Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
403 am CDT Sat Oct 25 2014
314 am CDT
Short term...through Saturday night...
for the remainder of the night and into Saturday morning...the initial
concerns will be cloud cover...and impacts on visibility. A weak
middle level impulse that tracked across the region Friday evening
brought some scattered sprinkles. As the middle and high cloud pushes away
from the area...fog and low stratus are blooming. Northwestern Illinois and eastern
Iowa have already seen dense fog and low stratus form...and in the
moist airmass in place...with dewpoints in the low 50s...expect that
the area of fog/stratus should increase...though the lowest
conditions should be along and west of the I-39 corridor. East of
I-39...any dense fog should be limited to sheltered...low-lying
Through the morning hours...a weak frontal boundary will cross the
area...with surface high pressure building across the Central Plains while
upper level ridging also builds out of The Rockies and across the
Central Plains. The upper ridging will set up a subsidence pattern
across the area...leading to mostly sunny skies. Low pressure
moving through southeastern Canada...combined with the high pressure over
the plains will set up a generally northwesterly gradient across northern IL/in.
Daytime warming should allow any residual fog/stratus to dissipate
by late morning...while the ample sunshine and larger scale
subsidence should allow temperatures to quickly rise into the lower
70s across the southern portions of the County Warning Area...while location closer to
the WI border will be a bit cooler...but still significantly warmer
than normal...with highs in the upper 60s. Steady northwesterly winds at the
surface and aloft should prevent any kind of lake breeze from forming
and the warmer air should cover the area up to the Lakefront. While
dewpoints will be in the 50s for the morning hours...deep mixing
should draw much drier air to the surface...with dewpoints dropping into
the upper 30s for the afternoon hours. So...overall...Saturday is
shaping up to a very pleasant day...with temperatures around 10
degrees above normal. The surface high pressure will build across the
Mississippi Valley and into the Midwest overnight tonight while the
upper ridge slowly progresses as well. With a drier airmass in
place tonight...the chances for another round of fog and stratus is
unlikely...through the clear skies...light winds and lower dewpoints
should allow temperatures to radiate down into the upper 30s to
lower 40s. The Chicago area urban heat island should remain a
little warmer...with lows in the middle 40s.
Sunday through Tuesday...
guidance has been relatively consistent in the medium term featuring
middle-level ridging over the upper Midwest/Great Lakes region with a surface
ridge providing abundant sunshine and dry conds for the second half
of the weekend. However this ridge will only be temporary as the
middle-level heights begin to fall and a quasi-zonal flow begins to setup
for early next week. Fortunately it remains likely that Monday will
remain dry...with very mild temperatures expected. The surface ridge will be
shifting east of the forecast area...but should continue to keep
clouds to a minimum and precipitation west of the area. The combination of
this and an anomalous thermal ridge will allow temperatures to soar into
the middle/upper 70s...with a few 80 degree readings in the southern County warning forecast area
Broad surface trough over the central/northern plains will slowly
consolidate as this feature nears the Great Lakes Monday evening. There is
a small chance that a few showers/thunderstorms could skirt the
northern County warning forecast area Monday afternoon...but expect the bulk of the precipitation to occur
Post sunset. Middle-level trough will amplify across the Central Plains
Tuesday morning...as the surface low deepens across lower Michigan. Have
held onto the mention of thunder into Tuesday morning...as it appears
some instability/forcing will continue. Monday night temperatures will likely
not fall much...producing a mild overnight. Then for Tuesday with
continued cloud cover/precip...expect temperatures to remain in the upper
50s/low 60s. Depending on the timing of the frontal passage Tuesday...temperatures could
begin to fall Tuesday afternoon into the low 50s.
Wednesday through Friday...
for the second half of the week it appears the pattern will be
transitioning towards a cooler and somewhat active setup. Ensembles
continue to forecast middle-level ridging developing across the western
Continental U.S....allowing broad troughing downstream for the Great Lakes
region Wed/thur. Then Friday the 500mb ridge may begin to further
amplify as a potent shortwave is prognosticated to approach the Pacific-northwest and
ridging remains across the North Atlantic. Troughing will linger
across the Great Lakes Friday. Temperatures will trend back towards
seasonal...with perhaps Friday being sub-seasonal with highs in the upper
40s/low 50s. Periodic chances for showers throughout the extended
//Ord and mdw concerns...updated 09z...
* MVFR ceilings likely overnight...IFR ceilings a possibility.
* MVFR visibility possible overnight.
* Improving ceiling/visible Saturday morning.
* Breezy west-northwest winds...with gusts to around 20kt.
A weak surface trough and middle level impulse will cross the region
overnight. Latest satellite imagery indicates jet cirrus and some
middle level cloudiness over the area...but the higher level cloud is
pushing east of the region...and locations that have lost the
higher level cloud cover are already seeing lowering stratus and
fog. Anticipate that ceilings will gradually lower with the western
terminals...rfd/dpa lowering first and then Ord/mdw/gyy a little
later. Still have some concerns for IFR ceilings developing...with
higher confidence in IFR conditions at rfd/dpa...but have enough
confidence in IFR ceilings at Ord/mdw/gyy to at least include in a
tempo group. There is some concern for LIFR cigs/vis...especially
at rfd...which will have the longest period of time without the
higher level cloud. Also...as the weak surface trough axis crosses the
area...winds decreasing to blw 5kt could also help with denser fog
development away from the Chicago area urban heat island.
With ample sunshine expected...conditions should quickly improve
after sunrise and the remainder of the day should see VFR
conditions. The main issue for the remainder of the day will be
winds. Following the passage of the weak surface trough...high
pressure building over the northern plains will set up a strengthening
west-northwesterly-northwesterly gradient over the area. With ample mixing through the
afternoon...winds should become more gusty...with gusts to around
20kt through sunset.
//Ord and mdw confidence...updated 09z...
* high confidence in ceilings lowering to MVFR overnight. Medium
confidence in IFR ceilings developing.
* Low to medium confidence in MVFR visible overnight.
* High confidence in improvement to VFR and west-northwesterly-northwesterly winds
Saturday...with gusts to around 20kt in the afternoon.
//outlook for Ord/mdw for 12z Sunday-12z Friday...updated 00z...
Sunday...VFR. Slight chance rain showers/thunderstorms and rain Sunday night. Southeast winds.
Monday...VFR by day...shra/tsra likely Monday night. Breezy
Tuesday...chance shra/tsra. MVFR possible. Southwest-west winds.
Wednesday and Thursday...VFR. West-southwest winds.
229 am CDT
Low pressure over James Bay along with a surface ridge centered over
the northern plains has created a tight pressure gradient between
the two systems...which is positioned over the lake. This has
resulted in breezy southwest winds over the lake with speeds
generally between 20-25kt. Across the northern half of the lake
the gusts may near 30 knots this morning. As high pressure slowly
builds in from the west across the lake...wind will turn
west/northwest and gradually diminish late tonight/early sun. High
pressure will become centered over the lake sun...then continue to
slide east by late Sun night. This will allow only a temporary
lull in the winds...before winds turn south/southwest and begin to
increase. Another low pressure is poised to develop across the
Central Plains Monday afternoon...and slowly lift northeast
towards Lake Michigan Monday night/early Tuesday. This low will then
deepen Tuesday afternoon as it lifts northeast across lower Michigan.
This could produce a period of west gales Tuesday night/early Wednesday...then
another ridge of high pressure builds back into the western lakes
towards the middle of the week.
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