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fxus63 kdvn 200603 
afddvn

Area forecast discussion
National Weather Service Quad Cities Iowa Illinois
103 am CDT Fri Sep 20 2019

..aviation update...

Update...
issued at 753 PM CDT Thu Sep 19 2019

Radar images over the past hour have shown scattered showers
developing over northwest Illinois, mainly in Stephenson County,
with more recent activity along the I-88 corridor from the Quad
Cities to Sterling. These appear to be occurring along weak
boundaries from convection earlier today, triggered by a weak
upper level shortwave axis showing up on the 00z rap analysis from
central WI south-southwest along the MS river. With relatively
stable mid level lapse rates (4.8 deg c/km 700-500 mb lapse rate
on the 00z kdvn sounding), these should remain just showers and
trend eastward out of the area with the upper forcing early this
evening.

Through the rest of tonight, forecast confidence is low in the
potential for thunderstorms and heavy rain. While there is indeed
plenty of moisture, as evident by the 1.74 precipitable water on kdvn 00z
sounding, forcing remains lacking. In the near term, there will be
shortwave ridging following the current trough, an initial weak,
low level jet pointing more toward areas west of the MO river and
a lack of focused low level convergence in the forecast area.
Recent runs of the hrrr have backed off to now having only
isolated showers overnight then widely scattered activity after
sunrise.

Have updated the forecast for a dry evening, outside the isolated
showers early in northwest Illinois. Overnight, will keep only
low pops for scattered showers and thunderstorms along and north
of the I-80 corridor well after midnight into early Friday
morning. Heavy downpours will still be possible, but lacking any
strong signal supporting an organized mesoscale convective system or back-building storms,
the potential for flash flooding looks low tonight.

&&

Synopsis...
issued at 311 PM CDT Thu Sep 19 2019

18z surface data has a weak boundary from the morning storms running
from north central Illinois, to near Quincy and then across northern
Missouri. Dew points were in the 60s from the Great Lakes into the
northern plains with 70s from the Ohio Valley into the Central
Plains down to the Gulf Coast.

&&

Short term...(this evening through friday)
issued at 311 PM CDT Thu Sep 19 2019

Through sunset isolated showers and possibly a thunderstorm will
continue before dissipating.

Quiet conditions are then expected from sunset through mid-evening
across the area. Attention then turns to the late evening and
overnight hours.

The remnant boundary from the morning storms will likely be the
focus for new storm development tonight.

Trends from the very short term models suggest isolated showers and
storms should begin developing by late evening just south of I-80
and should increase in areal coverage through sunrise Friday.

The potential is there for another round of heavy rain given pwats
are around 2 inches which is in the 99th percentile of atmospheric
moisture.

At this time no new Flash Flood Watch will be issued. However there
is a very real possibility that a new Flash Flood Watch will be
needed. The overall scenario will be one of nowcasting. That is we
will have to watch and see where storms develop and how they evolve
over time.

Right now any area from the I-80 corridor on north may be at risk
for heavy or excessive rainfall.

For Friday, storms that develop overnight will persist into the
morning hours with the northern half of the area favored for rain.
The risk of heavy rain will also be there with any storms during the
morning.

By late morning and into the afternoon the nocturnal storms will
dissipate and there may be a brief period of quiet weather. Isolated
diurnal storms should begin developing across the area by mid to
late afternoon.

Long term...(friday night through thursday)
issued at 311 PM CDT Thu Sep 19 2019

Synoptic and mesoscale models both accurately depict a excessively
moist air mass with persistent lift will be found over the
Midwest through the end of next week. This pattern will not likely
result in rain in every period, but it will frequently provide
rainfall chances, and all of those seem to contain the threat of
heavy rainfall. The airmass in place, or the once advecting into
our region has been consistently forecast with precipitable water values around
2 inches. That is always a signal for heavy rainfall, and in this
case, it is solidly in the 99% percentile, or about 200% of
normal. That's not mesocale feedback, but the synoptic set up!

The tropical connection is a given, as it's already in place as
imedla's remnants are interacting with our midwestern moist
advection, this is what happened last night to our west and north,
and eventually into our north 1/2 this morning. That process should
not break down during periods of moist advection, until Sunday
evening. So, this looks like a slam dunk in needing a Flash Flood
Watch, but with the short term offering up a more mesoscale areal
extent of flash flooding, I am not inclined to issue an
exceptionally long lead time watch through the weekend at this
point. Though one could certainly be issued tomorrow for Friday
night through Sunday afternoon.

I am not overly excited about severe weather in this pattern, as the
extensive moist low levels are likely to become filled with low
clouds through the end of the event, and this should limit low level
cape. Like a typical tropical air mass, while we won't have
explosive cape, we may well have in inexhaustible supply of "skinny
cape" with the atmosphere very close to the moist adiabatic profile.
The ability to generate cold pools will not be efficient, due to
this expected profile. I expect the severe weather to be tied
directly to areas of low level convergence, which at this point,
appear mainly to the southwest of Iowa.

&&

Aviation...(for the 06z tafs through 06z Friday night)
issued at 1255 am CDT Fri Sep 20 2019

The very moist atmosphere may lead to fog and MVFR conditions,
including at mli, dbq and Cid overnight into Friday morning.
There is a low potential that fog could result in conditions as
low as IFR, especially at dbq toward sunrise. Isolated showers and
thunderstorms will be possible for much of Friday, but the
probability of occurring is too low to include in the forecasts
and VFR conditions with light southeast to south winds will be
the rule into Friday evening.

&&

Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&

$$

Update...sheets

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