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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
342 PM CDT Friday Jul 31 2015

issued at 330 PM CDT Friday Jul 31 2015

A weak cold front linked to an upper level trough moving through
Ontario reached roughly from central lower Michigan...SW across Southern
Lake Michigan...northern Illinois and just south of the I-80 corridor across
eastern Illinois and Iowa at middle afternoon. Due to Gulf moisture suppressed
well to the south and west...and the somewhat dry air in the middle and
upper level northwest flow...the front was acting mainly as a surface dry
line with little cloud cover over the forecast area. Regional radars
showed thunderstorms breaking out along the western end of the east-west
boundary across Nebraska...where the weak boundary was meeting up with
returning Gulf moisture. Stronger mixing behind the front was
resulting in gusty northwest winds and dewpoints in the 50s across the
north...while dewpoints were in the upper 60s to around 70 across
the south. Temperatures were mainly in the lower to middle 80s...with a
few upper 80s immediately along the boundary.


Short term...(this evening through saturday)
issued at 330 PM CDT Friday Jul 31 2015

Main challenge is the low potential for thunderstorms throughout.
Tonight...broad convergence along the boundary...expected to be
roughly along or south of the Iowa/MO border into west central Illinois...may
result in at least isolated thunderstorms overnight as negative middle
level thetae lapse rates overspread the area late tonight. This
convection is also indicated by some of the high res convective
allowing models...but generally staying south or SW of the forecast
area. Will thus continue with only slight chances for showers and
thunderstorms due to the low confidence. A separate area of at
slight chance probability of precipitation is carried in the far north...where weak
convergence along a surface trough may be sufficient enough under
the increasingly unstable middle levels to trigger elevated convection
late tonight into early Saturday morning. Otherwise...skies should
remain mostly clear tonight with mins as low as near 60 in the
driest air across the the lower to middle 60s elsewhere.

Saturday...a weak surface ridge and dry northwest flow aloft should provide
another dry day...similar to today but slightly warmer with lighter
winds turning more southerly by evening. With a similar airmass...
and accounting for warming aloft...will have highs from the lower to
middle 80s northeast to the upper 80s to near 90 SW. Higher humidity
levels will begin creeping into the southwest and west...where
developing light south winds will return dewpoints to upper 60s to
near 70. Weaker mixing over central and northeast should keep values
in a slightly more comfortable lower to middle 60s range through most
of the day.

Long term...(saturday night through next friday)
issued at 330 PM CDT Friday Jul 31 2015

Saturday night and Sunday...bulk of 12z model runs suggest wave
embedded in northwesterlies and associated low level front will act upon
more optimum return flow for storm cluster generation to the
northwest across northeastern Nebraska/eastern Dakotas and Minnesota Sat evening.
Mesoscale convective system forcing tools and other convective support parameters maximize
more across Minnesota and into WI where resultant mesoscale convective system will more likely
propagate across late Sat night into Sunday morning. But other tailing
forcing ahead of the incoming front and pre-frontal warm air
advective support should generate sctrd showers/storms across the
local area possibly into the northwestern third of the County Warning Area before
midnight with areas further to the east still capped and dry. But
this should erode from west-east after midnight for increased storm
development into early Sunday morning. But latest forecasts suggest the
better late night low level jet convergence occurs to the northeast across
WI...with veering low level flow locally. Shear profiles during this maximum
coverage time after midnight marginal and only see the chance for
isolated severe storms at this time. Locally heavy rain where storms
clusters manage to occur may be the primary threat.

Sunday very uncertain with the same old scenario of how long
convective debris and outflow lingers during the day to limit
heating and instability build up. The main incoming front not
prognosticated to push east across the MS river until generally around middle
Sunday evening or so. Deeper shear profiles more robust by Sunday
evening...and if convective debris can clear enough to allow full on
pre-frontal heating much of the County Warning Area may warm into the middle to upper
80s...with some sites hitting 90. This combined with pooling surface
dewpoints at least into the lower 70s may make for sbcapes of 3500-4500+
j/kg Bay late afternoon. Thus there could be more potential for
stronger storms by Sunday evening as opposed to Sat night. Current
assessment makes for the scenario where a line of storms develop on
the front or where front interacts with lingering outflow/warm
frontal boundaries right overhead of the central to eastern County Warning Area
Sunday evening...with them then taking off across the north half of Illinois
Sunday night. All modes of severe weather may be possible if this
scenario unfolds... with damaging wind primary.

Monday through Wednesday...most of Monday may be a Post-frontal dry
and less humid day under persistent northwesterly flow aloft. But
return flow will look to organize nocturnally up the plains and MO
River Valley Monday night. There will be a chance for the resultant
convection to bleed into portions of the dvn County Warning Area as the Monday night
into early Tuesday morning period progresses. Longer range indications
and medium range model ensembles indicate warm dome gathering
strength and bulging north up The Rockies and Western Plains into
the middle week period. This could place the dvn County Warning Area on this feature/S
northeastern flank and dumping zone for ridge-riding mesoscale convective system/S or under
a portion of the potential ring of fire pattern. The 12z GFS shows
no Mercer and keeps the storm clusters coming especially during the
evening and overnight periods...while the 12z European model (ecmwf) is more bullish
with Great Lakes l/west troffiniss-northeasterly push that the storm track
gets shunted west and south of the local forecast area by later Wednesday. But
with low confidence...daily chance probability of precipitation will have to ride at this point.

Next Thursday and Friday...the 12z European model (ecmwf) would bring mainly dry and
below normal temperatures to the region until Friday night when the pattern
starts to break down from the west and flattened flow Ushers an mesoscale convective system
across Iowa. The 12z GFS keeps the storm track going until later Friday
when it undergoes pattern amplification with seasonably deep Great Lakes
troffiness and Sharp Ridge across The Rockies...allowing dry weather and
well below normal temperatures to spill down across the upper MS River Valley
for the start of the weekend. Again with such model discrepancies
will have to keep at least low chance probability of precipitation into the end of the extended.


Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1212 PM CDT Friday Jul 31 2015

A weak cool front moving through this afternoon will result in
little more than a period of wind gusts near 20 kts this
afternoon. Otherwise...VFR conditions will continue tonight
through early Saturday with mostly clear skies and unlimited
visibilities. The probability for typical early morning fog or
haze is very low.


Dvn watches/warnings/advisories...


short term...sheets
long term...12

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