Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 10/28/17
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
853 PM CDT Fri Oct 27 2017
843 PM CDT
No real big changes this evening, however, did make some slight
adjustment to the pop/wx mainly for locations north of I-80 where
there may be a slight increased chance of sprinkles or flurries
tonight through early Saturday morning. Overall, conditions still
appear marginal for precip development tonight, given initial
light ascent overhead and with lacking deep moisture. Nonetheless,
have seen reports of both sprinkles or flurries already this
evening. With a slight increase in forcing through the night with
the arrival of a vort lobe just to the northwest, could see some
slight increase in coverage.
Deeper moisture and lacking crystals have been in question this
evening but some indications this evening are leading to believe
there could be more flurries observed than sprinkles. Latest
AMDAR soundings showing moisture present in the -8 to -10 C layer
and although crystals are more than likely to be lacking in this
temp range, it is not out of the question. This presence of
crystals is also noted on latest GOES R imagery, and all
supportive of latest short term guidance trends of the slightly
higher chance of flurries tonight. However, still not expecting
any accumulation tonight.
224 PM CDT
Blustery and colder weather will persist through the near term,
with the potential for a few sprinkles or flurries late tonight
into early Saturday morning, mainly over far northern IL.
Surface low pressure was analyzed over the Upper Peninsula of
Michigan early this afternoon, in association with a closed upper
low over WI. A secondary surface trough/cold front extended from
the low through western WI and into eastern IA, beneath the main
upper trough axis. This secondary cold front will push east across
the forecast area overnight into early Saturday, with a little
deeper wrap-around moisture spreading a few sprinkles or flurries
into the area mainly after midnight tonight. Forecast soundings
depict moisture up to about 750 mb, with the coldest temps at the
top of the layer approaching -10C. This suggests a lack of ice
nucleation for producing snow, though any deeper cloud layer would
likely support ice crystals and a potential mix of light
rain/snow. Precip is expected to be light, with no accumulation,
but a few wet flakes could be in the air over parts of far
northern IL toward morning.
Otherwise, cloudy skies and blustery westerly winds will persist
in the tight cyclonic surface pressure gradient with the slowly
departing low. Clouds/winds will keep temps from dropping off too
much overnight, generally in the lower and middle 30s. Similar
conditions favor undercutting MOS numbers for max temps Saturday
as well, with afternoon highs about 40-42 rather than low-mid 40`s
from MET/MAV guidance.
320 PM CDT
Saturday night through Friday...
No significant changes to the long term forecast with trends for
much below normal temperatures for much of the period, with some
warming expected toward the end of next week.
The upper level long wave pattern continues to trend toward a broad
upper trough covering all of the CONUS except for the west and east
coasts. Between the lowering heights from the broad upper trough
and flow trending to nwly aloft, expect temperatures through mid
week to be well below normal, with highs in the 40s in the 30s. The
coldest night should be Saturday night when temperatures across much
of the area west of the Indiana state line will be in the upper
twenties to low 30s. However, the urbanized areas of the Chicago
Metro area will likely remain above freezing, but will include Cook
County in the Freeze Warning since the outer neighborhoods should
drop to or below freezing. Toward the end of next week, the longer
range guidance is indicating a trend to rising heights aloft and
more zonal, wly flow aloft. This should allow for some moderation
to temperatures, trending back to seasonal normals of upper 50s to
arnd 60 F and lows in the upper 30s to low 40s. There will be
periodic chances for some light pcpn through the extended forecast
period with shortwaves dropping southeast out of the nrn plains and
scntrl Canada. Timing of individual waves has some uncertainty due
to timing differences among the models, so have carried generally
low chance to slight chance PoPs. There could be some light rain
showers or light snow showers or a mix, depending on how timing of
the short waves lines up with diurnal temperature trends. Regardless
of p-type, QPF amounts will be low.
For the 00Z TAFs...
Did not make big changes to the TAFs as west southwest wind
expected to continue, while the current VFR ceilings lower to MVFR
overnight. Am monitoring the possibility for a slightly earlier
time frame of this transition, and have already moved up the
timing by an hour. Low end MVFR still appearing more likely across
the RFD area, with a brief window of IFR ceilings not out of the
question. Continued a dry forecast for late tonight into early
Saturday morning. However, could see some locations seeing a small
window of either sprinkles or flurries mainly during early
Saturday morning. Ceilings expected to slowly lift through the
day, with VFR returning by Saturday afternoon.
320 PM CDT
Low pressure is lifting northeast from northern Wisconsin and the
central upper Great Lakes region. A secondary surge of cold air
will push across the lake late tonight and early tomorrow, setting
up an extended period of brisk winds shifting from swly this evening
to nwly by tomorrow morning. Winds and waves hazardous to small
craft will likely persist through the day tomorrow for the Illinois
nearshore waters and into tomorrow night for the Indiana nearshore
waters. A weak ridge of high pressure moving across the lake will
allow winds to diminish over the lake Saturday night and Sunday.
Another low will track across the upper Great Lakes Sunday and then
become absorbed into a larger low over sern Canada Sunday night.
early Monday. The cold front associated with this low will move
across the lake early Monday morning. Another surge of cold air
following this front may bring some westerly gales Monday afternoon
ILZ023-ILZ032-ILZ033-ILZ039...11 PM Saturday to 9 AM Sunday.
LM...Small Craft Advisory...Winthrop Harbor to Gary IN until 7 PM
Small Craft Advisory...Gary to Michigan City IN until 4 AM Sunday.
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