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Office: DVN

FXUS63 KDVN 181805

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1205 PM CST Mon Dec 18 2017


Issued at 330 AM CST Mon Dec 18 2017

Sfc trough axis has been slowing moving from west to east through
E Iowa/NW Illinois overnight. Ahead of and within the trough,
winds were light and dewpoints were in the 30s which caused areas
of fog to form.

The lowest visibilities as of 3 AM, down to 1/4 mile in some
cases, were in the counties along and east of the Mississippi
River (north of Monmouth, IL). Immediately west of the trough,
sfc winds have increased out of the SW and visibilities have
improved to better than 3 miles for most locations.

Have a graphicast out to highlight most likely area for dense fog
through early this morning, but do not think a Dense Fog Advisory
will be necessary because the lowest visibilities at any one
location will only last for 1-2 hrs.


.SHORT TERM...(Today and Tonight)
ISSUED AT 330 AM CST Mon Dec 18 2017

Today and Tonight

Low stratus is forecast to thin and eventually clear out form west
to east through the day. GOES-16 imagery is already depicting a
large area of clear skies expanding over parts of the NW forecast

For this afternoon, SW winds and some sun will allow temps to
reach into the lower 40s N/NE to the lower 50s south. For the
overnight period, temps are only forecast to fall into the lower
30s north to upper 30s far south due to steady SW winds
maintaining a mixed boundary layer. Uttech

.LONG TERM...(Tuesday through Sunday)
ISSUED AT 330 AM CST Mon Dec 18 2017

Mild December weather continues until late this week, then a large
scale pattern change will bring much colder weather and periodic
snow chances from the weekend into next week. In the transition, the
late week storm system continues to trend on a more northwesterly
track, setting up the forecast area for primarily a rain event, with
a possible transition to light snow as it exits late Friday. From
there, temperatures will get progressively colder with some models
suggesting highs in the teens or much colder by Christmas Day.

Tuesday through Thursday, a mostly zonal flow, tracking storm
systems well north and south of the area will continue above to
above normal temperatures. Thursday, will continue to carry low
chance pops across the northwest, where models continue to suggest
isentropic lift in an inverted trough developing ahead of low
pressure lifting out of the southern plains. While the timing and
thermal profiles are low confidence, there would be a potential for
mixed precipitation or possible sleet at the early morning onset
over our far northwest. Developing warm advection at the surface and
aloft would support possible light rain north of I-80 during the day
with the south remaining dry.

While there remains some model timing differences with the storm
system lifting out of the southwest Thursday night through Friday
night, the general consensus has the surface low tracking across
east central Iowa into northwest Illinois. This would favor the bulk
of accumulating snow occurring north of the forecast area across
northern IA, southern MN into WI. Looking at timing in the 00z runs,
the 00z GFS is the fastest, followed by the ECMWF, then the slower
and much more dynamic system shown by the Canadian, which a notable
outlier as it nearly bombs a secondary low from the southern Plains
into IL and the Great Lakes by Saturday morning. Precipitation is
likely to transition from rain Thursday to snow in the northwest
Thursday night, with this rain to snow translation in the wrap
around cold air then taking place from northwest to southeast Friday
into Friday night, when the faster solutions have it exiting the
region. Also, during this period, there is likely to be a dry slot,
into at least the south sometime early Friday. Thus, overall
confidence in the depiction of rain vs snow is low, but the synoptic
setup points toward any measurable light snow mainly northwest of a
Cedar Rapids to Freeport axis.

Saturday and Sunday, a longwave trough enveloping an arctic airmass
sweeps down into the central U.S. This will result in sharply colder
temperatures, with forecast highs in the mid 20s to mid 30s Saturday
and 20s Sunday possibly too optimistic as 850 mb temperatures drop
to the teens below, to possibly 20s below zero Celsius. Low
confidence chances for snow arrive Sunday as the setup is favorable
for waves of at least modest lift interacting with the arctic air.

Beyond, it is worth noting an anomalously strong arctic high shown
by the GFS and Canadian models over the northern plains spilling
into the local area from Christmas Day into the middle of next week.
Depending on the snow cover, this could limit highs to the single
digits and teens with overnight lows below zero.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday Afternoon)
ISSUED AT 1158 AM CST Mon Dec 18 2017

Low clouds will continue to clear the east early this afternoon.
followed by some scattered high clouds. A W/SW breeze around 12kts
will diminish slightly early this evening and become more westerly
late tonight. The big question is fog potential again this
evening. Some of the model guidance is hinting at some light fog
overnight. Have gone with 6SM BR at each site, as confidence is
very low in any widespread MVFR visibilities.




LONG TERM...Sheets

Office: LOT FXUS63 KLOT 181738 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1138 AM CST Mon Dec 18 2017 .UPDATE...1029 AM CST Areas of light rain/drizzle along a SW-NE corridor from KVYS (Peru) northeast to Ohare will drift southeast in the coming hours. There is some decent dry advection from the west where a lower dewpoint air mass exists aross western IL and eastern IA. Expect at least partial clearing across the area this afternoon. Clouds will be harder to clear across areas southeast of a Pontiac to Gary line being more removed from the drier air. A while there is a better chance of clearing later tonight, there could be some reforming stratus or lingering stratus, especially for the aforementioned corridor. KMD && .SHORT TERM... 330 AM CST Through tonight... The main concerns are with areas of fog and drizzle this morning and then cloud cover trends this afternoon along with the impact of these trends on temperatures. A warm front is lifting north across far northern Illinois this morning, with light to moderate southwest winds. Patchy to areas of fog and drizzle (with even pockets of steadier drizzle) are in place early this morning, with RFD reporting 1/4 SM visibility at this hour. Based off observations just to the west and southwest of the RFD area, it appears that the dense fog is likely to be short-lived, so not planning any headlines for it. Any lingering fog should erode by the mid to late morning hours, with patchy drizzle gradually sliding east, and a completely dry afternoon favored. Returning to cloud cover trends, GOES-16 advanced nighttime microphysics RGB indicates a hole in the low stratus poking into northeast IA, but otherwise solid coverage of it extending north of the hole and south of it into northern MO. Cloud cover trends are as usual tricky. Have overall leaned a bit more pessimistic than much of the guidance. Boundary layer flow driving the movement of the stratus is not expected to significantly pick up until after dark, remaining out of the west-southwest 10-20 kt range through that time. Thus, anticipate a slow movement of the cloud deck, keeping much of the daytime hours OVC. Temperatures have gradually risen through the night, so the mild starting point will still enable low 40s highs along the WI border counties and mid to upper 40s everywhere else. Should the thinking on clouds be incorrect, a quicker clearing could result in going highs being a bit too cool. Southwest winds will pick up tonight ahead of a cold front and become gusty at times, along with some cloud cover. Therefore, temperatures are unlikely to drop much. Leaned closer to raw model guidance for lows with this in mind, and a bit above previous forecast, with mid 30s to around 40 indicated. Would not be surprised if some locations remain above 40 through the night. Castro && .LONG TERM... 129 AM CST Tuesday through Sunday... Big changes on tap during the long term period in the hemispheric circulation over the north western hemisphere with a transition from a more zonal split flow, to a more phased and highly amplified meridional flow by the weekend. The result will be a change from unseasonably mild conditions this week to a colder pattern heading into the weekend, with a potentially wet, stormy period as we make the transition. As is always the case, the devil will be in the details, which get murkier and more uncertain by Friday into the holiday weekend. Tuesday, a cold front trailing southwest from a powerful cyclone tracking across central Canada will move across the area during the day. Moisture and forcing will be lacking with this front, so not anticipating much more than a wind shift, with even the colder air lagging behind the front. Should start the day Tuesday unseasonably warm, so given the expected sunshine, it looks like many areas could make a run at reaching/climbing above 50 degrees Tuesday. Behind the weak cold front, look cooler temperatures Tuesday night through Wednesday night, but even those cooler temps still look to be a few degrees above average. Pattern should get more interesting heading into the end of the work week as strong shortwave digs into the southwest U.S. and is progged to close off a rather deep tropospheric circulation. Lee side cyclone will develop Thursday with surface warm frontogenesis taking place east from that low into the mid MS Valley Thursday. Initially, looks like most of our CWA could be north of the warm front during the day Thursday. By Thursday night, the sfc cyclone will move northeast to the southern Iowa/northern Missouri with the associated warm front progged to lift north across the CWA Thursday night. Expect rising temperatures Thursday night behind the warm front with most areas likely well into the 40s if not lower 50s by Friday morning. Cold front should move across the area Friday afternoon, but not before most (if not all) of the CWA warms into the 50s Friday. Could see a few showers Thursday night, mainly northern CWA ahead of the warm front and then again Friday into Friday evening ahead of the cold front. Forecast confidence decreases substantially at this point as medium range models have been trending farther west with the axis of the highly amplified long wave trough. The latest 00z operational runs have continued this trend and that has potentially big implications not only on temperatures over the weekend but also precip chances. Unlike earlier runs which had the heart of the arctic air mass spreading into the area in the wake of the Thursday night/Friday morning's shortwave, it now holds the heart of the Arctic air farther west. While that would keep temperatures more moderately cold this weekend, the bigger implication is that it would keep us far closer to the baroclinic zone and potential storm track. If we do end up close to the baroclinic zone over the holiday weekend, that would bode well for those dreaming of a white Christmas, as any shortwave energy riding up the eastern side of the deep long wave trough could easily lay down some accumulating snow. Important to keep in mind that medium range models have yet to really lock into a solution and until they do, further refinements to the forecast could be needed. If you have travel plans across the Midwest over the holiday weekend, it would be advisable to pay attention to later forecasts as there is potential for wintry precip making travel dicey, but too soon to say to what extent and where. - Izzi && .AVIATION... For the 18Z TAFs...1136 PM CST... IFR cigs transitioning to MVFR and specific clearing times, along with LLWS overnight. Areas of drizzle/light rain will drift southeast of the terminals this afternoon. Ceilings lift to low end MVFR behind the drizzle. There is a small area of clearing across western Illinois that will likely drift into the terminals this afternoon, but there are indications that the MVFR cloud shield behind the clearing line will become at least scattered to broken. A more significant push of drier air from the west should eventually allow for a for clearing of the lower clouds after 3z or so, but there could still be some pockets of stratus lingering through the overnight hours. The low level jet will increase tonight ahead of a cold front. Marginal LLWS conditions will be in place and so have left the conditions as is in the TAF but shortened the window. There will no significant impacts with the front other than a subtle wind shift from SW to W. KMD && .MARINE... 416 AM CST West-southwest winds will increase through today, reaching 30 kt on the north this afternoon. As a cold front trailing from strong low pressure near James Bay approaches the lake this evening, southwest winds will increase to lower end (35 kt) gale force on the north half of the lake. There may be a very brief lull in the gales as the front sweeps across the lake early Tuesday morning, but they will return quickly out of the west-northwest by the late morning and continue through Tuesday evening. Have issued a Gale Warning to cover this. While a few/occasional gale gusts can't be ruled out on the south half tonight and Tuesday, prevailing gale speeds/gusts appear less likely, so again the warning is only for the north half. A Small Craft Advisory is in effect for the nearshore waters for the brisk southwest winds ahead of the front tonight and the west winds behind the front on Tuesday. Gusts to 30 kt are possible along portions of the Indiana shore late tonight into Tuesday morning. Conditions hazardous for small craft are again possible for the Indiana shore Tuesday night as winds turn northwest and then north late before diminishing with high pressure moving over the region. The next period of concern for strong winds over the lake is with an area of low pressure late in the week. Confidence is still low in the track and strength of this system, with gale force winds most likely Friday night into Saturday behind it as much colder air returns to the region. Castro && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...6 PM Monday to noon Tuesday. Small Craft Advisory...LMZ744-LMZ745...6 PM Monday to 3 PM Tuesday. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO
Office: ILX FXUS63 KILX 181744 AFDILX Area Forecast Discussion National Weather Service Lincoln IL 1144 AM CST Mon Dec 18 2017 .UPDATE... Issued at 1103 AM CST Mon Dec 18 2017 Skies should remain cloudy across most of the area and this should keep temps at bay through the afternoon. Any areas where clearing takes place could see some warmer temps and current forecast has a good handle on that. Rest of forecast looks good, so will not be issuing an update at this time. && .SHORT TERM...(Today and tonight) ISSUED AT 315 AM CST Mon Dec 18 2017 08z/2am surface analysis shows overcast conditions across central Illinois. The lowest ceilings and drizzle have been primarily confined to locations west of the Illinois River and along/east of the I-57 corridor. Based on satellite/radar trends as well as latest HRRR ceiling/visby forecast, think patchy drizzle and fog will persist through mid-morning before dissipating and shifting further east into Indiana. The main short-term forecast challenge will then focus on how quickly skies will clear later today into tonight. Current IR satellite imagery shows the back edge of the low clouds well to the west from central Iowa to eastern Kansas. Both NAM and RAP forecast soundings suggest partial clearing could occur west of the Illinois River as early as midday...with clearing progressing slowly eastward to near I-57 by late afternoon. Models tend to clear skies too quickly this time of year: however, given the 10-15mph W/SW winds expected to prevail throughout the day, think they seem quite reasonable. As a result, will start the day overcast across the board...with partly to mostly sunny skies developing west of I-57 this afternoon. Further east toward the Indiana border, skies will remain mostly cloudy until early this evening. High temperatures will be in the upper 40s and lower 50s. Low temperatures tonight will drop into the upper 30s under mostly clear skies. .LONG TERM...(Tuesday through Sunday) ISSUED AT 315 AM CST Mon Dec 18 2017 Warmest day of the forecast period still appears to be Tuesday, as ample sunshine and mild W/SW winds ahead of an approaching cold front push high temperatures well into the 50s. Once the front passes, a slightly cooler airmass will spill into the region, allowing temps to drop into the 40s on Wednesday. The cooler weather will be short-lived however, as winds shift back to the S/SW and temperatures once again climb into the 50s for Thursday and Friday. A significant storm system is still slated to arrive by the end of the week, but its onset has been delayed by about 6-12 hours. 00z Dec 18 GFS and ECMWF are in fairly good agreement with a cold front pushing through Illinois during the day Friday, while the GEM holds the front to the west until Friday evening. Given slowing trend, think Thursday/Thursday night will remain dry...with rain chances developing across the area on Friday. Will need to keep an eye on how quickly the front departs late Friday, as any lingering rain showers will change to a few snow showers behind the departing front Friday night. Little or no snow accumulation is anticipated. After that, a trend toward much colder weather is evident over the upcoming weekend into next week. An area of weak lift could potentially bring a period of light snow to portions of central Illinois on Sunday: however, confidence in a mesoscale feature on the models that far out is quite low. At this point, the main take-away for the weekend will be the colder weather...with high temps falling into the 30s by Sunday...and even further early next week. && .AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon) Issued at 1144 AM CST Mon Dec 18 2017 MVFR cigs will be at PIA/BMI/SPI this afternoon and then clouds/cigs will scatter out and become clear for late afternoon. DEC and CMI will start at VFR cigs around 3.5kft but drop back down in an hour or two into MVFR like the others. They will also scatter out late this afternoon. All sites from late this afternoon and through the night will remain clear or just have scattered high clouds this evening/overnight. Clear skies are also expected tomorrow as high pressure trys to build into the area behind a weak sfc trough. Winds will be out of the southwest this afternoon with gusts to around 20-24kts. Winds will decrease this evening but still remain around 10kts through the night. As sfc trough moves through the area tomorrow winds will become westerly. && .ILX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...Auten SHORT TERM...Barnes LONG TERM...Barnes AVIATION...Auten