000 
fxus63 klot 212005 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois 
305 PM CDT Thu Mar 21 2019 


Short term... 
207 PM CDT 


Through Friday night... 


An upper low will continue to dig south across the Ohio Valley 
today with increasing breaks in the cloud cover expected locally 
in the wake of the low late this afternoon into the evening. 
Meanwhile, a cold front is draped from Lake Superior west across 
northern Minnesota this afternoon and is progged to drop across 
the forecast area tonight. Winds will back to westerly this 
evening, then veer back to northerly behind the front overnight. 
Expect an increase in clouds once again with frontal passage, but 
moisture appears too shallow to support any precipitation. 


High pressure will build into the upper Midwest on Friday and 
eventually spread across the local area by late Friday night. A 
diminishing north breeze will be in place through the day with 
veered flow coming off of Lake Michigan through much of the day. 
This will set the stage for a decent temperature gradient across 
the city. Inland, temperatures are expected to warm into the mid 
to upper 40s, while cold onshore flow near the lake will suppress 
temps in the mid 30s immediately along the lake front. 


Low level cyclonic flow and steepening low level lapse rates 
during the day will result in partly to mostly cloudy skies 
through the afternoon with cloud cover most extensive in the east. 
It's possible a few locations could see a brief sprinkle or light 
shower, especially in Northwest Indiana where modest mid level 
height falls associated with an upper low digging across the 
central/eastern Great Lakes grazes the area, but most locations 
will be dry through the day. Assuming skies clear out quickly late 
Friday evening, will have to watch for potential fog development 
overnight under the ridge axis. 


Deubelbeiss 


&& 


Long term... 
304 PM CDT 


Saturday through Thursday... 


Main concern for the long term forecast period will be the next 
significant chance for precipitation Sunday into early Monday. 


The high amplitude, short wavelength progressive pattern will 
gradually transition to a broad long wavelength, low amplitude 
pattern by early next week. In the mean time, the next chance for 
significant pcpn is expected for early Sunday through early Monday. 
The responsible feature is a southern Pacific short wave moving into 
the Southern California coast. The long range guidance maintained 
relatively good agreement with the track/timing/strength of the 
system, so confidence is relatively high for pop/wx trends 
associated with this system. The general trend should be for the 
main short wave to weaken as it crosses the Central Plains Saturday 
night and into early Sunday as it starts to get absorbed into a 
broad, deep upper low centered over northern Quebec. At the time of 
pcpn onset Sunday morning, temperatures should be high enough to 
support all rain and with strengthening warm/moist advection in 
advance of the system, temperatures should reach into the lower 50s 
with dewpoints in the lower 40s. Longer range guidance is 
suggesting pwats of 0.75 to 1.0" with some positive bl cape and 
broad isentropic lift poking into, at least, the southern portions 
of the County Warning Area. So, there may be some chance for thunder with this 
system, especially for the far southern portions of the County Warning Area into 
central Illinois. However, should the track of the sfc low be shunted to 
a more southerly track by a strong sfc high over central Canada, 
then any thunder potential would be more confined to the south. So, 
will not introduce thunder into the forecast just yet, but it may 
need to be considered in later updates. Also, quantitative precipitation forecast may be a concern 
for portions of the rock and Pecatonica River basins that have been 
impacted by significant flooding. Latest river stage forecasts 
suggest that many locations on these rivers should drop below flood 
stage Monday or Tuesday. While is certainly some uncertainty to the 
quantitative precipitation forecast with this system, there are indicators that there could be 
enough rainfall to at least delay or slow down the falling trends on 
the rock and Pecatonica rivers. Interests along these rivers should 
monitor forecast updates. As the system passes off to the east 
early Sunday, colder air should filter back into the region, 
allowing for a transition from rain to snow early Monday morning as 
temperatures drop into the lower 30s for the northern portions of 
the County Warning Area. 


&& 


Aviation... 
for the 12z tafs... 


Some light showers continue across the eastern terminals early 
this morning. This activity will continue for another couple hours 
this morning before ending. However, expect IFR to low end MVFR 
cigs to be in place for much of the morning, before improvement 
occurs this afternoon. Otherwise, expect north-northwest winds to 
continue across the area through early afternoon today, before 
the winds back more westerly in response to the approach of 
another surface trough of low pressure. This trough of low 
pressure will then shift another cold front across the area later 
this evening, with northerly winds likely to set up again in its 
wake and persist into Friday. 


Kjb 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...Small Craft Advisory...lmz740-lmz741-lmz742...1 am Friday to 4 
am Saturday. 


Small Craft Advisory...lmz743-lmz744-lmz745...1 am Friday to 11 
am Saturday. 


&& 


$$ 


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