Area forecast discussion National Weather Service Chicago/Romeoville Illinois 1005 PM CDT Monday Jun 17 2013 Discussion... 929 PM CDT A scattering of showers with a few storms have seen an overall diminishing in coverage since 8 PM. Almost all new development in that time has been on outflow from preceding showers/storms...with the 00z dvn radiosonde observation indicating instability present from the afternoon mixed layer. Some festering on outflows is likely to continue although each successive redevelopment should be weaker. The area will remain under a favorable middle-level jet region into overnight...but there appears to be a break in short waves let alone the diminishing of the instability with the time of day. So continue with no showers forecast by 1 am. Outflows brought rain-cooled air along with the marine layer into far northeast Illinois that created a quick temperature drop. Lows still look on track for most areas...although did bring down the far northern two tiers of counties a couple degrees based on this lower starting point going into the night. If winds were lighter fog may be a concern in those areas that saw showers...but speeds will stay up tonight as the synoptic cold front works through the region. The GOES WV loop shows the next short wave being well-defined in southeast ND. This small "bowling ball" in the middle-level flow looks to track across the southwest forecast area during Tuesday afternoon. The amount of instability on guidance is strongly correlated to the surface dew point forecast...which could be high on both the NAM and GFS per observational trends and initialization /at least on the 18z runs/. Still enough to warrant a slight chance though...as there currently are storms with the feature in the Dakotas at this hour. Mtf //prev discussion... 251 PM CDT Tonight... visible imagery early this afternoon depicts a cirrus shield from southeast Iowa stretching northeast along I-80 in Illinois into northcentral Indiana. Water vapor imagery also indicated an abundance of upper level moisture associated with the clouds...however some subsidence was occurring late this morning into the afternoon hours as dew points at the surface were dropping into the middle/upper 50s across the County warning forecast area. Low level flow under the cloud shield was generally southwesterly...while further north winds were westerly. It appears that where skies are mostly sunny slightly better mixing was occurring...which was also aiding in the subsidence at the surface. The focus for later this afternoon/evening is on the approaching frontal boundary and potential for isolated showers/thunderstorms along/just ahead of this boundary. Visible imagery does show a Narrow Channel of a cumulus field developing across central wisc this afternoon...stretching northwest through minn. A weak middle-level wave was sliding southeast the upper Midwest...which will be the driver for some added lift into this cumulus field. As expected isolated showers/thunderstorms were developing under this channel of cumulus across central wisc/eastern minn. Middle-levels are much cooler...with steep lapse rates to the north across wisc. This suggests that a few thunderstorms could easily grow vertically and possibly produce some hail. Feel that most of the late afternoon/early evening will end up being dry for the County warning forecast area. Guidance continues to suggest that the best chance for any precipitation will likely occur after 00z. With much of the convective potential being diurnally driven...expect that around 2-3z the instability to greatly diminish. At this time will hold onto the slight chance of thunderstorms and rain wording through 3z. Across the north...then linger the thunderstorms and rain wording through 06z for the southeast County warning forecast area. Earlier thinking was that this boundary would end up being a back door cold front...and this appears to be the case based on current conds to the north. Cloud cover should be slowly thinning overnight to a p-cloudy sky...with winds turning northerly Post frontal passage. This will usher in much cooler air and allow temperatures to easily radiate into the middle/upper 50s. Confidence in temperatures...high. Confidence in precipitation/thunderstorms...low/medium. Tuesday through Wednesday... initial glance is that Tuesday would be dry...however the concern is on the southeast diving 500mb vorticity that will be arriving across the western County warning forecast area middle/late morning and pivoting east by the middle-afternoon hours. This may provide enough lift...coupled with diurnal cumulus Tuesday...and some moisture from the boundary to allow a few thunderstorms to develop. With temperatures aloft steadily cooling across the County warning forecast area...lapse rates will steepen and a few stronger storms could easily produce some pea size hail...brief wind gusts and heavy downpours. The focus appears to be along and south of a Rockford to Benton County line. Otherwise anti-cyclonic flow will continue to settle into the Great Lakes region...and keep a northeast flow at the surface. This will easily aid in the much cooler marine environment advecting inland across the County warning forecast area and keep surface temperatures in the low/middle 70s...except the far south/southwest where temperatures may still touch 80 degree in the afternoon. Locations immediately adjacent to Lake Michigan may have temperatures remain in the middle 60s Tuesday. Overnight temperatures will be much cooler in the low/middle 50s. A few locations along and north of a Rockford...Aurora...Waukegan line may have temperatures in the upper 40s early Wednesday morning. Then high pressure begins to drift east. While winds will be more easterly off of the lake...temperatures will be slightly warmer Wednesday afternoon in the upper 70s to low 80s. The exception will be areas adjacent to Lake Michigan...where temperatures will likely remain in the 60s to low 70s. Confidence...high. Thursday through Sunday... ensembles continues to point towards a pattern change for the end of the week into the weekend. Many guidance members are in agreement that the 500mb trough over the Great Lakes region will pivot east Thursday...with a strengthening middle-level ridge across the Central Plains replacing the departing trough Friday. The ridge does appear to flatten slightly...which may be an indication that a slightly active period may accompany the steady warming trend. 850mb thermal ridge continues to be rather warm for the weekend...with temperatures generally around 20-22 degree c. The ec/Gem have taken a slightly cooler approach with 18-20 degree c...however the overall theme is that temperatures will be steadily warming. The wildcard for maximum temperatures Sat/sun/Mon...will be low level moisture and cloud cover. Guidance continues to suggest dew points will generally be around 70 degree. This may slow the diurnal curve for the weekend...however temperatures should easily still warm into the upper 80s to possibly lower 90s. As a result have nudged temperatures warmer than guidance for the upcoming weekend. Confidence in temperatures...medium/high. Confidence in precipitation/thunderstorms...medium. Beachler && Aviation... //Ord and mdw concerns...updated 03z... * some uncertainty regarding gusts in the near term...thinking they will diminish as the night GOES along. Lenning //discussion...updated 00z... Showers passing near the terminals around 00z are anticipated to be short lived as afternoon heating diminishes. As of this moment...winds have turned north at Ord but are still west-southwest at mdw...though expected to follow the Ord example shortly. This wind direction is being influenced by the local lake breeze and outflow from neighborhood showers...but the overall trend is to remain out of the north. It is possible that a northwest direction could return briefly this evening...but elected in the forecast to stay with a northerly direction through the night...gradually turning northeast tomorrow as per model guidance and wind roses for this time of year. No major concerns with the weather for tomorrow. Lenning //Ord and mdw confidence...updated 03z... * medium to high confidence that gusts will be short lived tonight. * High confidence in prevailing northeasterly wind for much of the forecast period. Lenning //outlook for Ord/mdw for 12z Wednesday-12z Monday...updated 00z... Wednesday...VFR. Thursday...VFR. Friday...VFR early...chance of thunderstorms and rain overnight. Saturday and Sunday...periodic chances for thunderstorms and rain. Lenning && Marine... 245 PM CDT A stretch of relatively quiet weather is still in the forecast for Lake Michigan over much of the upcoming weak. A trough of low pressure and an associated weak cold front...extend from northern Maine west through Central Lake Michigan. As the low moves off to the east...the front will continue to move south down the lake...reaching the southern tip of the lake by late this evening. Winds will shift to northerly behind the front...with occasional gusts to 25 knots tonight. High pressure over the northern plains will spread east across the Great Lakes late tonight and tomorrow...with high pressure remaining the dominant feature over the lake through Wednesday night when the high will begin to slide off to the east. In the mean time...low pressure developing over the northern rockies Wednesday night and will move over the northern plains by Friday morning. With the high pressure to the east and the developing trough to the west...winds will become southerly by Wednesday or Wednesday night...through the weekend. && Lot watches/warnings/advisories... Illinois...beach hazards statement...ilz006-ilz014...1 PM Tuesday to 4 am Wednesday. In...beach hazards statement...inz001-inz002...1 PM Tuesday to 4 am Wednesday. Lm...none. && $$ Visit US at http://weather.Gov/Chicago (all lowercase) Follow US on facebook...twitter...and youtube at: www.Facebook.Com/US.Nationalweatherservice.Chicago.Gov www.Twitter.Com/nwschicago www.Youtube.Com/nwschicago