Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1008 PM CDT Tuesday Jul 22 2014
807 PM CDT
The highest Theta-E surface air mass of the year has advected into
the area in advance of a southward moving cold front advancing
toward the County Warning Area from southern Wisconsin. An axis of middle to even some
upper 70 degree dewpoints is pooling across northern Illinois just
ahead of the front. The very high Theta-E juxtaposed beneath an
impressive eml with very steep lapse rates is resulting in a classic
loaded gun situation with very strong instability that is currently
largely being held in check by the strong cap at the base of the eml
between 900-800 mb on the dvn evening sounding. Frontal
convergence alone has not been terribly effective at busting
through the cap thus far...though a few cells have made Stout but
relatively short lived efforts.
As the front moves through the County Warning Area this evening would not anticipate
much boundary layer stabilization to take place ahead of the front
due to the very high dewpoints in place leading to relatively
limited diurnal cooling. Water vapor imagery this afternoon and
evening has shown very well defined shortwave tracking
east-southeast from South Dakota now into northern Iowa and southern
Minnesota. As this feature continues east-southeastward and begins
to catch up to the surface front...the increasing large scale ascent
associated with the shortwave could result in a weakening of the cap
and an increase in storm coverage this evening along the front as it
moves into our County Warning Area. Confidence in this scenario is not overly
high...so not going to make any changes to probability of precipitation at this point...but
that is the reason that despite the paltry storm coverage thus far
that i've been reluctant to lower probability of precipitation. If cap does weaken and
sustained storms can form...the very strong instability and modest
effective shear would be enough to support at least a pulse severe
233 PM CDT
Forecast focus primarily on short term heat and humidity and
convective chances this evening. After thunderstorms tonight...we
will have a couple days of dry and cooler conditions...followed by a
return to a potentially more active pattern for the latter half of
the week and through the weekend.
The large upper ridge is being flattened by an upper wave
progressing through the northern lakes. Dewpoints are pooling out
ahead of the cold front approaching from the north and west...so
readings are generally well into the 80s to around 90 and dewpoints
in many areas into the 70s where it starts to feel quite
uncomfortable...even all the way to the Lakefront. Lingering convection
from earlier today is sinking south and weakening along the Iowa/MO
border. Recent thunderstorm development has been ongoing in
central and eastern Iowa out ahead of the cold front. The storms
are trying to grow against the cap but have overall been
relatively unimpressive on satellite to this point. Latest Storm Prediction Center/rap
mesoanalysis shows an area of very high mixed layer instability in
eastern Iowa and extending into far northwest Illinois where dewpoints
are in the low to middle 70s. Unfortunately in this situation the best
moisture convergence is not juxtaposed with the better height
falls and stronger winds aloft associated with the disturbance
gliding by to the north.
Short term guidance still remains mixed to this point as to how
widespread the coverage will be...but any that do form still hold
the potential for strong winds...large hail due to the moderate middle
level lapse rates...and locally heavy rainfall as precipitable water values hover
above 2". What we can say with decent confidence is the timing of any
storms. Some hi-res guidance suggest a brief window of the capping
to be eroded in north central into NE Illinois in the 6-7 PM time
frame...moving through metropolitan area between 8-11pm...and then
shifting south and east into Northwest Indiana. Confidence is low on a full
line developing as some models indicate as the capping holds in our
southwestern areas where weaker low level convergence and upper
forcing will have a harder time with the cap. Still believe there
will be scattered activity during this period. Would like to be more
definitive with the probability of precipitation at this time frame but will generally
continue the 40-50 pop/scattered coverage for most areas as the cap
looks to keeps things in check at least at this point. Closer to
the lake we may get slightly better height falls later this
evening along with the front.
The front will be rather progressive...thus the window for storms
for any one area appears to be pretty short. By daybreak the front
should largely have passed through the entire County Warning Area.
There may be some lower clouds behind the front especially closer to
the lake in the onshore flow pattern. Cold advection continues
through the day thus expect some warming with solar insolation being
strong...but we will likely have a quicker drop off in the afternoon
and evening. Looks like a dry and seasonably cool day with northwest flow
aloft in place while the upper ridge in the center of the country
amplifies into Saskatchewan and Manitoba. Not much of a change in
the pattern for Thursday as high pressure moves overhead and shifts
east...maintaining cooler than normal conditions under onshore flow.
The upper disturbance that is moving by today will merge with a
large upper low in the Hudson Bay area that gets blocked by a ridge
it the Atlantic Basin. Meanwhile an upper low off the Pacific coast
will move through the Canadian prairies. Warm advection ahead of a
surface low ejecting out of the northern rockies on Friday looks to
trigger a round of showers and storms. A bit more uncertainty to the
evolution on the weekend as a surface wave associated with the
Pacific low is handled quite differently...thus some timing concerns
as to when the associated cold front slides through...which will
provide another opportunity for showers and storms.
Beyond that forecast confidence increases somewhat with
models/ensembles depicting a highly amplified flow pattern with a
large ridge in the west and an anomalous low in the east keeping the
region under northwest flow for cooler than normal conditions with periods
of clouds and showers.
//Ord and mdw concerns...updated 03z...
* line of scattered thunderstorms and rain moving through between 0345z and 0500z.
* Gusty north-northeast winds developing behind cold frontal
* Small chance for temporary MVFR ceilings overnight.
The line of scattered storms is moving southeast across the area
and will move through Ord and mdw prior to 05z. Coverage has been
enough to mention in Ord and dpa tafs. But any storms look to be
short-lived. Noted on kmkx and tmke radars is the secondary cold
front shifting winds to the north-northeast racing down the lake
at around 30 knots. This will likely move inland into northeast Illinois
and northwest in but possibly slow some. Have timed that as well
in taf amds.
Previous discussion...updated 00z...
A cold front draped across Central Lake Michigan through southern
WI and into Iowa as of 2330z will move southeastward this
evening...passing the taf sites between 0230z and 0500z. Along and
just ahead of this feature will be potentially enough convergence
for lift to overcome the Stout atmospheric cap. Coverage should be
scattered at best along the boundary. Feel thunderstorms in the vicinity was still the
proper way to handle it as we are able to focus in on frontal
timing much better now given the observational trends. The wind
direction will turn northwesterly behind the front and then north-
northeast within an hour or two of frontal passage. Strong pressure rises
should lead to periodic gusts even in the overnight hours. The
gusty winds will continue through the day Wednesday. While there
may be temporary due north winds early in the morning
Wednesday...a 020-040 direction is primarily expected for much of
Any MVFR ceilings overnight associated with the initial cold air
advection should be short-lived and are not expected to make it
into the morning rush hours...at least at the broken category.
//Ord and mdw confidence...updated 03z...
* medium in thunderstorms and rain impacting Ord and low yet if at mdw.
* Medium in timing of wind shift to north-northeast. High in wind
forecast for Wednesday.
* Low in any temporary MVFR ceilings in the couple hours after frontal passage.
//Outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
Thursday...VFR. Northeast winds.
Friday...chance of thunderstorms and rain during the night.
Saturday-Monday...periodic chances of thunderstorms and rain.
300 PM CDT
Low pressure over southern Quebec will continue to deepen as it
moves across Quebec tonight while high pressure moves off the middle
Atlantic coast. A strong cold front associated with the low will
push south down the length of the lake tonight. Moderately strong
south to southwest winds will veer to northerly following the
frontal passage with winds increasing to 30 knots with occasional 35
knots gale force gusts possible. A Small Craft Advisory associated
with the strong north winds will go into effect late tonight and
remain in effect through Thursday morning when winds and waves
will taper off as high pressure begins to build across the
Illinois...beach hazards statement...ilz006-ilz014 until 10 am Thursday.
In...beach hazards statement...inz001-inz002 until 10 am Thursday.
Lm...Small Craft Advisory...nearshore waters
until 10 am Thursday.
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