Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
543 PM CST Wednesday Nov 25 2015
307 PM CST
Tonight through Thanksgiving day...
Main forecast concern is with patchy fog/drizzle and gradually
increasing light rain potential. Local area is in between
expansive and strong high pressure sliding east off northeast
coast and low pressure/frontal trough spanning from western
Ontario to the Southern Plains. Broad disturbed southwest flow
aloft...with one middle/upper wave crossing area now and a few other
disturbances noted on west/v back to the southwest. Strong southwest
low level jet and associated warm air advection will continue through
Thanksgiving day...while at the surface steady and occasionally
gusty southerly winds will persist.
Clouds are expected to gradually lower tonight...and expecting
diminishing dewpoint spreads as temperatures and dewpoints slowly rise
after temperatures perhaps drop a few degrees in spots early this
evening. This could set the stage for some patchy fog and drizzle
overnight into Thursday morning. However...now have decreasing
concern in seeing more widespread/areas of fog. For
instance...upstream conditions across northern Iowa this
afternoon...which did have a deep snowpack and has temperatures in 40s
and 50s and dewpoints well into 40s. Would have expected to see
evidence of fog in this area. It seems that dewpoint spreads will
remain high enough...combined with steady south winds and much of
the snow melt already occurring...to preclude dense fog.
Considering that statistical guidance is also not hitting fog all
that hard...backed off to patchy fog mention in grids through middle
Thursday morning...but will need to monitor.
Approaching frontal trough will gradually improve low level
moisture convergence on Thursday...providing a better rain focus
slowly sliding southeastward with trough...and steadier light to
moderate rain across far northern Illinois late Thanksgiving morning
into the afternoon. Otherwise...until that point due to
aforementioned disturbed SW flow and low level jet...occasional/spotty light
rain...with chance to low-middle likely probability of precipitation carried. Temperatures
rising to around 50/low 50s by early Thursday will set the stage
for a mild Turkey day with highs in the middle 50s to around 60. This
will be despite overcast skies and occasional rain.
231 PM CST
Thursday night through Wednesday...
Main issue for the longer term forecast period will be the heavy
rain event that will be ongoing Thursday evening and continuing into
The main driver for the continued rainfall at the beginning of the
period will be a deep long wave trough over the western Continental U.S. With a
closed upper low centered near Salt Lake City at 00z Friday. With
high amplitude upper ridging from the western Gulf of Mexico through the middle
Atlantic region aloft and a slow moving cold front extending from
Northern Lake Michigan to the Texas Panhandle...a persistent fetch of deep
layer moisture will continue. The latest model guidance is in good
agreement on the general pattern...with a wide open Gulf of Mexico and
strong low level jet with southwesterly 850mb winds around 50kt pointing into the
Midwest. Along with a strong Theta-E ridge and a corridor precipitable waters of
1.5 inches which will be coincident with the frontal boundary and
low level jet...all indicators point to a classic heavy rain event. Precipitable waters of
this magnitude are around 3 Standard deviations above climatological
normals for late November for both the dvn and ilx sounding
locations. The trend for Thursday night into Saturday will be for a
slow progression of the surface front while the upper low remains nearly
stationary over the Great Basin...with a series of impulses ejecting
out of the southern rockies in fast...persistent southwesterly flow aloft. Before
all is said and done...the entire County Warning Area should see well over 1 inch of
areal average quantitative precipitation forecast...with locally higher amounts associated with the
shortwaves rippling through the southwesterly flow aloft. The latest
guidance suggests that the corridor of the highest quantitative precipitation forecast will extend
from southern WI through eastern Iowa...where upwards of 2 inches of areal
average quantitative precipitation forecast is indicated...but with some spread to model details
developing by Friday night into Saturday...there is still a chance
that the total quantitative precipitation forecast across northern Illinois/northwestern in could be higher should the
pattern set up slightly more to the east.
Temperature trends for the early portions of the long term forecast
period will be tricky and highly dependent on the timing of the
frontal passage. The latest guidance suggests that the front will
slowly push across the County Warning Area...reaching the Rockford area early Friday
evening and steadily pushing to the southeast...and not exiting the
County Warning Area until middle-day Friday...when the front should extend from northwestern Ohio
to central Illinois. A much colder airmass will filter in behind the
front...and maximum temperatures for Friday will likely occur during the
overnight hours Thursday night and fall through the day on Friday.
Below normal temperatures will continue into the weekend with highs
Saturday in the upper 30s to lower 40s and lows Sunday morning
ranging from the middle 20s to around 30f. Moderation of
temperatures is expected for Sunday and into early next week as the
upper low remains parked over the Great Basin and upper ridging
builds across the Mississippi Valley. Greater uncertainty is
introduced into the forecast toward the end of the forecast
period...for Tuesday and Wednesday...as the longer range guidance
begins to diverge on handling the evolution of the upper system over
the Great Basin and when it will eventually kick out to the east.
Given the rather extreme model spread that develops early next
week...have trended toward a middle of the Road approach and not
made any significant changes to the going forecast for early next
245 PM CST
Rapid snowmelt is underway as temperatures rise into the middle
40s to middle 50s across the region this afternoon and dewpoint
rise into the middle to upper 30s. Much of the snow should be
melted by the time that steady rain begins to overspread the
region overnight tonight. The latest guidance suggests an areal
average total quantitative precipitation forecast of up to 1.5 inches...with locally higher
amounts very likely...across region following the melting of snow
with water equivalent values of slightly in excess of 1 inch
across far northern Illinois where the deepest snow cover had
been. The frozen soil depth is very shallow and the melting snow
saturating the ground...additional infiltration will be difficult
and much of the rainfall should go to runoff. With streamflow
values running higher than normal for some streams and rivers
due a period of wet weather the past few weeks...there is concern
for some streams to potentially rise into minor flood stage.
Current forecasts highlight that this is possible across portions
of the Des Plaines...Fox...and Illinois River basins. The North
Branch of the Chicago river may also reach near 5 feet as
well...which creates some urban concerns in the city of Chicago.
Other basins will have elevated streamflows. Expect ponding of
water on area roadways may be possible as well. Have continued esf
at this time with details in specific rainfall amounts and timing
to be refined as the event approaches.
//Ord and mdw concerns...updated 00z...
* low level wind shear developing this evening and continuing overnight.
* Ceilings steady to gradually lowering later tonight below 2500 feet
after midnight and below 1000 feet Thursday morning.
* Visibilities lowering to 3-5sm by 09-10z... and to IFR after daybreak
* Spotty light rain/drizzle becoming more widespread late
Brisk south winds will continue this evening ahead of a cold
front over western Iowa. Expect gusts to tail off as surface winds
decouple and a strong low level jet streak noses east. With such
low level wind shear will develop this evening and continue overnight with winds
at the 2k feet level increasing to around 50kt by 10 PM. Warmer and
more moist air will continue to stream north into the area
overnight helping to lower ceilings gradually. Soupiest air
arrives Thursday after daybreak and expect a 6-8 hour period of
IFR visibilities in fog and IFR ceilings with rain/drizzle during the day
//Ord and mdw confidence...updated 00z...
* medium to high with low level wind shear.
* High with wind speed/direction.
* Medium with ceilings and visibilities forecasts and timing.
//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...
Thursday night...IFR likely LIFR possible with periods of rain.
Friday...MVFR likely with rain. Gusty north-northeasterly winds.
Saturday...VFR. East-northeasterly winds.
Sunday through Tuesday...MVFR possible. Slight chance of rain.
355 PM CST
Southerly gales will continue on the northern portion of the lake
through late this evening and then will gradually ease as a frontal
trough approaches the lake. Brisk southerly winds will continue on
the southern portion of the lake through Thursday evening and speeds
will generally continue to be hazardous for small craft. Cold front
will sink down the lake Thursday afternoon...with winds shifting
northerly and quickly increasing behind the front to 25 to 30 knots by
As colder air filters in behind the front...at least gale force
gusts are possible for much of the open waters and the Indiana
nearshore waters by late Thursday night...and diminishing from
north to south by early Friday evening. The Illinois nearshore
waters will have a lesser chance for gales.
High pressure will spread over the lake Friday night and
Saturday...bringing diminishing winds through the weekend and into
early next week.
Lm...Small Craft Advisory...nearshore waters
until 10 PM Thursday.
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