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fxus63 kdvn 202320 
afddvn

Area forecast discussion
National Weather Service Quad Cities Iowa Illinois
620 PM CDT Fri Sep 20 2019

..00z aviation update...

Synopsis...
issued at 248 PM CDT Fri Sep 20 2019

18z surface data has high pressure over the southeast United
States with a cold front entering the Western Plains. Deep
tropical moisture was moving north through the plains due to a
combination of the remnants of Imelda and Lorena, Gulf moisture
and a tropical connection developing from mario. Dew points were
in the 60s across the Great Lakes with widespread 70 dew points
from the Gulf Coast into the northern plains.

&&

Short term...(this evening through saturday)
issued at 248 PM CDT Fri Sep 20 2019

Some isolated diurnal showers and storms may develop late this
afternoon through mid-evening before dissipating. A vast majority of
the area will remain dry through midnight.

After midnight the first pulse of tropical moisture moves into the
area along with an upper level disturbance. Showers and storms will
develop and/or move into the area and overspread nearly all of the
area through sunrise Saturday. Thunderstorms that develop will be
very efficient in producing rainfall so locally heavy rain will be
possible.

On Saturday widespread cloud cover will help suppress temperatures.
The initial wave of rain will move through during the morning with a
new wave of rainfall arriving by late morning and through the
afternoon.

Like Friday night thunderstorms will be very efficient in producing
rainfall in the tropical atmosphere. Locally heavy rainfall will be
possible during the day with the stronger thunderstorms.

The widespread cloud cover and high atmospheric moisture is expected
to put a damper on any severe risk. If severe storms were to occur
they would be very isolated with damaging wind the most likely risk
due to precipitation loading in the downdrafts.

Long term...(saturday night through friday)
issued at 248 PM CDT Fri Sep 20 2019

The weekend heavy rainfall event still appears to be on track. The
Gem, GFS, European model (ecmwf) continue to show synoptic rainfall support for heavy
rains, as exceptional moisture levels in the moist conveyor converge
over eastern Iowa and western Illinois Saturday night through Sunday
afternoon. I plan no changes to the Flash Flood Watch, beyond some
working changes already have been made. The forecast remains largely
unchanged, except, after Saturday evenings convection, we are less
likely to see actual torrential rainfall rates, rather than a
widespread continuous moderate to lower end heavy rate for hours and
hours. Thus, messaging will switch from the 1-2 inch per hour
potential Saturday afternoon and evening, to one where 0.20 to 0.30
is more likely, but may continue for 12+ hours. This may in fact be
a flash flood event, that transitions to a more widespread areal and
river flood event. By Sunday, thunder may become less likely at all,
with strong synoptic lift arriving, producing widespread rain.

I talked with Storm Prediction Center early today, and we agreed that the severe weather
threat seems limited to locations near the frontal boundary in the
initial convective flare up Saturday afternoon to the west, and by
the time storms move east we're likely unable to get much of a cold
pool with the exceptionally high moisture levels through a deep
layer. Low level shear is strong, so maybe if there is any threat, a
weak tornado cannot be ruled out Saturday night.

In the week ahead, I left the grids as the blend arrived, with
numerous periods of low pops. It is certainly concerning that the
models re-Load the western synoptic trof, which will certainly fill
the entire Midwest and plains with Gulf moisture again at the end of
the period, into week 2.

&&

Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 619 PM CDT Fri Sep 20 2019

VFR conditions will continue through late tonight with a low
potential for isolated showers and thunderstorms. Deeper moisture
arriving from the south is expected to bring in widespread low
ceilings and scattered showers and thunderstorms after 10z.
Prevailing MVFR and possible IFR conditions are expected through
Saturday morning with rain ending. A gradual improvement back to
MVFR or VFR is expected in the afternoon with another round of
thunderstorms likely late in the afternoon, handled with prob30
wording for now.

&&

Dvn watches/warnings/advisories...
Iowa...Flash Flood Watch from Saturday evening through late Sunday
night for Benton-Buchanan-Cedar-Clinton-Delaware-Des Moines-
Dubuque-Henry Iowa-Iowa-Jackson-Jefferson-Johnson-Jones-
Keokuk-Lee-Linn-Louisa-Muscatine-Scott-Van Buren-Washington.

Illinois...Flash Flood Watch from Saturday evening through late Sunday
night for Bureau-Carroll-Hancock-Henderson-Henry Illinois-Jo
Daviess-McDonough-Mercer-Putnam-Rock Island-Stephenson-
Warren-Whiteside.

MO...Flash Flood Watch from Saturday evening through late Sunday
night for Clark-Scotland.

&&

$$

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