Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
322 PM CDT sun Apr 20 2014

Synopsis...
issued at 320 PM CDT sun Apr 20 2014

Latest surface analysis was indicating a general west-northwest-to-ESE oriented
boundary draped across northern Iowa and to the northern fringes of the
dvn County Warning Area. Deep dry mixing under mostly sunny skies have allowed the
entire County Warning Area to warm into the upper 70s to lower 80s...with humidity
levels in the 30s and 20s...a taste of California weather. Showers and
thunderstorms continue to fester in 850 mb mb temperature and 850 mb-h7mb thta-east
advection/convergent zone just to the north of the dvn County Warning Area...also
aided by weak vorticity aloft along the Iowa/Minnesota border.

&&

Short term...(this evening through monday)
issued at 320 PM CDT sun Apr 20 2014

Tonight...elevated thta-east advection and convergent fields on
accepted model forecasts support more elevated shower and some
thunderstorm development across southwestern Iowa into north central
and northeastern Iowa this evening...along with lift off approach of
another vorticity out of the Southern Plains. But with ongoing dry sub 700 mb
mb layer locally...could see a scenario where much of the dvn County Warning Area
stays dry through at least midnight...except maybe the far northwest
and northern County Warning Area where some decaying/weakening activity makes it in
from the west. Will adjust probability of precipitation accordingly. These trends may
persist to well after midnight until an elevated warm air advection
Wing with some more punch can try and drive some showers across the
northwestern half of the County Warning Area to the MS river through 12z Monday.
Southerly breeze and cloud debris should hold lows up in the middle to
upper 50s in most of the County Warning Area...but thinner clouds to clear/dry areas
of the northeast may get cooler.

Monday...some phasing in sluggish split flow pattern just upstream
of the area still on track to usher a frontal system through the
area from west-to-east with this feature possibly still lingering
across the far eastern County Warning Area by 00z Tuesday. Low to middle level moisture
return into this feature enough to fuel widespread showers still at
question with most models still probably overdoing low level moisture/
saturation by midday Monday. But even undercutting those amounts
some and looking at amount of forcing/lift progressing across the area
still should produce widespread sctrd showers across the County Warning Area as Monday
progresses/possibly not into the east central and southeastern County Warning Area
until afternoon/. Some cape/MUCAPE values evident as well to fuel
thunderstorms some of which may produce small hail in the afternoon.
But overall severe threat still appears limited for Monday. Will side
with a little bit of the warmer guidance values in case showers
having trouble getting going/deeper saturation holds off until
afternoon...low to middle 70s. Right now it appears bulk of County Warning Area to get
anywhere from 0.25-0.40 inches of rain by late Monday afternoon...but
of course any localized areas that manage to get under a decent
thunderstorm or have a few storms move over the same location could
get at least three quarters of an inch. On the other hand...could
see some areas missing out especially east of the MS river. ..12..



Long term...(monday night through next sunday)
issued at 320 PM CDT sun Apr 20 2014

With the cold front well to the east by evening...Monday night will
feature mostly clear skies and seasonably cool temperatures as high
pressure builds into the region from the northern plains. Will keep
token slight chance probability of precipitation for showers early in the evening over the
far east to account for possible slower frontal timing. Dry air and
near full sunshine Tuesday will lead to deep mixing and have
reflected the model trends with warmer highs in the lower to middle
60s. These highs may still be too conservative given the poor recent
verification showing a cool bias with these dry airmasses.
Likewise...dry air will favor larger than normal diurnal swings into
the overnight with lows in the middle 30s to lower 40s Tuesday night.

Wednesday through Friday...an organized surface and upper level low
is shown rolling through the upper MS valley. 12z models continue to
vary in the timing...track and strength of this feature. Confidence
remains low with the potential for showers and thunderstorms in the
warm advection regime out ahead...especially for the daylight hours
Wednesday...where models may again be overestimating the low level
moisture profiles. Will maintain chances for showers and isolated
thunderstorms for Wednesday with highs from the upper 50s to middle 60s.
These numbers would not likely be warm enough if precipitation
does not materialize. Thunderstorm chances look much greater for
Wednesday night on the nose of a strong low level jet and advancing
Gulf moisture...and then also for Thursday where the model consensus
has the primary system pushing through. Likely wording for
thunderstorms follows this progression...mainly in the west
overnight and then over the entire area Thursday. Friday...the European model (ecmwf)
and GFS have the surface low exiting over the upper Great Lakes...
which would lead to a breezy and dry day with near normal
temperatures. However...with a potential for the low to hang back as
advertised by the outlier Canadian model...slight chances for showers
are carried through the day across the north.

Next weekend remains a low confidence period due to large model
differences. The European model (ecmwf) has the next strong system moving in from the
southwest with widespread moderate rain likely as early as
Sunday...while the GFS keeps a ridge overhead. This keeps a mention
of rain for especially Sunday with temperatures likely to be well
below normal either way. 11

&&

Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1239 PM CDT sun Apr 20 2014

Will bank on elevated showers and some thunderstorms remaining to
the north of Cid and dbq through this evening in area of some
thta-east convergence and middle layer instability. Otherwise VFR
conditions until late tonight when some MVFR fog of 4-6sm may form
after 3 am CDT in vicinity of the taf sites. A few showers ahead of
incoming frontal system may get in the vicinity of Cid and dbq after
2-3am CDT as well. Sctrd showers and a few thunderstorms to become
more widespread Monday morning ahead of slow incoming frontal
passage. Some ceilings lowering to MVFR as well...along with ongoing
4-6sm visibilities with fog and showers Monday morning. South to southwest
winds of 10-16 kts and higher gusts this afternoon to become
lighter tonight into early Monday morning. ..12..

&&

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

$$

Synopsis...12
short term...12
long term...11
aviation...12