000 
fxus63 klot 210536 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois 
1136 PM CST Wed Feb 20 2019 


Short term... 
153 PM CST 


Through Thursday night... 


With the freezing precipitation threat coming to an end across 
the region, the main forecast focus is with the gusty west- 
southwesterly winds expected to develop across the area this 
evening. Given the current observation trends, we should be able 
to drop the rest of the Winter Weather Advisory early this 
afternoon. 


Surface low pressure over southern Minnesota will shift east- 
northeastward over Northern Lake Michigan this evening. As it does 
so, our surface winds will shift from the south to the west- 
southwest and become gusty up to 35 mph this evening following a 
frontal passage. These wind speeds will abate late tonight into 
Thursday. The arrival of the frontal boundary early this evening will 
also bring an end to much of the drizzle and fog that is 
currently in place across the area. 


Quiet weather with seasonal temperatures in the 30s is expected on 
Thursday as surface high pressure sets up over the area. While we 
should loss the low-level cloud cover tonight, higher level clouds 
look to result in partly cloudy to partly sunny skies during the 
day Thursday. 


Kjb 


&& 


Long term... 
309 PM CST 


Thursday night through Wednesday... 


Active weather will continue for the long term forecast period. Main 
concerns will focus on potential ice accumulation in freezing rain 
late Friday night into Saturday morning and then strong, gusty winds 
late Saturday night into Sunday night. 


Quiet weather will continue into the beginning of the long term 
forecast period as broad upper riding covers the ern 2/3 of the 
Continental U.S.. the next system to impact the region will be a Pacific 
shortwave dropping south into the Desert Southwest Friday and then 
quickly lifting out over the Central Plains. The long range 
guidance is in good agreement on the 500mb pattern with this system, 
with only minor differences in the timing/intensity of the upper low 
as it lifts across Missouri Saturday evening. Unfortunately, the 
same cannot be said for the associated sfc low, with the models 
showing a fair spread, especially for the track of the low center. 
So, have based the sensible weather features and impacts from this 
system on the blended solution, which will need to be refined in 
later updates. However, the main idea is that as the sfc low lifts 
newd, a warm front setting up across the lower Ohio Valley Friday 
night will eventually surge north through the day on Friday. A ridge 
of high pressure will remain across the upper Great Lakes, limiting 
the northward progression of the sfc boundary, but above the 
boundary layer, the models are in good agreement depicting strong 
warm, moist advection. Latest thoughts are that a shallow layer of 
colder, sub-freezing air will remain in place across the northern, 
portions of the cwa, while warmer sfc air may reach up to, roughly, 
the I-80 corridor. Pcpn should spread across the area late Friday 
night, with the freezing rain likely north of the I-80 corridor and 
all rain to the south. The surface freezing line is the main 
question mark at this time, and will be highly defendant on the 
exact track and speed of the main sfc low. Consequently, there is 
some uncertainty on the coverage and duration of ice accumulation 
with the freezing rain. This situation certainly bears watching. 
Based on the latest guidance, expect somewhat of a non-diurnal 
surface temperature trend, with sfc temps rising through the late 
night hours, and then rising above freezing during the morning hours 
allowing the pcpn to transition over to all rain. 


The next area of concern will be strong, gusty winds following the 
passage of the associated cold front. While there are some 
differences among the models in the exact track of the surface low, 
there are some strong signals that the sfc low will continue to 
deepen as it lifts northeast through the central Great Lakes early 
Sunday. A strong pressure rise/fall couplet will accompany the 
frontal passage with +/ 20 mb per 6 hr and +/- 12 mb per 3 hr 
pressure tendencies indicted in the longer range guidance. Also, 
model soundings show 50 to 60 kt west-northwest winds at the top of 
the mixed layer on Sunday. Depending on how deep mixing may occur, 
there will be a chance for wind gusts in excess of 45 mph possible 
and a Wind Advisory or High Wind Warning may be needed. A large 
temperature swing is likely with the passage of this system. Ahead 
of the front, temperatures are expected to reach highs ranging from 
around 40 f along the Illinois/WI border to the low 50s for the southern 
portions of the County Warning Area. Highs on Sunday should only be in the lower to 
middle 30s. 


Yet another system is forecast to move across the region early next 
week. At this distance, there are still large differences in the 
strength and track of this next system. While there are some 
indications that another round of snow is possible, but model spread 
in the long range guidance is too large to have any confidence in 
any specifics. 


&& 


Aviation... 
for the 06z tafs... 


No significant forecast concerns this period. Gusty southwest 
winds will continue to slowly diminish overnight with westerly 
winds 10-15kts Thursday. Speeds will diminish under 10kts Thursday 
evening and become northwest...eventually shifting light 
northeast early Friday morning. 


Cigs are slowly lifting to low VFR late this evening with clearing 
across western Illinois expected to move across the terminals in the 
next few hours. High clouds will continue through Thursday 
afternoon with a possible mid deck Thursday night/Friday morning. 
Cms 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...Small Craft Advisory...nearshore waters until 9 am Thursday. 


&& 


$$ 


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