Scientific Forecaster Discussion
fxus63 klot 212330
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
630 PM CDT Thu Sep 21 2017
224 PM CDT
Late September heat remains the main focus of the forecast in the
near term. New record high temp already set today at o'hare with
a 94 degree reading there.
Midwest remains along the periphery of highly amplified upper
level ridge axis, downstream of a deep long wave trough across
western noam. 12z radiosonde observation from dvn indicated 850 mb temp of +20c, and
a 925 mb temp of +26c, resulting in low-mid 90 degree temps across
the region. Residual high cloud cover from decaying mesoscale convective system over
southeast Iowa/northeast MO has kept temps slightly lower across
western parts of the cwa, though a decrease in the denser cirrus
has temps approaching 90 there as well this afternoon. Of course
the weak surface pressure gradient in the vicinity of the
stationary front just north of the cwa has allowed a lake breeze
to push inland across far northeastern IL, keeping temps in the
mid-upper 80's for areas near the lake.
Recent runs of the hrrr indicate the potential for isolated
thunderstorm development across mainly the southern parts of the
cwa late this afternoon. Experimental esrl hrrr also hints at this
though with much lower coverage. It appears these models may be
focusing on a remnant mesoscale convective vortex from the Iowa/MO convection, though no
distinct circulation is noted in water vapor imagery. While
current rap mesoanalysis depicts 3000 j/kg of MLCAPE and little
cin across the southern tier of cwa counties, weak flow and
fairly shallow cu field per GOES-16 vis imagery suggests these
models may be quite overdone with thunderstorm potential.
Maintained less than 20 percent pops through sunset, but may have
to add an isolated mention if cu field becomes more impressive or
Another mild night is in store for the region with persistent
light south winds and unseasonably high dew points. Overnight mins
should remain in the 70's in most locations, which will also give
a warm head start for temps on Friday. Less cloud cover and
similar low level thermal field should allow highs in the low
90's in most spots again Friday afternoon, again at or near record
levels for September 22nd. (Records for tomorrow are 92 for
Chicago, 93 for rockford) weak southeasterly lake breeze should
again keep temps in the 80's right along the lake.
Friday night through Thursday...
218 PM...the models continue to show slight thermal cooling this
weekend but this will likely have little if any noticeable
affects as overnight lows remain in the 60s...low 70s
downtown...and highs in the upper 80s/near 90 Saturday/Sunday.
Bumped temps up a few degrees to near current guidance values.
Lake breezes are possible both days which will keep the immediate
Lakeshore areas cooler but confidence is low regarding how strong
these lake breezes may be and how far inland they will move.
Forecast highs are cooler near the lake but trends will need to be
monitored as these time periods approach. Highs may still reach
the upper 80s before temps cool and winds shift off the lake.
There is still some potential that the models are cooling the
airmass too fast and if that were to occur...highs could still be
a few degrees warmer than currently expected this weekend.
However by early next week it does appear that the entire airmass
will cool enough for highs to remain in the 80s. Additional cloud
cover...perhaps a more broken afternoon cu field...may also help
put a lid on high temps...keeping them in the 80s.
A cold front will move across the area Tuesday night into
Wednesday next week. While an isolated shower is possible any
afternoon from Sunday Onward next week...this front will be the
next best chance of rain for the entire area. However...not
confident on how widespread precip will become and its possible
just scattered precip in a narrow time window moves across the
area ahead of the front. Though a slower front...that arrives
later into Wednesday could allow for more daytime heating and
thus better precip coverage. This front will allow much cooler air
to spread across the area with highs back into the 70s. A second
push of colder air is possible late next week. Cms
for the 00z tafs...
Lake breeze came close to Ord, but has begun to retreat and no
longer poses much of a threat of making it through this evening.
Will probably be another lake breeze tomorrow, but opting to go
with persistence for now and not include a wind shift at the
terminals. Otherwise, other than possibly some patchy ground
fog/MVFR visibility at dpa early Friday morning, not anticipating any
significant weather at the terminals through this taf cycle.
218 PM...a weak cold front has stalled over south/central
portions of Lake Michigan this afternoon. This front will lift
back north as a warm front tonight...shifting light winds back
southerly. A large trough of low pressure will remain nearly
stationary over the plains through early next week maintaining
southerly winds over the lake. This trough/front will move across
the Great Lakes region Tuesday night into Wednesday shifting winds
northwest with a period of 15-25 kts expected. Cms
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