boundary expected to be in the region it would likely
serve as a trigger but it may be south of the area tonight. Upper
ridging will build in Saturday which would bring another push of
warm air and coupled with the boundary would probably drive more
thunderstorms and rain development which may be favored Saturday morning. Will leave
the taf dry beyond this afternoon given the uncertainty but have
decent confidence that thunderstorms and rain will occur at some point.
//Ord and mdw confidence...updated 12z...
* high confidence that any IFR would be brief.
* High confidence in light SW winds becoming northeast this
afternoon. Medium-high confidence in speeds under 10 knots. Medium
confidence in timing of shift to northeast.
* Medium confidence that there will be some isolated/scattered rain showers/thunderstorms and rain
development this afternoon.
* Low confidence in timing of best rain showers/thunderstorms and rain chances this evening
through Saturday morning but medium confidence that additional
activity will occur during this time.
//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...
* Saturday night through Thursday...periodic chances for
thunderstorms and rain/rain showers with occasional MVFR cigs/vis.
349 am CDT
An elongated area of low pressure stretches from southern Minnesota
into the Southern Plains with a weak warm frontal boundary
stretching from northern Indiana back into central/northern Iowa.
This front will move little into Saturday. Morning storms have
caused winds to be a bit chaotic across the far southern part of the
lake where south to southeast winds will turn more east and
northeast into the afternoon. Low pressure will organize across the
central and northern plains later Saturday and move to western
Ontario Sunday allowing winds to turn southerly and increase in
speed toward Monday. The low will then track to the northeast early
next week dragging a frontal boundary southward which may stall out
somewhere across the lake allowing for variable winds. Another low
is expected to lift northeast middle week bringing a stronger cold
front across the lake which would bring a shift to north and
northwest winds. Still early for details but this scenario would
bring a period of stronger wind speeds thanks to cold
advection...not unlike similar cold frontal passages we have seen
the past few weeks.
Illinois...dense fog advisory...ilz005-ilz006 until 10 am Friday.
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