Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
200 PM CST Friday Dec 26 2014
258 am CST
High cloudiness will continue to stream into the region through at
least early afternoon resulting in periods of cloudiness and
sunshine before beginning to shift more to the southeast of the
region as core of upper jet shifts farther southeast. Despite the
presence of scattered-broken cirrus still anticipate an unseasonably warm day
with highs soaring well into the 40s and probably cracking 50
degrees in some areas given the mild start...continued warm air
advection and local 925/850mb temperature climatology. Unseasonably mild
conditions continue tonight in advance of the front with southerly
winds transporting 40f+ dewpoints north into the region and helping
hold temperatures in the 40s through the night. Along with the higher
dewpoints will likely arrive stratus and the threat of some light
rain or drizzle...especially late. Quantitative precipitation forecast fields in the global models
seem to generous with amounts given the corresponding forecast
soundings are dry above a shallow near surface moist layer...so not
anticipating much more than very light rain or drizzle if any precipitation
Cold front will sweep across the region on Saturday and probably
provide for at least a period of some scattered showers with
it...but continues to not look like an all day washout by any means
with many areas (especially south) possibly escaping with very
precipitation. Timing of the front will be key to how warm temperatures get with
models having slowed the timing of the front a bit and given the
very warm start to the day have nudged highs up a bit over mainly
the southeast half of the County Warning Area. Temperatures should begin a slow fall behind
the front but with air mass more of Pacific origins not really
looking for any major drop in temperatures.
258 am CST
Saturday night through Thursday...
Seasonable temperatures and dry conditions look to be on tap for Sunday and
Monday before Arctic air begins spilling south into the area Tuesday
into middle week. Medium range guidance has varied a bit with the
magnitude of the cold air mass in our region...but do continue to
focus The Heart of the cold air west over the plains. Lack of snow
cover here and upstream a ways will likely allow air mass to modify
before arriving...but none the less the prognosticated 850/925mb temperatures
support highs/lows much much colder than we have grown accustomed to
and possibly struggling to even reach 20f for highs by midweek. Low
temperatures could dip into the single digits outside of the Chicago urban
heat Island...A far cry from the upper 20 degree lows the area has
averaged thus far this December. Prospects for measurable snow with
the arriving Arctic air mass through midweek look pretty bleak...and
if we do manage to close out December without any measurable
snowfall in Chicago it would be the first time since 1912 that this
has happened and only the second time in Chicago recorded history.
//Ord and mdw concerns...updated 20z...
* MVFR ceilings developing this evening and lowering to IFR overnight.
* Chance of light rain or drizzle overnight into Saturday morning.
* Gusty south/southwest winds this afternoon.
* Strong/gusty westerly winds late Saturday morning into Saturday
A weak frontal boundary extends from northeast Wisconsin southwest
into the plains. As this boundary slowly moves east today...new
low pressure will develop over Missouri and lift northeast across
northwest Illinois Saturday morning and then continue northeast
across Lake Michigan Saturday afternoon. Southerly winds will
continue today and tonight ahead of this low with some gusts into
the upper teen/lower 20kt range possible this afternoon. Winds
will shift to the west as the low departs and quickly increase
late Saturday morning into Saturday afternoon...when there could
be a few hour period of gusts to 30 knots...before relaxing back
into the middle 20kt range through sunset Saturday evening.
Ceilings ranging from 2kft to 4kft are spreading north into southern
Missouri and southern Illinois late this morning. This cloud cover
is expected to continue north across the terminals late this
afternoon into this evening then slowly lower...eventually into
IFR overnight. Overall confidence in this trend is high but the
timing and specific ceilings will likely require tweaks with later
forecasts. As the low approaches...its possible that ceilings could
lower into LIFR...especially at rfd. Ceilings will then lift into MVFR
as the low departs and possibly become VFR Saturday afternoon.
Confidence regarding precipitation overnight into Saturday morning is
somewhat low. Precipitation amounts look fairly light but could be a
period of very light rain or drizzle. Opted to maintain current
rain/drizzle mention for now. Visibilities will also likely lower
into MVFR as low level moisture increases and if drizzle becomes
prevailing...in addition to the approaching low pressure...visible
could easily drop to 1-2sm with some dense fog possible. Cms
//Ord and mdw confidence...updated 20z...
* high for ceiling trends...medium for timing and specific heights.
* Medium for light rain or drizzle overnight/Saturday morning.
* High for winds through the period...medium for timing changes. Cms
//outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...
Saturday night...MVFR ceilings with breezy northwest winds.
Monday...schc -shsn. MVFR possible. North to NE winds.
Tuesday...VFR. Gusty northwest winds.
Wednesday and Thursday...VFR. Breezy SW winds at times.
234 am CST
High pressure over the southeast stretching up the Atlantic Seaboard
will remain stationary today before shifting out over the Atlantic
on Saturday. Meanwhile a low pressure trough axis extends from the
southern High Plains through western Wisconsin. The trough axis will
shift over the northern half of the lake today which will ease the
current gradient over the lake that is driving some stronger
southerlies...especially over the southern half of the
lake...where winds to 30 knots can be expected through middle morning. A
secondary low moving through the trough axis will reach Central
Lake Michigan after daybreak Saturday then northeast of Lake
Huron Saturday night...deepening as it does so. Modest high
pressure emerges behind this system...but will drive a period of
westerly winds to 30 knots with a modified cold front. Stronger high
pressure will move out of Canada beyond this time frame...bringing
colder north- northwesterly flow over the lake which may become
gusty at times. Before the high moves into the northern plains...a
weak trough and cold front will pass over the lake by
Monday...followed by another weak trough mid-week.
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