Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 261140

Area Forecast Discussion
National Weather Service Quad Cities IA IL
640 AM CDT Mon Jun 26 2017


Issued at 310 AM CDT Mon Jun 26 2017

06Z surface data has high pressure across the Plains with a
dissipating frontal boundary from the southern Plains into the Ohio
Valley. Dew points were in the 40s from the northern Plains into the
upper Midwest with 50s from the Great Lakes and Ohio Valley into the
central Plains.


.SHORT TERM...(Today and Tonight)
ISSUED AT 310 AM CDT Mon Jun 26 2017

Below normal temperatures will continue Monday.

Through sunrise, the first of two upper level disturbances will move
into the area. This disturbance should bring some sprinkles or very
light showers west of the Mississippi between highways 20 and 34.

During the daylight hours, the second upper level disturbance will
move through the area. This second disturbance is expected to
produce a more significant band of rain that affects the southern
half of the area. The combination of cloud cover and cool
temperatures raises questions regarding the potential for any
thunderstorms. Profiles of the atmosphere does indicate one or more
layers of unstable air aloft which should be just enough to allow
some embedded isolated thunderstorms to develop, especially during
the late morning and afternoon hours.

Areal coverage of the rain will decrease late this afternoon as the
disturbance exits the area with all but the extreme south areas
clear of rain by sunset.

After sunset, quiet and dry conditions will be seen across the area
as high pressure builds into the Midwest. Light winds and clearing
skies should allow some patchy fog to develop, especially in those
areas that see rain during the day.

.LONG TERM...(Tuesday through Sunday)
ISSUED AT 310 AM CDT Mon Jun 26 2017

Forecast focus remains on active pattern mid to late week.

Tuesday-Tuesday night...
Gorgeous weather on tap, as upper level heights build over the
region in wake of persistent low pressure finally loosing its grip
and shifting to the eastern third of CONUS. At the surface, a
large expanse of high pressure passing through the region during
the day will lead to abundant sunshine, light winds, low humidity
and splendid temps from the mid 70s to near 80. Upper level
ridging will remain largely in control for much if not all of
Tuesday night leading to dry conditions. At the surface, the high
will shift east with southerly return flow becoming established
and increasing along with some increase in WAA clouds overnight.
This combination will support warmer lows (albeit still quite comfy)
in the mid 50s to around 60.

Confidence remains high on evolving active pattern during this
timeframe as the region resides in favored convective spawning
grounds near the main belt of westerlies within fast zonal flow,
and on northern periphery of main heat dome (from the Southwest US
to Central/ Southern Plains) characterized by seasonable temps and
higher humidity levels. While confidence is high on an active
pattern, it is not as high on the day to day details which will be
largely governed by prior nights convective trends and mesoscale
details as typical for summertime and difficult too pin down this
far out. Having said that, details for Wednesday are coming into
more focus and revealing Wednesday afternoon and night as the first
window of active weather. Good agreement exists with models on
shuttling a progressive and moderate amplitude shortwave (remnants
of upper low currently off the northern CA coast) across the region.
This energy will aid an anomalously strong southwesterly LLJ (40-
45 kts; 3-4 standard deviations above normal per 12z/25th NAEFS),
which will in turn lead to impressive and strong moisture advection
with integrated water vapor transport 2-4 standard deviations above
normal lending to PWATs around 2 inches. Combination will lead to
organized convection shifting eastward from SD/NE into the Mid-
Mississippi Valley by Wednesday afternoon with some potential for
strong/severe storms given 30-40 kt 700 mb winds owing to favorable
bulk shear. Thermodynamic environment will favor potential robust
rainfall rates of 1-2 inches/hr within the strongest cores. The
southwesterly LLJ will remain aimed into the region Wednesday night
until the shortwave passes, thus leads to concerns for backbuilding
with some areas seeing repeated storms and potential for localized
swaths of several inches of rain and increased risk for flash flooding.
Also, potential exists for rather windy conditions Wednesday into
Wednesday evening given anomalously strong LLJ (40+ kts) and
tightening pressure gradient between Upper Midwest low and Mid-Atlantic

Signal appears to be strengthening for less coverage of showers and
storms for a good part of Thursday with subsidence in wake of
departing shortwave. However, by later Thursday afternoon into Friday
morning there is a signal for this being the next round of active
weather, as another moderate amplitude shortwave is shuttled through
the region in fast zonal flow while at the surface a weak cold front
sags into the area and eventually stalls out. Wind fields are not
shown to be as strong, but thermodynamic environment aided by warmer
temps could make this one to watch for strong/severe storms. In
addition, the LLJ is shown to veer with main integrated water vapor
transport aimed to the south/east of cwa, but in close proximity to
where additional heavy rain will be possible and perhaps also a concern
for flash flooding if it falls over areas previously saturated from heavy

Still fair amount of disagreement on the pattern with GFS/ECMWF showing
NW flow pattern, and slightly cooler and somewhat drier conditions.
Nonetheless this would be a dirty NW flow being still unsettled and
harboring potential for mainly diurnal driven convection with any
perturbations embedded within the flow. Meanwhile, Canadian model is
showing semi-zonal to SW flow aloft, which would maintain warmer and
closer to normal temps and potential for another decent event later in
the period. Forecast blend looks to be decent merge of these two scenarios.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday Morning)
ISSUED AT 635 AM CDT Mon Jun 26 2017

Two passing upper level disturbances will bring separate rounds
of rain showers to the area through 00z/27. VFR conditions are
expected with the possible exception of isolated MVFR conditions
in the stronger showers. Although not included in the 12z TAFs,
isolated TSRA are possible. After 00z/27, high pressure will build
into the Midwest. The light winds, clear skies, and wet ground may
allow some patchy fog to develop after 06z/27.




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