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fxus63 kdvn 181737 
afddvn

Area forecast discussion
National Weather Service Quad Cities Iowa Illinois
1237 PM CDT Wed Sep 18 2019

..aviation update...

Update...
issued at 1028 am CDT Wed Sep 18 2019

Some accas and cu in northeast Missouri corresponds nicely with
steep 850 mb-700 mb lapse rates. Rap trends move these steep lapse rates
into southeast Iowa and northeast MO by mid to late afternoon. These
lapse rates in combination with weak vorticity advection, negative
Theta E lapse rates, some 850 mb-700 mb Omega and temperatures in the
upper 80s may be enough to initiate isolated diurnal showers and
storms. Hrrr trends in simulated reflectivity are also pointing to
the possibility of a few diurnal storms late this afternoon in
southeast Iowa.

Thus will be updating the forecast to reflect the possibility of a
few diurnal storms late this afternoon in southeast Iowa and
northeast Missouri that will dissipate with sunset.

&&

Synopsis...
issued at 335 am CDT Wed Sep 18 2019

The local area continues to sit in an area between high pressure to
the east and low pressure to the west. Overnight have seen
substantial convection across northwest Iowa into Minnesota ahead of
the cold front currently from the central Dakotas into northwest
Kansas. Moist surface conditions are ongoing locally, with some
reduced visibilities down to 5sm seen early this morning across
parts of the area. Also, continue to watch tropical system (now
named imelda) over eastern Texas which contains a great amount of
moisture and could provide some extra moisture to parts of the local
area later in the week.

&&

Short term...(today and tonight)
issued at 335 am CDT Wed Sep 18 2019

The slight bit of visibility reduction seen across the area this
morning (5-7sm) should go away by mid morning as the surface heats
up and widens the temp/dewpoint spread. Much of the area is without
stratus this morning so would expect generally sunny skies this
morning, with the exception of some high level clouds streaming
across the region ahead of the trough to the west.

The pressure gradient is a bit tighter than yesterday so expecting
slightly higher winds today, but still not strong blowing around
10kts. As peak heating occurs, decent mixing of the boundary layer
is expected, but with winds aloft not being too strong, may just see
mixed layer winds and gusts around 15kts.

Good warming is expected today and highs should be a bit higher for
most areas today due to the lack of low level clouds. This brings
heat indices up into the upper 80s and low 90s for locations from
Independence, Iowa to the Quad Cities and areas south and west.

As the cold front to the west moves toward the area through the day
today, convection should develop ahead of it across portions of west
central into north central Iowa. Have included low chances for
storms across the northwest parts of the forecast area beginning
late this afternoon which expand to the southeast through tonight.
Confidence is not high on timing of storms, but chances increase as
the night progresses. Any storms that do move across the area will
have the potential for heavy rain as precipitable water values
exceed 1.70 inches by late this afternoon. Additionally, warm cloud
depth is around 4000 M so storms will be efficient rainfall
producers. Wpc does have our northwest areas in a marginal risk for
excessive rainfall for tonight into Thursday which does look
appropriate.

One thing to keep an eye on today is for the potential for some
isolated to scattered showers/thunderstorms across our west and
southwest forecast areas. Looks like the time around peak heating
could see some development of diurnal convection as instability is
plenty and convective temperatures could be reached. The area with
this potential would also be at the nose of the Theta-E ridge which
does have a bit of low level moisture transport into it. Models do
have surface moisture pooling in this area with dewpoints in the low
70s and there are a few cam's indicating this potential. Confidence
is on the low end, so currently kept the forecast dry, but will have
day shift monitor if there is any need to put chances in.

Long term...(thursday through tuesday)
issued at 335 am CDT Wed Sep 18 2019

Thursday

Models have come into better agreement on a line of storms reaching
the north-central and northwest forecast area late Wednesday
night into Thursday morning, following a north-to-south aligned
instability gradient. The biggest question is have far south the
storm make it before they fall apart. Href members show the
convective outflow boundary pushing all the way into the southern
County Warning Area by midday with convection weakening and becoming more
isolated in nature near and south of I-80. The main concern is
heavy rain rates due to slow individual cell motion (per corfidi
vectors) and high pwats near 1.7 inches. For this reason, there
is a marginal risk for flash flooding in the northwest County Warning Area.

Thursday afternoon through Friday

Rain cooled air and residual clouds from the morning convection could
hold Max temps down especially over the northwest half of the forecast
area. Thus, lowered highs by several degrees below the model blend
in this area which produces a gradient from the upper 70s northwest to
upper 80s in the far south. Depending on where the outflow
boundary stalls out, isolated storms may redevelop during peak
heating and have 20-30% chances in the forecast to message low
areal coverage.

Low pressure strengthening in the plains will reinvigorate the
southerly flow over the upper Mississippi Valley resulting in
another very warm and humid day with highs in the mid 80s on
average, potentially even a few degrees warmer depending on the
amount of sun. Aside from weak isentropic lift, not seeing much
forcing for showers and storms, so if anything develops it would
be diurnally driven and coincide with peak heating. Coverage is
expected to be low.

This weekend

Forecast remains on track for the weekend. A mid-level trough
will shift out of the northern rockies into the upper Midwest on
Saturday into Saturday night. The European model (ecmwf) and CMC favor Saturday
evening and Sunday as the most likely timing for widespread
showers and storms ahead of and along a cold front. The GFS is a
bit of an outlier with its faster eastward progression of the
front and timing of widespread rain. If anything the European model (ecmwf) has
slowed down even more, now keeping a threat for heavy rain through
Sunday night especially south of I-80.

This will be a period to watch for heavy rain as an impressive
1.8-2.0"+ precipitable water plume/atmospheric river punches out of the western
Gulf into the upper Mississippi Valley. Naefs is showing
integrated WV transport values near 3 Standard deviations above
the mean for the middle to end of September. Those with interests
along area rivers, especially tributary rivers, should keep up
with the latest forecasts through the weekend. At this time,
model consensus favors the southern third of the area to receive
the heaviest rainfall (1-2"+ possible). Early next week we'll
finally get a break from the unseasonably warm and humid
conditions as temps fall back to near normal levels. Uttech

&&

Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1234 PM CDT Wed Sep 18 2019

VFR conditions will be seen through 06z/19 as a frontal boundary
approaches eastern Iowa. Isolated diurnal rain showers/thunderstorms and rain are possible
21z/18 to 01z/19 across southeast Iowa that may be in the vicinity
of kbrl. After 06z/19 VFR conditions should continue outside of
of shra/tsra. MVFR conditions are expected to develop by 12z/19
at kcid/kdbq with shra/tsra.

&&

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

$$

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