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Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 517 AM CST Mon Jan 1 2018


AREA FORECAST DISCUSSION



National Weather Service Chicago/Romeoville, IL
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Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 517 AM CST Mon Jan 1 2018

.SHORT TERM... 245 AM CST

Through tonight...

Forecast concerns continue to focus on the dangerous cold/wind chills across the area through the period.

A very impressive arctic surface high, with a central pressure around 1050 mb, is centered over the Plains early this morning. This is resulting in a very cold morning not only locally, but across much of the central CONUS, where temperatures were as cold as -38 in portions of southwestern North Dakota. Closer to home, temperatures as of this writing ranged from a balmy -1 in downtown Chicago down into the lower to middle teens below zero across my far southwestern areas. While temperatures are running a couple of degrees warmer than forecast, wind speeds are still u Paround 10 to 15 mph across the area, which is still resulting in wind chills of 20 to 35 below zero (coldest readings in my west). Therefore, the current wind chill advisory and warning appears to still be on track, so no changes are planned.

After the cold start to day temperatures do look to recover into at least the lower to middle single digits above zero under sunny skies. I did warm temperatures a couple of degrees over the previous forecast. This was mainly due to the fact that the arctic airmass across the central CONUS is likely to undergo some decent subsidence warming today. For example, 925 mb temperatures on yesterday evening RAOBS over the northern Plains and Upper Midwest were in the -20 to -28 degrees C range. Today, these temperatures will be a good 5 to 10 degrees C warmer across the region. Because of this, I think temperatures this afternoon (why still very cold) will be a bit warmer than previously forecast. Therefore, records for the coldest high temperature for today may survive another year.

Tonight the aforementioned arctic high is expected to begin to weaken some as it shifts over the Mid-Mississippi Valley. This track of the high will likely result in some light westerly wind component over much of the area again tonight, albeit weaker than the current winds. In spite of the modifying arctic airmass over the region today, temperatures tonight are still likely to fall into the single digits to teens below zero. These temperatures will again result in wind chills down into the -20 to -30 degree range. At the current time it does not appear that many areas will have wind chills fall below -30. For this reason, we plan on continuing with the current wind chill advisory. However, if wind chills do end up a bit colder than currently forecast, some of the area could need to be upgraded to a wind chill warning in later forecast updates. Either way, it will be another cold night across the area.

KJB

&&

.LONG TERM...

Tuesday through Sunday...

320 am...Main forecast concern remains very cold temperatures through the end of this week.

The center of high pressure will be south of the area Tuesday morning as it slowly weakens over the mid Atlantic Tuesday night. While still cold...models are in good agreement with modest warm air advection Tuesday into Tuesday night...as another clipper system moves across the northern lakes region Tuesday night. So while lows Tuesday morning will still be in the -10 to -15 range over much of the area...warmer in the metro area...guidance has been consistently showing temps warming into at least the lower teens for much of northeast IL with perhaps the more outlying areas only in the upper single digits. Temps may drop a few degrees Tuesday evening but also may remain steady and continue to slowly rise into Wednesday morning. Then a cold front will move across the area Wednesday morning with another blast of cold air with potentially gusty northerly winds Wednesday. So temps are most likely going to be slowly falling Wednesday...especially in the afternoon. The going forecast had much of these temperature trends from Tuesday morning through Wednesday morning and made no significant changes.

The above noted cold front will likely have the potential for some snow showers with it Wednesday morning though guidance/models aren/t showing much currently. Added slight chance for mainly northern/northeast IL for some snow showers. Lake effect snow will then develop downwind of the lake into northwest Indiana. Wind directions wouldn/t favor northeast Porter County until Wednesday evening/night when at least some accumulation is possible.

Another high pressure will move across the Plains Thursday and then into the western lakes by Friday. This will setup another period of very cold temperatures with overnight/morning lows below zero for much of the cwa for Thursday, Friday and Saturday mornings with perhaps temps near zero/low single digits above zero for the metro area. Wind chills may approach -20 Thursday and Friday mornings depending on how close the high moves and wind speeds...so additional wind chill advisories may be needed during this time period.

Models continue to be in general agreement with temps slowly moderating next weekend. However...that trend to warmer appears to be slowing and also shrinking in duration. Confidence is low with how the northern/southern streams may or may not phase as well as how much warm air can push this far north and if precipitation is occurring at that time. Though from this distance enough consistency from the models for pops and have maintained all snow for now. If warmer air does spread north...given how cold the ground has been...by that time going on 2 weeks...mixed preci Pcould be a concern. As mentioned above...duration of any warmer air now looks brief...as the gem/gfs/ecmwf all show another high pressure and colder air returning early next week. cms

&&

.CLIMATE...

245 AM CST

Records that may be in jeopardy for the first day of 2018:

January 1 Record Low      January 1 Record Low Max
Chicago          -10 (1969)                   5 (1969)
Rockford         -13 (1924, 1968)             4 (1974)

MTF/KB

&&

.AVIATION... For the 12Z TAFs...

No significant aviation concerns next 30 hours, with VFR conditions and west-northwest winds around 10 kts through the period.

KJB

&&

.MARINE...

320 am...High pressure over the Plains today will slowly weaken as it moves east to the mid Atlantic region Tuesday night. Low pressure will move across Lake Superior Tuesday night and then into Quebec Wednesday with a trailing cold front moving down Lake Michigan Wednesday morning. The gradient will tighten ahead of this front with west/southwest gales developing Tuesday morning across the north and then by Tuesday evening across the south. Winds will shift northerly behind the cold front and gales are still possible but confidence is low behind the front. Another high pressure will build across the western lakes region Friday and move south of the lake Saturday. With the very cold air...freezing spray will be likely during periods of stronger winds and gales. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES... IL...Wind Chill Warning...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-      ILZ012-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039      until noon Monday.

     Wind Chill Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-      ILZ012-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-      ILZ039...noon Monday to noon Tuesday.

     Wind Chill Advisory...ILZ006-ILZ013-ILZ014-ILZ022 until noon      Tuesday.

IN...Wind Chill Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 until      noon Tuesday.

LM...Small Craft Advisory...IN nearshore waters until 3 AM Tuesday.

     Gale Watch...LMZ777-LMZ779...3 PM Tuesday to 3 AM Wednesday.

&&

$$

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NWS Chicago Forecast Discussion 530 AM CST Fri Dec 1 2017

.SHORT TERM... 320 AM CST

...Today through Monday night...

Meteorological winter will enter quite gently as dry and very mild weather, with highs 10-20 degrees above seasonal normals, persists through the weekend under the influence of zonal flow aloft. Friday night and Saturday morning could see a period of cloudiness as a weak shortwave passes aloft, but the more significant weather holds off until later in the weekend or early next week.

By Sunday night a strong low level jet develops over, or perhaps just west of, the area ahead of a longwave trough digging deeply into the western CONUS. The GFS/ECM solutions show a 50+ kt jet at 850mb bringing a warm front through Sunday night and PWATs in excess of an inch from Sunday night through Monday night, which would be at the upper end of the climatological range for this time of year. Forcing from this warm and moist advection could even support thunderstorm chances Monday night, but the more widespread chances of rain and possible TSRA would come Monday evening as a strong cold front swings through the area and brings an end to the mild pattern.

The warmest of these four days appears to be Monday behind the warm front that pushes through Sunday night and ahead of the cold front expected to arrive Monday night.

Lenning

&&


NWS Chicago Forecast Discussion 323 AM CST Fri Dec 1 2017



.SHORT TERM...
320 AM CST

...Today through Monday night...

Meteorological winter will enter quite gently as dry and very mild
weather, with highs 10-20 degrees above seasonal normals,
persists through the weekend under the influence of zonal flow
aloft. Friday night and Saturday morning could see a period of
cloudiness as a weak shortwave passes aloft, but the more
significant weather holds off until later in the weekend or early
next week.

By Sunday night a strong low level jet develops over, or perhaps
just west of, the area ahead of a longwave trough digging deeply
into the western CONUS. The GFS/ECM solutions show a 50+ kt jet at
850mb bringing a warm front through Sunday night and PWATs in excess
of an inch from Sunday night through Monday night, which would be at
the upper end of the climatological range for this time of year.
Forcing from this warm and moist advection could even support
thunderstorm chances Monday night, but the more widespread chances
of rain and possible TSRA would come Monday evening as a strong cold
front swings through the area and brings an end to the mild pattern.

The warmest of these four days appears to be Monday behind the warm
front that pushes through Sunday night and ahead of the cold front
expected to arrive Monday night.

Lenning

&&

.LONG TERM...
320 AM CST

Long Term...Tuesday through Friday...

A significant pattern change has been widely advertised for next
week. Behind a cold frontal passage Monday night, Tuesday highs
may only drop to near normal values around 40 degrees but the
brisk cold advection will be a shock after this mild spell. A
secondary cold front Tuesday night will help temperatures fall
further toward midweek with highs only reaching the lower 30s...or
less...Wednesday and Thursday. Things of course become less
certain at this time range, but Friday could be just a bit milder
as models suggest flow could turn back to the southwest for a
period. Then again, it could be just as cold as the previous two
days if the cold advection persists.

At this point it does not appear that any of these reinforcing
frontal passages will be associated with much moisture, so snowfall
does not look like a major concern with this first round of cold
winter temperatures next week.

Lenning

&&

.AVIATION...
For the 06Z TAFs...

VFR conditions and light winds expected through the TAF cycle.

- Izzi

&&

.MARINE...
320 AM CST

Mild and moderate southerly to westerly flow will be in place today
into Saturday before relaxing on Sunday, then the primary near-term
period of concern for the lake comes Sunday night with strong
southerly gales. These winds do not relax much before turning
westerly behind a cold front Monday night and persisting as gales
into Tuesday.

Lenning

&&

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

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AREA FORECAST DISCUSSION
National Weather Service Chicago/Romeoville, IL
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NWS Chicago Area Forecast Discussion 1054 PM CST Thu Nov 30 2017



.SHORT TERM...
1200 PM CST

Through Friday...Weather is quiet through Friday. Occasionally gusty west-northwest winds behind this morning`s cold front will ease late this afternoon and even more tonight as high pressure across the plains expands across the southern Great Lakes. With the light winds and clear skies, expect a colder night tonight, with lows in the 20s away from downtown Chicago. The high will shift east of the area Friday afternoon, allowing generally light southwest winds to develop. Temperatures will therefore climb back close to today`s readings in the upper 40s to low 50s. Expect some passing high clouds given the progressive flow aloft.

KMD

&&