Scientific Forecaster Discussion
fxus63 kdvn 211732
Area forecast discussion
National Weather Service Quad Cities Iowa Illinois
1232 PM CDT Sat Sep 21 2019
issued at 1149 am CDT Sat Sep 21 2019
The moist conveyor is being synoptically lifted over the County Warning Area
today, and this has already resulted in a persistent band of
moderate to heavy rainfall. While intense rates are rather
isolated and brief, the band is going to produce an half to one
inch rain from southwest to northeast through 18z, and could
easily do that again this afternoon. I have not changed tonight
and Sunday much, but in general, the widespread nature to the
heavy rain should increase. Thus, a hybrid threat for flash
flooding is growing, from both storm clusters, and the synoptic
heavy rain threat. Rates seem to be less of a threat than overall
rain totals themselves.
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 18z tafs through 18z Sunday afternoon)
issued at 1227 PM CDT Sat Sep 21 2019
The widespread heavy rain event is taking place and is centered on
eastern Iowa to northern Illinois as of 18z. The nearly continuous
band of moderate rain with embedded heavy rain and isolated
thunder will remain over the area through Sunday afternoon.
Flooding is likely, but also, widespread IFR conditions, much like
a winter storm would provide. Strong south winds of 15 to 25 kts
are expected this afternoon, but this should decrease to under 15
kts by tonight, as it remains southerly to southwest. Visibilities
may be generally lower tonight due to both heavy rain and fog in
the moist air.
issued at 1144 am CDT Sat Sep 21 2019
Heavy rainfall event ongoing across the area. The event has been
forecast relatively well. Concerns with flash flooding along with
river flooding still look valid across the area. Flash flood
guidance remains low, especially north of i80. With this area
seeing heavy rainfall the last week or so, rivers are already
high. Especially in the rock and Pecatonica basins. High pwats
and the low ffg, this area should see plentiful of runoff.
Current River forecasts now include rainfall through 12z Sunday.
Additional rainfall is expected between 12z Sunday to 00z Monday.
This additional rainfall will lead to an increase in river
forecast crest heights with the 00z river forecasts tonight. Those
with interests along area rivers should continue to pay attention
to forecasts and possible warnings.
Iowa...Flash Flood Watch from 7 PM CDT this evening through late Sunday
night for Benton-Buchanan-Cedar-Clinton-Delaware-Des Moines-
Illinois...Flash Flood Watch from 7 PM CDT this evening through late Sunday
night for Bureau-Carroll-Hancock-Henderson-Henry Illinois-Jo
MO...Flash Flood Watch from 7 PM CDT this evening through late Sunday
night for Clark-Scotland.