Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 220424
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1124 PM CDT WED SEP 21 2016

...AVIATION UPDATE...

.UPDATE...
Issued at 622 PM CDT Wed Sep 21 2016

While all eyes are on that HP supercell to the north of Waterloo,
we`ve been doing updates here to lower our threat of rainfall. The
same cap that`s held that storm`s progress from racing straight
southeast also squashed our ongoing vigorous updrafts that existed
over southeast Iowa around 330 PM. Thus, I have maintained some
low pops northwest to account for a right turn of the supercell,
but otherwise expect a dry evening in all locations. Overnight, if
we get a complex over central and northeast Iowa, we could see
storms propagate into the northwestern CWA after midnight.

&&

.SYNOPSIS...
Issued at 330 PM CDT Wed Sep 21 2016

Latest sfc obs and Vis satellite loop were indicating ringing
outflow boundary lined with enhanced CU from northeast of Macomb
Il, up north of Burlington IA and to northwest of Waterloo. This
feature matching up with Main warm front in northwestern IA and
far southwestern MN. Large ambient temp spread from north to
southwest acrs the CWA...low to mid 70s to around 90. Water Vapor
loop indicating another upper short wave trof/vort max acrs
central Neb, rolling northeastward riding the upper ridge axis
acrs the central MS RVR Valley.

&&

.SHORT TERM...(This Evening through Thursday)
ISSUED AT 330 PM CDT Wed Sep 21 2016

Tonight...Will have to keep isolated to widely scattered POPs along
the outflow boundary during late afternoon and into the early
evening, then expect a decrease with loss of heating. Most of the
storm clusters currently northwest of ALO should drift to the
northwest and north of the DVN CWA through evening as well, but will
keep low POPs for potential additional development making it into the
far northwestern CWA through at least 01z this evening. Then most of
the latest run model MCS generation tools as well as conceptual H85
MB flow point to another evening of storm cluster/MCS formation acrs
the southern third of MN and western IA/Mo RVR Valley. Will have to
watch for storms from both of these generation regions eventually
bleeding down into portions of the local area mainly after midnight
into early Thu morning. Will target CHC POPs in the northwestern
third or so of the CWA late tonight into Thu morning, but upstream
evolution will have to be monitored to see if POPs need to be raises
and expanded further to the east/southeast.

Most models suggest a weakening mode as they/convective storm
clusters enter the local area getting away from more optimum
support/shear profiles to the north, but there may be some lingering
overnight CAPE and low to mid lapse rates to allow for a few storms
riding a cool pool to produce gusty winds. Locally heavy rain also
possible of at least a half inch to an inch of rainfall possible
into the northwestern CWA before storms weaken further. But current
thinking suggests still not enough to warrant matching up with the
ongoing upstream Flash Flood Watches in the northwest for now. See
what happens tonight and reassess tomorrow. Lows in the mid to upper
60s.

Thursday...For now will keep POPs confined mainly to the
northwestern and northern third of the fcst area for lingering
activity out of tonight and new development possibly festering on
it`s remnants or more outflow boundaries. Again, POPs may have to be
expanded east and southeast of these areas of concern if incoming
storm clusters want to feed further south on higher thermodynamics.
POPs for the afternoon for isolated to sctrd new development on
remnant boundaries much like today, but best storm support shifts
back to the northwest/north of the region by late afternoon and Thu
evening.  Will use a model blend for high temps with convective
debris and outflow boundaries a concern again making for high temp
variability acrs the forecast area. If not much in the way of
clouds/storms make it down acrs the area or linger well into the day
Thu, these advertised highs may be too cool.    ..12..

.LONG TERM...(Thursday Night through NEXT Wednesday)
ISSUED AT 330 PM CDT Wed Sep 21 2016

Overview: Very warm Friday and Saturday by late September standards.
Another wet period possible Sunday into early next week - heavy rain
is possible but it is too early to determine amounts and duration at
this time.

Thursday Night...Models in good agreement on warm front location
remaining quasi-stationary along the MN/IA border into southern WI.
If the forecast does not change, the focused area of showers and
thunderstorms would be mostly north of the CWA. Outflow boundaries
sinking southward could easily fire off isolated to scattered
storms, so have 20-50% PoPs across the northern two-three tiers of
counties (highest north of highway 20 and west of the Mississippi
River). Lows are forecast in the mid 60s.

Friday and Saturday...Very warm and humid for the end of September.
Forecast highs are near 80 F across the north but in the mid to
upper 80s further south - about 3-5 degrees below daily record
highs. 850-500mb ridging amplifies over the Central U.S.
northward into Central Canada ahead of a deep upper-low over the
Central and Northern Rockies. Anomalously warm ~850mb temps in the
upper teens Celsius will cap the atmosphere, limiting chances for
precipitation. Also, synoptic forcing and areas of low-level
convergence are forecast to stay well north/northwest of E Iowa and
NW Illinois.

Sunday...Cooler temperatures are likely as a cold front and large
upper-low move in from the WNW. ECMWF/GFS in reasonable agreement
with timing of initial wave of showers and storms on Sunday. PoPs
increase to 40-60%, highest western third and may go higher in
future updates if inter-model consistency continues.

Monday into Tuesday...Model solutions quickly diverge with respect
to the evolution of a large upper-level low in the Central U.S. The
ECMWF offers the most ominous solution, digging the upper trough
furthest to the south, all the way down to the Gulf of Mexico. If
this occurs, eastward progression is halted, and a deep tropospheric
layer of WV transport develops on the eastern flank of the low.
There is an impressive Atmospheric River signal from the Gulf
northward into the Midwest. This scenario could result in impactful
rainfall amounts from a flooding standpoint; however, it is not
backed by other models, so have to message low confidence on the
rainfall forecast for now. Certainly something to monitor over the
coming days. Uttech

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday Night)
ISSUED AT 1120 PM CDT Wed Sep 21 2016

A dry overnight with southeast winds and VFR weather is expected,
though powerful thunderstorm complex north of Waterloo will need
to be watched carefully, and it could move toward eastern Iowa
after 10Z, as it weakens. This is not confident, but has enough
potential that a vicinity shower has been included toward morning
at DBQ and CID. Otherwise, another warm and mainly dry day is
forecast for southeast Iowa and Illinois Thursday.

ERVIN


&&

.HYDROLOGY...
Issued at 1020 PM CDT Wed Sep 21 2016

There is an ongoing flash flood event upstream in the Cedar, Iowa,
and Wapsipinicon basins. The NCRFC has come out with QPF heavily
influenced river crests now over major for several tribs in Iowa,
and flooding does now appear likely, but the exact values of crest
are not certain. So, I`ve issued a set of flood watches for many
points along the Wapsi, Iowa, and Cedar. The Cedar at Cedar Rapids
crest is well into the major category, possible, but not yet
confident, thus the watch. Once this water is on the ground, we
can gain far more confidence.
ERVIN

&&

.DVN Watches/Warnings/Advisories...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

UPDATE...Ervin
SYNOPSIS...12
SHORT TERM...12
LONG TERM...Uttech
AVIATION...Ervin
HYDROLOGY...Ervin



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