Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281140

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 AM CDT Thu May 28 2020

640 AM CDT

Pushed out a quick update to account for northward progress of
well defined short-wave/vort noted on water vapor imagery across
central IL and IN. See no reason why fairly stratiform showers
with embedded moderate rates with this on radar mosaic won`t just
lift northward across most if not the entire CWA this morning into
the early afternoon. Thus, have a period of categorical Pops in
the updated gridded database. Lightning should be limited, but may
be possible on the southern flank of this activity as a plume of
steeper low and mid level lapse rates lift northward. Not thinking
there will be much of a flood threat with this rain, but couldn`t
rule out some minor ponding on roadways in any embedded heavier
rainfall rates. Updated text products have also been sent.



330 AM CDT

Through Friday...

The main concern is the potential for heavy downpours from showers
and scattered thunderstorms this afternoon into the early evening.
Moisture ingredients will be in place to support efficient
tropical-like rainfall rates. Thus for any area that receives a
longer duration of heavy rainfall rates, there will be a risk for
localized flash flooding. However, guidance trends the past
couple model cycles have shifted the heavier rainfall axis and
higher flooding potential primarily to the west and northwest of
the CWA. Certainly do not want to rule out the potential for
localized flash flooding given the very wet antecedent conditions
and the possibility that mesoscale convective evolution shifts
higher confidence flash flooding potential into the CWA. The WPC
Day 1 ERO maintained slight risk probabilities into the northwest
1/3 of the CWA, which is reasonable. This includes the Rockford
area, far north/northwest Chicago suburbs, and the Rock, and
northern Fox and Des Plaines River basins. A Marginal Risk ERO is
in effect for the rest of the area.

Early this morning, we`re watching showers streaming northward
from just north of the Ohio River in association with a short-
wave/vorticity maxima on east side of stacked mid-upper level low
pressure centered over northwest Arkansas, both nicely depicted on
GOES-East Simple Water Vapor RGB. This should bring an uptick in
shower coverage this morning, though unlikely to have any
lightning due to lacking instability, focusing over eastern half
of CWA. There are additional vorts that should result in
increasing shower coverage in the northwest CWA, with a threat for
isolated (slight chance wording) embedded thunder as some
lightning strikes have been noted recently closer to this area.

The main area to watch for how things may evolve this afternoon
into this evening is from northeast Iowa, southeast MN into
western Wisconsin. This area is already seeing widespread
convective coverage at this early hour and will be in the
vicinity of gradually southeastward progressing cold frontal
boundary. With deep layer flow parallel to the frontal boundary,
potential for training convection is greatest likely a bit of a
southeast jog from current convective placement as outflows steer
effective frontal boundary a bit southward.

Air mass ahead of the front this afternoon will be muggy (upper
70s to around 80 temps and dew points well into 60s) and uncapped
but only weakly unstable with MLCAPE likely generally less than
1000 j/kg amidst weak 5.5 to 6C/km mid- level lapse rates. With
sufficient but not overly strong forcing as upper low opens up to
our south and steers more subtle waves northward, we should see
scattered to numerous showers streaming north- northeastward in
mean flow with thunderstorm coverage a bit questionable given
limiting factors above, but probably in the isolated to scattered
range. PWATs of 1.5 to 1.8", about 200% of normal, will support
heavy downpours, but footprint will likely be on the smaller side
with individual convective cells, so any flooding threat would
probably be highly localized.

Northwest CWA as mentioned earlier is the area to watch for a
possibly higher flash flood thread, closer to the frontal zone
during peak heating. If outflows from convection to the northwest
can surge the effective frontal zone farther southeastward, then
the scenario depicted by the majority of guidance of at least
temporary training ahead of the front, could shift farther
southeastward and threaten the northwest CWA. This is currently
not the most likely scenario, but observational trends will need
to be monitored closely. We did hold onto an area of higher end up
to categorical PoPs to account for this possibility. Overall, the
global ensemble probabilities of 1"+ rainfall amounts shifted
northwest out of the CWA and the HREF mean is generally under 1",
with the official NDFD QPF totals coming down to about a half inch
to just shy of an inch northwest. With the high PWAT air mass and
deep warm cloud depths, certainly can`t rule out pockets of 1"+
rainfall from thunderstorms really anywhere in the CWA.

As the front gets some better southward momentum this evening and
we lose instability diurnally, thunder coverage should wane
quickly, with a transition to post-frontal rain/showers into the
overnight. This regime could still have some moderate rainfall
rates at times, but a much lesser concern for flooding given lack
of thunderstorms by the late evening and overnight. Post-frontal
showers will continue to push southeast into Friday morning and
should end by mid day or so if not sooner. There will be rapid
clearing behind the front for a brief period of clear skies, but
as decent cold advection gets going, there should be a blossoming
of scattered to perhaps broken cumulus during peak heating. Much
drier air mass with dew points in the 40s to around 50 will be a
needed break from the several days of humid conditions. Highs
should reach the lower to mid 70s inland, but only mid to upper
60s lakeside as a lake breeze shifts inland during the afternoon.



243 AM CDT

Friday night through Wednesday...

Quieter conditions look to prevail at the start of the long term
period as Canadian high pressure builds across the Great Lakes
region. Lingering mid-level moisture will support the development
of fair weather cumulus in the afternoon, but precipitation-free
conditions are expected through the weekend and potentially even
through Monday. Persistent onshore northeasterly winds will likely
hold temperatures down near 60 degrees across our lakeshore
locales, while upper 60s/near 70 degree readings will be more
prevalent inland over the weekend.

Southerly return flow will quickly become re-established on Monday
as the sprawling 1024+ mb high builds eastward towards the Mid
Atlantic. Global guidance is in good agreement developing an
expansive area of rather stout low-level warm advection in
response to strengthening southwesterly 850 mb flow. However,
forecast soundings still look extremely dry through Monday, and
the main core of steeper mid-level lapse rates (as it stands
currently) are forecast to remain well west of our region.
Currently thinking that Monday should remain largely dry, although
can`t discount some shower activity under the lingering subsidence
inversion. By Monday evening/overnight, coarse resolution guidance
does offer a more coherent signal for some increased shower and
thunderstorm chances as the mid-levels moisten and a low-level jet
nocturnally increases.

Extended guidance then supports the potential for a briefly more
active period of northwest flow aloft through the middle and end
of next week as several mid-level perturbations drop southward out
of Canada. Obviously will have to contend with the typical timing
differences for a while at this range, but the general idea seems
to support building moisture and instability and at least modest
amounts of deep layer shear to support some potential for
thunderstorms and a returning threat for heavy rainfall.



For the 12Z TAFs...

Concerns this forecast period include:

* MVFR cigs spreading northward this morning with a potential for
  showers and brief threat for TS
* Timing the main window favoring scattered showers and storms
  this afternoon
* Northwest wind shift this afternoon and early evening
* Transition to mainly light showers behind a cold front this
  evening and overnight

One mass of showers will continue to organize in the vicinity of a
cold front across Iowa and Wisconsin this morning which will
eventually sag southeastward into the region later today. In the
meantime, monitoring a compact vort lobe swinging out of
Kentucky which should bring with it an increasing chance for
mainly MVFR showers later this morning to the Chicago-area
terminals. There may be a brief window for TS as this initial
batch of precipitation comes to an end, roughly during the 17z-19z
timeframe although current thinking is that the best chances for
this will remain immediately south and west of the TAF sites. MVFR
cigs will prevail this morning as well, with gradual improvement
to VFR anticipated (away from RFD) this afternoon.

There may be a brief lull in activity into early this afternoon
before additional scattered showers and a few thunderstorms
develop with heating. Like yesterday, the overall coverage of
thunderstorms may not be quite high enough to warrant on-station
TSRA chances, so have elected to carry a brief VCTS with a TEMPO
for SHRA during the expected window of increasing coverage. At
RFD, this transition between late-morning showers and afternoon
precipitation associated with the cold front may be more seamless
and have attempted to depict this with the outgoing 12z TAFs. On-
station TSRA potential at RFD will be a bit higher due to the
closer proximity to the front and slightly better instability
profiles today.

Once the front passes, expect any embedded TS chances to quickly
diminish with precipitation trending to mainly light showers. For
now, have left conditions VFR (except at GYY downwind of the
lake), but MVFR conditions will at least be periodically possible
this evening and overnight. However, will let later shifts add
this detail as shower trends become a bit more clear.





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