Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/28/19

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 905 PM CST Fri Dec 27 2019

.UPDATE...
855 PM CST

Main updates this evening were to lower temperatures a bit
overnight and to add some fog wording to the grids, especially
south and east of I-55/57.

Skies have remained mainly clear most of the evening which has
allowed temperatures to drop off pretty quickly towards their
dewpoints across much of the area. That is changing, however, with
the approach of a modest mid-level wave which is spreading
additional mid-level cloud cover northward. An expansive area of
low stratus is percolating at this hour immediately east of the
CWA, and this all looks to slosh west and eventually northwestward
later tonight as the low-level wind field begins to veer as
surface high pressure slinks off to the east. The northern-most
portion of this low cloud deck is just that--low clouds, but
visibilities have been inching downwards with southward extent
where low-level moisture is deepest. Unsure exactly how the
incoming mid-deck will impact visibility reductions, but elected
to toss in some fog wording into the grids after midnight with the
greatest coverage expected mainly along and east of I-55/57 where
near-surface winds will remain lightest. With temperatures near
and below freezing, any fog/mist would be freezing fog which could
cause some slick spots on elevated surfaces, although just how
thick fog may get remains in doubt. Will continue to monitor
trends this evening.

The aforementioned wave to our south has resulted in the
development of mid-level radar echos which have, thus far, not
resulted in any precipitation due to the dry 850-600 mb airmass in
place. ILX`s 00z sounding showed a bit more saturation in thay
layer, but recent AMDAR soundings through this layer are still
fairly dry. Noting a very modest mid-level f-gen circulation with
this feature with some attendant decrease in 500-600 mb lapse
rates. Will leave the forecast precip-free given the dry mid-
levels, but have tossed in some silent <10% PoPs south of I-80.

Finally, another subtle wave is lifting out of Arkansas which
could help encourage some precipitation development closer to I-39
across our western counties after daybreak on Saturday. With
temperatures already dropping into the mid-upper 20s, could be a
potential for a very brief rain/freezing rain scenario but with
temperatures pushing north of freezing pretty quickly, don`t
envision this causing any notable travel issues.

Carlaw

&&

.SHORT TERM...
315 PM CST

Through Saturday night...

Several forecast concerns through Saturday night, including
patchy fog potential later Saturday morning through the afternoon,
followed by widespread rain/showers and embedded thunderstorms
Saturday night.

The pesky stratus deck that kept our temps cooler today finally
eroded to along and southeast of I-55 as of this writing.
Expecting it continue to slowly erode southeastward, though with
low level winds aloft diminishing. However, if it doesn`t, it may
tend to expand back north/northwest tonight as cloud steering
winds turn to southeasterly. Otherwise, tonight should be quiet
with increasing cloud cover overall as surface high pressure
transits the area. Temperatures will ease back to the upper 20s
to mid 30s this evening and then slowly rise overnight thanks to
the increasing clouds, a bit of wind late, and strengthening warm
advection aloft.

On Saturday morning, attention turns to developing surface low
pressure over the southern High Plains in response to deep western
troughing and a strong upper level jet streak. In deep southwest
flow aloft over our area, a lead mid-level shortwave will clip
northwest and north central Illinois with an area of light
rain/showers, possibly mixed with intermittent drizzle late the
morning as low cloud cover rapidly expands back northward. Think
that temperatures will most likely have safely climbed to above
32F by the time any precip ensues, which would preclude risk for
any light freezing rain or drizzle.

For the rest of Saturday, the local area will be positioned north
of a developing warm frontal zone from the Plains surface low
that will only gradually lift northward through the day. After the
aforementioned morning wave in northwest 1/2 or 1/3 of CWA during
the morning, forecast soundings indicate that additional precip if
any would mainly be in the form of drizzle. Exception to this
could be the far northwest, where additional waves of light
rain/showers will be possible. Synoptic set-up tomorrow, with area
north of warm front, and dew points gradually ticking upward and
into the 40s, is typically favorable for some fog development.
Arguing against a significant dense fog risk is steady east-
southeast to southeast winds and lack of snow cover over the
region. Nonetheless, with temperatures likely staying only a
degree or two above the increasing dew points, felt it was
reasonable to add patchy fog mention. Of note, the 12z MET MOS
guidance was hitting the fog threat hardest in part of the area,
which is probably overdone.

There will be a strongly non-diurnal component to the
temperatures on Saturday, which is also classic to the synoptic
set-up. Temperature climb tomorrow will be gradual, with most
areas only topping out in the 40s by sunset, except low 50s well
south. Far northern areas may not get out of the 30s until early
afternoon. Daily high temps will occur in the early evening.

The surface low will strengthen into the low 990s mb Saturday
night as it tracks toward southern Minnesota. It is during this
time that large scale ascent will be at its strongest, along with
robust warm advection/isentropic ascent. Expect an area of
widespread rain/showers to spread quickly north- northeastward.
With PWAT values increasing to or over 1.25", near the December
maximum, this will support occasional moderate to heavy rainfall
rates. Mid-level lapse rates will remain meager, but the magnitude
of the WAA is expected to compensate and still yield a threat for
embedded thunderstorms, with a bit higher chance south. Finally,
as south-southeast winds remain elevated while warm front lifts
north, temperatures will steadily rise through the night, with 50s
for all or most areas prior to daybreak Sunday. This will set the
stage for another day of impressive warmth on Sunday.

Castro

&&

.LONG TERM...
305 PM CST

Sunday through Friday...

By Sunday morning, the center of the surface low associated with the
strong storm system expected to develop over the Central Plains
should have lifted to southern MN.  An associated surface cold front
will quickly push across the plains and should be moving crossing
the Middle Mississippi River into wrn IL by the beginning of the
period.  The latest longer range guidance remains relatively
consistent will the general details of the system, particularly in
the path and strength of the mid-level circulation center, though
there are still some minor timing differences for the frontal
passage.  These timing differences should have little impact on the
general trends and character of the system and impacts on the local
area.

In advance of the system, strong moist advection will drive
temperatures well into the 50s by daybreak, with temperatures likely
to rise to, or slightly above 60 F with dewpoints in the 50s before
the front pushes through the area.  Gusty south winds will set up in
advance of the system, with gusts of 25-30 mph.  Widespread rainfall
should still be ongoing Sunday morning,  but should diminish to
light rain or drizzle by mid-day as a mid-level dry slot overspreads
the region. The upper low is expected to meander around the upper
Midwest as surface system begins to occlude.

However, a secondary and somewhat strong vort max will rotate
around the swrn/srn periphery of the main upper low Sunday
evening. Temperatures should begin to drop off quickly as the true
cold air begins to filter into the area. This should bring
another round of quick hitting rain. Both the GFS/ECMWF have
enough column cooling as the pcpn is winding down for the rain to
mix with or, briefly, change over to snow before precip ends early
Monday morning. Marginal surface temps should limit what minor
accumulations may occur, if any, to grassy areas.

The cold core of the upper low will spread overhead Monday afternoon
and nwly flow aloft with deep layer cold advection should develop as
the low lifts to the Upper Great Lakes Monday night.  With weak
shortwaves rotating around the swrn periphery of the exiting upper
low, there will be the potential for some sct light snow showers or
flurries overnight into early Tuesday.  Conditions should be
seasonably cold highs on Tuesday in the lower to middle 30s.
Wednesday should be a bit warmer as upper ridging builds across the
area.  A modest warming trend will set up for Wednesday and through
the end of the period as southwest flow returns aloft as the upper
level pattern gets more active over the wrn CONUS with a sharp long
wave trough setting up.  A series of weak shortwaves lifting out of
the base of the upper trough and rippling through swly flow aloft,
bringing both a modest warming trend with periodic low chances of
pcpn.

&&

.AVIATION...
For the 00Z TAFs...

Main concerns through the TAF period:
* Potential for IFR or lower cigs/vis developing overnight into
  early Saturday morning
* Periods of light rain/showers and/or drizzle on Saturday, mainly
  at RFD in the AM and possibly farther east in the afternoon with
  IFR/near LIFR cigs.

Continue to have a fairly complicated TAF forecast through Saturday.
Hi-res guidance is trending toward increased fog/low stratus
potential early Saturday morning as the surface high drifts east
across the area and light surface winds turn southeasterly. Have
updated the TAF to hint at this trend. Things could very well be
even lower if fog/stratus does develop toward VLIFR. Temps will
also be low enough for some freezing fog potential.

As for precip, a first wave of light rain/drizzle should push
through in the morning across our northwestern zones that could
impact RFD. This still looks to remain northwest of the other
terminals. Drizzle will then develop by Saturday afternoon out ahead
of the main wave of precip. There is higher confidence in decreasing
cigs to IFR/near LIFR Saturday afternoon associated with this
precip.

Winds will be light and mostly variable through the night, becoming
east-southeasterly Saturday morning as the surface high shifts to
the east. By Saturday afternoon winds will begin to pick up to 10-
15KT with gusts to 20-25KT possible as the main system approaches
from the southwest.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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