Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 kdvn 211142 

Area forecast discussion
National Weather Service Quad Cities Iowa Illinois
642 am CDT Sat Sep 21 2019

..aviation update...

issued at 340 am CDT Sat Sep 21 2019

The "calm before the storm" as no pcpn occurring across the dvn
cwa early this morning, and only a few showers developing in
central MO. Shear, instability, and lapse rates are all quite weak
but pwat's are healthy as high as 1.8 inches in our southern cwa.
Over 2 inch pwat's were located in Arkansas and Oklahoma.

There is a perfect feed off the western Gulf with surface
dewpoints in the mid 70s in se/East Texas, with a 16 dewpoint at
h8 at koun.


Short term...(today and tonight)
issued at 340 am CDT Sat Sep 21 2019

Headlines: no changes to the Flash Flood Watch for tonight and

Today: cam's insist on showers and a few thunderstorms developing
this morning then a brief break, and then more showers and storms
later this afternoon. This due to forcing provided by a 40 kt low level jet
veering into the area, as low pressure tracks into NW Minnesota.
Southerly winds will be gusting to around 30 mph later this
morning into the afternoon. Storm Prediction Center has re-introduced a slight risk for
severe storms in our western counties for this afternoon and
evening. However, the severe threat appears minimal due to
tropical moisture in place and a "tall skinny cape" sounding
(which is more conducive to flash flooding vs severe weather).
With that said still can't rule out a few strong storms with gusty
winds, hail and frequent lightning. Temperatures will be cooler
today thanks to extensive cloud cover, with highs in the mid 70s
to lower 80s.

Tonight: widespread heavy rainfall and thunderstorms expected as
deep trough in the plains provides large scale ascent, a 40 kt low level jet
in place and pwat's as high as 2.25 inches (which is above the Max
percentile for september). Forecast soundings indicate a deep
warm cloud depth of 12k ft (more than sufficient to produce flash
flooding). Any thunderstorm will be capable of rainfall rates of 2
inches per hour which would cause flash flooding (especially
where soils are saturated). Even in areas that are drier any
training of storms for several hours would also cause flash
flooding. Therefore, the Flash Flood Watch will be in effect for
the entire dvn cwa. Needless to say anyone living near creeks,
streams or rivers, and also urban areas need to monitor this

Long term...(sunday through friday)
issued at 340 am CDT Sat Sep 21 2019

Will continue with periods of rain, potentially heavy at times, with
a chance of storms on Sunday as a slow moving cold front makes
inroads into an anomalously moist and precip efficient atmosphere
characterized by pwats over 150% of normal with a deep low level warm
cloud layer. In addition, Max Omega increasing with signal for a
possible frontal wave attendant to right entrance region of 100-130 kt
h25 jet from Midwest to the Great Lakes. Overall, not much change from
previous thinking of widespread 1-3 inch storm total rain potential,
with locally higher amounts possible especially from around the
Quad Cities south/east. Not only will flash flooding be a concern, but
river flooding will also be a concern in areas that experience the
heaviest rainfall. See the Hydro section below for more information.
The rain looks to end from west to east, beginning mid afternoon
Sunday continuing through Sunday evening with the passage of the cold

Monday through most of Tuesday look to be dry with surface ridging
and increasing heights building in. Near normal temperatures in the
low/mid 70s are anticipated Monday along with much lower humidity.
Tuesday will see warmer temperatures as we transition into the
return flow on backside of the departing surface ridge. Deeper mixing
aided by gusty south winds in tightening pressure gradient should
push highs on Tuesday above the blended guidance, and mostly into the
upper 70s to around 80, with potential to be even warmer with more
widespread lower 80s.

Chances for showers and storms look to return midweek with next cold
frontal passage. Ecm has ramped up quantitative precipitation forecast and signals another bout of
heavy rain (1+ inch) over the region. GFS is faster with front and
consequently lighter on quantitative precipitation forecast being further to our south with the
heavy rain signal midweek. Both models indicate north of 1.5 pwats
/2 Standard deviations above normal/ with some branching potential off
tropical systems in eastern Pacific. Bottom line, will have to monitor
the midweek time period in the coming days for boosting rain chances
and potential for more heavy rain.

Late week into next weekend...indications are another brief bout of
ridging and dry weather just beyond midweek. Beyond that, some large
model differences exist with respect to timing/strength/track of
shortwave energy, as a deep upper trough takes shape over the western
Continental U.S.. strong/deep southwest flow should lead to above normal temps,
and expect the wet pattern to persist with periodic rain chances
attendant to timing of ejecting shortwaves.


Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 641 am CDT Sat Sep 21 2019

Deeper moisture arriving from the south is expected to bring in
widespread low ceilings and scattered showers and thunderstorms
this morning. Prevailing MVFR and possible IFR conditions are
expected today with a possible break in the rain for a time this
afternoon. Then another round of rain and thunderstorms likely
late in the afternoon through much of tonight. South winds will
be gusting to around 25 kts this afternoon.


issued at 340 am CDT Sat Sep 21 2019

The potential exists for rainfall amounts of 1 to 3 inches from this
impending storm system, with locally higher amounts possible. This
heavy rainfall would be accompanied by a threat for flash flooding,
followed by a threat for river flooding through at least early to
mid next week. Most susceptible is portions of northeast Iowa and
northern Illinois, including the rock and Pecatonica River
basins, which have seen quite a bit of rainfall this month leading
to very saturated soils and highly sensitive antecedent
conditions in these areas. Elsewhere, spotty swaths of higher
rainfall has occurred but in general amounts have been much lower
compared to areas near the Highway 20 and Highway 30 corridors,
leading to near normal to lingering below normal soil moisture and
water levels. However, heavy rain of 1 to 3 inches in these areas
of the hsa would at the very least produce rapid within bank
rises especially if the heavy rain were to fall over a few hour
period, and could possibly result in new flooding on some area
rivers that experience the heaviest rainfall even where soil and
water levels are currently near to below normal. Still a lot of
uncertainty exists with rainfall amounts and location. Therefore,
anyone living along area rivers and streams, or in low lying flood
prone areas should pay close attention to water levels and the
latest forecast information. As a reminder, river forecasts only
include observed precipitation, plus forecast precipitation for
the next 24 hours.


Dvn watches/warnings/advisories...
Iowa...Flash Flood Watch from this 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 this evening through late Sunday night
for Bureau-Carroll-Hancock-Henderson-Henry Illinois-Jo Daviess-
McDonough-Mercer-Putnam-Rock Island-Stephenson-Warren-

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




National Weather Service Glossary of Abbreviations