Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
320 am CDT Tuesday may 26 2015

Short term...
319 am CDT

Through Tuesday night...

For the remainder of the overnight hours...conditions will remain
quiet across the County Warning Area as a series of short waves continue to track
north around the top of a persistent upper level ridge which extends
from the southeastern Continental U.S. Through the upper Mississippi Valley. This
upper ridge has been the feature which has controlled the path of
the series of shortwaves lifting out of the Southern Plains...and
will continue to do so through the day today. Southerly...moderately
strong...winds will persist through the night...and much of the day
allowing for surface dewpoints to remain consistent or possible inch a
little higher.

Main concern for the morning hours will be the track of a strong
short wave and associated surface low lifting northward through the
middle Mississippi Valley. This feature will predominantly impact
eastern Iowa and western Illinois by around daybreak. Through the morning...the
combination of isentropic lift in the warm sector of the
system...all of Illinois and into northwestern in...will lead to some
destabilization across the region. A surface well indicated
by the latest high res guidance...with lift through northern Illinois/in
through the day...coincident with a strong south-southwesterly low level jet of 45-
50 knots...with provide the potential for showers and strong to severe
thunderstorms across the County Warning Area. Based on wind profiles and middle level
drying...the primary concern with regard to severe weather will be
strong wind gusts. However...some of the high res surface wind guidance
suggest some slight backing of surface winds from 180-190 degrees to 160-
170 degrees...while winds aloft remain southwesterly. This slight backing of
the surface winds could provide enough directional shear through the
lower levels to bring an isolated tornado threat. Locally heavy
rainfall will also be concerned with precipitable waters reaching 1.75 inches
later this morning. However...flooding will likely not be a
significant concern as the southwesterly winds right off the deck will range
from around 45 knots above the boundary layer to around 90kt at upper
levels which should allow any of the stronger storms that do develop
to move quickly.

By early afternoon...the main short wave will lift into Wisconsin
while a surface trough tracks east across the area. This will lead to
winds at the surface veering to southwesterly...setting up a unidirectional wind
profile...effectively minimizing the severe weather threat to the
region. By late afternoon and into the evening the models are
advertising that a dry slow should nose into northern Illinois...effectively
cutting off the precipitation area into 2 separate over Wisconsin
and eastern Iowa and another more focused over Indiana. The northwestern
Indiana counties will still have a chance for some showers and
thunderstorms into the evening...while precipitation chances over northern Illinois
begin to wane. Much of the model guidance also indicated that the
shortwave should slow it's progression through Wisconsin as it
approaches the apex of the upper ridge. An associated surface
trough/cold front extending southwest from the surface low will linger
over eastern Iowa in the early evening before steadily pushing across
northern Illinois and into northwestern in overnight. Through the night...the
strongest upper level forcing will be well north of the County Warning any
lingering chances for precipitation overnight will be low and likely confined
to the northern tier counties of Illinois...along the WI border.



Long term...
229 am CDT

Wednesday through Monday...

Wednesday through Thursday...
open 500mb wave will be lifting northeast across the upper Midwest
into Ontario and eventually Quebec...with heights slowly increasing
Wednesday afternoon. Surface features remain disorganized Wednesday...with a boundary
slowly shifting east. Oper guidance differs on how quickly precipitation
will depart the region. Ensemble data suggests holding onto chance to
slight chance probability of precipitation mainly for the central/eastern County warning forecast area Wednesday/Wednesday evening.
Although with some hint of surface heights nudging is possible
that some dry air could filter in and kick precipitation east by midday and
allow for a mainly dry Wednesday. Surface winds will be turning southwest to
west...keeping an offshore flow for locations close to Lake
Michigan. Temperatures should yet again touch upper 70s to around 80 Wednesday...if
clouds and precipitation can push east by midday...some clearing would
allow surface temperatures to push a couple degrees warmer into the low 80s in
some locations. Late in the evening the gradient slackens
further...and could allow a late day onshore flow to develop for
areas adjacent to Lake Michigan.

Middle-level flow begins to flatten as a weak surface ridge becomes oriented
from western Quebec south through the eastern Ohio River valley.
This will generally allow a southerly flow...although the gradient
is not very strong. This should set the stage for a lake
breeze/onshore flow to develop Thursday. Timing could end up being
slightly earlier given the light surface gradient. Guidance has been
consistent in holding precipitation west/south of the forecast area a result of the weak surface ridging. So one more mild day
with highs warming back into the upper 70s/low 80s...however as
previously noted...a lake breeze is likely to develop. This will
bleed the much cooler marine air west across northeast Illinois and
downtown Chicago. If the timing is earlier in the day Thursday...temperatures
may not warm beyong the low/middle 60s along the Chicago Lakefront.

Flow then turns southerly Thursday night with the more progressive flow
in the middle-levels pushing the ridging east. A return flow moisture
will continue to be focused across the central Continental U.S. Thursday evening...with
a slow moving frontal boundary nudging east Thursday night towards
western Illinois. This frontal boundary will become energized as a robust
500mb vorticity maximum breaks south from central Canada late Thursday night.

This should allow for an atmospheric river to develop from the
Southern Plains stretching north through the middle-Missouri Valley
into the western Great Lakes early Friday morning.

Friday through early next week...
ensembles continue to forecast a robust trough sliding south from
central Canada Friday. This coupled with downstream middle-level ridging
across the southeast Continental U.S. Will strengthen the frontal boundary
that will be oriented from the Southern Plains stretching
northeast through the Great Lakes region into western Quebec.

This setup is ideal for the development of an atmospheric river with
abundant low level moisture and efficient precipitation producing

Oper guidance has been relatively consistent with bringing a series
of waves along the slow moving frontal boundary Friday aftn/eve...then
the boundary should slide east with winds quickly veering
north/northeast for Sat and ushering in much cooler air to the

Current indications are that highs Sat may struggle to warm beyond
the upper 50s across northeast Illinois/northwest in...with only low 60s
elsewhere. Strong surface ridge will remain north of the the
prior frontal boundary may become quasi-stationary for the weekend
along the Ohio Valley and continue to feed precipitation chances to the
southern County warning forecast area.




//Ord and mdw concerns...updated 06z...

* south-southwest winds of around 40 knots at 1000 feet
overnight...setting up low level wind shear.

* MVFR ceilings probable at times Tuesday with a chance for
temporary IFR.

* Periods of rain showers with periodic thunderstorms and rain from late Tuesday morning
through the afternoon.

* Near due south winds on Tuesday with periodic gusts possible
and some variability between southeast and southwest.


//discussion...updated 06z...

Gusty south-southwest winds have diminished to 10 knots or less and
become southerly. A low-level jet will develop overhead with
south-southwesterly-southwesterly winds of 40 knots to 45 knots flow at 1500-2000 feet until
around 12z. Some thunderstorms may develop through overnight ahead
of a front from northern MO through eastern Iowa and into WI. These storms
will only make slow progress eastward due to the strong south-southwesterly-southwesterly
winds aloft so...and storms that do develop should remain north
and northwest of area taf sites...though there is a chance that a
few storms could approach the rfd terminal area very late

As an middle level disturbance moving from the Southern Plains to
northern Illinois by Tuesday afternoon may bring another round of showers
with scattered embedded thunderstorms to the region. Upper level forcing
from the middle level disturbance and an upper level jet stream will
enhance the likelihood for rain showers and the chance for thunderstorms and rain...with the
most likely chance for activity from 15z through 02z. Some
uncertainty remains in pinning down the exact period for the
thunderstorms and will maintain the going tempo for late morning through
early afternoon. However...the precipitation activity could be shifted or
expanded in time.

Winds will be southerly on Tuesday...though direction may be a bit
variable around convective activity...though a generally southerly
direction should prevail through the night and much of the day
Tuesday. Some gustiness is possible through the entire day...with
the latest guidance suggesting that winds could strengthen and
become even more gusty through the night tomorrow night with winds
veering southwesterly as surface low pressure moves northeast across
southern Wisconsin.


//Ord and mdw confidence...updated 06z...

* high confidence in 40-45 knots south-southwesterly-southwesterly flow aloft overnight...but
medium in true low level wind shear occurring.

* High confidence in wind trends overnight and Tuesday morning.
Medium in wind trends tomorrow evening.

* Medium confidence in MVFR ceilings Tuesday morning with lower
confidence in duration and how low ceilings will drop.

* High confidence in showers and at least a period of thunderstorms and rain. Low
confidence in specific thunderstorms and rain timing and impact on the terminals
with multiple rounds possible between 15z and 02z.


//outlook for Ord/mdw for 12z Wednesday-12z Monday...updated 00z...

Wednesday...chance of thunderstorms and rain. West winds.
Thursday...slight chance of thunderstorms and rain late at night. Southeast winds.
Friday...increasing chance of thunderstorms and rain through the day and night.
Southwest winds.
Saturday...rain showers likely with a chance of thunderstorms and rain. Northeast winds.
Sunday...chance of rain showers. East winds.



229 am CDT

Warm moist air continues to feed north across the lake
today...which may help to further enhance any areas of fog that
have already developed. It is possible patchy dense fog may
develop...although winds may be just strong enough to limit the
coverage and intensity of the fog. A surface low pressure system
will be lifting north across eastern Iowa into central Wisconsin
this evening. The gradient will tighten across the
central/southern portions of the lake...which with warmer
nearshore waters expect some slightly better mixing to occur.
Further out across the open waters the water remains cold and
expect the stability to make it difficult to mix to the surface the
higher gusts. High pressure then arrives late Wednesday with light
winds developing. Then high pressure shifts east Thursday
night...with southerly winds developing again Friday. A frontal
boundary will approach Lake Michigan late Friday night then
steadily slide south along the lake. This will allow winds to
quickly veer from south to north/northeast Saturday. Along with
this quick wind shift will be cooler air sliding south over the
lake. Waves should easily build Saturday for the central/southern
portions of the lake.



Lot watches/warnings/advisories...


Visit US at http://weather.Gov/Chicago (all lowercase)

Follow US on facebook...twitter...and youtube at:

National Weather Service Glossary of Abbreviations