Scientific Forecaster Discussion
fxus63 klot 190707
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
207 am CDT Sat Oct 19 2019
207 am CDT
Today and tonight: a mid-level shortwave trough and surface low over
the central Great Plains early this morning will lift northeastward
across the mid-Mississippi River valley this afternoon and evening.
The system will be in a weakening trend as the mid-level trough
becomes increasingly stretched just northwest of the County Warning Area late this
afternoon/evening. However, enough energy should remain to interact
with brief pre-frontal moisture return to bring some isolated to
scattered showers to areas roughly northwest of I-57 late afternoon into
the evening. Coverage of the showers will likely be highest across
the western County Warning Area late afternoon before diminishing in coverage while
lifting north and east through the evening. Any shower activity
should clear the entire County Warning Area by around midnight.
Sunday: the cold front will likely have shifted just south and east
of the County Warning Area by daybreak, then stall or even slightly retreat
throughout the day. While guidance is in decent agreement that a
stratus deck will trail the front, there are varying degrees of
solutions on how quick it dissipates. The NAM is the most
pessimistic with removing the cloud deck, but with thicknesses of
less than 1kft and dry air above the deck, current thinking is that
the clouds will mix into a cumulus field by the afternoon. Temps
will hinge on this cloud cover, but 60s should be common across the
cwa, with an isolated 70 possible across the far south.
234 PM CDT
Sunday through Friday...
Next system of interest is a potent vort Max and associated area
of low pressure which is expected to pinwheel into the region
Sunday night and into Monday. Developing gusty southeasterly
surface flow ahead of this system will encourage additional
poleward moisture transport. That said, the best upper-level
dynamics associated with this system from an upper-jet perspective
look to remain displaced both immediately south and north of the
region. This, combined with a lack of appreciable surface-based
instability should help greatly curtail the threat for strong-
severe storms with this system. Greatest precipitation chances
look to move in into Monday morning and early afternoon hours
(with a threat for some embedded thunder), but this will be a
quick hit as the warm conveyor rapidly peels east of the area
through Monday afternoon and evening. As a result, while there
will be a window for some locally more moderate to even heavier
rainfall at times, the quick-hitting nature of this system should
keep any associated flood threat to a minimum.
The main concern with this latest system will be from the wind,
with a potential for wind gusts Monday and into Tuesday to rise
into the 30-40 mph range for a period. At this time, gusts look to
remain perhaps just under Wind Advisory criteria, but either way
looks like a period of rather breezy conditions to start off next
There could be some light wrap around precip, though this does
appear to just graze the northwest portion of our area. Given
the more amplified upper flow, brief colder air and generally
dry weather is expected in the wake of this system outside of
any NW flow shortwave activity that will drive another weaker
surface reflection/front across the region mid week.
Another system looks to impact the area during the later part of
next week, but still a lot of uncertainty with regards to
track/intensity and resultant impacts to our weather. It does
appear to bring another brief cool shot.
for the 06z tafs...
Mid-clouds will gradually spread into the region Saturday ahead
of a slowly moving cold front. Southeast winds will trend to south
or possibly slightly SW and generally around 10 kts. VFR mid-deck
will thicken up by Saturday evening, with the potential for a few
showers or sprinkles into rfd toward 23-00z, and for the Chicago
terminals after dark. As indicated above, dry air mass and
weakening forcing with a damping short wave disturbance do not
look to support a significant rain threat, especially for Chicago
terminals, but the main layer of forcing/moisture will continue
right through the area this evening. So we do expect some
rain/virga in the area in the evening. We will handle with a
prob30 for -ra.
Surface cold front will move through the terminals mid- late
Saturday evening, with a wind shift to the west-northwest, but
with not much push behind it. Upstream high pressure will then
shift into southern Wisconsin on Sunday morning, turning winds to
the northeast. We may have some stratus clouds late Saturday night
into early Sunday.
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