Area Forecast Discussion
Issued by NWS Quad Cities, IA IL
FXUS63 KDVN 211528 CCA
Area Forecast Discussion...Correction...
National Weather Service Quad Cities IA IL
1028 AM CDT THU JUL 21 2016
Issued at 1017 AM CDT Thu Jul 21 2016
Very light showers have developed in the accas field across
northwest Illinois. Based on satellite trends, additional showers
should develop south into Whiteside county this morning. Radar
trends indicate the showers should move to the east southeast.
UPDATE Issued at 734 AM CDT Thu Jul 21 2016
Looks like the MCS and attendant warm advection wing will remain
focused into Wisconsin rest of this morning. Precipitation
chances then will hinge on any redevelopment back to our north
and northwest along residual outflow boundary, and questions of
whether diabatic heating within strong instability will be
sufficient to weaken warm EML. If storms do develop to our north
and northwest later today into early evening then a favorable
environment exists for upscale growth and potential propagation
into cwa, with some severe risk along with very heavy rain with
PWATS above 90th percentile for July. Confidence is low and
thus will maintain lower to moderate chance pops across north
1/2 or so of cwa for this evening/tonight.
Otherwise...main concern is with the heat, with highs likely
topping out in the lower to middle 90s. This combined with
dew points in the mid 70s to possible some lower 80s, will make
for dangerous heat indices of 100-110+ degrees this afternoon.
Issued at 333 AM CDT Thu Jul 21 2016
Severe MCS is shifting ESE across northern Minnesota early this
morning. More vigorous convection and best severe threat
appears to be migrating eastward ahead of cold pool. Meanwhile,
cloud tops show signs of warming with echo tops decreasing on
southern flank as outflow undercutting storms leading some
weakening at this time. All eyes are focused on the evolution of
this convection as favorable juxtaposition of north to south
surface theta-e max, 850 mb thermal gradient 19-24C and
increasingly diffluent flow aloft along with high MUCAPE (3000-
4000+ j/kg) favor propagation to right of mean flow or SSE.
Either through sustenance or additional development on outflow
would support increasing threat of storms for portions of the
area today, especially north of I-80.
.SHORT TERM...(Today and Tonight)
ISSUED AT 333 AM CDT Thu Jul 21 2016
As mentioned, focus is with Minnesota MCS and evolution/trends
with potential impacts for increasing storm chances across
our area next 24 hours.
Increasingly diffluent flow aloft attendant to passing disturbance
into the Great Lakes, along with presence of moisture and
instability axis would seem to favor increasing storm chances next
24 hours. Hi-res CAMS including ARW, NMM and HOP-WRF have idea of
Minnesota convection and all show propagation or development south
into the region, with mainly just some timing differences.
Presence of warm EML adds some uncertainty on sustenance and
propagation into or near cwa, but aforementioned increasingly
favorable ingredients with potential for outflow boundary in/near
area and warm advection along with CAM solutions supportive of
raising pops and have capped into chance category north for now
with uncertainty on sustenance given warm EML across the region.
Will continue to monitor convective trends to our north for
additional changes to forecast. Increasing storm chances and
convective debris will also potentially wreak havoc with
temperatures today. South should be warmest with readings in
the lower to possibly mid 90s while most uncertainty exists with
the northern counties being closer to remnant outflow and greater
Tonight, depending on what happens today the area could be in
favorable MCS spawning grounds with increasing low level jet
and tail of Great Lakes forcing conducive for storms. Could see
development late evening and overnight with potential for upscale
growth, which would then tend to propagate to right of mean wind
or possibly to SSW across much of cwa. Have increased pops, but
may need to go higher depending on convective trends today.
Ample shear 0-6km 30+ kts and strong instability supportive of
some severe storms with mainly strong winds 60-70+ mph. Very
heavy rain will yet again be a real concern with anomalously
moist airmass and potential for backbuilding and mergers with
southwesterly low level jet.
.LONG TERM...(Friday through Wednesday)
ISSUED AT 333 AM CDT Thu Jul 21 2016
Main forecast concern for the long term remains the excessive heat
and any convection that may temper heat index values through
Saturday. At this time models continue to produce convection Friday
and Saturday. Depending on the timing of this convection, and
convective debris, heat index values across the northern CWA may
struggle to reach 105+.
Friday, sfc boundary still prog to drop south across the area. Stout
cap remains in place across the CWA however, OFBs from possible
overnight convection could serve as a foci for convective
development. Have slightly lowered surface temps. These temps are
just above the regional blender. Also painted high chc pops across
the region where the boundary is. Convective debris could limit
overall temps Friday. The GEM yesterday morning had high temps over
us in the mid 80s for Friday, implying convective debris shielding.
Now the corollary to this is excessive heat across the area.
Regardless of these uncertainties it is wise to keep the warning out
as this could be the first round of really warm heat indices this
Saturday, the proximity of the H5 flow again suggests that any
convection forming on the ring of fire will slide into our area.
Models have a low across the plains, which tries to drag the
boundary north. This boundary appears to pivot across our area as
convection fires along the warm front. This could also hamper our
temperatures into Saturday. Like usual for this time of year,
mesoscale processes that depend on the day before will drive that
days weather. Being so close to the steering flow suggests that the
forecast will be a challenge into the weekend.
Sunday...the heat will start to wane as the stubborn boundary moves
south through the area and we see temps around normal again. The
rest of the extended sees temperatures near normal and quiet for a
couple of days, before we return to a flow where we see convection
and a warm front across the area.
.AVIATION...(For the 12Z TAFS through 12Z Friday Morning)
ISSUED AT 641 AM CDT Thu Jul 21 2016
Low clouds and/or fog with patchy mvfr to ifr conditions will
lift with conditions improving to vfr by mid to late morning, and
remaining generally vfr through the rest of the taf cycle. Appears
now that the storms over far eastern Minnesota into Wisconsin will
stay largely to our northeast today. Outflow boundary from this
activity will pull up near northeast Iowa and convergence coupled
with a weak upper level disturbance may serve to focus some storms
to our north this afternoon. Which if occurs would have better
chance sagging into the area late today and evening, but warm air
aloft or capping inversion may limit this potential thus confidence
on this occurring is low. If this does not occur still chances
for storms would exist later tonight with veering low level jet and
brushing of forcing from Great Lakes disturbance. Winds will generally
be southerly at 5-15 kts.
IA...Excessive Heat Warning until 7 PM CDT Saturday for Benton-
Buchanan-Cedar-Clinton-Delaware-Des Moines-Dubuque-Henry IA-
IL...Excessive Heat Warning until 7 PM CDT Saturday for Bureau-
Carroll-Hancock-Henderson-Henry IL-Jo Daviess-McDonough-
MO...Excessive Heat Warning until 7 PM CDT Saturday for Clark-