Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1202 am CST sun Nov 23 2014
Update...631 PM CST
Made adjustments to the pop forecast for tonight as radar is
rather quiet right now. The wave and associated rain showers over
southwestern and central Illinois looks like they will be the best chance
for rain through tonight. Therefore kept higher probability of precipitation south of
I-80. Ceiling heights keep rising over northern Illinois which leads to
ME to believe drier air will limit rainfall. Left chance probability of precipitation in
across the County Warning Area late this evening as I do not have enough
confidence to go dry. Dewpoint depressions are in the single
digits across the board so still thinking areas of fog will
persist overnight. Not expecting dense fog at this point in time.
Looking toward tomorrow...timing of precipitation and fog look good so
did not make any changes.
Also adjusted temperatures a bit higher tonight and still may not
have gone high enough. With the thick cloud cover and southerly
winds...not expecting temperatures to drop very much overnight.
258 PM CST
For the remainder of this afternoon and through tonight...an
influx of warm...moist air will continue to stream into the upper
Midwest. Latest surface analysis shows dewpoints in the 50s over
Missouri while dewpoints across northern Illinois. Northwestern in have climbed into
the middle to upper 40s. Temperatures have also been on the rise
through the day with temperatures at or in excess of 50f as far
north as Midway Airport and Aurora as of 230pm CST. Temperatures
are expected to continue to rise into the early evening under the
persistent warm...moist advection under a strong southwesterly low level
jet with 850mb winds up to 45kt...temperatures will likely hover
around the 50 degree mark through the night. Additional light rain
or drizzle will persist through the evening and overnight...with
areas of fog possible.
258 PM CST
More significant rainfall is setting up for Sunday and into
Sunday night as a strong southern stream shortwave lifting out of the
south High Plains phases with another shortwave lifting from the
southwest Gulf Coast region. These system will result in a
deepening surface low of 984mb near St Louis by Sunday evening. The
models continue to advertise precipitable water values around 1.2 inches...which
is at the 99th percentile for precipitable waters for the region. This phasing
of the two southern stream systems combined with the unusually high
precipitable water airmass will combine to focus steady rainfall across the
region through the day on Sunday and into Sunday evening...with a
corridor of the heaviest rain over the northwestern portions of the
County Warning Area...generally north of a line from Peru to McHenry...where
upwards of a half inch per 3 hour period is possible. Later Sunday
night...the phased southern stream system will begin to phase with a
northern stream shortwave...with low pressure rapidly deepening as it
lifts through the upper Great Lakes. While the guidance is
relatively consistent on the track and timing of the deepening
low...the GFS is a much stronger outlier in the guidance
envelope...deepening the central pressure to around 966mb by midday
Monday...while much of the other guidance only deepens the low to
around 980mb. While the GFS solution may be too deep...likely
drawing colder air into the system faster than the other
models...or tapping more northern stream energy...there will still
likely be a deep low developing over the upper Great Lakes Sunday
night into Monday...whose circulation pattern will cover a large
expanse from the east-Central Plains to northestern Quebec. A strong westerly-
northwesterly gradient will set up on the southwestern periphery of the
system...leading to strong...gusty winds across the County Warning Area from early
Monday morning through late Monday evening. The strongest winds
should occur from late morning through late evening...with
sustained winds of 25-30 miles per hour and peak gusts of 35-40 miles per hour.
However...if the system evolves a little deeper that the consensus
guidance and deepens closer to the outlier GFS solution...winds
across the area could reach Wind Advisory levels of sustained
winds of 30 miles per hour or higher or gusts of 45 miles per hour or greater.
Also...as the surface low begins to lift out to the northeast Monday
afternoon...much cooler air will begin to filter across the region.
As the pattern transitions from one of strong warm advection Sunday
night to strong cold advection by Monday morning...following the
passage of a strong associated cold front...lingering precipitation will
begin to transition from rain to snow. Since the cold air should
extend through the entire atmosphere...there should be a more
straightforward transition from rain to snow with little chance for
any freezing rain or sleet. There cold be some issues with water
flash freezing on elevated surfaces as sub-freezing air quickly
filters across the region by late Monday afternoon.
The strong cold advection will persist through the night with
temperatures dropping to the lower to middle 20s by daybreak Tuesday
morning. The persistent cold advection will allow for little
diurnal warming with highs on Tuesday expected to be in the upper
20s to around 30f over the far southeastern portions of the County Warning Area. There be some
moderation to temperatures on Wednesday as weak surface ridging crosses
the area allowing temperatures to rise into the middle 30s.
Wednesday should be the warmest of the extended period. A series of
strong northern stream shortwaves is expected to carve out a broad
longwave trough over eastern Continental U.S. From the Mississippi Valley to the
East Coast. The upper level pattern should decrease in amplitude
through the extended period...but the local area is expected to
remain under general northwesterly flow aloft...keeping a persistent fetch of
cold air across the area. By late next week...high temperatures will
struggle into the lower to middle 20s...while overnight lows drop
into the lower to middle teens...with the possibility of the northwestern
portions of the County Warning Area...including the Rockford area dropping into the
single digits Friday and Saturday morning. The series of shortwaves
rippling through the northwest flow should bring periodic chances
for some light snow...but confidence is low on the exact timing.
Confidence is a little higher that any snow that does occur should
be light as there will be little environmental moisture or surface
forcing. Any snow generation will have to rely on middle-level forcing
and meager moisture content.
//Ord and mdw concerns...updated 06z...
* MVFR ceilings through this morning. IFR possible Sunday morning and
likely by late afternoon.
* VFR visibility expected outside of any showers through Sunday
* Isolated-scattered shower chances increase
overnight...especially just south of the terminals. Better
chances late Sunday morning through the afternoon. Steadiest
rain by evening.
* South-southwest winds becoming gusty Sunday morning. Should turn
southeast Sunday afternoon then back south Sunday evening.
* Low level wind shear possible tonight with 45 knots winds at 2000
feet above ground level.
Ceilings continue to deteriorate from west to east early this
morning...with lower MVFR ceilings knocking at the door closer to
krfd...while southeast of kikk to kgyy ceilings will continue vary
with VFR dominating and occasional MVFR through the overnight
hours. IFR conditions may sneak into our western areas overnight
but not seeing much below 1000 feet ceilings yet. Low level wind shear
is also possible for the coming hours.
Have not made major changes to the current taf theme. Expect a
wave of rain this morning between 12z and 18z from south to
north...with activity more scattered in nature in the main
chicagoland terminals and points east...while areas west hold a
better chance of moderate rain and IFR conditions. Expect MVFR to
hold elsewhere. The main forcing for precipitation best coincides
with deeper moisture in the 22z-6z time frame where ceilings and
visibilities should easily fall to IFR/LIFR. Winds will also shift to a
southeasterly direction through the afternoon and early evening. After
6z winds will shift more southerly and rain will ease somewhat.
That said ceilings will generally remain in IFR/LIFR with a period of
lighter rain/drizzle with an additional period of showers toward
//Ord and mdw confidence...updated 06z...
* high in MVFR prevailing through midday. Low-medium confidence in
falling to IFR before Sunday afternoon. High that IFR/LIFR will
prevail from late afternoon through early Monday morning.
* Medium-high in VFR visibility outside of any showers overnight/Sunday
* High that coverage of showers will generally be low through
tonight...with most activity just to the south. High that
coverage will increase late Sunday morning and especially late
* High in wind trends through Sunday morning. Medium confidence
* Medium in low level wind shear potential.
//outlook for Ord/mdw for 12z Monday-12z Saturday...updated 00z...
Monday...chance of rain and snow. Strong west to southwest winds.
Tuesday...MVFR ceilings possible. Westerly winds.
Wednesday...chance of snow and MVFR. South to southwest winds.
Thursday...snow showers and MVFR possible. Northwest winds.
Friday...VFR probable. West to southwest winds.
231 PM...strong low pressure will move along the U.S. Canadian
border tonight and begin to weaken as it reaches Western Lake
Superior Sunday afternoon. Another low will develop over the
Southern Plains tonight and move north to Southern Lake Michigan
as it deepens and absorbs the low over Western Lake Superior. This
low will then deepen further as it reaches Eastern Lake Superior
Monday morning. There still remains some differences among the
models regarding how strong this low will become. The deepest of
the models has a pressure of 28.5 inches midday Monday. Confidence
is increasing for middle/high end gales so will be going with a gale
watch for Monday and Monday night. Storm force gusts or prevailing
winds also remain possible. Winds steadily diminish Tuesday as
high pressure moves across the southern lakes and Ohio Valley.
Another low is expected to move across the lakes region Wednesday
into Thursday...bringing northwest winds to 30kt on Thursday. Cms
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...4 am Sunday to 7 PM Sunday.
lmz874-lmz876-lmz878...6 am Monday to 3 am Tuesday.
lmz868...noon Monday to 3 am Tuesday.
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